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Condenser and Circulating Water System

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Condenser and Circulating Water System Powered By Docstoc
					Condenser and Circulating
     Water System




             G. V. HARSHE
                   B.E. (Mech)
            Dy. Director, NPTI (WR)
                   NAGPUR




NATIONAL POWER TRAINING INSTITUTE
    (Ministry of Power, Government of India)
            NAGPUR - 440 022, India



                                 MARCH, 2004
                                 INDEX


CHAPTER NO.                          TOPIC            PAGE NO.
CHAPTER 1     THE CONDENSER                             01
1.1           Introduction                              01
1.2           Principle of a Condenser                  01
1.3           Purpose of Condenser                      02
1.4           Functions of Condenser                    03
1.5           Classification of Condenser               04
1.6           Theory and Arrangement of Condenser       08


CHAPTER 2     DESIGN AND CONSTRUCTIONAL FEATURES OF     10
              CONDENSERS
2.1           Introduction                              10
2.2           Construction of the Condenser             12
2.3           Russian Design Condenser                  17
2.4           Condensers for KWU Machines               20


CHAPTER 3     CONDENSER PERFORMANCE                     27
3.1           Introduction                              27
3.2           Pressure Condition in the Condenser       27
3.3           Effect of Varying the Back Pressure       28
3.4           Effect of Air on Back Pressure            29
3.5           Scale Formation                           30
3.6           Condenser Efficiency And Performance      31
3.7           Case Study                                33


CHAPTER 4     METHODS OF IMPROVING CONDENSER            39
              PERFORMANCE
4.1           Introduction                              39
4.2           Condenser Cleaning                        40
4.3           On Load Cleaning                          41
4.4           Condenser Leaks                           42
CHAPTER 5   CIRCULATING WATER SYSTEM        50
5.1         Introduction                    50
5.2         Types of C.W. System            51
5.3         Design Aspects of Major Items   54
5.4         Chlorination                    65
5.5         Circulating Water Pumps         65


            GLOSSARY                        72


            REFERENCES                      73
                                    Chapter 1
                            THE CONDENSER
1.1     INTRODUCTION

        The steam condenser plays a measure role in all modern thermal power
stations. The main purpose of condenser is to condense the exhausted steam
from low pressure turbine and convert it in to water i.e. condensate, so that it
could be reused in Boiler as feedwater. The condensing equipment plays the role
of cold source in the thermo-dynamic cycle of steam turbine installation. The
decrease of temperature of cold source increase the thermal efficiency of cycle.

        The increase of economy of steam turbine installation is affected by two
major factors. Firstly achieving with high thermal efficiency of the cycle itself and
secondly by operational perfection of different components of the set. The former
is affected by increase of initial parameter as well as decrease of back pressure of
steam. The increase of initial parameter has been a fashion and pressure of 300
atmospheres and temperature upto 5650C have already been achieved in western
countries but the main obstacle in this direction is suitable and cheap heat
resistant steels. With decrease of back pressure, the temperature of exhaust
steam is decreased which in turn decreases the quantity of heat given to the cold
source and thereby increasing the thermal efficiency.

        The above goal is achieved by condensing the exhaust steam from the
turbine in the condenser.

1.2     PRINCIPLE OF A CONDENSER

1.2.1   Volume of Steam

       If water is put into a closed vessel and heated, a quantity of heat known as
sensible heat is required to bring the water to boiling point and if further heat is
added to convert the water into steam this is known as latent heat.

       The volume of the steam formed is far greater than that of the water and
consequently the pressure in the vessel rises. thus the application of the latent
heat has caused an increase in pressure. (See Figure 1.1 and 1.2).

1.2.2   Removal of Heat

       Now reverse the process and remove some heat by cooling the vessel.
During this cooling the latent heat is removed from the steam which is reduced to
water (or condensed) with a consequent fall in pressure. (see figure 1.2)

      This removal of latent heat happens on a very large scale in a turbine
condenser.




                                         [1]
      Fig. 1.1 : Heat increases pressure     Fig. 1.2 : Cooling decreases pressure

1.2.3     Condenser Pressure

        The condenser is an airtight vessel where the steam exhausted from the
turbine is cooled and condensed. The condensation is so complete that the
pressure inside the condenser is reduced below that of the atmosphere and this
condition is referred to as the vacuum in the condenser.

       To maintain this low pressure condition it is essential that any air or other
incondensible gases, passing in to the condenser with the steam must be
continuously removed and, in addition to condensing the steam, the condenser
must separate, these gases from the steam for discharge by an ejector or air
pump.

1.3       PURPOSE OF CONDENSER

i)        To create and maintain vacuum in the turbine :

        By using a condenser it is possible to get higher output and increase
thermal efficiency of the cycle. Due to condenser it is possible to expand the
steam in turbine to negative pressure or vacuum so the work out-put of turbine
increases. Also due to creation of vacuum in condenser steam flow from last
stage of turbine to condenser is established. In fact it provides steady operation
of turbine maintaining stable conditions at turbine back end.

ii)       Conservation of Pure feed water :

       Very large quantities of steam pass through a turbine, it would of course,
be not only very wasteful but impracticable to allow this vast amount of steam to

                                           [2]
be exhausted to atmosphere. By using condenser the exhaust steam is converted
to water which is removed from the condenser for continuous use in the power
station heat cycle.

iii)   Deaeration of Make-up water

        Due to leakage and necessary blowing down of boilers some of the water
used in the power station heat cycle is lost and must be replaced. This water,
which is known as make-up water, is generally supplied from reserve feed water
tanks and, being in contact with the air, contains dissolved oxygen. If this oxygen
were not removed it would cause corrosion in boilers and pipework. The best way
of releasing this oxygen is to bring the water to boiling point and for this purpose
the condenser can be employed. The make-up water is introduced into the
condenser where it is brought to boiling point and the dissolved oxygen released
ready for removal together with any air and other gases which may be in the
condenser.

1.4    FUNCTIONS OF CONDENSER

       The primary function of the condenser are

i)     To provide the lowest economic heat rejection temperature for the steam
       cycle.
ii)    To convert the exhaust steam to water for reuse in feed cycle.
iii)   To collect the useful residual heat from the drains of the turbine feed
       heating plant and other auxiliaries.

        The aim of the C.W. system is to maintain a supply of cooling medium to
extract the necessary heat, in order that the condensing plant can meet it’s
objectives. It achieves this by the use of effective screening equipments
circulating water pumps, valves and cooling towers (where necessary).

      In addition to the condenser satisfying the primary function, it’s design
must be capable of meeting the following objectives :

i)     To provide the turbine with the most economic back pressure consistent
       with the seasonal variations in C.W. temperature or the heat sink
       temperature of the C.W. system.
ii)    To effectively prevent chemical contamination of the condensate either
       from C.W. leakage or from inadequate steam space gas removal and
       condensate dearation.

      The aim of the design is to ensure that these objectives are met within the
framework of the following practical consideration :

i)     Economies of size, space, and pumping power.
ii)    Ease of maintenance and construction.




                                        [3]
1.5     CLASSIFICATION OF CONDENSER

        Condenser is classified (1) Based on heat rejection (2) According to
position or arrangement (3) According to cooling water flow.

1.5.1   Based on Heat rejection :

      Steam can be condensed by using either the jet or surface type
condenser.

Jet Condenser

        The simplest method is to mix the steam with a spray of water in a closed
vessel. The water will remove the heat from the steam by direct contact and the
steam will condense. This method is used in the jet condenser which is illustrated
n figure 1.3

        In a power station the condensate is returned to the boiler and must be
absolutely pure. If a jet condenser were used the cooling water, which is mixed
with the condensate would have to be equally pure. Because very large quantities
of cooling water are required, this type of condenser is not a practical proposition
for power plant.




                             Fig. 1.3 Jet Condenser
Surface Condenser
       Where water is available in large quantities it is usually very impure, for
example, sea water and river water, but such impurities have little effect upon its
cooling properties. This suggests a condenser with two entirely separate water

                                        [4]
system, steam being condensed on the outside of surfaces which are kept cool by
an abundant supply of water flowing on the inside.

        Such an arrangement is known as a surface condenser and the cooling
surfaces consists of small diameter tubes as shown in figure 1.4. In this case the
purity of the cooling water does not matter because apart from any leakages
which may occur it is never in contact with the condensate.




                          Fig. 1.4 Surface Condenser

1.5.2   Based on Positions or Arrangement :

       According to the position condenser are classified as : (1) Undenslung
Axial or (2) Transverse condenser. (3) Integral condenser (4) Pannier or side
mounted condenser.

Underslung -Axial or Transverse Condenser :

      In this condensers are mounted below the L.P. Turbine. The condenser
may be located axially w.r. to turbine shaft. In some machines condenses are
mounted under the turbine at right angles to the turbine shafts (See fig. 1.5).




                                       [5]
             Fig. 1.5 Underslug -Axial or Transverse Condenser

Integral Condenser :

      In this condensers are arranged around the L.P. Turbine cylinders as
shown in fig. 1.6




                         Fig. 1.6 Integral condenser.

Pannier Condenser :

      In this condenser are arranged at each side of the L.P. turbine cylinder,
known as Pannier condenser (See the fig. 1.7).




                Fig. 1.7 Pannier or side Mounted condenser.

                                      [6]
1.5.3   Based on cooling water flow :

       As per the cooling or circulating water flow condenser can be classified as
(1) Single flow (Single pass) (2) Double flow (Double pass) (3) Three pass
condenser.

         When the cooling water makes only one journey across, this is known as
a single pass condenser (Fig. 1.8b). If the cooling water makes two journeys then
it is known as a double pass condenser. (Figure 1.8b). In this case the water in
the bottom half of the tubes will be flowing from front to back and in the top half
from back to front. Fig. 1.8b shows a method of venting for a 2 pass condenser.
A 3" air vent is fitted to each shell from the highest point on the return water box.
The air is vented to the cooling water outlet main and prevents air bubbles in the
second pass. The inlet pass is protected by drilling say 4 one inch holes in the
inlet water box divisional wall between passes 'x'. More emphasis is given on
single pass and double pass condenses in the next chapter.

        Similarly with a three pass condenser the water makes three journeys
across. A single pass design which gives a long narrow condenser, suits the large
modern turbines and can be mounted axially under the machine in line with the
turbine shifts. The steam distribution is line with the turbine shafts. The steam
distribution is not as good in a single pass as in two pass condenser.




                  Fig 1.8 Single and Double Pass Condenser.
                                         [7]
1.6     THEORY AND ARRANGEMENT OF CONDENSERS

1.6.1   Exhaust Steam From Turbine

        The exhaust steam from the last stage of the turbine has a higher
temperature than the cooling water in the tubes. Hence, there will be a flow of
heat from the steam to the water. The steam, when it flows from the turbine to the
condenser is already at saturation temperature and, therefore, all the heat
removed by the condenser is latent heat.

1.6.2   Heat Transfers Through Tubes

        Heat transfer through the tubes is complicated by the fact that on the
inside and outside surfaces there are various films which offer a resistance to the
flow of heat.

         Consider part of a tube surface as shown in Figure 1.9 on the outside of
this condensate film there may sometimes be a film of air and gas. These two
films are continually swept off the tube by the rush of steam. The condensate film
soon forms again but the air and gas film takes longer. On the inside of the tube
there is a stationary film of cooling water that remains on the metal surface even
though there is a flow through the tube. In addition the tube metal offers a very
slight resistance to the flow of heat through it.

        The individual resistances of the various films and the tube metal combine
to give one total resistance. The values of the various resistances can be
calculated and it is found that the resistance of the tube metal is very small
compared with the others. If it were possible to go from cooling water to steam.
Measuring the temperatures at the boundaries of the various films. Figures similar
to those shown in figure 1.10 would be obtained. This shows that to overcome
resistance to heat flow, a difference in temperature is needed and the greater the
resistance the greater must be the temperature difference.




                   Fig 1.9 : Films on condenser tube surface




                                        [8]
         Fig 1.10 Temperature Difference across condenser tube wall

1.6.3   Terminal Temperature Difference (TTD)

        The temperature difference between the exhaust steam and the cooling
water is least at the top of the condenser where the cooling water leaves. Here the
cooling water has its highest temperature. This particular temperature difference
is very important and is given a special name. It is called the terminal temperature
difference. The important point is that any increase in this terminal difference
leads directly to increase in the saturation temperature of the exhaust steam and
a higher back pressure.




                                        [9]
                                    Chapter 2
         DESIGN AND CONSTRUCTIONAL FEATURES OF
                      CONDENSERS
2.1       INTRODUCTION

        In most of the thermal power stations, surface condenser either single flow
or double flow are used, hence this chapter deals with constructional details and
other features of surface condensers.

        Surface condenser is the cold end in the thermodynamic cycle of a steam
turbine installation. The main purpose of the condenser is :

     -       To create high vacuum, thus allowing maximum heat drop in the
             turbine and thereby improving the efficiency of the cycle.
     -       To re-use the pure condensate which is at higher temperature than the
             ambient temperature thus minimizing the cost of generating steam to
             required parameters. Mostly condensers can be divided into following
             two categories :

a)        Single Pass Condenser : When cooling water enters through front water
          box and comes out through the rear water box, there being no reversal of
          water flow, it is said to be single pass condenser. See Fig. 2.1.




                          Fig. 2.1 Single Pass Condenser


                                         [10]
                        Fig. 2.2 Double Pass Condenser

b)     Double Pass Condenser : In double pass condenser water enters
       through the front water box and flows through the half of the total tubes to
       rear water box and then through the other half of the tubes back to the
       other half of the front water box and then to the outlet pipe. See Fig. 2.2 of
       a 210 MW KWU type condenser.

       The end views of above two types of condensers are shown in fig. 2.3 and
2.4 respectively.




       Fig. 2.3 End view of                         Fig. 2.4 End View of
     Single Pass Condenser                        Double Pass Condenser
                                        [11]
       The prime factors that need to be considered for developing a design of
the condensers are :

   -       Quality of cooling water, type of C.W. system.
   -       Cooling water inlet temperature.
   -       Space available in turbine foundation.

2.2     CONSTRUCTION OF THE CONDENSER

2.2.1   General Arrangement :

        In general appearance the surface condenser has not changed a great
deal from earlier designs, but operating experience has led to may improvements,
particularly with the tube arrangements. Fig. 2.5 shows a typical condenser
indicates names of the various parts.




                           Fig. 2.5 A typical condenser

2.2.2   Condenser Shell

       Shells are of welded steel construction and are ribbed externally and
braced internally to ensure that they are rigid. Where turbines have two exhausts,
separate shells may be connected to each and joined by a balance pipe.

        The condenser of a modern machine is so large that it could not be
transported from the works as a single component. The main shell is fabricated of
mild steel plate in sections which are joined together for final assembly at site by
welding. It is possible to subdivide a single shell so that the water circuit is in two


                                          [12]
parts. One half of the tubes can then be cleaned whilst the other half is in
commission.

2.2.3   Tube Configuration :

        Tube layout plays very important role in condenser performance. In a 210
MW Russian condenser it is basically finger type (See Fig. 2.6) which provides
uniform steam distribution. This arrangement gives a lower pressure drop and at
the same time provides an effective means of removing condensate from the
collecting plates along the support plates, thus avoiding dripping of condensate on
next row of tubes and as a result improving the heat transfer. The design
provides a conveying steam passage and an efficient air removal system. To
avoid under cooling a portion of steam constantly sweeps the condensate surface.




   Fig. 2.6 Layout of tubes for 200/210 M.W. Condenser (Russion design)

2.2.4   Differential expansion between tubes and shell :

        To cater for differential thermal expansion between shell and tubes a
carbon steel bellow is provided in the shell. This reduces the stress on tube to
tube sheet joint. Where the expansion bellow is not provided tube to tube sheet
joint is to be thoroughly analyzed for the stresses which could develop during
operation.
                                        [13]
2.2.5   Tube to tube sheet joint :

       Usual method for making tube to tube sheet joint is roller expansion (See
Fig. 2.7). Quality of expansion determines the seepage of cooling water into
steam space which has bearing on scaling of boiler tubes, steam purity and salt
deposits on turbine blades which effects the life of boiler and efficiency of the
plant.




                          Fig. 2.7 Tube expansion joint

       It is, therefore, necessary to ensure very high quality of expansion. The
holes should have good finish and minimum ovality. The expansion should be
carried out by torque controlled expanders. The expanders should be set to
achieve approx. 7 to 10% wall thinning. Before carrying out expansion, retaining
beams should be used for large condensers and predetermined sequence of
expansion should be followed to prevent buckling of the tube sheet during
expansion.

         The other methods of fastening the tubes to tube sheet are by providing
ferrules and packers. This method is costlier in comparison to roller expansion
and used for small condensers where tube replacement is considered frequent
(like in marine services).

2.2.6   Size of tubes :

         The tubes generally used for industrial condensers are 19 mm outer
diameter. For large condensers tube size is 22 mm or 25.4 mm outer diameter.
Sometimes 30 mm tubes have also been used. Bigger size of tubes involve less
drilling work and therefore less manufacturing cost. However, condenser is
compact if smaller size tubes are used.


                                       [14]
2.2.7    Floating of condensers on springs :

       The method of floating of condenser depends upon the allowable thrust
which can be allowed on turbine foundation. If condenser is floated for its dry
weight, the foundation is subjected to downward load because of operational
weight of water. At the same time there is an upward thrust because of thermal
expansion of condenser.

      In case condenser is floated for its working weight, it is subjected to
upward thrust because of thermal expansion. Also upward thrust comes when
condenser is dry.

       All these forces need careful analysis before the floating of condenser is
decided.

2.2.8    Types of condenser supports :

Usually condenser is supported by one of the following two ways:

   i)       Spring supports: Condenser is supported on springs which allow for
            movement of condenser during operating because of thermal
            expansion /vibration. There are two types of spring supports as shown
            in the fig. 2.8 i.e. Rod type and continuous type.
   ii)      Solid supports : The condenser supports are fixed with respect to
            vertical axis. Provisions are made in condenser supports to allow for
            differential thermal expansion in horizontal plane.         For vertical
            movements a flexible joint is provided in the super-structure. This joint
            is usually of rubber having shape like a dog bone. A stainless steel
            expansion bellow is also used in place of rubber expansion joint.




               Fig. 2.8 Condenser support - spring arrangement

                                         [15]
2.2.9   Spacing of support plates and effect of tubes hole clearance :

        The support plates in condenser are to be located to avoid vibration of
tubes for all operating conditions including HP-LP bypass. HEI standard gives the
method of calculation of the spacing between support plates. The amplitude of
vibration depends upon the clearance between tubes and the hole. Large
clearance also might change nodal points and frequency of vibration.

      With excessive amplitude of vibration the tubes strike against each other
and subsequently fail besides creating noise. The clearance should therefore be
minimum (approx 0.5mm on diameter) but adequate to assemble the tubes.

2.2.10 Self draining of tubes :

        To avoid pitting corrosion during long outage of condenser it is necessary
to have the tubes which are self draining. Basically following two methods are
available :

i)      Bowing the tubes :

       The holes of support plates are staggered such that the tubes are slightly
bowed with highest point at the centre. This is for circular shell which has
roundish configuration. For rectangular shell packing has to be given of off set the
support plates. See Fig. 2.9 for bowing of tubes in 210 MW Russian condenser.




                             Fig. 2.9 Bowing of tubes

ii)     Inclination of condenser :

        The supports are in such a way that condenser as a whole rests in an
inclined position nearly ½0.

      Bowing of the tubes is simpler and also results in uniform loading of
condenser supports.



                                        [16]
         Inclination of the condenser results in non-uniform loading of supports
because of shift of centre of gravity. This could create an undesirable couple
acting on turbine. The depth of water in hotwell is also not uniform. Besides it
has little more problems during manufacturing, as maintaining ½ 0 inclination of
certain parts during fabrication is rather difficult.

2.2.11 Water Box Profile :

         The water boxes should be designed to ensure even distribution of cooling
water and at the same time to keep the pressure drop to a minimum. To achieve
this, diverging inlet sections have been provided in our design. A suitable depth of
water boxes has been provided to enable even water distribution.

2.2.12 Design of Exhaust hood and Expansion below

        The exhaust hood design has ensured streamlined flow and even
distribution of exhaust steam throughout the tube length, minimizing the entry loss
to condenser.

       The location of expansion bellow on C W pipes and their layout should be
care fully decided. Erroneous design of c w pipes and bellow orientation could
lead to undesirable thrust on turbine effecting its stability and hence
performance/availability.

2.2.13 Priming of cooling water side:

        Generally priming is done by having 25 mm gas pip bore at the top of rear
water box. But a priming ejector provided on water box to evacuate water box
and spaces at the inner bores of the tubes is a better proposition. This helps in
charging the water box fully with water and results in uniform distribution of water
within the tubes as air locks are eliminated.

2.3    RUSSIAN DESIGN CONDENSER

Constructional features :

        The condenser is of welded construction having two condensers each
connected with the exhaust hood of the turbine (see fig 2.10 & 2.11 for
longitudinal & end view of condenser). These two condensers have been inter-
connected by a bypass piping which permits cleaning and repair of one condenser
at reduced load.

       Each condenser has been subdivided into upper and lower halves. The
front water box shell, and rear water box constitute the lower half. Two end tube
plates and six support plates are located inside the lower body of the condenser.
The front water box has been divided into two parts to make the condenser two
pass design. The covers of water boxes can be removed for facilitating repairs
and replacement of tubes. Manholes have been provided for routine maintenance
and visual inspection along with venting and draining arrangement for individual
water box.


                                        [17]
        Condenser tubes are secured to the end tube plates by expanding and
flaring of tube ends which provides very good sealing arrangement against
penetration of circulating water into steam space.




                 Fig. 2.10 Longitudinal view of condenser




                     Fig. 2.11 End view of condenser

                                    [18]
       A steam dumping device is provided in both the condensers which helps in
condensation of steam from turbine bypass system. It is also useful during the
turbine start-up, load variation, etc. The condenser has been divided into a
number of parts for ease of manufacture, transportation and erection.

        In order to allow for expansion along the height, the condenser is
supported on springs specially designed to take its load. With this arrangement,
the weight of the empty condenser is taken up by springs, while loads due to
weight of circulating water and thrust of springs during expansion are borne by the
turbine foundation.

Important materials for condenser :

i)     Shell –mild steel plates – IS 226/2062.
ii)    Water box & end covers – mild steel – IS 226/2062.
iii)   Tube sheet-mild steel IS 226/2062 for soft water and naval brass for sea
       and brackish water.
iv)    Support plates – mild steel – IS 226/2062.

      In case of sea water or brackish water the water box has to be lined with
neoprene rubber lining or fibre glass lining.

Tube material :

i)     Admirality brass BS 2871 –Alloy CZ110 for soft water.
ii)    Aluminium brass BS 2871 –CZ111 for sea water and not so clean soft
       water from river, pond etc. Highly resistant to chloride attack.
iii)   Cupro nickel alloys – for polluted water. 90-10 or 70-30.
       Higher resistance to corrosion and pitting in operating and stagnant
       condition. Prone to corrosion (pitting) at lower cooling water velocity.
iv)    Stainless steel for highly corrosive and dirty water. Heat transfer co-
       efficient is less. Prone to pitting in operation at lower C W velocity and in
       stagnant condition. Chloride level in water should be below 250 ppm
v)     Titanium – Used when chloride level is higher than 250 ppm. Suitable for
       very corrosive, erosive and toxic water.

Composition of various tube materials :

1.     Admirality brass
       Copper                70%
       Arsenic               0.4%
       Tin                   1.25%
       Zinc                  rest
2.     Aluminium Brass
       Copper                76%
       Arsenic               0.4%
       Aluminium             2%
       Zinc                  rest


                                        [19]
3.      Cupro-nickel alloys :
                90 - 10                               70 - 30
        Nickel                  10%            Nickel              30%
        Iron                    1%             Iron                3%
        Manganese               1%             Manganese           3%
        Copper                  Rest           Copper              Rest

2.4     CONDENSERS FOR KWU MACHINES

         The function of the condenser is to condense the steam exhausted from
the L.P. cylinders and to produce and maintain as high a vacuum as possible in
order to increase the heat drop which can be utilized in the turbine. Depending on
local circulating water conditions, the vacuum is of the order of 0.03 to 0.121 ata.

Design Features :

       This condenser is a box – type surface condenser with a divided circulating
water system. The steam space is of rectangular cross-section to achieve
optimum utilization of the enclosed volume for the necessary condensing surface.
The constructional features are shown in Fig. 2.1 to 2.4. However, a typical cross-
sectional view of LPC and condenser joined together is shown in Fig. 2.12.




      Fig. 2.12 Section through L.P. turbine condensing unit (KWU design)
                                        [20]
Tube Spacing :

        The principal factors to be taken into consideration when determining the
tube spacing are low steam velocity between the condenser tubes, uniform
distribution of the steam over the whole condensing surface and equal pressure at
the top and bottom rows of tubes. A computerised design is used to optimize
tube spacing, tube cross section and condensing surface of the condenser.

        The tube spacing is broken up to a large extent by the arrangement of the
condenser tubes in bundle. Wide lanes from top to bottom are left between the
tube bundles so that the steam can also reach the lower rows of tubes without
incurring appreciable pressure loss. The steam then flows sideways from these
lanes into the tube bundles.

        The downward-leading steam lanes are inclined to the vertical. In few of
the lanes, condensate drain. Sheet running to half of the length of the steam
space are fitted to prevent condensate from the upper tubes dripping on to the
lower tubes and causing a film of condensate giving rise to sub-cooling. The
collected condensate passes to the condenser hotwell, A typical inclined tube
layout is shown in fig. 2.13.




                 Fig. 2.13 Condenser tube spacing (right half)
                                       [21]
       The necessary sub-cooling of the steam/gas mixture for air extraction from
the condenser is performed by two tube nests arranged at the centre arranged at
the centre, Because of their function, these two tube nests are shielded from the
steam flow except for an aperture at the bottom.

      Of late some improvements have been made and vertical type of lay-outs
as shown in fig. 2.14 are being used in 500 MW condenser designs.




          Fig. 2.14 Tube layout-vertical type for 500 MW condenser

Condenser Support :

        The condenser is supported on springs which allows free movement of
condenser during operation because of thermal expansion. Two spring elements
below each end of the tube support plate are arranged to support the condenser
uniformly through its length. Each spring element can have one, two or even three
springs depending upon the loading of condenser. Please see Fig. 2.15 for a
similar type of condenser mounting arrangement on springs.

Tube to Tube Sheet :

       The tubes are the most heavily stressed element of the condenser.
Consequently utmost care is taken in their fitting, method of support and fixing.
The tubes are expanded into tube plates of steel or brass. There is no danger of
leaks developing at the expanded joints due to differential thermal expansion
between the tubes and steam shell because the force which it is possible to
develop in the tube by the maximum temperature difference reached during
operation.

        Inside the steam shell the condenser tubes are supported at intervals by
support plates. These plates prevent any vibration of the tubes induced by the
flow of steam.
                                       [22]
             Fig. 2.15 Spring support for KWU design condenser

Sizes of Tubes :

        Different sizes of tubes have been used in various condensers designed
so far for KWU machines due to various reasons. These are as follows :

1.   Hasdeo T.P.S. – Korba            -           25.4   x 1.245   L = 6400
2.   NTPC Korba Condenser             -           19     x1        L = 9900
3.   Raichur T.P.S. Condenser         -           25.4   x 1.245   L = 10100
4.   Unchahar T.P.S. Condenser        -           25.4   x 1.245   L = 7600

Self Draining of Tubes :

        Most of the condensers have been installed with ½0 inclination for proper
draining of the tubes during long outages of the condenser. Korba hasdeo double
pass, Korba NTPC single pass and Unchahar double pass condensers have been
developed based on this concept while in Raichur condenser, like Russian design,
the tubes have been bowed upward by gradual lifting at the intermediate supports
so that these are automatically drained during shut downs of the condensers.

Water Box Profiles :

        There water boxes should be designed to ensure even distribution of
cooling water and also to keep the pressure drop to a minimum. Except Raichur
all the condensers for 210 MW KWU machines have been provided with domed
head. These domed shape water boxes are specifically suited for on-load wall
cleaning arrangement which has been provided with few of such condensers. The
condenser at Raichur Thermal power station has got rectangular water box with
flat covers in line with Russian Design.



Expansion Bellow on C.W. Side :



                                       [23]
        As per normal recommendations, two bellows one in horizontal and one in
vertical run on C.W. piping are provided so that there is no upward thrust on the
turbine. Normally this recommendation is given to either consultant or customer
since this piping in most of the cases is not BHEL scope.

Drains Inlet :

       The drains from L.P. feed heaters, turbine drains etc. are fed into the
condenser hotwell via flash vessels. The flash steam produced in the flash
vessels by the reduction in pressure flows into the condenser steam dome
through connecting pipes.

By-Pass Steam Inlet :

        During by-pass operation the initial steam is discharged directly into the
condenser via the combined by-pass valves. The necessary by-pass steam inlet
connections (steam throw-device-see Fig. 2.16) are welded into the condenser
dome wall. Each connection incorporates an orifice plate (diaphragm forging)
which reduces to approximately condenser pressure the steam which has already
been partially reduced in pressure; and by means of injected condensate cools it
to saturation temperature.




                          Fig. 2.16 Steam throw device

Hot Well :

       The condensate produced in the condenser and the drains entering via the
flash vessels collects in the condenser hotwell from where it passes to the
condensate pump. The level of condensate in the hotwell is regulated by a control
system according to the requirements of the plant. Special provision is made to
maintain the minimum suction head for the condensate pump. The hotwells of
condensers in large plants are normally divided. There is a partition in the middle
and outlet branches on each side. The purpose is to separate the condensate
produced by each half of he condenser nest for better identification of tube-leaking
zone. Conductivity measuring devices are fitted in each condensate outlet from


                                        [24]
the hotwell to give a warning of any leakage of circulating water into the
condenser. The affected half of the condenser can then be shut down.

        The condensers have been designed to take 60-100% dumped steam
during by-pass operation which consequently reduces the time of hot start of the
TG set and also prevents loss of condensate by preventing the blowing of the
safety valves.

        Substantial amount of site-assembly erection work is carried out at site in
these condensers as most of the individual components like bottom plate water
boxes, dome walls and side walls etc. are fabricated in the shop and dispatched
loose for assembly at site due to transport limitations.

       Few significant technical parameters of these KWU type of condensers are
reproduced below :

A.      SINGLE PASS CONDENSER :

-    Total surface area                          -   10232 M2
-    Total number of tubes                       -   17492
-    Length of tubes                             -   9900 mm
-    Size of tubes                               -   dia 19x1 mm
-    Tubes material
     a)     Condensing zone                      -   90/10 Cu. Ni.
     b)     Air cooling zone                     -   90/10 Cu. Ni. Or stainless steel
-    CW quantity required                        -   28570 M3/hr.
-    Weight of condenser including tubes         -   275 Tonnes Approx.
-    Length of condenser across turbine axis     -   13.5 M Approx.
-    Height of condenser                         -   8.5 M Approx.
-    Width of condenser                          -   6.7 M Approx.

B. DOUBLE PASS CONDENSER :

-    Total surface area                          -   9656 M2.
-    Total number of tubes                       -   19208
-    Length of tube                              -   6400 mm
-    Size of tubes                               -   dia 25.4 x 1.245 mm
-    Tube material
     a)     Condensing zone                      -   Al. brass or Adm. Brass
     b)     Air cooling zone                     -   70/30 Cu. Ni.
-    CW quantity required                        -   28500 M3/hrs.
-    Weight of condenser including tubes         -   310 Tonnes
-    Length of condenser across turbine axis     -   10.0 M Approx.
-    Height of condenser                         -   10.0 M Approx.
-    Width of condenser                          -   6.7 M Approx.




C. UNCHAHAR T.P.S. DOUBLE PASS CONDENSER :

                                        [25]
-   Surface area                               -   11495 M2.
-   No. of tubes                               -   19208
-   Length of tubes                            -   7600 mm
-   Size of tubes                              -    25.4 x 1.245 mm
-   Tube material (Condensing and Air          -   90/10 Cu. Ni.
    Cooling zones)
-   C.W. quantity                              - 27000 M3/hr
-   Weight of condenser                        - 345 Tonnes

D. RAICHUR T.P.S. DOUBLE PASS CONDENSER :

-   Surface area                               -   13845 M2.
-   No. of tubes                               -   17350
-   Length of tubes                            -   10100 mm
-   Size of tubes                              -    25.4 x 1.245 mm
-   Tube material (Condensing and Air          -   90/10 Cu. Ni.
    Cooling zones)
-   C.W. quantity                              - 27000 M3/hr
-   Weight of condenser                        - 460 Tonnes Approx




                                        [26]
                                    Chapter 3
                   CONDENSER PERFORMANCE
3.1    INTRODUCTION

        With the increasing size of the power plant more attention is being focused
upon the design of the exhaust end of the machine. The main control parameter
is the back pressure measured at the turbine exhaust since any deviation in this
directly affects the heat rate of the machine.

        The main factor in a good condenser performance is the maintenance of
the most efficient value of back pressure possible with the available cooling water
condition. Even the slightest increase of the back pressure has a considerable
effect on the heat consumption of the turbine.

3.2    PRESSURE CONDITION IN THE CONDENSER

        The low pressure in the condenser is induced mainly by the conversion of
steam to water, the latent heat of steam is being transferred to the cooling water
through the tubes. The lowest pressure will be at the bottom of the condenser
because there is very little steam at this point, most of it having been converted in
to water. The true condenser pressure is therefore taken as that at the bottom of
the condenser. The pressure at the top of the condenser must be greater then
that at the bottom by an amount necessary to provide a downward flow of the
steam.

       The difference in pressure, from the exhaust steam inlet at the top, to the
condensate take off at the bottom is known as the condenser pressure drop. It is
very important that the pressure drop is kept as low as possible because any
increase means a higher pressure at the top of the condenser and a higher back
pressure against the turbine. There is also a direct heat loss with greater
pressure drop as the condensate has a temperature lower than that of the exhaust
steam in proportion to the amount of pressure drop.

       If a condenser is so designed or operated that after condensing the steam,
the condensate is cooled below saturation temperature, then under cooling said to
be taking place. This causes a direct heat loss and can take place in most
condensers in winter when the temperature of the cooling water is low.

       To illustrate the important contribution made to the work done by
operating at vacuum, consider fig. 3.1. Steam is admitted to the turbine at the
pressure of 11 bar absolute as shown by P1. The volume of the steam is 0.177
m3/Kg. If after expansion in the turbine, it is rejected at a pressure P2 of 1 bar
absolute the volume will become 1.7 m3/Kg and work done will be represented by
the area under the cure between the limits shown by P1 and P2.




                                         [27]
        Fig. 3.1 Pressure Vs. Specific volume for Dry Saturated Steam

       If now the final pressure is reduced to 0.5 bar absolute the expansion will
continue to P3 and volume will be 3.3 m3/Kg. The extra work obtained per Kg of
steam is represented by shaded area. To achieve a comparable amount of extra
work at the inlet to the turbine, the stream pressure would have to be lifted from
P1 to P4, i.e. from 11 to 17.5 bars as shown by the cross-hatched area. So even
small change in the back pressure can cause considerable changes in work done
per Kg of steam. So, it is important for efficient operation of a unit that is back
pressure is always maintained at optimum level.

The factors which affect the back pressure of condenser or responsible for low
vacuum are :

       i)     Air ingress through leaking joints .
       ii)    Insufficient cooling water flow.
       iii)   Fouling of cooling water tubes of condenser.
       iv)    Malfunctioning of vacuum-pulling
       v)     Excessive thermal loading of condenser due to leaking drain valves
              and HP/LP bypass.

3.3    EFFECT OF VARYING THE BACK PRESSURE

        A large amount of the extra work is done by the team, when the back
pressure is reduced. However, the trouble is that as the back pressure improves
certain losses increase.

Those are mainly :
      1)     CW Pumping Power.
      2)     Leaving losses.
      3)     Reduced condensate Temperature.
      4)     Wetness of the steam.


                                        [28]
3.3.1   Increased CW Pumping Power

        Assuming that the CW inlet temperature is low enough, the back pressure
can be reduced by putting more and more CW through condenser tubes.
However, this will require more CW pumping power and the gain from improved
back pressure must be offset against extra power absorbed by the pumps. So
the CW pumps should be run only when the cost of running the pump is equal to,
or less than the gain in output from the machine.

3.3.2   Increasing leaving loss

        The steam leaves the last row at a velocity which depends upon the
conditions prevailing at the point. As this velocity is not utilized usefully, it is
represents a loss of possible work known as the leaving loss. So velocity steam
through fixed annulus is must also double. But leaving losses varies as square of
the velocity. So it will increase four times.

3.3.3   Reduced condensate temperature / increased bled steam

       The condensate in the condenser is at saturation temperature
corresponding to the back pressure. It back pressure is reduced, saturation
temperature will drop. When it enters first LP heaters it will be cooler than before
consequently more steam will automatically be bled to the heater. The extra
steam is no longer available to do work in the turbine will be deprived of some
work.

3.3.4   Increase wetness of the steam

        The lower the back pressure, the greater the wetness of steam. The extra
moisture could result in damage to the moving blade. Also with increased
wetness, volume of steam is reduced water droplets being heavier than steam
moves slowly. So the front edge of moving blades have to push the droplets out
of the way. This can cause damage to blades. Therefore, it is usual to fit satellite
erosion shields to the leading edge to reduce this damage. As a rough guide, it
can be assumed that every 1% wetness will reduce efficiency of associated stage
by 1%.

       The reduction in back pressure will result in net improvement in heat
consumption until a point is reached beyond which benefit due to improve back
pressure is outweighed by the losses and heat consumption increase.

3.4     EFFECT OF AIR ON BACK PRESSURE

      The reason why air has such an adverse effect on vacuum is often
misunderstood, so a few words on the subject will not be out of place.

       The 100 percent capacity of the air pumps is often of the order to 1/2000 of
the weight of steam entering the condenser per hour.

      Now, it is frequently (but wrongly) assumed that the back pressure suffers
because of the extra partial pressure of the air. That this is not so can be easily

                                        [29]
shown by calculation. Maxing even the maximum weight of the air, the air pumps
can handle with the steam in the condenser would do very little to increase the
back pressure because of the partial pressure alone.

         For example, if the back pressure is 34.474 mbar (vacuum) without any air
presently would only rise to 34.3485 mbar. The real trouble with air is that as the
steam condenses on the condenser on the condenser tubes the air (which is
incondensable) is left behind. If the quantity of air is mall the scouring action of
the steam and condensate will sweep the air is small the scouring action of the
steam and condensate will sweep the air off the tubes. However, if the air
quantity is significant things are different. Air is such an excellent heat insulator
that it only requires a film a few molecules thick to seriously interfere with the heat
transfer to the cooling water from the steam. Accordingly the vacuum suffers.

         Fortunately, it is easy to determine whether air is present in a condenser
by merely measuring the temperature of the contents of the air suction pipe to the
air pumps. With no air present this temperature is approximately the same as that
of the saturated steam in the condenser. When air is present this temperature
falls-the more air present the lower is the temperature.

3.5    SCALE FORMATION

        Scaling is the precipitation of hare and adherent salts of calcium and
magnesium on metal surface. These scales have very poor thermal conductivity
and heat transfer in condenser is affected very seriously. It is therefore essential
to control the scale formation on one hand and remove the deposited scale on the
other hand for good condenser performance. Some of the common scale are
calcium and magnesium carbonates and sulphatas, silicates and iron salts. Most
commonly encountered scale in normal cooling water system is calcium
carbonate, which is moderately soluble in water is present in almost all cooling
waters and get decomposed into calcium carbonate at higher temperature and ph.

        Calcium carbonate is soluble in water and gets precipitated on the tubes.
Calcium sulfate has higher solubility and hence less precipitation scale.
Magnesium salts have less scaling potential, as they are more soluble than
calcium salts and their concentration in water is usually slow. Process of scale
formation gets accelerated with increase in water temperature and pH or alkalinity.
Limiting cycle of concentration, using softening water for CW makeup, reducing
CW water pH to about 8.5, and using on-line tube cleaning system with sponge
balls - these are some of the measures that can be taken for controlling scale
formation.

        Typical scales removed from condenser tubes (copper-based alloy and
stainless steel) at Khaperkheda Thermal power station were analysed. The
results are presented in the table follows.




                                          [30]
Sr. Parameters                                          Scale from Copper-based
No.                                                    Alloy Tubes (21OMW Units)
 1 Physical appearance                                  Brown colored hard scale
 2 Loss on ignition at 900 Deg (for CO2                          39.55%
    estimation)
 3 Acid insoluble                                                  2.6%
 4 Metal oxides (R2O3) (comprising Fe2O3Al2O3 )                    2.4%
 5 CaO & MgO                                                      55.54%
 6 Sulphate as SO3                                                  Nil
 7 Phosphates as P2O5                                               Nil

        The analysis clearly indicates that the properties of scale are not a
function of tube metal but solely depend upon the quality of cooling water and
conditions like increased temperature and ph which are favorable to scale
formations and deposition on metal surface.

3.6    CONDENSER EFFICIENCY AND PERFORMANCE

        The condenser efficiency may be defined as ''the ratio of the difference
between the outlet and inlet temperature of cooling water to the difference
between the temperature corresponding to the vacuum in the condenser and the
inlet temperature of the cooling water."

        Because of the considerable effect that condenser performance can have
upon heat rate there is a need to apply a strict control upon its operation. Though
the main control parameter is the back pressure measured at the turbine exhaust
flange, since any deviation in this directly affects the heat of the machine, the
following parameter also have to be measured and recorded periodically.

A)     C. W. inlet temperature
B)     C. W. outlet temperature
C)     C. W. pump amperage and bus voltage
D)     Loss of pressure across the condenser
E)     Megawatt load on unit
F)     Main steam temperature and pressure
G)      Reheat heat temperature and pressure
H)     FW final temperature
I)     Whether all feed water heaters are in service, if not , which are not
J)     Condenser exhaust temperature
K)     Condensate temperature

        Generally, condensers are designed to operate at 85% cleanliness factor.
It is possible to draw curves for different C.W. inlet temperature designed exhaust
pressure at different cleanliness factors at a different MW loads. The steam flows
can be read out from heat balance chart or in case of any basic departure (like a
particular heater remaining out of service) a fresh heat balance can be drawn.
Once these curves are available, the performance of the condenser ca be easily
estimated any time . The C.W. pump current and pressure drop across condenser
would give fair estimate of the quantity of C.W. flow to the condenser in practice
checking of condenser tubes can be apprehended by people, but loss due to
scaling/deposition is not easily seen and need shut down inspection.

                                        [31]
3.6.1   Condenser Condition Graph

a)      Deviation due to CW inlet temperature

        Plot a line vertically from the actual CW inlet temperature to intersection
with the optimum CW rise. Then plot horizontally to the intersection with optimum
terminal difference (TTD) line, and then vertically downwards to cut the saturated
steam temperature line to obtain the corresponding back pressure. (Refer fig 3.1)
Hence the loss due to the high CW inlet temperature can be calculated by
subtracting the optimum value from the actual back pressure

b)      Deviation due to C.W. flow

        Plot a line from the actual CW inlet temperature vertically to the
intersection with actual CW rise Then plot horizontally to the optimum TTD, then
vertically downward to the satueration steam temperature to obtain the actual
back pressure. the difference between the actual back pressure and the optimum
back gives the loss due to the incorrect CW flow.

c)      Deviation due to air/dirty tubes

         The effect of the air and dirty tubes on the heat transfer is to increase the
TTD above optimum. As they both give the same effect they are lumped together
in this exercise pot from the actual CW inlet temperature to the actual CW rise and
then across the actual TTD line plotting vertically downwards to adjacent steam
temperature back pressure. So the deviation due to air/dirty tubes can be found
out.




                     Fig. 3.2 Condenser Conditioning Graph
                                         [32]
3.7         CASE STUDY

        The table below shows observation for condenser performance monitoring
carried out at different seasonal days for Unit No. 10f -210 MW

KHAPARKHEDA THERMAL POWER STATION (MSEB)

               Input data            Test -1          Test -2         Test- 3
                                    July 2001        Dec. 2001       May 2001
      CW inlet (0C)                    29               22             27.82
      CW outlet(0C)                   39.2              32              38.8
      CW rise (0C)                    10.2              10               11
      Vacuum (Kg/cm2)                 0.87              0.9             0.82
      Saturation
      temperature
                                      46.3             39.9                54.2
      corresponding to back
      pressure (0C)
      Terminal Temperature
                                       7.1              7.9                15.4
      Difference (0C)

Design Data.

            Condenser tubes specification

      1.      Tube diameter (outside diameter)                   25.4mm
      2.      Tube thickness                                     1.0mm
      3.      Tube diameter ( inside diameter)                   23.4mm
      4.      Tube length                                        7.5m
      5.      Number of tubes                                    19208
      6.      Cross sectional area per tube                      430.05m3
      7.      Surface area per tube                              0.60m2
      8.      Total surface area of tube                         11495 m2
      9.      Specific heat of CW                                1.00kcal/kg/0C
      10.     Density of CW                                      1000kg/ m3
      11.     Condenser vacuum                                   650.06mm of Hg
      12.     Condenser back pressure                            0.09kg/cm2
      13.     Sat. temperature at condenser back pressure        43.60C
      14.     Average temperature of CW inlet 30.500C            30.500C
      15.     Average temperature of CW at outlet                39.200C
      16.     CW temperature rise across condenser               8.790 C
      17.     Terminal temperature difference                    4.400 C



                                              [33]
Calculation for the TEST-1

1)     If all values of C.W. Inlet (C.W.), C.W. Rise (C.W.R) ,T.T.D. are as per
       design then
                C.W.I+C.W.R.+T.T.D. = 30.5 + 8.79 +4.4 = 43.7 = 89.11 mbar
2)     If only inlet water temperature is actual then
                (C.W.I)a.+C.W.R.+T.T.D.=29 +8.79 +4.4 = 42.2= 82.46 mbar
3)     If both the inlet temperature and temperature rise is actual then
                (C.W.I)a.+C.W.R)a.+T.T.D.=29 +10.2 +4.4 = 43.6 = 89.11 mbar
4)     If all parameters are actual then
                (C.W..I)a.+(C.W.R)a.+(T.T.D)a=29+10.2 +7.1 =46.3 = 102.4 mbar
       Now effect of various parameters is as a follows
#1)    Cooler Circulating Water improves vacuum by
                (82.46 -89.11) = -6.65 mbar = - 5 mm of Hg.
#2)    Dirtiness of tubes detoriates vacuum by
                (89.11 - 82.46) = 6.65 mbar = 5 mm of Hg.
#3)    Air ingress detoriates vacuum by
                (102.4 - 89.11 ) = 13.29 mbar = 10 mm of Hg.

Conclusion:

#1)    Inlet temperature is slightly lower than design value and hence little
       improvement in vacuum by 5 mm of Hg.

#2)    Tubes are dirty which detoriates vacuum by 5mm of Hg.

#3)    There is a little air ingress which causes vacuum detoriation by 10mm of
       Hg.

Calculation for the Test - 2 :

1)     If all values of C.W. Inlet C.W. Inlet (C.W.), C.W. Rise (C.W.R) , T.T.D.
       are as per desung then
                C.W.I.+C.W.R.+T.T.D. =30.5 +8.79+4.4 =43.7 = 89.11 mbar
2)     If only inlet water temperature is actual then
                (C.W.I)a. +C.W.R.+ T.T.D.= 22 + 8.79 + 4.4 =35.19 = 56.6 mbar
3)     If both the inlet temperature and temperature rise is actual then
                (C.W.I)a.+(C.W.R)a.+T.T.D.=22 + 10 +4.4 =36.4 =61.18 mbar
4)     If all parameters are actual then
                (C.W.I)a.+(C.W.R)a.+(T.T.D)a= 22+10+7.9=39.9=73.15 mbar
       Now effect of various parameters is as a follows
#1)    Cooler Circulating Water improves vacuum by
                (56.6 - 89.11)   =-32.5 mbar =-24.4 mm of Hg.
#2)    Dirtiness of tubes detoriates vacuum by
                (61.18 - 56.6)    = 4.58 mbar =3 mm of Hg.
#3)    Air ingress detoriates vacuum by
                (73.15 - 61.18) = 12 mbar = 9 mm of Hg.




                                      [34]
Conclusion :

#1)     Inlet temperature is slightly lower than design value and hence little
        improvement in vacuum by 24.4 mm of Hg.

#2)     Tubes are dirty which detoriates vacuum by 3mm of Hg.

#3)     There is a little air ingress which causes vacuum detoriation by 9mm of Hg.

Calculation for the TEST - 3

1)      If all values of C.W. Inlet (C.W.), C.W. Rise (C.W.R) , T.T.D. are as per
        design then
                 C.W.I.+C.W.R.+T.T.D.=30.5 + 8.79 +4.4 = 43.7 89.11 mbar
2)      If only inlet water temperature is actual then
                 (C.W.I)a.+C.W.R.+T.T.D. =27.82 + 8.79 +4.4 =41 = 73.5 mbar
3)      If both the inlet temperature and temperature rise is actual then
                 (C.W.I)a.+(C.W.R)a.+T.T.D.=27.82 + 11 +4.4 = 43.2 = 87.11 mbar
4)      If all parameters are actual then
                 (C.W.I)a.+(C.W.R)a.+(T.T.D)a=27.82+11+15.4 = 54.22 = 151.62
                 mbar
        Now effect of various parameters is as a follows
#1)     Cooler Circulating Water improves vacuum by
                 (73.5 - 89.11)    = -15.61 mbar = -11.74 mm of Hg.
#2)     Dirtiness of tubes detoriates vacuum by
                 (87.11 - 73.5)    = 13.61 mbar =10.23 mm of Hg.
#3)     Air ingress detoriates vacuum by
                 (151.62 - 87.11) = 64.51 mbar = 48.5 mm of Hg.

Conclusion :

#1)     Inlet temperature is slightly lower than design value and hence little
        improvement in vacuum by about 0.5 inches of Hg.

#2)     Tubes are dirty which detoriates vacuum by 10.23 mm of Hg.

#3)     There is a little air ingress which causes vacuum detoriation by around 2
        inches of Hg.

3.7.1   Notes on Results

1)      Losses due to high CW temperature.

        Provided that the cooling towers are performing satisfactorily this loss must
be accepted to some extent. It is possible, of course, to minimize the loss by
having an abnormal quantity of cooling water flowing through the condenser; this
gives a smaller cooling water temperature rise across the condenser then
optimum. However, the gain which results from this is almost cancelled out by the
additional pumping power required. Therefore, the increased turbine output
caused by improving the vacuum must be greater then the increased circulating
pump hour by required to justified these means of reducing the loss.

                                         [35]
2)     Variation of CW flow :

       This is a loss which can normally be eliminated. If the cooling water
temperature rise across the condenser is less then optimum, then the opening of
the condenser cooling water outlet valve should be reduced. This condition may
also be shown up when the condensate temperature is lower than the saturated
steam temperature. If the cooling water temperature rise is hardly effected by
opening (even abnormally wide) of the cooling water valves then the condenser
tube plates are probably fouled-assuming that there is no shortage of cooling
water.

3)     Dirty tubes.

       Operationally little can be done to eliminate the cause of this loss, as the
tubes must normally be cleaned when the unit is off - load.

        However, as soon as loss due to dirty tubes is determined, it should be
ascertained that chlorine injection to the affected condenser satisfactory. It may be
that the station chemist will wish to have the dosage increased; so he should
always be informed.

4)     Effect of air in condenser.

        Practically all the air entering the condenser does so through leakages into
the turbine spaces which are under vacuum and can have one or more of the
following ill effects on operation:-

       a)      Air entering to the outside to the condenser tubes adds
               considerable resistance to the heat flow. To overcome this, in order
               to maintain the flow of heat the exhaust temperature must rise. This
               is known as air blanketing.
       b)      The corresponding backpressure will rise as a result of increased
               exhaust temperature.
       c)      The condensate temperature in relation to that of the exhaust
               temperature in a similar manner to the pressure drop.

        In the case of a) the increasing heat transfer resistance will increase the
amount of heat that must be transferred and as the steam consumption is
increaser attempts to hold the turbine output constant would further aggravate the
situation.

5)     Partial Pressure.

       The reason for a) and b) are clear but some explanation is necessary for
c). Here the reason lies in a scientific law known as Dalton's law of partial
pressure, which states that “if a mixture of gases of vapor is contained in a closed
vessel each gas exerts a pressure equal to that which it would normally exert if
alone in the vessel. In other words each exerts a partial pressure and the total
pressure in the vessel is the sum of the partial pressures.



                                         [36]
        Consider these laws in relation to the condenser. At the top, the weight of
air present is very small compared with the weight of steam and the air partial
pressure can be neglected. The total pressure can be regarded as that due to the
steam alone. The steam temperature actually corresponds to the partial steam
pressure. At the top of the condenser, steam temperature will therefore
correspond to total pressure.

        At the bottom of the condenser, however, most of the steam will have been
condensed and there is a much bigger ratio by weight of air to steam. So, now the
air partial pressure is not negligible, hence, at the bottom of the condenser the
total pressure is greater than the steam partial pressure by an amount equal to the
partial pressure.

        The condensate temperature corresponds to the steam partial pressure at
the bottom of the condenser. It will therefore, be lower than if calculated from the
total pressure.

       This is shown numerically in the following examples :

                                       Pressure (mm of Hg )     Temperature (0C)
 At the top of the condenser, due to
                                                 38                     33
 steam alone.
 Pressure drop through condenser.               Say 5                   --
 Total pressure at the bottom of the
 condenser made up of partial                    33                    30.5
 pressure of steam & air.
 Partial air pressure at bottom                Say 2.5
 Partial steam pressure at bottom                30                     29

         Thus the total losses in temperature are now 70C and there are due to air
in the example shown is 20C. These emphases the importance of prevention of
the air leaks and the removal of them condenser.

6)     Velocity of steam.

        The velocity of incoming steam is the main factor in the forcing the air
towards the bottom of the condenser. Because of these velocities, the steam
sweeps over tubes and drives the air away before it. In this way the tubes are kept
free of air, which is kept. Moving towards the air outlet. It must not be allowed to
recalculate or find a stagnant corner. During this tine the steam has been
condensing so the air concentration increased towards the bottom of the
condenser.

        When the air and the any uncondensed steam mixed with it reach the
bottom of the condenser they come within the range of powerful effect of ejector
draws this mixture under a baffle which encloses a nest of the tubes in the lowest
temperature cooling water zone. In fact the water temperature at the outlet from
this section might be only 50F higher than the water inlet temperature.




                                        [37]
       In this air cooler section any steam remaining in the mixture is condensed
and the air cooled. The reason for cooling the air is to reduce the volume and
enable the ejector, which operates by volume, to remove a greater weight of air.

       The actual take off from the air cooler is usually placed about three tubes
drawn the top of the air cooler section. This is to prevent air being reheated
through contact with the baffle plate which has relics much hotter steam on the
side.




                                       [38]
                                   Chapter 4
   METHODS OF IMPROVING CONDENSER PERFORMANCE
4.1    INTRODUCTION

       The main scope for short term control of turbine efficiency is in the
optimization of condenser vacuum and auxiliary power consumption. C.W.
pumping power may account for upto 15% of steam auxiliary consumption and is
therefore an appreciable saving when a pump is shut down. This, however,
impairs the condenser vacuum and reduces turbine efficiency. It is required
therefore to determine whenever the shutting down of a C.W. pump will produce
greater saving than the corresponding loss in turbine efficiency or, consequently,
whenever the starting up of a pump will bring about a more than compensating
increasing in turbine efficiency.

       The effect of a change in the number of C.W. pumps in service depends
upon the initial vacuum and the number of machines and pumps on the system.
Also the change in heat rate corresponding to a given vacuum change depends
upon whether the vacuum rising or falling; the question of putting a pump into
service must, therefore, considered separately from that of shutting one down.

       Figure 4.1 illustrates typically changes in heat consumption due to
depreciation in vacuum and the equivalent heat of pumping power.

       The intersection of these curves indicates the critical "breakeven" vacuum
changes which are plotted in figure 4.2. If the vacuum changes which is greater
then the critical value the pump ought not to have been shut down and vice-versa.
These curves give no guidance as to when to experimentally increase or decrease
the no. of pumps in service. It has been found possible by comparing the actual
and critical vacuum change for a wide variety of pump, machine, initial vacuum
and load condition to derive a relationship between CW temperature differential
and vacuum. (See fig. 4.3).




      Fig. 4.1 Turbine heat consumption plotted against vacuum change

                                       [39]
                         Fig. 4.2 Critical vacuum chart




        Fig. 4.3 CW temperature differential plotted against vacuum

4.2    CONDENSER CLEANING

        When condenser tube plates become fouled, with debris CW flow is
reduced, CW temperature differential rises and there is a deterioration of vacuum.
The heat rate corresponding to actual vacuum may be readily determined from
correction curves. However, it is necessary also to know what the vacuum and
corresponding heat rate would have been if the fouling had not taken place. By a
suitable range of test with clean condenser the normal CW temperature
differential corresponding to machine and pumps in service, with fully open CW
valves, may be determined. (The effect of initial CW temperature is negligible).
The corresponding vacuum and heat rate may then be derived.

         By throttling in CW inlet valves to simulate fouling the change in vacuum
corresponding to various degree of fouling may also be derived for various states
of initial vacuum and then corresponding increased in heat rate derived. The form


                                       [40]
of a family of curves relating heat rate to load in shown in figure 4.4 in which the
curve applies to mild fouling except curve no. 1 applies to heavy fouling.




                 Fig. 4.4 Turbine heat rate plotted against load
                   for various degrees of condenser fouling

       It is also necessary to know the cost of cleaning to restore efficiency. For
on load cleaning this involves evaluation of :

1)     The rise of vacuum for any initial vacuum with one side of a condenser
       isolated and drained, the dirty side remaining in commission.
2)     The vacuum which would exist when the second side of the condenser is
       being cleaned, the already clean side being back in commission.
3)     The duration of cleaning operation.
4)     The incremental labor cost involved (this may be expressed in terms of the
       BTU of heat that could have been purchased with the additional money
       spent on labor).
5)     The load reduction is necessary for safe operation with one half of
       condenser isolated & drained. The cost of cleaning will obviously be
       influenced be the loading condition of the station. In the extreme case, if
       the turbine is to be shutdown over night or at the week end the cost of heat
       loss due to running at reduced load owing cleaning will be eliminated.
       Partial load reduction will alleviate this loss but the optimum redistribution
       of station load during condenser cleaning on one machine must be
       established.

4.3    ON LOAD CLEANING

        The desirability of keeping the condenser tube clean on the C.W. side is
obvious, as dirt is synonymous with interference with heat transfer. One particular
source of trouble arises from slime deposition. River water contain algae another
organism which find the habital inside a condenser tube amenable, and so they
settle on the tube surface.



                                        [41]
       The solution of the problem is to make the environment inhospitable, which
is done by intermittently dosing the C.W. going to condenser with chlorine. The
residual free chlorine at the C.W. outlet must be carefully monitored to ensure that
no contamination occurs if it is return to river. Chlorination is very effective,
although quite expensive.

        An alternative to chlorine injection for keeping the tubes clean is to use on
load mechanical cleaning. A modern method has been developed where by the
condenser tube are cleaned continuously while the unit is on load. (See the fig.
4.5) Foam rubber balls of diameter slightly in excess of the tube bore are
circulated, trapped and the recirculated in the circulating water system. In this
way the tubes are continuously cleaned & dirt & scale formations are not allowed
to build up.




                 Fig 4.5 On - load condenser cleaning system

4.4     CONDENSER LEAKS

        There are two leakage problems associated with all surface condenser is;
these are air leakage, and C.W. leakage into the steam and condensate space. A
third leakage problem which can arise is the leakage of cooling water into the
turbine hall.

4.4.1   Air leakage

       Air leakage into the condenser is one of the main causes of poor vacuum.
Air can leak into the condenser from any part of the condenser system which is

                                         [42]
under vacuum, but leakage at the condenser itself is usually via the flange is
associated with the condenser mounting, such as at the condenser flash box and
the gauge glass. This type of leakage very often occurs after the prolonged
period of two shift operation, when the temperature cycling causes failure of the
flange joints.

4.4.1.1 The effect of air in the condenser

        Air is poor conductor of heat. As air collects in the steam space of the
condenser it tends to form a blanket around the tubes and so increase the
resistance of the heat transfer path. This means that the temperature at which the
steam has to be increased to transfer the latent heat of condensation from the
steam to the circulating water result in a poorer vacuum. This is a similarity effect
to that caused by dirty tubes.

       Excessive air in the condenser also tends to under cool the condensate.
Dalton's law of partial pressure states that the pressure of mixture of gaseous
substances is the sum of the pressure which each would exert if it alone were
present in the vessel. These individual pressures are called partial pressures.

        At the top of the condenser the quantity of steam is far in excess of that of
air and the pressure at the top of the condenser can be taken as being equal to
the partial pressure of the steam. As the steam passes over the tubes and
condenser the air which is present becomes more significant and the pressure in
the lower part of the condenser is the sum of the partial pressure of the steam and
the partial pressure of the air.

4.4.1.2 Detection of air leakage

a)     ON. Load Detection :

       The traditional method of locating air leaks when the turbine is on load is to
pass a lighted taper round the joints which are suspected of having a leak. The
flame of the taper is drawn towards the place where the air is being drawn into the
condenser.

       This is a time-consuming technique as the taper has to be passed slowly
over every area where a leak is suspected, and the presence of draughts can
make this a very frustrating job.

        A quicker way of locating leaks is to spray the suspected area with freon or
other halogen gas. This is then drawn into the condenser and sucked into the air
extraction equipment. If a lighted blow lamp is placed with its flame above the air
discharge port on the air extraction equipment the normally blue flame will change
to orange when the halogen is emitted.

        A more modern development of this method is the use of halogen gas
detectors. These are inserted into the air discharge line from the air extraction
equipment and a meter registers when a halogen gas passes the detector. A
suitable gas (such as freon) is sprayed round the suspected area until the detector
registers.

                                         [43]
        The disadvantages of these systems are :

        a)     The operation needs two men: one man spraying, and the other
               watching the blow lamp or inkicator.
        b)     Time must be allowed to elapse after each spray so that, if there is
               an indication, the operator knows which area that has been
               sprayed contains the leak.

b)      Off. Load Detection :

        Off - load leak searches are carried out by filling the condensate system
and steam space with water to a level below the turbine blades. Care must be
taken to ensure that the condenser supports have first been set in the correct
position to cater for the extra load in the condenser.

        Fluorescene is added to the water, and if any leakage takes place the
fluorescene can be detected by the use of an ultra-violet lamp. Leakage is
detected by this method, not only at the condenser mountings, but also on the
low pressure feed heaters also.

       In KWU turbines, vacuum leak detection is done by filling water up to one
meter above the top row of condenser cooling water tubes. This procedure is
called vacuum tightness test. During this test, CW side of condenser is kept
completely empty.

       In this procedure, however, leakage points one meter above the tube nest,
remain undetected. Previously, "FREON GAS " detection instrument was
employed for the same. This instrument is currently not manufactured due to
environmental degradation by the Freon gas.

        The following sections bring out the procedure of detecting leakage points
in above mentioned undetected area by steam pressurization. The purpose of
this test is to check the leakage in those areas which will remain under vacuum
even at a load of 80 to 100% on the machine. This procedure is to be adopted
during commissioning process.

4.4.2   Circulating water leakage

       The two kinds of CW leakage, internal and external. Internal leakage into
the steam and condenser space is the most important of these two.

4.4.2.1 Internal leakage

        Leakage of cooling water into the condensate can be caused by several
fault, but the main causes are-

        a)     Tube to tube plate fixing leaking
        b)     Internal corrosion and erosion of the tubes.
        c)     External corrosion of the tubes.
        d)     Fatigue and stress cracking of the tubes


                                        [44]
          We shall look at each of this in some detail to see exactly how the leakage
occurs.

a)        Tube to tube plate fixing

       The condenser tube has to be securely fastened into the tube plate to
make a watertight joint and yet has to be allowed to expand relative to the tube
plate.

       There are two types of tube fixing; by the use of ferrules, and by tube
expansion in the tube fixing by fellows the tube is fixed at both and by ferrules.
There being a clearance which allows expansion of the tube to take place.
Ferrules are screwed into the tube plate and the seal is made by packing rings,
which are usually made of lead and fibre and are leak resistance.

        The expanded or rolled in method is a better method of fixing. It gives
better hydraulic configuration is simple, and has good sealing properties. The
relative expansion which may occur between the tube and the condenser shell is
usually catered for by fitting bellows section between the tube plate and the
condenser shell.

b)        Internal corrosion and erosion:

      Internal corrosion and erosion of tubes can be a serious source of leakage
and can lead to tubes having to be blanked off.

c)        External erosion:

      External erosion of tube results from water impingement. Where baffles
have worn away, or have by mistake been omitted only get from the condenser,
make-up or drainage water can impinge directly onto the tube.

d)        Fatigue and stress cracking:

        Fatigue cracking can take place when tubes have been subjected to
vibration. Cracking takes place at positions adjacent to the tube of support plates
and is mainly transcrystalline within the structure of the metal.

         Stress cracking takes place in the metal where unequally distributed static
stresses exist or are applied to it. Cracks due to stress cracking are usually inter
crystalline and within the metallic structure and viewed under microscope, and be
seen to be branching in character.

       Condenser tube leaks have also been caused by broken turbine blades
being rejected into the condenser. Moreover, if there is any vibration of the
tubes, the tubes may come into contact with one another, so causing them to
racks after a while and give further leaks.




                                          [45]
The effect of C.W. leakage into the condensate :

        Leakage of cooling water into the condenser steam side can have serious
consequences. The C.W. carries impurities with it into the condensate system;
the most detrimental are those containint chloride, such as NaCl. These
impurities are then carried forward into the boiler.

        The presence of chloride in the boiler water constitutes a potential hazard,
principally because acid chlorides can be formed and boiler tubes corrosion can
result. The higher the boiler pressure the greater is the danger. It is, therefore,
very important that C.W. leakage should be detected, the source of leakage
located, and the leaks rectified.

The initial indication of a tube leaks

        Fortunately, the impure water has a property which can be utilized to
detect it. The impure water conducts electricity better than the pure condensate
and is said to have a higher conductivity. If the conductivity of the condensate is
monitored a change will be detected when a leakage of C.W. occurs.

        The practical advantage of condensate conductivity measurement is that it
indicates changes, not only in the actual value but also in increases above the
normal running value. To the plant operator, this often gives the first indication of
condenser leakage.

Detection of Leaking Tube

       There are several methods of locating the leaking tube, and new methods
are continually being tried. The principal methods of leak location are as follows :

a)     The simple manometer :

        Fig. 4.6 shows a simple manometer, which can be manufactured by the
station chemist. One end of the condenser tube is plugged and the manometer is
inserted into the other end. The leaking tube will suck the liquid out of glass
because of the vacuum in the condenser. This method is very effective, but can
be time consuming.

b)     The Blanket Effect :

       In this method the tube plate is covered by thin plastic sheeting or by foam.
The leaking tube will tend to pull the foam or sheet into it.

c)     Sonic detection :

        As air is drawn into the leaking tube it creates a supersonic whistle. This
wistle is detected by a microphone placed in the entrance to the tube, and the
resulting signal is amplified.




                                         [46]
d)     Bubbler leak detectors :

        Fig. 4.7 shows a conventional bubbler. This can be used for on-load
detection and its method of operation is similar to the simple manometer.

        If one end of the tube is blocked with a bung, the vacuum of the condenser
pulls air through the bubbler when it is inserted in the other end of the tube.

        A more advanced type of bubbler called MEL bubbler can be used both for
on-load and off-load detection and is very suitable for use with pannier and
integral condensers.

        Fig. 4.8 illustrates the principle of the MEL bubbler. The tube to be tested
is plugged, connected to a reference vessel and vacuum pump, and there the
system is evacuated. The pump is then isolated from the system and, after a
short time, to ensure that the pressure in the tube being tested and the reference
vessel have equalized, the balance valve is closed. From then on any air leaking
into the tube under test will be indicated by a stream of bubbles issuing from the
end of the tube in the jar or water.

        If on-load location is not successful it may be necessary to take the turbine
off load to locate the leak. On some older turbines it is possible to enter the steam
space and locate the leak directly as the CW sprays into the steam space, but
usually this is not possible on a modern turbine.

        Where underslung condensers are fitted it is possible to fill the steam
space with condensate containing fluorescene and examine the tube plate with an
ultra-violet lamp to find the leaking tube.

       As we mentioned previously, the condenser support springs must be
jacked up before the steam space is filled with condensate.

       Where pannier or integral condensers have been fitted this method cannot
be used if the steam space is filled with water the low pressure cylinder could be
under water.

The Double-Tube Plate

         On modern turbine plant extensive use is being made of the double-tube
plate in an attempt to reduce the effect of leakage at turn fixings. Fig. 4.9
illustrates the principle of the double-tube plate.

        The interspace A can either be under vacuum (in which case leakage will
be into space) or it can be fed with condensate under pressure a leakage from the
system. Alternatively, the conductivity of the drainage from the interspace A can
be monitored; an increase indicating a leaking tube fixing.




                                         [47]
4.4.2.2 External Leakage

       External Leakage from condenser water boxes and joints is usually due to
metal removal by erosion or corrosion. Erosion is the physical removal of metal
by excessively turbulent water (particularly when it contains air bubbles), or by
water carrying grit or other suspended solids. This makes particularly susceptible
those places where water has to change direction quickly, such as water boxes, or
in areas of excessive turbilance due to the throttling action of valves. Leakage
path erosion between the impeller eye and casing of large CW pumps may
necessitate the use of wearing rings at this point. An external leakage source
may also be a broken or loose anode in a cathodic protection system.

        Corrosion is the result of electro-chemical action, which can be reduced
but cannot be entirely eliminated. Cast iron condenser waterboxes are particularly
affected by sea water, which dissolves the iron content of metal, leaving behind
weak and porous graphite in original shape. The application of protective coatings
and cathodic protection adoption help to reduce electrolytic corrosion. Painting
gives some protection to condenser water-boxes, although adequate surface
preparation and coverage is difficult to achieve; severe localized corrosion may
occur where there is a defect in point film. Natural or synthetic rubber coatings
are more successful and have a longer life, although initial cost is high. An
unprotected water box, however, provides some protection for copper alloy
condenser tubes by limited cathodic protection mechanism.             Conversely,
successful coating of waterboxes accelerates corrosion, elsewhere, particularly at
tube ends. Thus, cooling should extend a short distance into tubes, or plastic
inserts may be placed in tube ends.
                                       [48]
         Cathodic protection is based on the principle of a corrosion cell; if two
dissimilar metals are placed in electrolyte, corrosion of the more electro-negative
one (anode) takes place in preference to the other (cathode). In cooling water
systems the iron components from the anodes and the copper alloys (tubes) from
the cathodes. If a third electrode, more electro-negative than the iron and the
copper alloys, is added to the system and is electrically connected to the other two
electrodies, the new electrode corrodes in preference to the iron, or the copper
alloys. This system is know as the sacrificial anode type of cathodic protection
(Fig. 4.10), as the third electrode is sacrificed to save the original (iron) electrode.
Improved cathodic protection, requiring less maintenance (replacement of
sacrificial anodes) and able to protect a larger area, is provided by an impressed
circuit type system. It uses the same principle, but an inert or semi-inert material
(e.g. platinum coated titanium) is deliberately made anodic to existing material by
passing low voltage direct current through it into the electrolyte (Fig. 4.11).




                                          [49]
                                   Chapter 5
                 CIRCULATING WATER SYSTEM
5.1    INTRODUCTION

        The cooling water circulating (CW) system is probably the most important
auxiliary system in a power station. Without a supply of cooling water to the
condensers, a condensing turbine cannot be operated. It is therefore essential
that the CW system is reliable.

         A 210 MW turbine rejects large amount of heat to the cooling water with a
CW temperature rise of between 80C and 100C. A lot of money can be saved by
skilful operation of the CW system to give optimum conditions in the condenser.
Fig. 5.1 shows where the major losses occur in 210 MW unit.

        The CW system has to be designed so that it is flexible for economic
operation, and reliable to give good availability. The basic aims of the designer
are to provide :

(a)    A guaranteed supply of water to all running sets at all times.
(b)    Ready and efficient control of water quantity to give optimum power station
       efficiency under all conditions of power station load and water temperature.
(c)    A balanced supply of water to all sets without recourse to unnecessary
       ‘throttling’.
(d)    Minimum maintenance needs easily carried out.
(e)    Minimum overall capital and operating costs consistent with the above
       requirements.

In this chapter we shall deal more with the design aspects of CW systems and
plant items.




               Fig. 5.1 Major Losses in Thermal Power Station


                                       [50]
          In the thermal power station we require the cooling water for :

   i)        Condensers.
   ii)       Generators.
   iii)      Lubrication oil coolers.
   iv)       For bearing cooling.
   v)        Air conditioning plant.

        Among all above circulating water required for condenser is comparatively
very high. In the condenser about 60 to 65% at steam is condensed, quality of
circulating water for 210 MW unit is around 15500 m3/hr.

          Circulation water required in condenser depends upon

   i)        Steam condition (Turbine exhaust)
   ii)       Unit size
   iii)      Use at regenerative feed heating cycle.

5.2       TYPES OF C.W. SYSTEM

      In various thermal power station different C.W. systems are used.         It
depends upon the availability of cooling water and atmospheric condition.

There types are :

   i)        Once through or Direct system or Open system.
   ii)       Recirculation or closed system.

        In the once through system water is taken from the source either pond or
river or sea, it is introduced condenser and after condensing the steam water is
discharged in the source at suitable location. See in fig. 5.2.




                    Fig. 5.2 Once through or open C.W. system



       Wherever cooling water is not available in ample quantity the recirculation
system is used in the thermal power station. In this system C.W. discharge from
the condenser is cooled by cooling towers either wet type or dry type and again
same cooled water is reintroduced in to the condenser. In this system we have to

                                           [51]
add some quantity of make up water into system on the part of evaporative
losses. See the fig. 5.3




                   Fig. 5.3 Recirculation or closed system




       The economics, design constriction and functional requirements of the
C.W. system and associated plant components are discussed in detail in the
sections which follow.

        Fig. 5.4 indicates the component terminology for the two typical
arrangements of Recirculation cooling system and once through cooling system
and it is intended to be used for reference purpose. The following paragraphs
briefly describe the functional requirements of some of the C.W. system
equipments.

Screening Plant : The screening plant must remove any debris from the cooling
water which is large enough to block the condenser or auxiliary cooler tubes. It
must be easy to keep clean even during periods of excessive debris.

C.W. Pumps : The cooling water (C.W.) pumps must circulate the water against
system resistance, or pumping head, or pumping head, under all the condition
encountered at a particular site. To ensure efficient and flexible C.W. pump
operation, valves are usually provided to allow any combination of pumps,
condensers and cooling towers to operate together.

Cooling towers : The purpose of cooling tower is to cool the hot C.W. discharges
from condensers, so that it could be reused for the same purpose.




                                      [52]
Fig. 5.4 Terminology for condenser and C.W. system




                       [53]
5.3     DESIGN ASPECTS OF MAJOR ITEMS

5.3.1   Cooling towers

        When power stations are built beside rivers which cannot supply sufficient
water to condense the turbine exhaust steam by using a once-through system,
cooling towers are used in conjunction water with a closed-circuit system to cool
the circulating water.

5.3.1.1 Principles of operation

        Fig. 5.5 illustrates the operation of a cooling tower. Cooling water is
pumped from the turbine condenser by the tower pump to the cooling tower.
Inside the tower the water passes through sprinklers, and sprays out in find drops.
The water then falls as droplets, passing over pickings where it is made to present
a greater surface area to the cooling air. The water then falls into the cooling
tower pond.

       Air is drawn in near the bottom of the tower, either by natural draught or by
a fan. The air passes up the tower and cools the water is it does so. Any water
droplets which have been carried up by the air are removed by the water droplet
eliminator screen.

5.3.1.2 The theory of cooling

       As a water droplet falls through the tower, air flows past it and cooling
takes place in three ways :

(a)     A small proportion of heat is lost from the droplet by radiation of heat from
        its surface.
(b)     Approximately a quarter to one-third of the heat transferred is by
        conduction and convection between the water and the air the amount of
        heat transferred depends on the temperature of water and air.




                       Fig. 5.5 : Cooling Tower Operation.
                                         [54]
(c)    The remainder of the heat transfer is by evaporation. As the air
       evaporates some of the water into water vapour, the vapour takes with it
       the latent heat of evaporation. The remaining water therefore has a lower
       heat content than it had originally, and is also at a lower temperature.

        The amount of evaporation which takes place depends on a number of
factors; these include the total surface area the water presents to the air (the
reason the packing design is so important), and the amount of air flowing. The
greater the air flow, the greater the cooling achieved.

5.3.1.3 Types of cooling towers

       There are several types of cooling tower tasted on two air and water
systems. They can be natural or forced draught cooled, and can be wet towers or
dry towers. Fig. 5.6 and 5.7 illustrates some of these types are shown.

Natural draught cooling tower

        The modern natural draught tower is usually of the concrete hyperbolic
pattern. The term 'hyperbolic' refers to the fact that the side of the tower has the
form of a hyperbola. In this type of tower, air moves upwards, because of the
chimney effect created by the difference in density between the warm moist air
inside the tower and the colder, denser air outside. Hyperbolic towers are best
suited to regions with high humidity, populated areas, and where land prices are
high the height of the exhaust from these towers helps to prevent the formation of
fog along the ground.

Mechanical draught - cooling water

        Mechanical draught towers, using fans either to force or to induce the
movement of air, first came into use in the 1930s. In a forced-draught tower, the
fan is at the bottom and pushes the air up through the tower. I n an induced
draught tower the fan is at the top and pulls the air up. One of the main problems
with forced draught towers in recirculation; vapour leaving the tower at low velocity
tends to re-enter the tower, with the result that the wet bulb temperature of the
entering air is increased and the performance of the tower is impaired.

        A combination of natural and mechanical draught cooling can be seen in
the assisted draught tower. The fans, in this case, help to increase the air flow.
Fig. 5.8 shows such a tower with cross-flow packing (which we shall explain leter).




                                         [55]
Fig. 5.6 : Natural draught wet cooling tower




   Fig. 5.7 : Induced draft cooling tower




                    [56]
                    Fig. 5.8 : Assisted draught cooling tower

        With this arrangement it is estimated that a single tower will provide the
cooling for at least 660 MW of plant and although its base diameter will be about
140 m (450 ft), its appearance from a distance will be little different from a single
natural draught tower with a capacity of 250 MW. Compare this with a 500 MW
unit which requires two natural draught towers 115 m (375 ft) high by 90 m (300 ft)
diameter.

Dry cooling towers

        In cooling tower made their first appearance in Hungary during the year
1950, and later it was used in 1962 by CEGB in U.K. for one of their unit of 120
MW capacity. Fig. 5.9 show a schematic layout at a dry cooling tower system. In
principle, it is a simply a water to air surface heat exharger, like a motor-car
radiator, the air being induced to flow through the radiator by the tower chimney
effect.




                      Fig. 5.9 : Dry cooling tower system.

       In the closed circuit, cooled water from the heat exchangers in the cooling
tower is sprayed through nozzles into the condenser, where exhaust steam form
the turbine is condensed by direct contact. The cooling water and condensate
mixture passes to the CW pump, which delivers most of it through the discharge

                                         [57]
culvert to the heat exchangers : the remainder (about 1 ½ per cent) is taken by the
extraction pumps and delivered through the feed heating system to the boiler. As
you will appreciate, all the water used is to condensate quality.

5.3.1.4 Tower shape and construction

         Air entering a cylindrical tower is carried in towards the centre of the tower
as it rises up through the tower until the man of air reaches a minimum diameter,
as shown in fig. 5.10. The minimum diameter is called the vena contracta, and it’s
dimension depends on the radius of the tower at the air opening (R), and the ratio
of the height of the air opening (h) to the radius.




             Fig. 5.10 : Air flow through a cylindrical cooling tower

        From a strictly thermal point of view, the cooling tower shape could be
cylindrical, but there is no thermal advantage to be gained by making the throat of
the tower of greater diameter than the diameter of the vena contracta.
Considerable savings in materials and costs can be gained by tapering the shell to
the diameter of the vena contracta. One more advantage is that the hyperbolic
shape stiffens the concrete shell against wind forces – an added safety factor.
Windening out the tower after the throat also helps to stiffen the shell against wind
forces.

         In Germany, cylindrical shells are used, the main advantage being that
sliding shutters can be used in the construction, which makes for speedy
construction. In British conditions the extra steel and concrete required is not
considered to be economical. In Britain, tower shells are built with concrete cast
in situ; the shuttering is supported on the concrete case a few days earlier, so
dispensing with the great quantity of scaffolding which was formerly employed.
The entire weight of the shell and wind reaction load is supported by reinforced
concrete columns. These columns are inclined in the same plane as the bottom
of the shell and provide a support between the base rings of the shell and the
foundation. The cold water basin beneath the tower may form the foundation, or it
may be separate. The basin is sometimes provided with a conical bottom to
collect silt from the cooling water.

5.3.1.5 Packings

                                          [58]
       The water to be cooled is allowed to flow over the packing and, in so
doing, presents a large surface area to the air stream. It is in the packing where
most of the cooling takes place. For this reason, it is worthwhile examining in
more detail the design and construction of the various types. The ideal
requirements of any packing are that it should :

   (a)     Have a very large surface area for a given volume of material.
   (b)     Be structurally strong.
   (c)     Be chemically inert.
   (d)     Not support scale.
   (e)     Not be susceptible to attach by micro-organisms.
   (f)     Have a low resistance to air flow.
   (g)     Have a low weight per unit volume.

       There are two types of packing which are both named after the way they
provide a large surface area to the water; they are splash packing, and film
packing, and are shown in fig. 5.11.




               (a) Film packing                   (b) Splash packing
                  Fig. 5.11 : Types of cooling tower packing



       Splash packings, as you may have guessed, break the falling water
droplets into smaller water droplets, by breaking them up as they splash on to the
packing. The packing may be up to 6 m high, and the spacing of the splash bars
may be 150 mm horizontally and 230 mm vertically.

        The area of wetted surface of the splash bar also has some effect on the
cooling rate. A rectangular cross-section gives the highest surface area for a
given volume of material, but a triangular cross-section has more strength and has
less resistance to air flow.

                                       [59]
       Film packings spread the water into a thin film, so forming a large surface
area for a given volume. These packings must have good adhesive properties so
that the water will stick to them, and then (because water has a low surface
tension) a thin film will be formed (see Fig. 5.12).




                     Fig. 5.12 : Formation of thin water film

Materials for packings

        Spalash packings and some types of film packings have traditionally been
made from wood. The timber is unplanned, so giving a greater surface area. The
wood used is from one of the varieties of fir tree grown as far north as possible -
that for the most severe duty coming from within the Arctic circle. The growth of
the wood is slower in trees grown within the Arctic circle, and so their annual rings
are closer, making the timber more durable when wet.

        One of the main disadvantages of wood packings is that they are subject
to fungal attack. Chemical inhibitors can be used to stop this attack, but these,
like the older but still popular creosote, tend to leach out of the timber, by
evaporation, or are washed out by the water. This has been overcome by the use
of water-borne salts such as 'Tanalith C' and 'Celcure' which combine with the
timber to form insoluble compounds which are not leached out by the circulating
water.

         Through timber packings have advantages, the limitation in their active life
and the increasing cost of timber has led to a search for new packing materials
and it is replaced by asbestos cement.

        A more efficient design is the use of a double layer of corrugated sheets
fabricated from asbestos cement, and this is the design which is now in use. The
material is relatively inexpensive, non-flammable and is not subject to fungal or
chemical attack.

         Glass has also been used as a packing material at some power stations,
but its use is not widespread.


                                         [60]
         As in so many other fields considerable attention has been paid in recent
year to plastics. Fig. 5.13 shows a typical plastics plate packing. One of the
main disadvantages not yet overcome is its poor mechanical strength. Work is
still progressing to determine how the various plastics can best be used and also
to see if better materials can be found. The advantages of plastics as packing are
that they :

   (a)     Give a light, easily supported pack.
   (b)     Are inert in acid or alkaline water conditions.
   (c)     Do not leach away to form a sludge.
   (d)     Do not support scale.
   (e)     Will not support animal or vegetable life.
   (f)     Are not subject to electrolytic action.
   (g)     Are comparatively cheap.
   (h)     Are easily formed.




                  Fig. 5.13 Plastic plate cooling tower packing

Cross-flow packings

        In the cross-flow packing water falls vertically, by gravity, over the packing,
while air flows horizontally, as we showed earlier in fig. 5.14. The performance of
the tower improves as the height of the packing increases. In the cross-flow tower
the height of the packing can be increased without affecting the length of the air
flow path and, therefore without increasing the draught loss.




                                          [61]
                  Fig. 5.14 : Cross-flow packing arrangement

5.3.2   Screens

        Circulating water, whether it is taken from a river or from the sea, contains
matter which could easily block condenser and cooler tubes. Among the things
most likely to cause blockages are leaves, fish, wool, and other industrial waste.
Most of the water drawn in is used in the condenser and, therefore, all the water
passes through a screen, immediately before the CW pump, this screen has holes
of such a size that it will allow only those particles which will easily pass through
the condenser to enter the system. After the water has passed through the
moving screens, it still contains some particles that would block the smaller tubes
in the auxiliary plant coolers, such as the air and oil coolers on electric motors,
and the oil coolers on the variable speed couplings. Water which is tapped off the
main CW system to be used in the auxiliary coolers is passed through strainers
with smaller perforations.

        So that large floating debris, such as logs and oil drums, are not sucked
into the system with the possibility of damage to the moving screens, fixed coarse
screens are mounted at the main intake. These take the from of vertical bars, an
dare spaced up to 300 mm (1 ft) apart. If anybody should fall into the water at the
main intake these fixed coarse screens also prevent them from being drawn into
the system. Fig. 5.14 shows the relative positions of the screens and strainers in
the CW system.




                           Fig. 5.15 : Screen positions




                                         [62]
5.3.2.1 Fixed coarse screen

       These screen consists of vertical steel bars which are placed close enough
together to stop large debris entering, but because these screens are not
continuously cleaned the bass have to be so for apart that smaller debris will not
build up and cause a blockage. The bass may upto 300 mm apart. As the
cleaning of the screens is carried out by raking, it is important that these should be
a minimum at cross members.

        The screen bars are usually manufactured of mild steel, but are given a
protective coating, possibly galvanized, but more probably a bituminous paint or
an epoxy based paint.

5.3.2.2 Moving Screens

        The open traveling or band screen is shown in fig. 5.16 with a band screen
the civil engineering costs are much lower than with the previously mentioned
screens. It is used where there is a high tidal difference and a smaller
requirement for water. Unfortunately, because of the many moving parts, the
maintenance costs are must higher; these costs must be considered in relation to
the savings in the civil engineering costs.




                        Fig. 5.16 : Band or moving screen

        The screen consists of two chains suspended by chain rollers running in
two sprockets fixed to a head shaft and guided by fixed tracks. The screen panels
are fastened to the inner links of the chain and are made up as in the cup screen
design. Wash water sprays and trash collecting buckets and baskets are provided
as in the cup screen design. It is very important to keep the load on the band
screen to a minimum because a high differential pressure across the screen at
low tide can cause severe buckling of the screen. It is important, therefore, to
keep the screen clean. To assist the sprays in this task, the screen can be driven
at varying speeds up to 0.15 m/s (30 ft/min) but because of the wear on the
moving parts, periods of running at high speeds should be kept as short as
possible, and whenever possible the screen should be stopped.



                                         [63]
       As a safety device against the screen being permanently damaged by
buckling, a by-pass door is sometimes provided which opens on receipt of a high
screen differential pressure signal. This allows unscreened water into the
condenser. Although this system saves the screen from permanent damage, it
can result in blocked condenser end plates and tubes.

       This type of screen also requires more operator attention, not only
because the screen has to be regularly inspected for screen blockage, but also
because someone has to make sure that the rollers are running centrally on the
sprockets and in the guide tracks.

5.3.3   Strainers

        The strainer mesh has to be sufficiently fine to stop small particles that
have passed through the CW screens from blocking the small auxiliary coolers.
The mesh size is approximately 4.00 or 4.75 mm (5/32 or 3/16 in) diameter. The
strainer bodies are made from cast iron or fabricated mild steel while the mesh is
brass or stainless steel.

        Because of the fineness of the mesh, it is important that the strainers are
kept clean. A differential pressure gauge is fitted across the strainer so that an
operator can tell when it is becoming blocked. The strainers are arranged in pairs,
in parallel, and the inlet and outlet valves of each strainer are connected so that
the valves on the stand-by strainer are opened as the valves on the strainer in
service are shut.

        An alternative type of strainer is the basket strainer where, instead of the
double reel, there is a basket which can be lifted out for cleaning when necessary.
This type is now being superseded by the double reel strainer.

        Strainers can also be made to have a continuous back-washing system,
which reduces the work required from the operator and is useful where a strainer
might be required in a relatively remote part of the power station. Fig. 5.17 shows
a rotating disc strainer which is having back-washing arrangement.




                        Fig. 5.17 : Rotating disc strainer

                                        [64]
         In this type of strainer, a small portion of the cleaned water passes back
through the strainer disc, which is motor driven; an alarm sounds if the motor fails
or if the screen differential pressure becomes too high.

5.4     CHLORINATION

        Chlorination of cooling water is required to be done, because water from
river or sea contains algae, mussel and bacteria. These may choke the cooling
water tubes and reduces the plant efficiency. The atmosphere inside condenser
is highly positive for the growth of bacteria which reduces the heat transfer rate.
To control and kill the small living organism like mussel and marine fouling,
chlonination is done periodically or as it is required. Chlorine can be dozed at the
cooling towers or at the site of course or primary screen. To ensure the cooling
water is free from bacteria and living organism, residual chlorine content is
checked at the C.W. outlet of condenser. Generally it should be 1ml/m3 (1 ppm
Vol/hr). The amount of chlorine needed to prevent marine fouling is larger than
that required to prevent-slime buildup, specially in the spring and summer month
and continuous chlorine dozing may be required. Bacteria, slime and algae are
rapidly killed by chlorine.

5.5     CIRCULATING WATER PUMPS

5.5.1   GENERAL

         Two types of C.W. pumps are used in thermal power stations. First is
horizontal split casing double-entry designs and second is vertical spindle bottom
inlet single-entry type. The horizontal pumps, while easy to maintain, require a
large floor space, and the weight problems on the larger units necessitated the
use of multiple casing castings. The alternative vertical spindle pumps, now
generally specified, have been developed for both metal casing and concrete
volute applications, with the choice depending on the overall economics of the
installation. They have significant advantages compared with the horizontal
designs in terms of the compactness and cost of mechanical, electrical and civil
engineering work.

        Considering the hydraulic designs the specific speed is limited to 1.6. This
reduces the risk of both head/flow characteristic instability and high powers at low
flows. It also gives a good efficiency spread over the range of operating flowrates.
The head/flowrate and absorbed power characteristics are now controlled by the
following specification :

              Pumps are designed so that the discharge head falls continuously
               as the flowrate is increased from zero to rated capacity by a
               minimum of 1% for any 15% increase in discharge flow.
              The pumps are capable of sustained running at any point on their
               head/flow curve from 25% to the maximum runout condition.
              The input power rises continuously as the flow is increased from
               zero to the rated flowrate.

      Circulating water pump speeds are normally in the range 150-300 r/min.
Low speed motors, suitable for direct coupling to the pumps, are expensive and it

                                        [65]
is usually economically justifiable to include a step-down gearbox between the
pump and motor. The combined efficiency of a 1000 r/min high speed motor and
associated gearbox can be similar to the low speed multi-pole motor. When
comparing capital costs of the two options, the costs of extra cranage equipment
and the additional civil work to support the heavier motor have to be added to the
direct drive pumpset costs. The two main advantages of the direct drive option
are its simplicity, which can lead to higher availability, and the elimination of any
noise control requirements arising from the high speed motor / gearbox
combination.

       For flowrates over 10 m3/s, main cooling water pumps are now based on
the concrete volute type. Below around 6 m3/s, it is impractical to consider
concrete volutes because of physical access limitations in the mechanical seal
area, and metal casing designs have been used.

5.5.2   HORIZONTAL SPLIT-CASING PUMPS.

        This design of pump has the major advantage that the top section of
casing and the rotating element can be removed without breaking either the pipe
joints or disturbing the alignment. While this feature gives good accessibility for
both overhaul and maintenance work, the horizontal design has a number of
disadvantages apart from the large floor area required. In particular, the
preference for installing the unit with the impeller eye below minimum water level,
to eliminate the need for complicated priming equipment, results in expensive civil
excavations. The physical size of the large metal casings has also brought about
both manufacturing difficulties and flexing problems with the complicated
assembly.

5.5.3   VERTICAL PUMPS

5.5.3.1 Vertical metal-casing pumps

       Volute casing designs are used for pumping duties up to around 8 m3/s.
On low head coastal applications, standard gear-driven units may be prefferd,
while on the higher head inland cooling tower applications, it has been possible to
use a direct motor drive while still retaining the same equivalent specific speed as
the equivalent gear-driven units.

        With this design, the pump forms part of the piping and its intake is from a
relatively unsophisticated suction chamber; it generally has a smaller civil
engineering cost than the equivalent concrete volute design. There is not
complicated volute to construct, and as the pump has a smaller overall diameter
than the concrete volute, the pumphouse floor area is smaller. On the debit side,
the bowl pump arrangement has a much higher mechanical pumping equipment
cost.

        Maintenance of the bowl pump is also difficult and the crane normally has
to be sized to take the total pump weight for installation and overhaul. As the
pump is withdrawn vertically for major overhaul work, a suction isolating valve is
not required. This however leads to a relatively long pump to facilitate vertical
isolation. One major technical disadvantage of the bowl pump is the need for

                                         [66]
submerged shaft bearing bushes. These are not readily accessible and can have
a relatively short life in silt-laden water.

         Casting problems have tended to limit the size of both the volute and bowl
pump designs. The standard use of cast iron necessitates the inclusion of
substantial allowances on coastal units to allow for corrosion/erosion effects over
the life of the station.

5.5.3.2 Concrete volute pumps

        Figure 5.18 shows the sectional arrangement of a typical concrete volute
pump which follows the same hydraulic principles as the conventional metal
casing designs except for the volute which is formed on-site on the concrete
foundations. The pump casing consists of permanently-embedded top and
bottom seating rings separated by the cutwater and stay vanes. These are
carefully positioned over the inlet duct. The assembly then acts as both a sealing
ring for the top cover and also as a central location around with the precision-
made volute shuttering framework is assembled prior to concrete pouring. The
volute shuttering is normally provided by the civil contractor to the pump
manufacturer's dimensions, and can be re-used on all pumps. Figure 5.19 shows
the various stages of construction of the volute. From experience, it has been
found that no special protective coating is required on the concrete to guard
against the effects of the water being pumped. Both wear and life expectancy of
the concrete are excellent and no problems have been reported so far.




      Fig. 5.18 Bottom inlet single-entry impeller concrete volute pump
                                        [67]
        A fabricated steel bearing bracket is mounted on top of the pump cover
and bolted down and dowelled into position to secure correct alignment. The
speed reducing gearbox is mounted on top of the bearing bracket, with the unit
incorporating the pumpset forced lubrication system. The rotating assembly is
supported by a combined thrust and journal bearing which can be located within
the gearbox. Access to the bottom bearing and mechanical seal is gained from
within the tubing.




                   Fig. 5.19 Construction of concrete volute



5.5.4   GEARBOXES

        Gearboxes, when included on vertical pump units, must be of the coaxial
type wit the pump, motor and gearbox centerlines positioned vertically above each
other to ensure an equal load distribution on the support ring.

        Coaxial gearboxes may be either of the multilayshaft or epicyclic designs.
With recent epicyclic designs, it has been possible to combine the gearbox and
the pump by mounting the impeller on the low speed output shaft of the gear unit
(Fig. 5.20). This arrangement eliminates the need for one journal bearing, which
in turn reduces the overall height of the pumpset leading to a more compact
arrangement.

         A further reduction in pumpset length is achieved by combining the thrust
collar into the coupling hub, as shown in fig. 5.20.

        The gearbox is equipped with a complete forced lubricating oil system to
provide a continuous flow of oil to the gear internals and the thrust and journal
bearings throughout the pumpset. The oil system includes both gear-driven and
standby motor-driven oil pumps, and the piping and non-return valve layout allows
delivery of oil to the bearings and gears, whichever direction the pump is rotating.




                                        [68]
Fig. 5.20 CW pump gearbox




 Fig. 5.21 Pump shaft seal



            [69]
5.5.5   SHAFT SEALS

       Mechanical seals are included to prevent water escaping from the casing
along the pump shaft. Split type seal designs are used (Fig. 5.21). All the
components which are subject to wear, are split into two sections to permit
inspection and replacement of worn components to be carried out without any
major dismantling of the pumpset.

        The seal needs to provided with a clean supply of flushing water which
should be filtered water taken from the pump discharge. Figure 5.22 shows a
typical seal flushing arrangement on a coastal station, where the filtered water is
normally taken from the pump discharge for flushing purposes while the pump is
running.

        The seal assembly also includes an additional inflatable static seal. This is
operated by air and forms a watertight joint around the shaft to allow dismantling
of the main seal unit without dewatering the pump. A secure source of air is
required and the use of a standby accumulator eliminates the need for the station
air supply accumulator eliminates the need for the station air supply to be
available at all times.

       Any water which leaks into the access well of the concrete volute pumps is
removed to the drainage sump by either a small submersible pump or an air-
operated ejector system. These can be controlled from float switches or an
adjustable timer.




                 Fig. 5.22 Typical seal water flushing system.

                                         [70]
5.5.6   MATERIALS

        For fresh water applications, grey cast iron is suitable for pump casings an
delivery mains. However, for seawater conditions, severe erosion and corrosion
of the cast iron occurs. Although generous allowances can be included in the
material thickness, these may not prevent the need for replacement of the cast
iron parts during the life of the station.

        The use of austenitic Ni-resist cast irons for the casing material of cooling
water pumps where severe conditions exist is now increasing rapidly. This
material, particularly in its spheroidal graphite form, offers proven superior
resistance to attack, with negligible wear after several years' operation.

         On concrete volute pumps, the built-in metal parts must be suitable for the
life of the station. Again, while gray cast iron an be used for inland fresh-water
sites, austenitic Ni-resist cast iron is required for seawater applications.

        Circulating water pump impellers are supplied in stainless steel with
renewable eye rings to cater for the erosion effects in this close clearance / high
velocity area. Both 13/4 chrominum-nickel steel and 18/10 chromium-nickel
austenitic stainless steel have been used successfully in freshwater and seawater
applications.

        Pump shafts are normally made of carbon steel and are fully protected by
stainless steel sleeves through the waterways. Special attention is required at the
sleeve/sleeve and sleeve/impeller joints to prevent ingress of water onto the shaft
surface, which can cause corrosion fatigue. On horizontal pump designs,
overlapping sleeves and radial O-rings have been introduced to cater for the
effects of shaft static and dynamic deflections. Mounting the mechanical seal on
top of the shaft sleeve permits seal refurbishment without slackening the sleeve
nuts, with resultant loss of axial compression.




                                         [71]
GLOSSARY

a)   Atmospheric pressure : Above the surface of the earth there is a layer of
     atmosphere or air and the weight of this air upon the earth surface exerts a
     pressure is called an atmospheric pressure.
b)   Gauge pressure : An instrument, which record pressure above
     atmospheric pressure is called a pressure gauge, and the reading of the
     gauge is called gauge pressure.
c)   Absolute pressure : It is a sum of atmospheric pressure and gauge
     pressure.
d)   Vacuum : A pressure below atmospheric pressure is called vacuum.
e)   Condenser vacuum : The difference between the atmospheric pressure
     and the pressure measured at the steam inlet and expressed in mm of Hg
     at a temp of 320F.
f)   Absolute pressures in the condenser : The difference between the
     barometric pressure and the condenser vacuum and expressed in mm of
     Hg.
g)   Heat head : The difference between the inlet circulating water temperature
     and the temperature corresponding to the absolute pressure at the steam
     inlet to the condenser.
h)   Temperature rise : The difference between the outlet and inlet circulating
     water temperature.
i)   Terminal temperature difference (TTD) : The difference between the
     temperature corresponding to the absolute pressure at the steam inlet to
     the condenser and the outlet circulating water temperature.
j)   Condensate depression : The steam after condensing forms condensate,
     which is also under vacuum, collects in a hotwell at the bottom of the
     condenser and requires pumping out. The condensate depression is the
     difference between the temperature corresponding to the condensate in
     the hotwell and the temperature corresponding to the absolute pressure of
     the steam at the inlet to the condenser. It is the actual number of degrees,
     the condensate is subcooled and must be maintained within closed limits
     since subcooling reduces the saturation pressure and hence the suction
     pressure on the intake of the condensate pump, reducing its performance.
k)   Dry bulb temperature : It is a temperature measured by an ordinary
     thermometer.
l)   Wet bulb temperature : If a wick wetted with water and air surrounds the
     bulb of an ordinary thermometer passed around it, water at surface will
     begin to evaporate.         The vapourisation takes heat-latent heat of
     vapourisation and the wick is cooled. The temperature finally reached
     when cooling stops is known as wet bulb temperature.
m)   Cooling tower approach : The temperature difference between WBT of
     incoming air and outgoing temperature of water is known as cooling tower
     approach.




                                     [72]
REFERENCE

     1)   CEGB Manual on Turbine and Auxiliary
     2)   B.E.I. Manual on Turbine and Auxiliary
     3)   BHEL Course Material.




                               [73]

				
DOCUMENT INFO
Description: Chapter 1 THE CONDENSER 1.1 INTRODUCTION The steam condenser plays a measure role in all modern thermal power stations. The main purpose of condenser is to condense the exhausted steam from low pressure turbine and convert it in to water i.e. condensate, so that it could be reused in Boiler as feedwater. The condensing equipment plays the role of cold source in the thermo-dynamic cycle of steam turbine installation. The decrease of temperature of cold source increase the thermal efficiency of cycle. The increase of economy of steam turbine installation is affected by two major factors. Firstly achieving with high thermal efficiency of the cycle itself and secondly by operational perfection of different components of the set. The former is affected by increase of initial parameter as well as decrease of back pressure of steam. The increase of initial parameter has been a fashion and pressure of 300 atmospheres and temperature upto 5650C have already been achieved in western countries but the main obstacle in this direction is suitable and cheap heat resistant steels. With decrease of back pressure, the temperature of exhaust steam is decreased which in turn decreases the quantity of heat given to the cold source and thereby increasing the thermal efficiency. The above goal is achieved by condensing the exhaust steam from the turbine in the condenser. 1.2 PRINCIPLE OF A CONDENSER 1.2.1 Volume of Steam If water is put into a closed vessel and heated, a quantity of heat known as sensible heat is required to bring the water to boiling point and if further heat is added to convert the water into steam this is known as latent heat. The volume of the steam formed is far greater than that of the water and consequently the pressure in the vessel rises. thus the application of the latent heat has caused an increase in pressure. (See Figure 1.1 and 1.2). 1.2.2 Removal of Heat Now reverse the process and remove some heat by cooling the vessel. During this coolin