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									Block 14 Condensate Recovery                 Flash Steam Module 14.6

Module 14.6
                               Flash Steam

The Steam and Condensate Loop                               14.6.1
         Block 14 Condensate Recovery                                                                   Flash Steam Module 14.6

                                                   Flash Steam
         The formation of flash has already been discussed in Module 2.2, ‘What is steam’, and a major
         flash steam application has been covered in Module 3.13, ‘Heat recovery from boiler blowdown’.
         This Module will provide a brief reminder of these earlier Modules; discuss how flash steam is
         formed, and focus on how flash steam can be used effectively to improve steam plant efficiency.

         What is flash steam and why should it be used?
         ‘Flash steam’ is released from hot condensate when its pressure is reduced. Even water at an
         ambient room temperature of 20°C would boil if its pressure were lowered far enough. It may be
         worth noting that water at 170°C will boil at any pressure below 6.9 bar g. The steam released by
         the flashing process is as useful as steam released from a steam boiler.
         As an example, when steam is taken from a boiler and the boiler pressure drops, some of the water
         content of the boiler will flash off to supplement the ‘live’ steam produced by the heat from the
         boiler fuel. Because both types of steam are produced in the boiler, it is impossible to differentiate
         between them. Only when flashing takes place at relatively low pressure, such as at the discharge
         side of steam traps, is the term flash steam widely used. Unfortunately, this usage has led to the
         erroneous conclusion that flash steam is in some way less valuable than so-called live steam.
         In any steam system seeking to maximise efficiency, flash steam will be separated from the
         condensate, and used to supplement any low pressure heating application. Every kilogram of
         flash steam used in this way is a kilogram of steam that does not need to be supplied by the
         boiler. It is also a kilogram of steam not vented to atmosphere, from where it would otherwise be
         The reasons for the recovery of flash steam are just as compelling, both economically and
         environmentally, as the reasons for recovering condensate.
         How much flash steam is available?
         If use is to be made of flash steam, it is helpful to know how much of it will be available. The
         quantity is readily determined by calculation, or can be read from simple tables or charts.
         Example 14.6.1 - Consider the jacketed vessel shown in Figure 14.6.1
         The condensate enters the steam trap as saturated water, at a gauge pressure of 7 bar g and a
         temperature of 170°C. The specific amount of heat in the condensate at this pressure is 721 kJ / kg.
         After passing through the steam trap, the pressure in the condensate return line is 0 bar g. At this
         pressure, the maximum amount of heat each kilogram of condensate can hold is 419 kJ and the
         maximum temperature is 100°C. There is an excess of 302 kJ of heat which evaporates some of
         the condensate into steam. The quantity of steam is calculated in the following text.

                 Ball valve
                                                                  Air vent

         Constant pressure                                                              Condensate at 7 bar g
         steam at 7 bar g                                                               hf = 721 kJ / kg

                                                                                                 Condensate at 0 bar g
                                                                                                 hf = 419 kJ / kg
                Steam at 7 bar g

                                                                             Excess heat at 0 bar g
                                                                             = 721 - 419 kJ / kg
                                                                             = 302 kJ / kg

                                   Fig. 14.6.1 Excess heat in condensate produces flash steam

14.6.2                                                                                        The Steam and Condensate Loop
Block 14 Condensate Recovery                                                                                                        Flash Steam Module 14.6

The heat needed to produce 1 kg of saturated steam from water at the same temperature, at
0 bar gauge, is 2 257 kJ. An amount of 302 kJ can therefore evaporate:

                                                          NJ RI VWHDP SHU NJ RI FRQGHQVDWH

From each kilogram of condensate in this example, the proportion of flash steam generated
therefore equals 13.4% of the initial mass of condensate.
If the equipment using steam at 7 bar g were condensing 250 kg / h, then the amount of flash
steam released by the condensate at 0 bar g would be:
                                           0.134 x 250 kg / h of condensate = 33.5 kg / h of flash steam
Alternatively, the chart in Figure 14.6.2 can be read directly for the moderate and low pressures
encountered in many plants.
The example shown in Figure 14.6.1 is depicted in Figure 14.6.2 and shows that 0.134 kg of
flash steam is produced per kg of condensate passing through the trap.


                                                                                     ar g

                                                                                                   ar g
                                                                             ar g

                                                                                     ar g

                                                                                                            ar g


                                                                                                                    0 ba
                                                                                    2.0 b
                                                                           2.5 b

                                                                                                 1 .0 b
                                                                                        1 .5 b

                                                                                                          0 .5 b



        Pressure on traps bar g









                                       0    0.02         0.06            0.10               0.14                   0.18      0.22
                                                                                      0.134 (See Example 14.6.1)
                                                                  kg Flash per kg condensate
                                                                Fig. 14.6.2 Flash steam graph

The Steam and Condensate Loop                                                                                                                      14.6.3
         Block 14 Condensate Recovery                                                         Flash Steam Module 14.6

         Sub-cooled condensate
         If the steam trap is of a thermostatic type, the discharged condensate is sub-cooled below saturation
         temperature. The heat in the cooler condensate will be slightly less, and the amount of flash
         steam produced would be less.
         If the trap in Example 14.6.1 discharged condensate at 15°C below the steam saturation
         temperature, then the available heat in the condensate would be less.
         Example 14.6.2 Consider condensate discharging at 7 bar g and with 15°C of subcooling
           Temperature of saturated condensate at 7 bar g = 170°C
                                          Amount of sub cooling = 15°C
         Temperature of sub-cooled condensate at 7 bar g = 155°C
         From steam tables:
                  Amount of heat in condensate at 155°C = 654 kJ / kg
           At 0 bar g, saturated condensate can only hold = 419 kJ / kg
           Surplus heat in saturated condensate at 0 bar g = 235 kJ / kg
                                         Heat in steam at 0 bar g = 2 257 kJ / kg
                                                                      N- NJ
                                        Proportion of flash steam
                                                                      N- NJ
         Proportion of flash steam from the condensate = 0.104 (10.4%)

         Therefore, in this example, condensate discharging at a temperature lower than the saturation
         temperature has reduced the proportion of flash steam from 13.4% to 10.4%.

         Pressurised condensate
         Example 14.6.3 Consider the condensate in Example 14.6.1 discharging to a flash vessel
         pressurised at 1 bar g
         If the return line were connected to a vessel at a pressure of 1 bar g, then it could be seen
         from steam tables that the maximum heat in the condensate at the trap discharge would be
         505 kJ / kg and the enthalpy of evaporation at 1 bar g would be 2 201 kJ / kg.
         The proportion of the condensate flashing off at 1 bar g can then be calculated as follows:
                                    Heat in condensate at 7 bar g = 721 kJ / kg
            At 1 bar g saturated condensate can only hold = 505 kJ / kg
           Surplus heat in saturated condensate at 1 bar g = 216 kJ / kg
                                         Heat in steam at 1 bar g = 2 201 kJ / kg

                                                                       N- NJ
                                        Proportion of flash steam
                                                                       N- NJ
         Proportion of flash steam from the condensate = 0.098 (9.8%)
         In this example, if the equipment using steam at 7 bar g were condensing 250 kg / h of steam,
         then the amount of flash steam released by the condensate at 1 bar g would be
         0.098 x 250 kg / h = 24.5 kg / h of flash steam.
         Therefore, the amount of flash steam produced can depend on the type of steam trap used, the
         steam pressure before the trap, and the condensate pressure after the trap.

14.6.4                                                                              The Steam and Condensate Loop
Block 14 Condensate Recovery                                                                  Flash Steam Module 14.6

The flash steam recovery vessel (flash vessel)
Flash vessels are used to separate flash steam from condensate. Figure 14.6.3 shows a
typical flash vessel constructed in compliance with the European Pressure Equipment
Directive 97/23/EC.
After condensate and flash steam enter the flash vessel, the condensate falls by gravity to the
base of the vessel, from where it is drained, via a float trap, usually to a vented receiver from
where it can be pumped. The flash steam in the vessel is piped from the top of the vessel to any
appropriate low pressure steam equipment.

                                                 Flash steam out

                    Condensate in

                                                    Condensate out
                       Fig. 14.6.3 A typical flash vessel constructed to European standards

Sizing flash steam recovery vessels
To size a flash vessel, the following information is required:
o   The steam pressure before the steam trap(s) supplying the vessel.
o   The total condensate flowrate into the flash vessel.
o   The flash steam pressure in the flash vessel.
Using this information, together with a flash vessel sizing chart (see Figure 14.6.4), the size of the
vessel can be determined. Example 14.6.4 demonstrates flash vessel sizing, using a chart.

The Steam and Condensate Loop                                                                                14.6.5
                      Block 14 Condensate Recovery                                                                                       Flash Steam Module 14.6

               Example 14.6.4 Determine the size of a flash vessel to suit the following conditions:
               The pressure onto the steam traps is 12 bar g with a total condensate flow of 2 500 kg / h. The
               flash steam from the vessel is to be supplied to equipment using low pressure steam at 1 bar g.
               1. From the ‘Pressure on steam traps’ axis at 12 bar g, move horizontally to the 1 bar g flash
                  steam pressure curve at point A.
               2. Drop down vertically to the condensate flowrate level of 2 500 kg / h, point B, and follow the
                  curved line to point C.
               3. Move right from point C to meet the 1 bar g flash line at point D.
               4. Move upwards to the flash vessel size and select the vessel.
               For this example, an FV8 flash vessel would be selected.
                                                                    Flash steam pressure bar g
                                                                           7 65 4 3 2 1
          Pressure on steam traps bar g


                                                     12                            A                   Example


                                                      8                                                          Flash vessel size





                                                           0 2 4   6 8 10 12 14 16 18 20%

                                                                                                                                                              Flash steam pressure bar g
                                                    250                                                                                                 0.5
                                                    300                                                                                                 1
                                                    400                                                                                                 1.5
         Condensate or blowdown flowrate kg /h

                                                    500                                                                                                 2
                                                   1 000                                                                                                5
                                                                                                 C                                   D
                                                   2 000
                                                   3 000
                                                   4 000
                                                   5 000

                                                  10 000
                                                  15 000
                                                  20 000
                                                  30 000
                                                                                Fig. 14.6.4 Flash vessel sizing chart

                 Requirements for successful flash steam applications
                 If full use is to be made of flash steam, some basic requirements must be satisfied:
                 o                               It is essential to have a continual supply of sufficient condensate from applications operating
                                                 at higher pressures, to ensure that enough flash steam can be released for economic recovery.
                 o                               The steam traps and the equipment they are draining must be able to function satisfactorily
                                                 against the backpressure applied by the flash system.

14.6.6                                                                                                                       The Steam and Condensate Loop
Block 14 Condensate Recovery                                                                     Flash Steam Module 14.6

o   Care must be taken when attempting flash steam recovery with condensate from temperature
    controlled equipment. At less than full-load, the steam space pressure will be lowered by
    the closing action of the steam control valve. If the steam pressure in the equipment approaches
    or falls below the specified flash steam pressure, the overall amount of flash steam formed
    will be marginal, and one must question whether recovery is worthwhile in this instance.
o   It is important that there is a demand for low pressure flash steam that either equals or exceeds
    the flash steam being produced. Any deficit of flash steam can be made up by live steam from
    a pressure reducing valve. If the supply of flash steam exceeds its demand, surplus pressure
    will be created in the flash steam distribution system, which will then have to be vented to
    waste through a surplussing valve.
o   It is possible to utilise the flash steam from condensate on a space heating installation - but
    savings will only be achieved during the heating season. When heating is not required, the
    recovery system becomes ineffective. Wherever possible, the best arrangement is to use
    flash steam from process condensate to supply process loads - and flash steam from heating
    condensate to supply heating loads. Supply and demand are then more likely to remain in-step.
o   It is preferable to actually use the flash steam close to the high pressure condensate source.
    Relatively large diameter pipes are used for low pressure steam, to reduce pressure loss and
    velocity, which can mean costly installation if the flash steam has to be piped any distance.
Control of flash steam pressure
Another consideration is a method of controlling the pressure of the flash steam.
In some cases, flash pressure will find its own level and nothing more needs to be done. When
supply and demand are always in-step, and particularly if the low pressure steam is used on the
same equipment producing the high pressure condensate, it is only neccessary to pipe the flash
steam to the low pressure plant without any other control.
Figure 14.6.5 shows the application of flash steam recovery to a multi-bank air heater battery,
which is supplying high temperature air to a process. Condensate from the high pressure sections
is taken to the flash vessel, from where the low pressure flash steam is used, to preheat the cold
air entering the battery via the frost coil (preheater). The surface area of the preheater section,
and the relatively low temperature of the incoming air, will mean that the low pressure flash
steam is readily condensed.

                  Temperature control valve

        High pressure steam supply

                                   Flash steam

            Air flow


        Flash vessel bypass line

                                                 Flash vessel
Low pressure

                         Fig. 14.6.5 Flash steam recovery on a multi-bank air heater battery

The Steam and Condensate Loop                                                                                   14.6.7
         Block 14 Condensate Recovery                                                                   Flash Steam Module 14.6

         Depending on operating temperatures, the flash steam will condense at some low pressure,
         perhaps even sub-atmospheric. If site conditions and layout permit, the flash vessel and the
         steam trap draining the preheater should be located far enough below the preheater condensate
         outlet to give enough hydrostatic head to push the condensate through the trap. If this is not
         possible, pumping traps can be used to drain both the preheater coil and the flash vessel.
         Steam condensing in the preheater at sub-atmospheric pressure will generally mean that a vacuum
         breaker is required on the flash steam supply to the preheater. This will prevent the pressure in
         the battery becoming sub-atmospheric, thereby assisting condensate flow to the trap. Drainage
         from the preheater trap is induced by gravity flow.
         Figure 14.6.6 shows an application where the flash steam system is kept at a specified constant
         pressure by steam fed from a reducing valve. This ensures a reliable source of steam to the low
         pressure system if there is a lack of flash steam to meet the load.

         Typical applications for flash steam
         Flash steam supply and demand in-step
         This gives maximum utilisation of the available flash steam. The air heater battery discussed in
         Figure 14.6.5 is one such system, but similar arrangements are practical with many other
         applications such as space heating installations using either radiant panels, or unit heaters.
         Figure 14.6.6 depicts a system where a number of heaters are supplied with high pressure steam.
         The condensate from approximately 90% of the heaters is collected and taken to a flash recovery
         vessel. This supplies low pressure steam to the remaining 10% of the heaters.
         With this system, the total heat output of the system is marginally reduced, as 10% of the heaters
         are operating at a lower steam pressure. However, it is rare to find an installation that does not
         have a sufficient margin of output above the normal load to accept this small reduction.
         Sometimes a problem arises where the use of available flash steam may require more than one
         heater but less than two. It would be better in this case to connect two heaters to the flash steam
         supply, rather than vent the excess flash steam off to waste. Two heaters together will usually pull
         the flash pressure down to a lower level, even to sub-atmospheric levels. To cope with this, the
         supply of flash steam can be supplemented with live steam from a pressure reducing valve.

                                                     Pressure reducing valve set

         High                High pressure heaters                             Low pressure heaters

                       High                                                                pressure
                     pressure                                                                traps
                                                                            Flash vessel
                                         Flash vessel bypass line
                                                                            Trap set                             condensate
                                         Fig. 14.6.6 Flash steam supply and demand in step

         Another example where supply and demand are ‘in step’ is the steam heated hot water storage
         calorifier. Some of these incorporate a second coil, fitted close to the bottom of the vessel adjacent
         to where the cold feedwater enters.
         Condensate and flash steam from the trap on the primary coil is passed directly to the secondary
         coil. Here, any flash steam produced by the drop in pressure across the trap is condensed, while
         giving up its heat to the feedwater. A typical arrangement is shown in Figure 14.6.7.

14.6.8                                                                                        The Steam and Condensate Loop
Block 14 Condensate Recovery                                                                                           Flash Steam Module 14.6

                                                                               Hot water out


                               Primary coil trapset
                                                                                               Primary coil

                                                                                               Secondary coil acting
                                                                                               as a flash cooler

                                                                                                      Return water in
      Low temperature condensate

                               Fig. 14.6.7 Secondary flash steam coil in a storage calorifier

Another example of this idea is shown in Figure 14.6.8. Here, a normal steam-to-water calorifier
drains condensate through a float trap to a smaller shell-and-tube heat exchanger (called a flash
condenser), in which the flash steam is condensed to sub-cooled condensate. The unit is fitted
such that the secondary flow pipework is in series with both calorifier and condenser. This enables
the secondary return water to be preheated by the condenser, thereby reducing the demand for
live steam in the first instance.
If the condensate in the flash condenser is likely to be sub-atmospheric, a mechanical pump is
required to lift the condensate to any higher return line. The motive steam exhausting from the
pump is itself condensed in the flash condenser. The pumping of the condensate is then achieved
at virtually no cost.
Consideration must be given to the pump filling head in that it needs to be greater than the
pressure drop across the flash condenser tubes under full-load conditions. A minimum head of
600 mm will usually achieve this.

                                                                                 Secondary flow

                                                                      Heating calorifier

                     Temperature control
                                                         Steam trap                            Secondary
                                                                                               flow path

                                                                                                   Shell-and-tube heat
             * Balance line                   Air vent                                             exchanger (flash condenser)
                                                                 Receiver                           Secondary return
                       Condensate return
                                                                                   Filling head > 600 mm

                                 Fig. 14.6.8 Packaged calorifier and flash condenser unit

The Steam and Condensate Loop                                                                                                         14.6.9
          Block 14 Condensate Recovery                                                              Flash Steam Module 14.6

          Flash steam supply and demand not in-step
          The arrangement in Figure 14.6.9 is an example of flash steam recovery where the supply and
          demand are not always ‘in-step’.
          Condensate from three jacketed pans and a drain pocket releases flash steam, but it can only be
          used to augment the supply of steam to the space heating installation. This is quite satisfactory
          during the heating season, as long as the heating load exceeds the availability of flash steam.
          During the summer season the heating equipment will not be in use, and even during spring and
          autumn the heating load may not be able to use all the available flash steam. The arrangement is
          not ideal, although it is quite possible for the steam savings made during the winter to justify the
          cost of the flash steam recovery equipment.
          Sometimes, surplus flash steam must be vented to atmosphere, and, as indicated, a surplussing
          valve is more suitable for this purpose than a safety valve, which usually has a ‘pop’ or ‘on / off’
          action and a seat arrangement designed for infrequent operation. The surplussing valve will be
          set so that it begins to open slightly above the normal pressure in the system. When the heating
          load falls and the pressure in the system begins to increase, the pressure reducing valve supplying
          the make-up steam closes down. A further increase of pressure, perhaps of 0.15 to 0.2 bar, is
          then allowed before the surplussing valve begins to open to release the excess flash steam.
          A safety valve may still be required if the surplussing valve fails. It must be set to open at a
          pressure between the surplussing valve set pressure and the system design pressure. It is usually
          convenient to fit the safety valve onto the flash vessel.
          Occasionally, during summer conditions it may be preferable to bypass the flash system with a
          manual valve (not shown in Figure 14.6.9). The condensate and its associated flash steam will
          then pass directly to a condensate receiver, where the flash steam will be vented to atmosphere.

                               Pressure reducing                                                           Surplussing
                                     valve                  Low                                               valve

                                                                             Condensate          Flash steam


                                                                        Condensate                vessel

                                         Fig. 14.6.9 Flash steam supply and demand not in-step

14.6.10                                                                                   The Steam and Condensate Loop
Block 14 Condensate Recovery                                                                       Flash Steam Module 14.6

Boiler blowdown heat recovery applications
Continuous blowdown of boiler water is necessary to control the level of TDS (Total Dissolved
Solids) within the boiler. Continuous blowdown lends itself to the recovery of the heat content of
the blowdown water and can enable considerable savings to be made.
Boiler blowdown contains massive quantities of heat, which can easily be recovered as flash
steam. After it passes through the blowdown control valve, the lower pressure water flows to a
flash vessel. At this point, the flash steam is free from contamination and is separated from the
condensate, and can be used to heat the boiler feedtank (see Figure 14.6.10).
The residual condensate draining from the flash vessel can be passed through a plate heat
exchanger in order to reclaim as much heat as possible before it is dumped to waste. Up to 80%
of the total heat contained in boiler continuous bowdown can be reclaimed in this way.

Cold                                                     Level
water                Make-up tank                      controller


                                                                      Boiler feedtank

                        supply to

                          Flash vessel
                        Float trap                                     valve              Boiler


                               Fig. 14.6.10 Typical heat recovery from boiler blowdown

The Steam and Condensate Loop                                                                                     14.6.11
          Block 14 Condensate Recovery                                                                Flash Steam Module 14.6

          Spray condensing
          Finally, consideration should be given to those cases where flash steam is unavoidably generated
          at low pressure, but where no suitable load is available which can make use of it.
          Rather than simply discharge the flash steam to waste, the arrangement in Figure 14.6.11 can
          often be adopted.
          This arrangement can be useful where the condensate receiver vent cannot be piped to outside,
          and where the presence of flash steam would be detrimental if left to discharge in a plant room.
          A lightweight stainless steel chamber is fitted to the receiver tank vent. Cold water is sprayed into
          the chamber in sufficient quantities to just condense the flash steam. The flow of cooling water is
          controlled by a simple self-acting temperature control, adjusted so that minimal amounts of flash
          steam appear from the vent. The process will use roughly 6 kilograms of cooling water per
          kilogram of flash steam condensed.
          If the cooling water is of boiler feed quality, then the warmed water is added to the condensate
          in the receiver and re-used. This will continue to make water savings throughout the year.
          If the cooling water is not suitable for recovery, the spray pipework can be installed as shown by
          the dotted arrangement. The cooling water and condensed flash will then fall to waste.

                               Vented to atmosphere

                                                                                         Water in

                                                                                temperature              Alternative
                                                                                control                  arrangement

                                                      Condensate receiver
                                                                                              Condensed water
                                                                                                 to waste
                                                                                      Overflow with ‘U’ seal
               Pumped condensate

                                                        Centrifugal pump
                                     Fig 14.6.11 Flash steam condensing and water saving by spray

14.6.12                                                                                   The Steam and Condensate Loop
 Block 14 Condensate Recovery                                                          Flash Steam Module 14.6


1. What is the difference between live steam and flash steam?
a| Live steam is made from water, flash steam is made from condensate                         ¨
b| Live steam is always hotter than flash steam                                               ¨
c| Live steam is made by adding heat to water, flash steam is made from heat already
   contained in water                                                                         ¨
d| Live steam is always at a higher pressure than flash steam                                 ¨

2. What percentage of flash steam is made from condensate at 10 bar g passing into a flash
   vessel at 0.5 bar g?
a| 12%                                                                                        ¨
b| 13%                                                                                        ¨
c| 14%                                                                                        ¨
d| 5%                                                                                         ¨

3. What is the effect on the production of flash steam from sub-saturated condensate?
a| The flash steam produced is less than that with saturated condensate                       ¨
b| The flash steam produced is more than that with saturated condensate                       ¨
c| There is no effect at all                                                                  ¨
d| Live steam is always at a higher pressure than flash steam                                 ¨

4. With reference to Example 14.6.1, what would be the proportion of flash steam produced
   if the flash pressure were 2.5 bar g?
a| 3%                                                                                         ¨
b| 6%                                                                                         ¨
c| 8%                                                                                         ¨
d| 10%                                                                                        ¨

5. In a steam system, the trap pressure is 15 bar g, the flash pressure is 0.5 bar g, and the
   condensate flowrate is 1300 kg / h. Which flash vessel is required?
a| FV6                                                                                        ¨
b| FV8                                                                                        ¨
c| FV12                                                                                       ¨
d| FV16                                                                                       ¨

6. What is used to top-up the flash pressure?
a| A safety valve                                                                             ¨
b| A larger condensate flow                                                                   ¨
c| A pressure surplussing valve                                                               ¨
d| A pressure reducing valve                                                                  ¨

                                  1: c 2: c, 3: a, 4: b, 5: b, 6: d

The Steam and Condensate Loop                                                                         14.6.13
          Block 14 Condensate Recovery             Flash Steam Module 14.6

14.6.14                                  The Steam and Condensate Loop

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