Doc - 15 Combined Cycle Power Plant Operation

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TRAINING MANUAL PROJECT 388.5 MW Combined Cycle Power Plant DOC No. IBDC/ L&T/ VCCPP/ 15 DOC. TITLE CCPP Operation Page No. Page 1 of 21

Combined Cycle Power Plant Operation General considerations Power plant operation includes mainly the following tasks: • start-up and shutdown of the power plant and of all of its ancillary systems, • monitoring of continuous operation, • adjustment of operation to the requirements of the load dispatch center or to the

requisites of the customer, • control of disturbances, • maintenance of the readiness for service of the plant during downtimes (shutdown

heating operation or preservation). In performing these tasks, both the codes of practice and the specific operating instructions and regulations of the plant are to be observed. The configuration of combined cycle plants may be different, but in principle there are some basic procedures which are determined by the start operation of the gas turbine itself and, as far as the water/steam part is concerned, they are similar to the start up procedures of the water/steam-circuit of a conventional plant. Guidelines for start-up operations Readiness: Observing the responsibilities and complying with the technical rules, the operating staff brings about the mechanical and electrical readiness for operation of the individual plant parts. This includes checks and controls, which have to establish • whether all assembly and repair works have been completed properly, • whether the construction and field assembly personnel has left all plant parts, • whether all construction material and assembly equipment (ladders, scaffolds etc.)

have been removed from the plant parts, • whether all components have been cleaned properly, and • whether all mechanical, electrical and I&C connections have been made.

Upon successful completion of all these checks, • manholes, access openings etc. are to be closed, • blanking disks (piping blinds) are to be removed, • pipes and tanks are to be flushed or purged, if necessary, and then to be filled with

the operating medium (water, air, gas, oil etc.), • within the scope of the trial runs, the direction of rotation of drives and the end

position and limit switches are to be checked. During this mechanical preparation work, also the readiness for operation in the field of electrical and I&C equipment is prepared. If the mechanical as well as the electrical and I&C clearance is issued, the plant parts can be started up in the technically proper order. Start-up conditions The type of procedure to be followed for starting up a plant after a layup period depends on the material temperatures of the plant components particularly of thick-walled components reached during the time elapsed since the plant had been shut down. Depending on the level of these material temperatures, a distinction is made between • cold start, • warm start, and • hot start.

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TRAINING MANUAL PROJECT 388.5 MW Combined Cycle Power Plant DOC No. IBDC/ L&T/ VCCPP/ 15 DOC. TITLE CCPP Operation Page No. Page 2 of 21 These temperatures apply to both turbine components and boiler components. A cold start is to be carried out usually after a longer layup period, e.g. after an inspection/overhaul of the plant. If the layup time is considerably shorter, e.g. one weekend, the plant can be started up again following the warm start-up procedure. If the plant had been shutdown only a few hours ago, a hot start can be carried out. At Vemagiri Plant the star-up preconditions are: Hot start – defined by shutdown period of less than 8 hours. Warm Start - defined by shutdown period of 8 to 48 hours. Cold start - defined by shutdown period of more than 48 hours. Start-up of the power plant includes also the systematic putting into service of all auxiliary and ancillary systems. In modern power plants, the individual start-up steps have been combined into functional groups which are processed automatically. The functional groups contain also the stipulated start-up times to be observed in order to avoid high temperature gradients. On the other hand, it is to be considered also that any start-up process entails start-up heat loss and therefore the start-up procedure should be carried out as rapidly as possible. The start-up process is thus a compromise between the endeavours to keep start-up losses as small as possible and the observance of prescribed maximum temperature gradients in the plant components. Steps of start-up The start-up process includes the steps enumerated hereinafter (start-up sequence in principle). For the specific power plant, the manufacturer's operating instructions are applicable and the persons in charge of plant operation have to familiarize themselves with these instructions. Here are two basic, safety-related mnemonic phrases for start-up of a power plant: The following applies to all energies and material flows: Ensure discharge first, and then put supply installations into service To avoid damage to tubes by overheating in the boiler: Fire may be available – but water must be available To be referred: • Start-up curves of GTG & STG • Start-up curves for Combined Cycle supplied. • Water balance Diagram. • Heat & Mass balance diagram. • All latest revision of P&IDs.

Combined cycle startup general steps: • Ensure that startup power is available from the grid via 220 Kv/400 Kv switchyard

& via power transformers. • Ensure that all electrical distribution transformers are energized and necessary

voltage levels viz. 6.6 KV, 415 V supplies are available. • Ensure that UPS supply is available. • Check that battery back-up for UPS is in healthy condition. No alarm is existing.

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TRAINING MANUAL PROJECT 388.5 MW Combined Cycle Power Plant DOC No. IBDC/ L&T/ VCCPP/ 15 DOC. TITLE CCPP Operation Page No. Page 3 of 21 • Ensure that all emergency D.C drives are available. • Ensure that emergency D.G sets are in “Ready” condition and on “Automatic”

mode. It is advisable to take a trial of DG sets one by one. • Ensure that enough raw water is available in the storage area. • Ensure that enough DM water is available in the DM storage tank. Also ensure that

DM water is having proper quality in the storage tank i.e. pH & conductivity. • Start the Pre-treatment plant. This comprises of the following steps:

Fill the clarifier upto normal working level. Filling of clarifier can be done by necessary dosing of chlorine, coagulant and polymer, whichever are applicable or by lime & alum dosing.

Start filling the clarifier water storage tank and fill upto normal working level. Fill-up the service water tank upto normal working level.

If adequate DM water is available in the DM storage tank, then DM plant can be started later on.

• Normalize the Fire Hydrant & Spray system for fire protection. Ensure that jockey

pumps, motor driven hydrant & spray pumps and Diesel Engine hydrant & spray pumps are on “Automatic” selection. It is advisable to check the auto startup of these pumps by draining the pressure switches / by opening the valves at site.

• Start plant compressed air system – Instrument air and service air. Normalize the

instrument and service air lines. Check the operation of some valves as a test. Ensure that dew point monitor of instrument air is in line & dew point is having sufficient margin above condensation temperature.

• Start HVAC system for plant air conditioning. Plant Start Up Curve (proposed)

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TRAINING MANUAL PROJECT 388.5 MW Combined Cycle Power Plant DOC No. IBDC/ L&T/ VCCPP/ 15 DOC. TITLE CCPP Operation Page No. Page 4 of 21 Following general check list to be followed before startup of a HRSG (With general mistakes that often happen): Considering COLD START UP – More than 48 hours of shutdown. For a longer shutdown (> 6 days it is preferable to wet preserve the boiler up to superheater outlet valve with minimum 100 ppm hydrazine hydrate N2H4). Cold startup curve (Typical) which resembles the HRSG at Vemagiri. HP Circuit:

IP Circuit:

• All PTWs returned by maintenance department & closed by Shift Charge Engineer

(SCE). • All isolation tools and tackles have been removed by maintenance department.

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TRAINING MANUAL PROJECT 388.5 MW Combined Cycle Power Plant DOC No. IBDC/ L&T/ VCCPP/ 15 DOC. TITLE CCPP Operation Page No. Page 5 of 21 • If the HRSG is under wet preservation, drain down the boiler both superheater &

boiler drum(s), completely. • Line up the boiler with respect to the drain & vent valves (As` per operating

procedure – securing of drain & vent valves). • Line up the boiler with respect to the feed water circuit to HP, IP and LP. • Normalize dosing system & ensure normal operating level in dosing tanks. For too

long shutdown it is preferable to replace the chemicals as contamination can occur during several maintenance activities.

• Normalize all the instrument impulse lines, flush if required. • Normalize drum water level gauge glass. • Normalize all the transmitters & check the signals at OS. • Rinse entire superheater & water-steam circuit with DM water & refill the boiler

drum upto “Start up” level. There are a few ways for filling up the drum effectively without loosing time. One way to increase the pH of the DM water up to required pH level. This can be done by taking makeup water in the condenser directly and dosing of hydrazine and/or ammonia (in case of non- steel tubes in the heat exchangers, ammonia is not advisable). Keep the CEP in recirculation & check the pH on line. After achieving required pH, water can be fed to the boiler. Another way is to fill the drum for a stipulated time, say, 15 minutes and then start dosing hydrazine in the drum with higher concentration than required during operation. Certain calculated quantity of hydrazine will be dosed. This will help to mix the hydrazine uniformly giving good pH. The other way is to dose Morpholine or equivalent chemical in DM makeup line to deaerator thereby increasing the pH. In some cases HRSG filling pump is envisaged during design stage for quick filling of HRSG. During start-up and low load operation single element control loop based on drum level measurement which controls 30% level control valve (1LCVFW203VC). During normal operation HP Drum level is controlled by using single element control or 3 element control. The selection between the 1 element control and the 3 element control is based upon the steam flow. The control loop switches from 1 element to 3 element control when the measured HP steam flow reaches approximately 25% of MCR (82 TPH). 30% feedwater control valve will be forced closed when steam flow is greater than 30%. Of 82 TPH = 24.6 TPH If “operating level” is maintained then during start up some water has to be drained down to take care of swelling. This is unnecessary wastage of costly DM water. Following points in general can be considered for startup of HRSG Condenser preheater system: • Close preheater inlet control valve , 1CD200V • Close Preheater recirculation valves, 1TCVCD207VA & 1TCVCD207VB. • Close DA level control valve and bypass MOV to level control valve. • Start condensate extraction pumps and keeping another pump standby. • Open preheater inlet control valve slowly. • Start condenser preheater recirculation pumps, 1CD201PA OR 1CD201PB

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TRAINING MANUAL PROJECT 388.5 MW Combined Cycle Power Plant DOC No. IBDC/ L&T/ VCCPP/ 15 DOC. TITLE CCPP Operation Page No. Page 6 of 21 Deaerator circuit Line up deaerator with respect to the feed, steam and drain & vent system. Ensure that safety relief valve on the DA is not Gagged. Then open deaerator drum filling valve and fill deaerator upto startup level. Continue CEP to run for maintaining the DA level during boiler filling. If DA feed storage tank (FST) level 1 LT FW 200 A/B/C reaches is greater than high level, adjust DA drum level by opening Blow down manual valve. Open Condensate control valve 1 LCV 215 V. Set deaerator drum level control in automatic operation and maintain minimum required water level. The following checks are required: • Check & ensure that no “Gag” is remaining on safety valves. • Check that no “Blind” is remaining in the safety valve flange. • If safety valve servicing has been carried out, then it is necessary to float the

safety valves and secure the same (After startup). • IBR requirement also states that after final setting of the safety valves, requisite

height of collar to be provided so that intentional increment of set pressure is averted.

• Normalize all the electrical drives and confirm that “Healthy” signal is existing on the Operating Station (OS).

• Normalize all the pneumatic valves, check the stroke whenever required. This ensures freeness of control valves.

• Check for proper closure of all the manholes. Sometimes due to use of inferior quality of ropes/gaskets, overheating of manhole material occurs and material degraded.

• Remove all the isolation tags after normalization. Otherwise misunderstanding can happen.

• Normalize sample lines to SWAS panel. • Ensure that there is no blind remaining at the inlet connections of CBD or IBD

tanks. This can happen if during Annual Inspection of passing of the HRSG drain valve cannot be controlled, a blind is sometimes provided at the blowdown tank inlet flange, temporarily. PTW must be issued for this work.

• Ensure cooling water supply line is available for CBD or IBD tank. This is required to cool the tank during blowdown, which is frequent during startup.

• Ensure that HRSG Expansion indicators are free to move. When there is a bypass stack in the system (which is absent in Vemagiri), Check the operation of diverter damper (DD) whenever GT is not in operation & confirm the feedback at central control room (CCR). Considering Steam Turbine Generator (STG) is in “IDLE” mode and STG auxiliaries are stand still:

• Line up cooling tower for operation. • Ensure that CW basin level is sufficient. • Start one/two circulating water (CW) pump(s) and establish the CW flow. • arryout necessary venting in the CW lines. • Ensure that makeup water for CW basin is available. • Lineup and fill “close cooling water – CCW” system with DM water. • Ensure that necessary chemical parameters are maintained by proper chemical

dosing. • Normalize auxiliary cooling water (ACW) system. • Start STG lube oil system (If not in service) and put the STG on barring gear as

per recommendation from OEM & continue on barring as per OEM recommendation or as per operating experience.

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TRAINING MANUAL PROJECT 388.5 MW Combined Cycle Power Plant DOC No. IBDC/ L&T/ VCCPP/ 15 DOC. TITLE CCPP Operation Page No. Page 7 of 21

• Ensure all the electrical & pneumatic drives are normalized further. • Start and load required numbers of Instrument air compressors. • Ensure that HP – LP bypass station has been normalized in all respect. • Ensure that condenser evacuation system has been normalized in all respect. • Ensure that power supply to the steam turbine vacuum breaker valve is in

energized condition. Preferably power supply for this valve from emergency supply.

• Ensure that condensate system has been normalized in all respect. • Ensure that STG drain system has been normalized. • Line up regenerative system as applicable. (e.g. Gland steam condenser, etc.)

“HRSG READY TO START” PERMISSIVE ON OS. (Typical)

No HRSG trip HRSG not under conservation , Command from DCS Flue gas HRSG HTG release DA circuit HRSG HTG release LP circuit HRSG HTG release, HP circuit HRSG HTG release IP circuit HRSG HTG release, BD Tank circuit HRSG HTG release, CPH circuit HRSG HTG release Utiliites Heating release Duct burner system HTG release

Now HRSG is ready to take the heat of exhaust gas from GT. The HRSG heating release signal to be obtained to start the HRSG and following conditions in general are to be fulfilled. HP system conditions for HRSG heat release are the following: HP drum at start-up level HP feed water main isolation valve opened HP FW control valve isolation valve1 or 2 or isolation valve of startup control valve

opened HP FW pumps in operation Any of boiler recirculation pump running, HP Boiler recirculation flow greater than minimum. Both HP attemperation isolation valves closed.

IP system conditions for HRSG heat release are the following: IP drum level at start-up level IP feed water main isolation valve opened IP FW control valve isolation valve 1 or 2 or isolation valve of startup control valve

opened HP-IP FW pumps in operation Both IP attemperation isolation valves closed.

LP system conditions for HRSG heat release are the following: LP drum level at start-up level LP feed water control valve isolation valves opened Any of Boiler feed pump is running, Any of boiler recirculation pump running, LP Boiler re-circulation flow greater than minimum.

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TRAINING MANUAL PROJECT 388.5 MW Combined Cycle Power Plant DOC No. IBDC/ L&T/ VCCPP/ 15 DOC. TITLE CCPP Operation Page No. Page 8 of 21 During combined cycle startup sequence, the steps are executed automatically in a logical manner by the DCS. HRSG startup is executed as per recommended “Startup curve” from OEM. This can vary from OEM to OEM but the basic concept remains same: Main function is performed by the Diverter Damper (DD) wherever applicable. At Vemagiri DD is not provided. Once GT start command given and if all the start up permissive is satisfied then GT rolling will be started without Flame ON (called purging) and after purging sequence is over, before admitting hot gases in the HRSG, it must be checked there are no flammable gases (resulting from improper combustion in GT) in HRSG enclosure. To ensure this, the GT must blow cold air into the HRSG. This is done during the GT startup with the GT FLAME OFF. This is applicable for cold, warm and hot start of HRSG and is done as per the recommendations of the GT manufacturer. For starting of GT Static Starting Device (SSD) is used. SSD can have different mode of operation like: • Turn mode • Purge mode • Fast cooling mode • Wash mode Above modes are having different speeds as per OEM recommendation. After purging firing takes place and GT accelerates to FSNL (Full Speed No Load) condition. At approximately 80% speed, turbine “Blow-off”/Anti-surge valves close. At approximately 90% speed generator exciter breaker closes and builds up necessary voltage. Operator will select synchronization on “Automatic” mode and Synchronoscope will match the generator voltage, frequency & phase angle with that of prevailing grid condition. Once these conditions are satisfied, generator breaker will be closed and machine will be loaded up to minimum loading as per OEM recommendation. At Vemagiri Plant GT the static start system uses a Load Commutating Inverter (LCI) adjustable frequency drive as the starting means for the gas turbine. By providing variable frequency power directly to the generator terminals, the generator is used as a synchronous motor to start the gas turbine. As heat input increases in the HRSG with increased in GT load, evaporation start taking place. At this stage check the following. All drum vents are in open condition. All super heater vents and drains are in open condition. All steam lines drain are in open condition. HP/IP/LP bypass station is in ‘Ready condition’. HP economizer over protection valve initially in open condition and when HP feed

water flow reaches preset value, it closes automatically. During HP evaporator in operation, it opens when HP eco inlet pressure greater than preset value. As soon as Drum pressure reaches 2 bar, drum vents will be closed. The drum vent valve will remain close during normal boiler operation. Only during start up from cold condition the vent valve will be kept opened till about 2 bar (g) pressure and above which the same will be closed. The automatic closing order is pressure above 2 barg OR HRSG under conservation.

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TRAINING MANUAL PROJECT 388.5 MW Combined Cycle Power Plant DOC No. IBDC/ L&T/ VCCPP/ 15 DOC. TITLE CCPP Operation Page No. Page 9 of 21 Then, the gas turbine will carry out its automatic start-up sequence, including air-purging sequence. As soon as HRSG is ON, permissive to reheat live steam vent inching MOV open. Then IP startup inching MOV will be opened and % of opening will be limited by HRH live steam pressure. The LP startup vent inching type valve opens to exhaust. The startup vents will operate until the respective steam bypass station is available. The HRSG drain valves closes automatically according to process conditions. The reheat live steam startup vent valve and LP startup vent valve, closes as soon as hot reheat bypass station and LP bypass station is available. Chemical Dosing System Line up and start chemical dosing system: Normally Hydrazine (Oxygen scavenger) dosing to be done at the suction of HP/IP/LP boiler feed pumps, Tri sodium Phosphate dosing to be done directly in to the drum (For removing the salts which are not soluble at high pressure, silica and to some extent maintain PH) and ammonia dosing carried out at CEP discharge (CO2 scavenger and pH controller). Check the samples are free flowing to the SWAS panel. Check that all cationic resins are fresh or at least not at the end of life. Sampling points are: • IP drum water • LP drum water • HP drum water • LP superheater • Hot reheat. • HP live steam • IP saturated steam • LP saturated steam • HP saturated steam • HP/IP/ LP feedwater • Deaerator • LP BFP Suction Evacuation System: Precondition is that STG must be on turning gear operation. Line up the gland steam system for evacuation purpose. After achieving adequate HP steam pressure, vacuum pump / Ejector can be taken in to service for vacuum pulling from the system. After starting vacuum pulling seal the glands and put the gland steam pressure and temperature controllers on auto mode. Normally after achieving approximately 0.3 bara vacuum in the condenser, bypass operation can be started. However OEM curves to be followed. Bypass System: The next step is to start the bypass system for the following reasons: This station is equipped with 100 % capacity HP/IP/LP bypass system with following objectives. When STG trips i.e. 100 % load throw-off bypass system will divert entire steam in

to the condenser. Your system is designed to bypass 100% steam to condenser. When load is temporarily lost on the steam turbine. Quick building of steam parameters during startup of the turbine. It prevents rapid building of pressure in the HRSG.

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TRAINING MANUAL PROJECT 388.5 MW Combined Cycle Power Plant DOC No. IBDC/ L&T/ VCCPP/ 15 DOC. TITLE CCPP Operation Page No. Page 10 of 21 It facilitates continuous operation of the steam turbine at part loads with HRSG

operating at 100%MCR condition. A different aspect of using this feature is that, the boiler is started quite before the rolling of the turbine. The rolling operation of the turbine may possibly delay due to: • Insufficient vacuum in the condenser. • The time lag in achieving permissible differential temperatures of the turbine components. • The fault in turbine governing and protection system etc. For this short non-generation period it is nor advisable to shutdown neither the boiler nor the steam generated during this time to be vented to atmosphere. During this period HP/ IP/ LP Bypass System plays vital role by establishing the flow in superheater/ reheater without wastage of costly dematerialized water and fuel. Moreover, startup of any boiler / HRSG requires some predefined time as per the nature of the shutdown period. When HRSG steam attains a desired preset pressure level bypass valve will be opening and start steam dumping. Whenever preset steam flow is achieved the start up vent valve start modulating to close. Now continuously monitor the steam parameters and match the parameters with recommended parameters from STG manufacturer. Once the desired parameters are achieved steam can be admitted in to the steam turbine and rolling of the turbine can be started. Depending on the manufacturer, HP/IP and LP steam can be admitted in phases as per recommended start up curve. Make sure that all turbine casing drains, control valve drains are in open condition and not obstructed. Check the bearing metal temperatures and the vibrations of the machine, which must be well with in the limit. Modern state of the art machines are equipped with Turbine Stress Evaluator (TSE) / Turbine Stress Calculator (TSC) for safe run-up & loading from different start-up conditions. This is integral to the DCS system. Even differential expansion can be taken care of automatically by DCS. However a close watch by the operator is required and if necessary, manual interfere can be achieved by the operator. After necessary soaking period machine to be accelerated to full speed no load (FSNL). At approx. 90% speed of the machine, exciter breaker will be closed and necessary voltage will be built up. Operator can choose to synchronize the machine on automatic mode or on manual mode. In automatic mode, which is the safest mode, the Synchronoscope is automatically switched ON by the “Function Group” control and generator voltage, frequency and phase angle are matched with the prevailing grid condition. When this condition is satisfied, synchronoscope will issue command to “close” the generator circuit breaker. Immediately after closing of circuit breaker, generator will be loaded to spinning reserve load by the programme itself. Operator has to adjust the reactive power as per the loading or “Reactive power control” can be put on “Auto” mode. Operator should check the reactive power according to the generator characteristic curve supplied by the OEM. Turbine will follow pre-defined ramp rate as per manufacturer’s recommendation and will be loaded up to base load. Checks to be made for steam purity continuously and necessary steps to be taken for correcting the deviations. It is of utmost importance that steam parameters to be

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TRAINING MANUAL PROJECT 388.5 MW Combined Cycle Power Plant DOC No. IBDC/ L&T/ VCCPP/ 15 DOC. TITLE CCPP Operation Page No. Page 11 of 21 maintained or else in the long run turbine will be endangered due to various depositions and undue stress will built up. All HP & IP super heater drains is having isolation MOV and inching type MOVs. The isolation MOV is governed by permissive and interlock which is mentioned in detail in respective logic diagram. There two main parameters for opening and closing drain MOVs such as Drain pot level and live steam temperature. The inching MOV position is controlled by HP, IP live steam pressure. In case LP super heater drains is having one drain MOV which is inching type. The inching MOV is governed by permissive and interlock which is mentioned in detail in logic diagram. There two main parameters for opening and closing drain MOVs such as Drain pot level and live steam temperature. The inching MOV position is controlled by LP live steam pressure. For warm and hot start-up, the procedures are similar except that during these start-ups, all off site systems are in service already. Start-up in above conditions will be taking less time as machines will be warm or hot, as the case may be. Start-up sequence will follow the OEM start-up curves, which are subjected to modification during the commissioning time. The VEMAGIRI POWER PLANT startup will be co-ordinated with start-up of the various equipment and auxiliaries of the plant. Startup power will be supplied from the APTRANSCO Grid. Following is the sequence of start-up of the VEMAGIRI POWER PLANT equipment: • Power required for the startup of plant will be from APTRANSCO grid and both the

UTs (Unit Transformers) are considered in service to meet the plant auxiliary and starting power requirements.

• Start (cold, warm & hot) will be initiated only after the following plant/equipment conditions are met:

i) GTG Ready to Start ii) HRSG Ready to Start iii) STG Ready to Start iv) BOP/Offsite equipment Ready or in operation v) All instruments are charged and in service.

Required conditions for the each of the above equipment ‘Ready to start’ condition is explained below Plant equipment ready to Start Certain conditions, considered to be those required for commencement of the startup sequencing, must be met prior to initiation of the VEMAGIRI POWER PLANT start. These conditions are monitored by the GTG, HRSG, STG, and BOP/Offsite equipment sequence logic. Each condition can be fulfilled by manual operator action or by the actions of pre-set sequencing logic. When all the conditions are met, a portion of, or the complete VEMAGIRI POWER PLANT, is defined as being in a “Ready to Start” state. Following are some typical “READY” condition lists for the GTG, HRSG and STG. GTG Ready to Start - MCC breakers set in auto mode - Cooling water system ready - Fuel gas pressure adequate

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TRAINING MANUAL PROJECT 388.5 MW Combined Cycle Power Plant DOC No. IBDC/ L&T/ VCCPP/ 15 DOC. TITLE CCPP Operation Page No. Page 12 of 21 - Gas turbine/generator permissive to start systems ready - Evaporative cooler ON as required - HRSG ready All starting will be done automatically, with the operator to hold the start up sequence at either crank (pre-ignition) or fire (post-ignition, pre-accelerate) points of start up. An auto mode selection will result in a start without any holds. Before issuing the start command, or during startup, the operator may make the preselected load or base load. If a selection is made, the unit will automatically load to the selected point and control there. If no selection is made, the unit will load to a low load referred to as “spinning reserve” after synchronization. Turbine governor will automatically be regulated to maintain the megawatt setting assigned to “spinning reserve”. HRSG Ready to Start Typically ‘ready’ conditions have already been discussed earlier. Conditions listed below will be satisfied to fulfil the HRSG “Ready to Start” condition: - The HP, IP and LP drum levels normal - HRSG recirculation pumps in operation - The HP, IP and LP drum level controller set points within an allowable deviation of

the start-up levels. These are dependent on drum configuration and pressure at start (determined by HRSG vendor for each start as a function of pre-start drum pressure).

- HP/IP and LP feed water pumps are in service in minimum recirculation mode - There are no HRSG trip conditions - HRSG super heater steam shutoff and bypass valves are closed - HP and IP economizer vents are closed - HP/IP and LP feedwater shutoff valves are normally closed prior to start and opened

at the starting point. - HP, IP and LP superheater vent valves are closed - HP, IP and LP superheater drain valves are closed - HP, IP and LP intermittent blowdown valves are closed - Drip leg drain valves on HP and LP steam headers and bypass lines are in auto and

the valves are closed - GTG “No Trip” - Instrument air pressure OK STG Ready to Start Typically ‘ready’ conditions below will be satisfied to fulfil the STG “Ready to Start” condition: - HP stop valves, reheat stop valves are closed - LP admission valves are closed - Steam turbine motor operated and pneumatic operated drain valves are in auto

mode - Lube oil pressure normal - Lube oil tank level normal - Control oil pressure normal - Turning gear motor is running and turning gear is engaged - Steam seal system is in service - Condenser vacuum OK - CCW system in service - CW and ACW system – at least one pump running and in service

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TRAINING MANUAL PROJECT 388.5 MW Combined Cycle Power Plant DOC No. IBDC/ L&T/ VCCPP/ 15 DOC. TITLE CCPP Operation Page No. Page 13 of 21 Plant Load Control The purpose of the co-ordinated control scheme is to provide the operation of the Gas Turbine Generator (GTG), Heat Recovery Steam Generator (HRSG) and the Steam turbine Generator (STG) in coordinated manner. The co-ordinated control scheme has been implemented in DCS. The MW demand from the plant coordinated control system will be sent to GTG control system and will be executed within the GTG maximum/base load. The STG will follow the GTG-HRSG and will operate on the sliding pressure mode (valves wide open). However, if the demand exist even after the GTG reaches the max/base load the additional MW demand should be met by the supplementary firing in HRSG. GTG load will be restricted to 50% load in the event of only one (1) CW pump operation. Plant Load Control Modes Plant Load Control Without HRSG Supplementary Firing The plant load control system will be provided to control the output of the entire combined cycle modules so as to meet the load demand as set by the operator in DCS operator station. For meeting the load demand, the plant load coordinated control system will regulate the power generated by the gas turbine generator unit while forwarding about 2/3rd (can be adjusted) of the demand to the gas turbine generator (GTG) considering the 1/3rd (can be adjusted) power demand will be compensated by the steam turbine generator (STG). The plant coordinated control loop will compare the total generated power output to the grid with the load demand signal set by the operator and accordingly generate set point signals for the load controllers of the gas turbine. The set point signals shall be limited by maximum (base load) and minimum preset load (house load) limits. The maximum loading rate permitted will be limited by the loading rate as determined by the gas temperature ramps which shall be preset in the control system. The set point for the GTG will be provided from the coordinated control loop as indicated in the co-ordinated control schematic. Plant Load Control With HRSG Supplementary firing The GTG base load can be considered as one of the start permissive for the HRSG supplementary firing, so that the supplementary firing shall not be in operation during part load operation of the GTG. The MW demand signal generated from the plant coordinated control loop after GTG reaches its max/base load shall generate alarm in the plant DCS to indicate the additional power required to meet the plant MW demand. Based on the MW demand the operator has to make decision to bring the supplementary firing in operation considering minimum/maximum number of burners in operation & minimum/maximum heat input. Once burners system in service, the operator can set the burner system in Auto mode & the additional MW (power) demand signal shall be sent to the burner system from plant coordinated control loop to increase/decrease the heat duty, till the MW demand met. The burner system will operate within the minimum & maximum heat duty limitations. The supplementary firing shall be withdrawn/burners shut down once the demand is less than the minimum heat duty of the HRSG burner system & also GTG load demand reduced below the base load. HRSG Normal Shutdown: During normal shutdown of HRSG following procedure is followed in general: Case 1: Duct burners are ON (Auxiliary firing ON) • The duct burner load will be reduced gradually by modulating the fuel control valve till the minimum opening position is reached. • Duct burner is taken out of service upon reaching the minimum load point of the fuel control valve. (Called Turn Down Ratio)

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TRAINING MANUAL PROJECT 388.5 MW Combined Cycle Power Plant DOC No. IBDC/ L&T/ VCCPP/ 15 DOC. TITLE CCPP Operation Page No. Page 14 of 21 • Unloading of GTG will follow; this in turn will also start unloading of HRSG steam generation and STG load. At this stage it is advisable to take HP-LP bypass system in service as a precaution against imbalance in the temperature at turbine end. OR • The GTG unloading is continued till the minimum load / permissible steam temperature of the STG is reached, as per OEM guide line. • Shutdown STG by opening first the generator breaker either manually from a minimum load OR automatically by “Reverse power” protection. • By –pass stations comes into operation. • GTG is further unloaded till the minimum sustainable load of HRSG is achieved. This load is defined by either % of GT load OR % of MCR steam flow OR minimum steam temperature at superheater outlet. • As HRSG load goes down drum level control will follow the logical sequence and will be transferred to 1-element control. Also feed control station will be changed over to 30% from 100% depending on the logic. • As HRSG shutdown is initiated, IP & LP pegging steam to the deaerator will be closed via HRSG function group logic. At this stage operator should ensure that necessary NPSH is available at the suction of BFPs (A suction pressure variation will be indication). • It is advisable to operate boiler blow down (for a brief period) through bottom header at approximately 30% of the boiler load, as this will discharge some insoluble salts at higher pressure. • After reaching minimum sustainable load, GTG is also tripped the GT sequencer. • HRSG can be kept under “hot box up” if the shutdown is planned for a shorter period. This depends on the rate of depletion of the drum pressure. If drum pressure reaches less than 2 bar, startup and drum vents will open as per HRSG protection sequence. If the HRSG is hot boxed up, then special precautions to be taken during startup activities with respect to the opening of the line drains and vents. As in this case, sometimes, operators may “Force” close the drain valves. Case 2 : Duct burners are OFF (Auxiliary firing OFF) The procedure will be same as in case 1, except the following: • No shut down sequence will be initiated for duct burner firing logic. HRSG Quick Shutdown • A quick shutdown (Protective load shedding - PLS) for GTG will be initiated as per GT / HRSG protection matrix supplied by the OEM. PLS can be initiated from the GT protection OR can be initiated from HRSG protection. • GT is de-loaded at a much faster rate than the normal de-loading gradient during PLS. Normally during PLS operation if at any moment of time the cause of the PLS is reset, then GT sequencer will automatically stop de-loading of GT and will maintain “status quo”. Operator will interfere at this phase to load the GT depending on the prevailing situation. OR operator can also “Reset the PLS” from the operating station. This type of unloading creates more stress to the GT & HRSG parts. • After reaching the “Reverse power” zone, GT generator breaker will be opened and GT will trip on “Flame OFF” condition. OR depending upon manufacture’s design, GT can be on FSNL for a stipulated time (called “cooling time”) and then trips on “Flame OFF”. HRSG Trip: HRSG trips one of the following conditions, IN GENERAL : Hardwired HRSG emergency push button (from control desk) pressed. STG trip (signal from STG trip logics) with any of HP,IP & LP Steam bypass stations

closed, to prevent over pressure of HRSG.

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TRAINING MANUAL PROJECT 388.5 MW Combined Cycle Power Plant DOC No. IBDC/ L&T/ VCCPP/ 15 DOC. TITLE CCPP Operation Page No. Page 15 of 21 DA circuit HRSG trip. LP circuit HRSG trip. IP circuit HRSG trip. HP circuit HRSG trip. Flue gas HRSG trip. If condenser vacuum rises above trip limit & STG trip protection fails to trip the STG,

then GT trip command will be initiated which will in turn automatically trip the HRSG. (Please remember that the last protection of STG from high vacuum is the “Rupture diaphragm). Operation of the plant General considerations Trouble-free duty operation in the range from full load to partial load should be the most frequent operating condition of a fossil-fueled steam power plant. In this operating condition, there is maximum exploitation of the power plant at the lowest cost and thus the plant performs at the • best efficiency, • lowest specific fuel consumption, • lowest specific steam consumption, • lowest specific heat consumption, and • highest availability. Thus it is the task of the operating staff to ensure the operationability of all plant parts by regular inspection and maintenance, immediate notification of the technical management, and if possible elimination of irregularities and faults. Aims of operation The optimum operation includes not only trouble-free operation at the operating point of maximum efficiency of the overall plant (at approx. 80 to 100% of load) but also observance of other requirements with regard to • safety, • economic efficiency, • quality of products, and • environmental protection. Conditions and criteria of operation The most important criterion of the point of load at which the plant should be operated is provided via the load dispatch center from the consumers of the main products (electricity and/or heat). The power plant can only produce as much of its main products as the consumers demand. Thus it has also to be taken into account that, depending on the development of demand, power plants of different design types and efficiency are used by the companies at different times and are operated at different load points in order to reach an optimum overall economic result. It is also important to • reduce the quantity of fresh water required (for cooling as well as for water treatment), • minimize the waste water arising, and • minimize possibly arising waste.

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TRAINING MANUAL PROJECT 388.5 MW Combined Cycle Power Plant DOC No. IBDC/ L&T/ VCCPP/ 15 DOC. TITLE CCPP Operation Page No. Page 16 of 21 The environmental protection ordinances and the limits stipulated therein are to be complied with. Deviations from these standards are permitted only in very restricted exceptional cases. An economical management of the fuel and consumables and a proper and careful treatment of all plant parts must be as natural for the operating staff as is the continuing brushing up of its operating knowledge and capabilities. In the following list some actions are compiled which considerably reduce the specific heat consumption (and thus the exploitation of the fuel energy) and increase the availability and economic efficiency of the plant: • short start-up periods by ensuring flawless performance of all plant parts and strict observance of the operating rules and standards; • short shutdown periods; • adjustment of steam output of the steam generator to the steam demand of the turbine; • purposeful hooking up and hooking off of plant parts and equipment in order to reduce auxiliary consumption; • keeping the steam parameters within the limits in order to keep injection into the reheater low; • regular cleaning of heating surfaces and condenser pipes in order to maintain heat transfer and reduce head loss; • keeping the water quality of feedwater, boiler water, condensate (condensate polishing plant, chemical treatment) within the limits in order to avoid scaling and corrosion and to keep the necessary blowdown volumes as low as possible. Preventive inspection and repairs The operating staff has to observe attentively the function of all systems of the plant. This may be done by permanent observation of the monitoring devices in the control room but also by cyclical and periodical walk-around inspections of the installation. Based on the manufacturer's recommendations and on experiences gained with commissioning of the plant, control check lists should be prepared and used by the crew which helps to establish a systematic control. If a high availability of the plant is required, also preventive replacement of parts which have reached a certain degree of use may be advisable. Long-term availability also depends on the maintenance strategy which is to be defined by the power plant owner. Maintenance strategy Depending on the contractual and economic objectives he has to meet, the plant owner has to decide on the maintenance strategy of the plant. This includes programming of shutdowns for inspection, controls and preventive repairs, purchase and storage of spare parts and contracting out of maintenance work to outside companies. Normally, the maintenance strategy is an economic compromise between repair costs and losses caused by undelivered power.

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TRAINING MANUAL PROJECT 388.5 MW Combined Cycle Power Plant DOC No. IBDC/ L&T/ VCCPP/ 15 DOC. TITLE CCPP Operation Page No. Page 17 of 21 Controllable losses in combined cycle power plants. Controllable losses in Steam & Power generating systems fall in different categories. Before going into details let us go through the following facts quickly:

• The fundamental difference between electricity and other commodities is that, electricity cannot be stored in a large scale, in a practical manner.

• Storing of electricity is very expensive & hence should be produced whenever demanded.

• Large fluctuations in the demand during the day requires quick reactions from generating stations in order to maintain the balance between demand and production, thus reducing the chance of Grid disturbances.

Hence the major pricing is on reliable supply of electricity, efficiently delivered and quality maintained. However efficiency of plants also means that losses to be minimum, because in today’s market, the overall production cost is a key factor of success. Electricity must be offered at lowest cost and hence whole world is focusing on minimizing the losses. The major factor contributing to the efficient production of electricity and steam is HEAT RATE. All roads finally meet at this point and this is the deciding factor for the pricing. Before we proceed further, let us focus on the key issues, which directly / indirectly decide the amount of loss being incurred by the process. What is Performance? As per Webstar’s third new International Dictionary, the definition of performance is: • Performance is a measure of achievement as against expectations. • “Capacity to achieve desired results” – Performance is the outcome of effort /

activity. This implies that Plant performance improvement lies in the efforts given to maximize the transformation of energy/ minimizing the losses. The above two definitions suggests the two features that characterize performance considerations: • A set of expectations derived from the claim and promises (Qualitative) & a set of

results that one would like to realize (Quantitative). • Effectiveness refers to the extent to which the programmed requirements are met

and Efficiency refers how economically the resources are utilized when providing a given level of satisfaction.

We are concerned about the second part i.e. how economically the scarce resources are utilized i.e. Efficiency and reciprocal of efficiency is “Heat rate”. Type of Power Plants generally available for base load / peak load operations:

1. Diesel Generator Plants. 2. Steam Turbine plants. 3. Gas Turbine Plants. 4. Nuclear Plants. 5. Combined cycle Plants. 6. Co-generation Plants.

Each of the above plants has their own characteristics and to be dealt accordingly.

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TRAINING MANUAL PROJECT 388.5 MW Combined Cycle Power Plant DOC No. IBDC/ L&T/ VCCPP/ 15 DOC. TITLE CCPP Operation Page No. Page 18 of 21 Conventional steam power plants are suitable for use as coal burning plants in base load, if cheap coal is available. Whereas combined cycle power plants are more economical than steam power plants due to its higher efficiency & “lower specific price” and low gestation period. However, whatever the type of plant be, the important factors that control the whole system are: Heat Rate: This is the ratio of input and output. Availability: This is the amount of actual hours for which the plant is available to deliver energy OR the ratio between declared capacity and nameplate / installed capacity, calculated on annualizes basis. Hence all type of losses finally contributes to increase in Heat Rate & hence must be minimized. Following are the factors (Losses), in general, which have impact on Heat Rate of a combined cycle power plant and needs attention: Losses related to Gas Turbine: • Pressure drop across Gas Turbine Air Intake Filters (AIF): AI filters play an

important role in a gas turbine power plant. Efficient cleaning of air is a prerequisite for efficient operation of gas turbine compressor. Hence pressure loss in the AIF system must be minimized. An increase in pressure loss in the inlet system of approx. 1 Kpa results in an increase of 1.5 Deg C in the exhaust system.

• Gas Turbine Axial compressor blade fouling: This is a frequent phenomenon in gas turbines, as, for a gas turbine, major power is consumed by the compressor, hence any dirt / salt deposition on the compressor blades decreases the compressor efficiency & thereby turbine output.

To minimize these losses, followings steps are normally taken: I. Check the air intake filters regularly, clean by air pulsation (cleaning will not

be effective if air pressure is not sufficient), replace defective filters. II. Check for proper fitment, if small air gap is there at the base, air bypassing

will occur. III. Monitor gas turbine power drop from a reference point and set an on-line

and off-line compressor wash regime depending upon the practical operating experience.

A dirty compressor cannot deliver required quantity of air causing low GT power output and low axial compressor efficiency. Losses related to HRSGs The heat transfer in HRSGs involve losses which are associated with following major factors: • Physical properties of water-steam cycle & exhaust gases do not match causing

exergic and energetic losses. • Heat transfer surfaces cannot be infinitely large. • The inlet feed water temperature should be sufficiently high in order to prevent

back end corrosion. Following are the practical problems faced in a specific 156 MW Combined Cycle Power plant (This can vary from plant to plant): • Losses due to defective /faulty insulation should be minimized. In the above

mentioned plant, they have faced problem with defective insulation causing heat

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TRAINING MANUAL PROJECT 388.5 MW Combined Cycle Power Plant DOC No. IBDC/ L&T/ VCCPP/ 15 DOC. TITLE CCPP Operation Page No. Page 19 of 21

loss. It has been observed that insulating medium, if wet during erection in rainy season, causes a poor insulation & creates “Hot Spots”.

• Loss of heat also can occur in the top and bottom pent house, if not sealed properly, causing short circuiting of flue gases in the header section.

• In that plant, they have substantially reduced the Boiler blow down loss by innovatively modifying the boiler water chemical regime. They have not operated boiler blow down since last one and half months approximately, thereby saving a lot of makeup water. The basic understanding is that, as modern power stations are equipped with DM plants having highest quality of feed water, hence dosing of Tri-sodium phosphate in the HP drum has been reduced drastically and residual phosphate level in the HP drum has been bench marked as 500 ppb (HP drum pressure is 72 Barg). Also in the water -steam circuit, ammonia dosing has been stopped, as dosing of Hydrazine in the downstream of condensate pump is sufficient of taking care of scavenging of carbon-di-oxide, as Hydrazine is breaking down to ammonia in the HP drum. This has saved a substantial cost in the chemical and makeup water. This plant is operating with 0.5 to 0.8 % of cycle makeup.

• Initially they had experienced problem of high Cationic conductivity problem in the water-steam circuit and this had occurred due to interference from Ammonia. Safe Cationic conductivity is critical for modern steam turbine operation.

• Losses due to leakages in the valves, flanges etc are being taking care by either on-line sealing or a strategic shutdown planning, considering the generation target involved.

It is not always prudent to take shutdown of machines only to attend a minor / non-critical leakage or passing. Losses in the Steam Turbine and Condenser section includes but not limited to the following reasons: • Major loss of power output of a steam turbine is due to poor / bad vacuum in the

condenser. • There are number of reasons of poor vacuum in the condenser. • Condenser tube fouling contributes to poor vacuum. • Another reason of poor vacuum can be attributed to inefficient Steam ejector /

vacuum pump or in-leakage air into the condenser. Thus it is necessary to check the circuit under vacuum periodically along with steam ejector nozzles etc.

• Poor vacuum can also be attributed to improper or off-design functioning of the cooling towers and cooling water system. Hence it is necessary to check the performance of cooling towers and take necessary preventive maintenance for performance improvement.

• Proper treatment and maintaining threshold values in the circulating water system will aid in minimizing losses, as requirement of cooling tower blowdown will come down.

So in every part of the power generating station there lies a potential to minimize losses, in any form or by any way. Losses in the electrical systems also to be looked into details and process optimization to be done to minimize the losses. We should strive hard to improve the HEAT RATE by controlling losses or by optimizing the process. Part load operation of Combined cycle power plant. The efficiency of gas turbine mainly depends on clean and cool intake air. Also exhaust gas temperature and temprature of annular gap and axial gap between casing, nozzles and rotating wheel bucket are also limiting factors for receiving efficient output from gas

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TRAINING MANUAL PROJECT 388.5 MW Combined Cycle Power Plant DOC No. IBDC/ L&T/ VCCPP/ 15 DOC. TITLE CCPP Operation Page No. Page 20 of 21 turbine. When gas turbine operates with part load the EGT also remains on lower side, hence input to HRSG becomes less.

0

1000

2000

3000

4000

5000

6000

GT1 GT2 GT3

Power InMWHHeat ratekcal/kwh

From above performance curves it is evident that operating GTG in lower load increases the heat rate i.e. kcal/kwh, hence must be avoided unless operating condition makes it inevitable. More over lower EGT and gas flow invariably reduces the steam generation, and unless the drum pressure is maintained by controlling steam flow or by additional heat input by supplementary firing (can be done as GT exhaust contains 16 % oxygen by volume) a reduction in heat transfer would become forthcoming with higher stack loss. In combined cycle power plant where plant is in part load the operation, the optimized part load condition must be selected basis waste gas temperature through stack, number of expansion stages of STG and condenser vacuum. Reduction in steam pressure will cause steam chest valve to open more and more causing more steam flow demand to maintain the load and can cause, wet steam at LP stage causing mechanical vibration, overloading of condenser and reduction in vacuum if there is limitation in cooling water flow. All above events worsen the thermal efficiency ration of turbine hence invariably bring down overall thermal efficiency, Ideally the LP stage exhaust steam condition should be just as saturated dry steam in corresponding vacuum (- pressure) in condenser and in condenser only latent heat to be extracted to convert the saturated steam to saturated condensate at same pressure in condenser. In boiler side due to pressure drop saturation temprature will be lowered causing rapid evaporation and enhance possibility of priming if the variation of GTG and STG loads are frequent and not organized. With enhanced steam flow there would be less degree of superheat as the volume & temperature of GT exhaust gas flow remains less with part load. In the situation where part load operation is unavoidable the plant should be operated in boiler pressure control mode if supplementary firing is not intended. Where option available to enhance the power out put, the GTG should be selected for primary control of load variation to maintain the steam pressure with STG on standby load control mode. When operating part load condition of combined cycle power plant the Steam pressure and temprature must be maintained as required by design at the inlet of STG to avoid above-mentioned operational problem. The GTG load to be increased in case of power demand which in turn produce improve quality steam for STG, improving thermal efficiency ratio. The excess steam may be used for regenerative feed heaters,

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TRAINING MANUAL PROJECT 388.5 MW Combined Cycle Power Plant DOC No. IBDC/ L&T/ VCCPP/ 15 DOC. TITLE CCPP Operation Page No. Page 21 of 21 process steam, or educators to improve condenser vacuum, to have more work in LP stage etc. Lowering the initial steam temperature by desuperheating would result in reduction in overall thermal efficiency.