OPERATION AND MAINTENANCE of GAS TURBINES

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OPERATION AND MAINTENANCE of GAS TURBINES Powered By Docstoc
					        UNIT (5)
    OPERATION AND
     MAINTENANCE
          OF
     GAS TURBINES
1- INTRODUCTION
2- OPERATION
3- MAINTENACE
                            1- INTRODUCTION
      We can not go deeply in operation and maintenance of gas turbines because of
different types in their design. Each design has its own characteristics. Therefore
these subjects are discussed in a general way, and we advise persons dealing with
gas turbine to see the unit manuals sent by manufacturers.
      Usually, gas turbine works easily without troubles for long periods after
installation, tests, and overcoming difficulties at the site. Therefore, it needs almost
little care at its first periods of its life. Experience proved that gas turbines are better
than internal combustion engines and steam turbines. This is true; if it gets clear oil
and fuel and works within the capacity range, for 10000 hrs between medium
overhauls and the major overhauls are carried every 25000 hrs approximately.
Major components change, as blade changes, occur every 50000 to 100000 hrs on
working at part load. These times concern medium and big units, while small units
need much care and short periods between overhauls. It is important to carry routine
examinations, repairs and quick adjustments needed between overhauls to get the
highest efficiency of operation.
      Gas turbine power plants should have recording system of operating and
maintenance data since the first moment of its installation. Usually these units are
well protected against human mistakes or any other problems occurring during
operation stages. Protective devices trip the engine immediately at emergency cases
(which are, fortunately, rare), as flame extinction, fuel supply interruption or over-
fueling, compressor surge, over-speed, lubrication-fuel failure, control system
failure and other problems.




                                2- OPERATION
1- Starting:
         Operation of all gas turbines pass by interlocked sequence, during the
    action of protection devices. These devices prevent series mistakes and prevent
    starting if inadequate conditions exist. Despite the manual operation yet all
    modern units start automatically or semi-automatic, to reduce human mistakes
    to maximum. Starting automatically or manually, the first step is to operate the
    auxiliary and control systems that prevents turbine operation unless at necessary
    condition assure safety. Among these conditions, powering the control circuits,
    reaching the necessary oil quantity to these circuits and the throttles at minimum
    fuel flow, open of air intake and exit and others.

                                                  2
     In heavy rotor turbines, it is important to heat the oil to reduce its viscosity
to have suitable starting torque less than that of the starter. When the auxiliary
system work in a suitable way, the starting sequences begin if the unit work
automatically. If it works manually, starting depends on the operator who
pushes the bottoms and moves the arms and wheels. If the turbine is big, it may
have barring motor to move the shaft before engagement of the starting device
clutch. The clutch may be engaged by a large solenoid, pneumatic or hydraulic
cylinder. After this power is given to the starter and the shaft begins running
until reaching ignition speed at which the barring motor separates.
     Experience showed that the turbines using gaseous fuel should continue
running by the starter for few minuets before starting combustion in the
combustion chamber, to get red of any internal gas accumulation occurred
during previous trip.
     There are three types of igniters according to design of combustion
chamber, they are:
     - Sparkler electrode.
     - Glowing electrode
     - Torch ( ignites by a spark or glowing electrode)
     When the igniter is powered (this is done manually as it is not
automatically connected to the starter), the main fuel flows to burners and the
main flame appears and combustion continues. Combustion chamber must be
equipped by a mean to detect the flame. Usually, this mean permits to see the
flame through suitable lens and through electric signal to alarm lighting or
extinction of the flame. At this point, operator must be sure that flame began, if
not he must immediately stop the unit to avoid explosion of the turbine in the
next start.
     System of ignition stops after continuation of combustion and flame, for
certain time according the designer, especially if the unit works at humid and
rainy weather. Usually the igniter s are placed at moderate-temperature location
in the chamber, or to be equipped by a device to tire it to outside if it is placed in
the chamber core, when flame settles.
     When the flame settles the fuel increasingly flows to raise the temperature
of the gases entering the turbine to get output power.
     When the turbine, at low speeds, is not able to drive the compressor, as its
efficiency is low, it is important to let the starter continue driving the turbine
until reaching one third of its complete speed. At this point, the turbine can
drive the compressor by itself without the need of the starter. Then the turbine
reaches self-driving point. The speed increases until the minimum-governor-
speed value. It is important to control sharply fuel flow in this stage to avoid
overheating, or surge.
     It the unit is manually operated, then control of air/fuel ratio is not
important because acceleration rate depends on operator, who depends upon
apparatus and control desk instruments.
                                             3
        In all cases it advisable that the operator surveys starting through control
   panel to see lube oil pressure, temperature, speed and exhaust temperature. If
   the compressor stops acceleration followed by sharp rise in the temperature of
   gases entering or leaving the turbine, it is preferable to stop the unit because the
   compressor surges. Yet, the turbine is stopped by temperature control system.
        If the unit is equipped by lube-oil pump driven by the turbine, the auxiliary
   pump that works during starting separates at certain point during starting series,
   by the pressure switch. This switch remits this pump again to work if any
   failure, during operation, happens to that main pump. Alarm occurs to attract
   operator attention. Timing device adjust starting sequences, in all automatic
   operating units, and majority of manual starting units, as the overall timing is
   necessary to prevent starting sequences done by control devices, if any failure
   exists. For example, exceptionally low-calorific-value fuel prevents reaching
   self-driving condition that makes the turbine stay working under the effect of
   the starter to infinity, unless the operator stops the unit if he notice the length of
   time taken.

2- Availability:
   a- Definition:
     It is important that the operator knows the turbine availability, to know the
     turbine ability when needed to be operated. It is calculated on certain period
     by measuring the time of the turbine operate with respect to this period.
                        hrs, available to run
      Availability =                          x 100
                       hrs could be available
     Example:
     360 hrs are needed to maintain a turbine every 8000 hrs, the available time in
     which the turbine can operate = 8000-760 = 7240 hrs.
     Then the availability is = (7240/8000) x 100 = 91%

  b- Factors affecting availability:




                                                      4
1- Fuel type:
  Type of fuel affects strongly the time of maintenance, and by consequence
  the availability, Fig. (1/5).

2- Starting frequency:
   It is the ratio between number of starting the turbine to the operation time.
  If a turbine works during 10 hours for only one start, then starting
  frequency is 1/10. Figure (2/5) shows the relation between the starting
  frequency and the time needed for maintenance. Increasing the operation
  time reduces the starting frequency that reduces maintenance time.
  Therefore, the availability increases.




There are two ways of operation:
    a- Normal start: When the turbine takes the base load. This lets the
       turbine operates long periods. In this case warming up time is normal
       and relation between maintenance time and starting frequency is as in
       Fig. (2/5).

     b- Fast start: When the turbine operates as emergency unit or black start
        or in the peak load. In this case it is needed to operate and load the
        turbine quickly, which means short (or no) heating up period. This
        affect turbine safety and needs approached maintenance periods and
        long maintenance time due to high stress in fast starting, Fig. (3/5).




                                         5
3- Loading:
   In normal operation, the turbine works at base load, but in emergency it
   may operate at loads above base load, or excess load. In this case, it is
   subjected to abnormal stresses that needs approached maintenance and
   inspection periods. That increases the maintenance time and by
   consequence reduces the availability, Fig. (4/5).




                                       6
     4- Environment:
        Turbines existing in desert places face dust and sand or to salty
        environment, that affects compressor blades due to erosion and corrosion,
        and by consequence long maintenance time. This reduces availability.

     5- Maintenance and operating procedures:
         carrying maintenance in time and solving operation problems as well as
        continuous surveying turbine operation keep the turbine safe for long
        period and reduce maintenance time, this increases the availability.

3- Pre-start operation (part 1):
  Preparation a turbine to operation is divided into:
  a- Pre-start inspection: it is necessary to be sure of all turbine requirements and
     devices when the turbine is in base load and take the necessary time for this
     inspection. When working as black start the turbine should be ready for
     inspection and operation at short time.

  As an example, the single shaft turbine shown in Fig. (5/5).

  b- Safety precautions:
     1- Operator should be ware of safety rules, for example ear muffs for ear
        protection.
     2- The operation selector switch should be on off to prevent sudden




        operation during inspection.


     3- Co2 extinguisher system should be stopped before inspection by putting
        the mechanical gag to prevent working and filling the turbine with co2 gas
        during the existence of any person in the unit, Fig. (6/5).
                                              7
   4- In sometimes the operator should wear special custom as gloves on
      dealing with acids or alkaline liquids

c- Inspecting the turbine section:
   1- Inspect the switch gear room, where generator protection devices and
      protective relays in set not in trip
   2- Inspecting generator air filter section and compare louver arm position
      that lets cold air to the generator, Fig. (7/5 a) and the thermostat reading,
      Fig. (7/5 b). The unit must be clean of tools, cloths or any solid bodies
      near air filter.




                                            8
3- Inspection of exciter and exciter brushes and the neutral transformer, Fig.
   (8/5).
4- Inspection of gearbox for any oil leak and the ground brushes.
5- Inspection of turbine section for cooling water from the cooler.
6- Inspection of control air section, fuel lines and fuel dividers for any leak.
7- Inspection of fuel oil pump section, gearbox, high pressure-oil filter and
   pressure difference before and after lube-oil filter, Fig. (9/5).
8- Be sure that the turbine low-running speed by the turning gear.




                                         9
4- Pre-start operation (part 2):
  a- Inspecting the remaining turbine sections:
     1- Inspection of lube-oil section to be sure of pump pressure and oil pressure
        at bearings by gauges shown in Fig. (10/5).




     2- The starter is a diesel engine, so inspect the engine and the back of gauge
        panel as in Fig. (11/5), and noticing any leakage. Inspect the fuel oil stop
        valve, Fig. (12/5 a). Tightening of filter cover, Fig. (12/5 b).
     3- Inspect gearbox section to be sure of no leakage of in cooling oil pump.
        Be sure of over speed device, Fig. (13/5).



                                             10
   Check the water level in the cooling system, Fig. (14/5), and the lube-oil-
   sump level gauge, Fig. (15/5).

4- Inspect again co2 room and return the mechanical gag as in Fig. (16/5).




                                       11
b- Checking the local control room:
   The last part to be inspected is the control room. Survey the following:

   1- All AC and DC switches are in safe position for operation.
   2- Check the batteries connection and solution level inside them are correct.
   3- Check of safety of all wires and connections inside control panel.
   4- The alarm annunciator panel, Fig. (16/5) must be clear and free of any
      alarm signal before starting.
   5- Push to check the safety of signal-lamps test.
   6- Operating switch must be put on auto, and lamps shown in Fig. (17/5) are
      lightening. That at left indicates validity of all auxiliaries, middle lamp
      indicates free of trip causes, and the right lamp that all requirement are
      available and unit is ready to start.

                                           12
5- Start up operation
  a- Typical automatic gas turbine start-up sequence:
     Figure (18/5) which is a relation between time and speed of the turbine shows
     the sequences of operation.
     1- Begin by starting the turbine using the starter device until reaching 20% of
        the rated speed.
     2- Ignition begins by the spark to begin combustion of the fuel/air mixture.
     3- On reaching stable flame, the igniter will be off, and the main flame of
        burners continues and turbine accelerates.
     4- After self-sustaining speed starter is stopped.
     5- Turbine accelerates by increasing fuel entering the combustion chamber to
        reach the operating speed.
     6- Synchronization is done automatically then loading


  b- Operator responsibility:
     Since the majority of unit has automatic starting sequences, the responsibility
     of the operator is to survey these sequences that happen in the exact time and
     in the convenient way. Usually the operation is controlled from remote or
     local locations. In all cases operation monitors should be surveyed.




                                             13
The first operation step is putting the operation selector switch on auto, then the
operator turns the master control switch to start. At that time, the three ready lights
will be off, Fig. (17/5). Signal lamp and sequence in progress light will be on, Fig.
(19/5). Auxiliary lube-oil pump starts automatically, then the starter increases unit
speed, then starting service lamp lights therefore the operator can watch unit speed
by the turbine rpm meter, Fig. (20/5).
    When the turbine speed reaches 20% of operation speed igniter gives the
required spark, then fuel valve opens and the flame lights, Fig. (21/5). Turbine
continues in that speed for sometime to warm up, during which operator watch
temperatures to avoid overheating. On reaching self sustaining speed, starter stops
and its lamp stays on until the starter cools then the lamp will be off.




                                               14
15
     Turbine speed automatically increases, then the gear pump lamp lights and
auxiliary lube-oil lamp will be off. By this operation is completed, after which
synchronizing and loading begin either automatically or manually according to the
system. Then the operator puts the operation selector switch on remote to control the
unit from the main control room, the above is the normal operation sequences.
     There is another type of operation which is the fast start in which warming up
time is too short, and unit loading takes place automatically in fast way. This is done
in emergency cases that needs fast operation and fast service.


6- Power operation:
   Steps done by the operator during operation and reaction on emergencies are
   discussed.

   a- Operating checkouts:
      Taking care and inspection and checkout of the unit must be continuous
      during starting and in service, which is similar the pre-start checkouts, except
      regarding gauges and indicators. The following should be followed:

      1- Inspecting the turbine section: Watching any leakage and any accidental
         problems. In switch gear room, survey any device may trip the unit, and in
         exciter section, notice brushes. In the load gear section, notice the ground
         brushes that may cause fire if spark and oil leak occur, stop the unit
         immediately. Notice oil pressures of all pumps and pressure difference on
         filter to check filter cleanness. Check gearbox and co2 house to be ready at
         any time for operation, cooling-system water level and lube-oil level
         indicator.

      2- Control room checks: watching all indicator readings of volt, current,
         load, temperature of gases and any monitor reading.


   b- Emergency operation: It is possible that faults will happen that must be
      carefully noticed before starting, other wise catastrophic event would occur.
      For example, vibrations ma occur and excessive vibration signal will be off,
      Fig. (22/5). In this case, automatic trip of the unit takes place and the operator
      should notice-stopping sequences, which are, flame light off and speed
      reduction. In this way, safe stop occurred. If trip is not done automatically,
      the operator must stop the unit by moving main switch to off, then he may
      restart the unit following the steps to know the vibration cause. This is called
      crank start where he checks the starting sequences. He begins by operation
      selector switch on crank, moving the master control switch to start and notice

                                               16
any vibrations, which may be due to blockage of bleed-off valve or
compressor dirties.




The problem may be more difficult than this as bowed rotor or bearing failure
represent. That is why to let the turbine out of service to be checked by




                                       17
      maintaining it if the vibration cause is not noticed during two crank starts.
      During crank start, if the vibration does not happen continue starting
      sequences until flame lamp is on, Fig. (23/5), then he puts selector switch on
      auto.




7- Shutdown
Automatic shutdown sequence occurs by gradual reduction of loading to zero, and
running during a short time for cooling, and shutdown is completed by fuel-flow
stopping to the nozzles. Operator should follow up the shutdown sequences. Starting
the shutdown begins by lightening of the sequence-in-progress light and turning off
the sequence complete light, Fig. (24/5). This means that shutdown signal, coming
from main control room, is received. If that signal is not received, the operator puts
the selector switch on auto and turns main switch on off. The operator should notice
the megawatt meter to be sure of turbine unloading, Fig. (25/5). The operator must
check the local breaker light to be sure off its separation. If this was not done
automatically, the operator checks the breaker trip fuses in the switch-gear room,
then separates the local breaker. Emergency stop must not be done as it bypass
cooling that may damage the turbine. On separation of the local breaker, the turbine
runs for awhile to be cooled and when the speed reach certain low value (about
4000 rpm) fuel stops flowing to nozzle and the speed decreases to zero. At this
point, flame light continue for awhile, (as in this example it takes its signal from
exhaust temperature), and when the speed reaches approximately 1000 rpm, exhaust
temperature reduces and flame light will be off. If the flame light takes the signal
from the flame detector, it will be off when the flame is off. When shutdown is
completed the three ready lamps light.



                                              18
                              MAINTENACE
Maintenance may be divided into three main types:

1- Running attention and services:
   Tacking care and surveying the turbine during operation do not need stopping it,
   as compressor and air intake cleaning. On cleaning the compressor reduce the
   load and spray the washing agent, which depends on fouling type, on the first
   stage of the compressor according to the operation and maintenance manual.
   Some solid particles can take help in cleaning rate but it may block the cooling
   air passages and erode the blades. It is recommended to record compressor speed
   or delivery temperature to be sure of the effect of the cleaning process. If the
   temperature of compressor delivery and the operation time are plotted, then the
   needed time for cleaning can be found, Fig. (26/5).




   Intake filter maintenance is the first protection against dirt and dust accumulation
   on compressor blades. Electrostatic filters are more suitable, but the are
   expensive and bulky. Moving screen filters are famous in which the packing is
   on the form of screen that continually rotates passing in oil path to clean it.
   Surveying all monitors, gauges and signals are very important during operation
   and following the operation manual is compulsory.

                                               19
2- Semi-overhauls:
  Tow or three-day maintenance is considered simple that depends on unit design
  and operation conditions between major overhauls. Aero-engine turbines, for
  example, need only checking combustion system and some auxiliary systems by
  replacing the assemblies that have troubles. Some large unit have split casing to
  inspect the glands, bearing and the blades of compressor and turbine.
   Simple overhauling takes care of combustion system and checking flame tubes,
  nozzles, fuel supply system and lines and ignition system.
  Figure (27/5) shows a louvered type flame tube that last long life




3- Major overhauls:
  It should be done under fine supervision, as it needs dismantling of all unit parts
  of the unit then re-assembling it, after changing the bad parts. As the details of
  major overhauls differ from unit to another, yet the common features are
  discussed. The turbine parts that inspected during the major overhauls are:

  a- Turbine rotor blades:
     Check the cracks and change the blades that eroded the make careful rotor
     balance to avoid running vibrations and problems.


                                             20
b- Turbine casing:
   In case of split casing some pinches may found at joint face that harden
   separation, therefore take care of the casing shape and the fixed blades on it.
   Imperfect cooling may cause cracks in the casing that are treated by welding
   and to know the causes of the cracks.

c- Turbine rotor:
   Accurate spigot location is very important as any displacement of the rotor
   may cause unacceptable vibrations, Fig. (28/5). Change the rotor if it has any
   cracks, therefore stringent test is carried, especially the rotor bore that may
   have stresses. Also, the rotor rim must be tested and take care of bolting on
   assembly to avoid blots overstressed or loosened on heating up or on loading.




d- Temperature coloration:
   This may be good indication for the conditions by which the turbine is facing
   of heating and cooling that may cause internal stresses.

e- Fretting:
   High friction may cause welding of parts or their corrosion

f- Cracks:
   Cracks of other parts rather than blades or rotor may cause damage due to
   flowing of pieces of these parts that may damage the blades.
                                           21
g- Erosion:
   Over-washing, or using in effective air filtration in bad weathers may cause
   erosion to compressor blades.

h- Corrosion:
   Using liquid fuels cause ashes that may corrode the blades, Fig. (29/5). Fuel
   can be properly treated to reduce the corrosion effect.




i- Rotor journal bearing:
   Scratches or hair cracks on the white-alloy surface may be acceptable. It is
   damaged due to bad lubrication, oiling, or heating up. The thrust bearings are
   examined for overheating and distortion.
                                          22
   j- Heat exchangers:
      Internal cracks of heat exchanger walls are common especially at welded
      points in corners. They must be hydraulically tested by compressed air at
      pressure equals 1.5 the working pressure to estimate the time during which
      the pressure may reduce and compare it with that of the factory.




                   MAINTENANCE SKILLS:
Maintenance needs special precautions and skills to carry the required sequences
perfectly. Checking the manuals is important never the less the following the
common maintenance skills are discussed as follows:

   a- Dismantling of the unit:

      1- Separate the unit connections and close all valves and cutout circuits.
         After taking off the metal covers, unscrew the horizontal joint bolts. Plug
         all open pipes and flanges, then unscrew the dowel bolts using lever pipe.

      2- Preparation steps:
         • Be sure of the number of the bolts to be unscrewing and collecting all
            nuts.
         • Be sure of disconnection of all joints for oil and sealing air before
            raising the upper half of the casing.
         • Take off the bolts connecting throttle valve flange to separate it from
            the cylinder.
         • Take off the turbine casing and gland casing to be able of inspection of
            the blades and bearings.

      3- Removing cover:
         Removing the unit cover represents a big problem. A hook and wires
         passing through the eyebolts can raise it. Figures (30/5 to 33/5) show the
         different ways to raise it. Some turbines include lifting beam and cables.




                                             23
24
25
26
   4- Check the condition of spindle in place:
      Check the rotor after taking off all covers to fix the blades and other
      inspections as:
      • Coupling check: Take away the fixing studs of the coupling and check
         alignment.
      • Measuring the clearance: Axial clearance must be checked (0.01 –
         0.015 inch).

   5- Removal of spindle:
      usually lifting beam and cables are used to raise the spindle. The cables
      are used at the spindle ends, as they are the only usable places. Be sure
      that the spindle be horizontal all the time.

   6- Inspection of spindle after remove:
      It is important to inspect the shaft from all sides as follows:
      • Cleanliness from soft and oil deposits and hard scales.
      • Inspect erosion especially in the last rows.
      • Look for cracks and blade lashing wires.
      • Inspect the shrouds.
      • Inspect the raised wedges.
      • Inspect for blade friction.
      • Inspect for thin blade edges.
      • Inspect for strange objects attached to the blades.

   7- Cleaning the spindle:
      Clean the spindle and blades by jet of air carrying particles of aluminum
      oxide, after covering bright surfaces on the spindle


b- Bearing care:
   Inspection and cleaning the bearings is the most important duty of the
   maintenance department.

   1- Design hint about bearing:
      • Journal bearing:
        It is a cylindrical hole lined by white metal, inside it the spindle rotates,
        Fig. (34/5). It is used in heavy radial loads. Therefore, it carries the
        rotor beside the dynamic forces generated. They may be classified as
        hydrodynamic operation bearings, because their ability to carry heavy
        loads comes due to the thin oil film wedge caused by the relative
        movement between the shaft journal and the liner, Fig (35/5). Figures
                                            27
   (36/5) shows the pressure distribution in the oil layer between them,
   and Fig. (37/5) shows standard types.




• Thrust bearing:
  It is composed of a ring or collar rotates against fixed plate or surface,
  Fig. (38/5). It is used to carry axial forces generated at compressor and
  turbine blades. In addition, hydrodynamic effect occurs due to the oil
  film shown in Fig. (39/5). The oil pressure is shown in Fig. (40/5),


                                    28
from which it is noticed that the least oil layer, and by consequence the
highest temperature in the bearing occurs at ends.




The way of oiling the thrust bearing is identical to that of journal
bearing. The oil comes from the bottom centerline of each bearing
housing, running in the passages around the external surface of the
linear then into the radial slots on the rear face from which to the inner
surfaces. There are many kinds of bearings, for example that of
General Electric, Fig. (41/5).

                                  29
2- Bearing maintenance:
  - Journal bearing-liners
     • Bore diameter
     • Outside diameter
     • Babbitt surface
  - Journals
     • Journal diameter
     • Journal surface
                             30
         • housing

    - thrust bearing-base ring assembly:
        • Base-ring outer diameter
        • Babbitt surface
        • Oil dam clearance
        • Land taper
        • Axial clearance
    - Thrust-bearing runners
        • Face runout
        • Runner surface

    - Thrust bearing housing


c- Care of flexible coupling:
   1- Thrust bearing housing
   2- Flexible member of coupling
   3- Gear type coupling
   4- Maintenance of flexible joint




1- Spare parts




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