Download - Case Studies by Chohan Sir

GT load control

• 3 modes: Individual, base and preselected mode. • The loading/ unloading rate in individual mode is twice

faster than that in other modes. • Therefore following practice is adopted:

– Loading/unloading is done in preselected or base mode.– After reaching the desired load set point, the machine is kept in

individual load mode.

• Benefits:– Lower stresses on the machine during loading and unloading.

(base and preselected load mode).– Quick response to fluctuations in grid frequency during normal

operation (Individual load mode).

Liquid fuel operation

• In case of GT tripping on naphtha, Heavy Purging is required.

• Before initiating Heavy Purge sequence, we are force-opening the HSD stop valve and manually flushing the fuel lines with HSD, ensuring complete purging of naphtha.

• Benefits:– Avoiding the risk of major/ minor explosion due to

naphtha accumulation

Fuel system and Guide Vane

• GE frame 9E support dual fuel system.• The fuel to the turbine is regulated by

Stop/Ratio Valve – VSR.Gas Control Valve – GCVLiquid fuel bypass control Valve – VC3

• The Inlet Guide Vane – IGV provide pulsation protection and used during partial load operation.

Components and Actuating Medium

• The control valves are comprised of valve body, electro-hydraulic servo valves and hydraulic cylinder.

• The IGV has piston arrangement.• The valves and IGV are modulated by hydraulic

system.• The hydraulic system utilizes lubrication oil. • The Lubrication Oil – Primary operating Medium.• Reliability, smooth and precise functioning of

control valves and IGV depend on quality of primary oil.

• Servos are very sensitive toward contaminations.

Gas Fuel : Gas Control valve and Stop/Speed Ratio Valve

96 SR1, 96 SR2: Stop Ratio valve LVDT – A position feedback device.96 GC1, 96 GC2: Gas control valve LVDT.90 SR: Stop ratio valve servo valve.65 GC: Gas control valve servo valve.FH7: Hydraulic “In-Line 25 micron Filter” provide adequate protection for small high-pressure

system up to 5000 PSI.

Liquid fuel bypass control Valve VC3 Scheme.

• VS1: Liquid fuel stop valve.

• VC3: Liquid fuel bypass control valve.

• 65 FP: Liquid fuel bypass control valve servo valve.

• 96 FP1, 2: Liquid fuel bypass valve position transmitter LVDT.

• FH 3: Precise 2 Absolute PALL make filters.

GT Lubrication System Schematic

Hydraulic Oil System

The Servo Valves

• Electro-hydraulic converters.

• Operate control valves and IGV.

• The servos are supplied by M/s Moog.

• F 771-K208 VC3.

• F 771-K202 IGV.

• F 771-K200 GCV and VSR.

• Total population is 16 Number.

Impact of failure of servos

• Failure of control valve servo causes unavailability on related fuel.

• Failure of IGV servo may cause unavailability of entire machine.

• Unpredictable behavior of the machine.• IGV servo problem can lead to load hunting.• Identification and rectification is complex and

time consuming process.• Direct monitory loss of Rs 1,65,000/- per servo.

Failure History of Servo

• Almost zero failure till the year 1996.• Total failure till date = 13 Nos.

Liquid fuel = 03 Nos.

IGV = 03 Nos.

GCV and VSR = 07 Nos.• Failure in the year 2000 = 07 Nos.• Approx. monitory loss per servo Rs 1,65,000/-

Services offered by M/s Moog

• Level one servicing – Repair of flow stage.– Strip and Ultrasonic clean the 2nd stage.– Cleaning the 1st stage for contamination.– Check for air gap dimension.– Replace 1st stage filter and “O” ring.– Check for magnetization and NULL setting.

Services offered by M/s Moog

• Level two servicing – Repair of complete servo/proportional valve.– Level One servicing + – Demagnetizing 1st stage.– Disassemble 1st stage.– Ultrasonic cleaning of 1st stage.– Nozzle alignment and assembly.

• Minimum Repair Charges – Rs 1500/-, Rs 16,000/-, Rs 20,000/-.

Inspection and Service Report.

• Out of Eleven only two require 2nd level servicing.

• Filter clogging and “O” ring damage was found in all the servos.

• Four servos repaired and serviced on urgent basis.

Contamination found in the Moog which has Contamination found in the Moog which has resulted in clogging of filter.resulted in clogging of filter.

Oil to be cleaned to NAS 5 or Below.Oil to be cleaned to NAS 5 or Below.

Testing of Lubrication Oil.

• Lubrication Oil = SIGTO – 32.

• Oil test as per OEM instruction manual GEK-32568A and EGT guideline CTI 97-09E.

• Viscosity, Gravity, TAN, MI are the parameters we are monitoring.

Oil Cleanliness Level.

As per the recommendations of M/s Moog Ltd and in response of frequent failure of servo valves, it was decided to get the lube oil tested from external agency and the matter was referred to M/s Pall Pharmalab Pvt. Ltd. The patch tests were conducted by M/s Pall Ltd. to ascertain the cleanliness level in terms of ISO – 4406 or NAS – 1638 class on oil samples from all four GT. The results were very disappointing.

Oil test results by External agency.

Serial No

Unit ID

Cleanliness Level

Maximum Particles/100 ml.

5 – 15 15 – 25 25 – 50 50 – 100 >100

01 GT-1A > NAS 12 >1024000

182400 32400 5760 1024

02 GT-1B > NAS 12 >1024000

182400 32400 5760 1024

03 GT-2A > NAS 12 >1024000

182400 32400 5760 1024

04 GT-2B > NAS 12 >1024000

182400 32400 5760 1024

The cleanliness levels observed were beyond the NAS range and it is not advisable even to run on any of the particle counter. Because the particle counter are very sensitive to the dirt particles.

M/s Pall after testing reported that there is a cake layer formed on the surface of the membrane. By looking under the microscope, they opined that it is due to very fine silt like particulate matter of less than five Microns.

M/s Pall suggested that these cleanliness levels observed are very disastrous to hydraulic system and components. As per the standard norms, if the system consists of servo valves and other critical components, the cleanliness level should be maintained about NAS 5 or less.

M/s PALL conducted the “Patch Test”.

Oil FiltrationTo clean the oil up-to NAS 6 or better in the tank

The management took the problem seriously and it was decided to take quick remedial action to improve the oil quality. The most economic and suitable solution at that time was adapted and M/s Pall Pharmalab Filtration Pvt. Ltd. was instructed to carry out the filtration using external efficient filters to achieve the oil quality up to NAS 6 or less in the reservoir. As per the contract awarded the filtration was conducted successfully for GT1B (GT-2A Under progress). The cost of the “Sigto-32” oil is Rs 85/Lt and approx. requirement per GT is around 12 KL. The oil taken from the drain point is forced circulated through highly efficient quality external filters.

Class Maximum particles/100 ml in specified size range (m)

5 – 15 25 – 25 25 – 50 50 – 100 >100

00 125 22 4 1 0

0 250 44 8 2 0

1 500 89 16 3 1

2 1000 178 32 6 1

3 2000 356 63 11 2

4 4000 712 126 22 4

5 8000 1425 253 45 8

6 16000 2850 506 90 16

7 32000 5700 1012 180 32

8 64000 11400 2025 360 64

9 128000 22800 4050 720 128

10 256000 45600 8100 1440 256

11 512000 91200 16200 2880 512

12 1024000 182400 32400 5760 1024

The NAS 1638 – ComponentsThe NAS 1638 – Components

Lube Oil filtration scheme

Drain Point

Lube oil Return Header

GT Lub Oil Tank

Capacity 12 KLNAS < 5.

Pall Particle Counter PFC - 200

External 12+6µ Filter

Set

12µ Absolute PALL part code HC 8900 FSK 16 H. (Rated with >= 200)

06µ Absolute PALL part code HC 8300 FSN 16 H & HC 9600 FKN 13 H.

PALL contamination analysis kit and PALL PFC-200 electronic particle counter.

Improvement observed of the order of NAS 4 in the tank for GT#1B and Mechanical impurities were 46 ppm (<50 ppm standard).

Problems during fuel changeovers

• Units used to trip on reverse power after unloading or exhaust temperature high

• Improper opening of GCV• Improper opening of LCV• Improper operation of flow divider• Passing of VCK2 and VCK1• Problem with servo valves• Oil quality was not proper• Inline filtration system was installed

Problems during fuel changeovers

• 25 micron coarse filters were replaced with 3 micron PALL filters.

• Fuel changeovers are now done at 70MW load to reduce the exhaust temperatures

• IGVis kept off during fuel changeovers.• Flow dividers are rotated once in a fortnight• Opening of liquid fuel purge valve is delayed by

30 sec after transferring from HSD to gas.

Tripping of GT due to damper not closing

• Unit trips if the damper closed limit switch does not pick up within 40 sec. after damper trip command

• Malfunction of limit switch• Slow closing of damper• Damper trip timings are checked during unit inspections

Tripping of boiler due to flow problems

• When one boiler is taken out of service, the steam flow through other boiler increases.

• Pressure drops and steaming occurs• Pumps are not able to develop flow.• This results in low flow through the re-circulation pumps

and these trip and results in tripping of ST• Primarily because the ST control valve closing (from 2

boiler to 1 boiler mode) was only after the header valves close

• Modified so that control valve starts closing as soon as the header valves start to close

Unloading of ST when coupling the second HRSG

• Coupling command after saturation temperature is achieved issues order to close HP bypass of the incoming boiler closes for pressure equalization irrespective of the steam flow.

• The HP bypass which was 40 to 50% open with low flow closes to 10-20% for equalizing the pressure.

• When coupling takes place, the low flow of this boiler shifts the pressure set point to a lower value.

• HP bypass of both boilers open and flow reduces• This results in unloading of ST.• Now, the coupling order is given only after the HP

bypass of the incoming boiler is open by 80%.

Naptha leakages due to pressurization of lines

• The valves in the pipelines ( tank filling lines and forwarding lines) used to be kept closed.

• Due to high ambient temperatures, Naptha used to get pressurised.

• Gaskets in pipelines used to burst resulting in naptha leakages

• One valve in each line towards any one tank is now kept open.

• Suitable relief valves have been introduced.

Waxing in HSD circuit

• Problem used to occur during winter• Filters used to get choked• Waxing of HSD was observed to be the cause of filter

choking.• Low ambient temperature HSD (for ambient temperature

< 4 deg C ) was procured to solve this problem • Wax content is less in the low temperature HSD

False trippings due to fire sensing or gas leak sensing

• False trippings on fire / gas leak detection• Due to burning of cables, earth faults occur in the wiring• Cables replaced with high temperature cables

Tripping of ST on low lube oil pressure during supply changeovers

• No shaft driven oil pump in ST• If the running pump trips, the standby pump cuts in on

auto.• However, the very low pressure switch picks up and trips

the unit• Nitrogen accumulator was installed to solve this problem

Tripping of ST during 6.6 kV changeovers

• There is quick as well as slow changeover in the 6.6 kV system.

• Quick changeover is fast (100 mS) and no effect on the system

• During slow changeovers (1.2 sec) the lube oil pumps of ST trips and standby pumps take start.

• Unit trips on very low lube oil pressure.• If slow changeover also fails, the DG set starts (40 sec).

However, the auxiliaries like CEX, CRF and APH pumps trip and ST trips.

Unloading and tripping of GT due to DC supply failure of switchyard

• When the DC supply in the switchyard fails, the relays sending information about the unit breaker and isolator positions drop.

• The speedtronic gets feedback that the unit is disconnected from the grid.

• The FSR setting gets reduced to FSNL value• The machine unloads and trips on reverse power.

Tripping of units during changeover of FLH pumps

• When a changeover command is given from the subgroup control, the standby pump starts and the running pump trips.

• However, the new pump trips on flow defect (low flow). The boiler trips in 15 sec.

• The subgroup gets disturbed and the running pump does not get a start command..

• Nowadays, the changeover of FLH pumps are avoided.• Subgroup on commands are repeatedly issued in case

some tripping occurs.

Unloading and tripping during fast rise of grid frequency

• Droop was initially 4%• This resulted in fast unloading of the GTs if frequency

rises.• Droop changed to 5%.• Maximum acheivable speed changed from 107% to

108%.

Tripping of units on line faults

• The back up earth fault relay on generator transformer used to pick up for heavy faults on transmission lines.

• The back up earth fault relay was desensitized• The settings of the characteristic angle of distance

protection relays of lines was modified

Tripping of units on voltage unbalance

• The units used to trip when the voltage unbalance relay picks up

• There was no actual unbalance• Snap on type lugs in PT secondary circuit were replaced

with bolted terminals.• PTs were draw out type with sliding contacts in

secondary circuit.• Sliding contacts were replaced with bolted type terminals

with flexible cables.

Misalignment of 220 kV Isolators

• Black cotton soil – swelling• The Isolator foundations tilting• Improper closing of isolators• Cross bracings on structures were installed

SF6 leakages from breakers

• Tilting of foundations due to soil swelling• Independent foundations of three phases were linked

together• The teflon seals at the joints were replaced with

improved design

Damage to neutral flexibles

• Overheating was observed in the neutral flexibles of generators during inspection of neutral chamber.

• The braids were found to be under sized.• The flexibles were replaced with those of bigger size

Failure of generator transformer

• A carbon shield is provided around the transformer core for equalizing the potentials.

• Buchholz relay operated while shutting down a unit.• Gas analysis and electrical tests indicated that the

problem is due to shorting of carbon shield • The paper insulation had got displaced and the carbon

shield was acting as a turn of winding.• The carbon shield started to burn.• The transformer was opened at site and the carbon

shield was replaced.

HRSG tube leakages

• Tube leakages and tube thinning was observed in the economizers.

• Frequent start stops• Part load operation leading to low approach temperature

and steaming• Drain valve was introduced between the two HP

superheaters.• Control of water chemistry to reduce dissolved oxygen

and impurities• Replacement of tubes with better material• HP recirculation was increased to increase the approach

temperature

Tripping due to low steam pressure when one boiler trips

• The damper of the boiler closes and the header valves close. • The steam flow from the running boiler used to increase as the

control valve is still wide open.• HP circulation pumps trip due to low NPSH• Problem was in the sliding pressure set point. • Shifting of sliding pressure set point from 2 boiler to 1 boiler

configuration from the closed limit switch of the header valves. • Now changes from not open limit switch of the header valves and

problem has reduced. • Further, if it is a planned shutdown, the load on the running GT is

kept at full load. Also the header valves are closed first and damper is closed later.

High vibrations in ST

• Abrupt peak in the bearing vibrations used to trip the Steam turbine.

• Misalignment observed over a period• Sinking of foundations doubted• Black cotton soil conditions• Bearings were replaced with tilting pad type of bearings

Heavy purge in gas turbines

• While carrying out heavy purge in one machine after it tripped on Naptha, a mild explosion occurred.

• The heavy purge procedure was then modified.• Warren pump clutch gets disengaged now during purge.

Failure of C computer of Speedtronic

• Communication link between speedtronic and centralog• Generator breaker information was through C computer• Unit unloads and trips when it was sensed that unit is

disconnected from grid when the C computer freezes.• Now this information as well as other important

information has been shifted from the C computer to R,S or T computers.

Experience Sharing

Experiences related toGas Fuel FiringLiquid Fuel FiringBoiler Coupling & DecouplingBoiler tube leakage

1. Unloading of Steam Turbine after 2nd Boiler Coupling

Problem Experience:The Steam Turbine is unloading

from initial load of 40-50 MW to 5-10 MW just after coupling

Coupling conditions:S/h Steam temp. > 475 Deg CPressure Diff. < 2.5 bar

Unloading of Steam Turbine Just after coupling of 2nd BoilerUnloading of Steam Turbine Just after coupling of 2nd Boiler

Unloading of Steam Turbine Just after coupling of 2nd Boiler

Unloading of Steam Turbine Just after coupling of 2nd Boiler

Observations:Opening of HP bypass valves of both boiler

Indicating higher pressure upstream of ST control valve

But the pressure observed was normal

The Variables :HP steam flow of incoming boiler

It was observed that the incoming boiler can be coupled if its saturation temp (475 Deg C) and pressure equalization is achieved

o Irrespective of HP Steam flow of the incoming boiler



Analysis:It was observed that the coupling command is

given as soon as saturation temp criteria is achieved

During that time HP Bypass opening seen was @40-50%

On giving coupling command the HP Bypass starts closing (10-20%) to increase the boiler pressure for equalization with other boiler. (10-20 %)

Closing of HP bypass valve causes flow to reduceWhen ultimately coupling takes place the low

flow of this boiler shifts the pr set point to a lower value thus opening HP bypass of both the boiler

Solution:Allow the HP bypass of the incoming boiler to

open more than 80 % to achieve @ 120 t/hr of steam flow

Then give coupling commandThe HP bypass will close to 40-50 % to increase

pressure to equalizationThe boiler will couple with flow of 80-120 t/hrTransition of ST control valve set point from

from one boiler flow to double boiler flow will be smooth

No unloading of ST after coupling


2. Tripping of 2nd boiler and hence Steam Turbine just after

decoupling of first boiler

Problem Experience:

After Decoupling of one boiler it was observed that the HP & LP circulation pumps of the 2nd running boiler were tripping on low recirculation flow causing tripping of the 2nd boiler also resulting in tripping of the ST on “both HRSG trip”

Tripping of 2nd boiler and hence Steam Turbine just after decoupling of first boiler


Observations made:The Diverter damper closing command is given first

then the header valves are opened for the boiler to be taken out

The motorized header valve takes long time in closing and some time even get struck also

Happening when the GT of the boiler to be kept running is at 70 % load

The HP/LP re-circulation flow of the 2nd boiler comes down and remains continuously below “low flow limit”locking the startup of the standby pump.

If at all the stand by pump is started the HP drum level reduces to low level trip value

The 2nd boiler also trips either on “ no re-circulation p/p running” or “HP drum level low”



The Variables:

Load on the other GTClosing or jamming of Header valve of the boiler to be

closedFor the boiler to be taken out whether Diverter damper

is first closed and then Header valve is closed or vice versa

Results of analysis of variablesGT load remaining high the impact is lessIf the valves closes the impact is lessClosing the header v/v first again reduces the impact

All the three reasons checked seems not the root cause



Further Observ.

The ST control valves closes down from 100 & to single boiler condition i.e, 10 % during decoupling

This takes place only when the header valves are fully closed (close limit switch)

The header valve takes 1.5 min to close and the ST control valve takes another 1-2 min to comes down to 11 %



AnalysisAs the control valves starts closing only when

the closed limit switch of the header valves are made So till the time the header v/s are fully closed

the control valves remain full open whereas the normal opening should be 10-11 %

The 2nd boiler steam flow increases ,The drum level comes down, the pressure in the drum varies and the recirculation pump flow comes down to abnormal value



SolutionThe closing of the ST Control valves from two boiler

operation to single boiler operation is now given from “not open” condition of the header valve

This closes down the ST control v/v to 10 % and prevents the 2nd boiler steam flow to increase

First the Header valve is given a closing command then its diverter damper is closed

A person is kept ready to manually close the header valve is its motor fails to do so

The time delay after low flow limit of HP recirculation pumps are incresed to 20 sec



3. WAXING IN HSD CIRCUIT

Problem Experience:

During Flushing of HSD circuit in Nov-Dec 1996, it was observed that when HSD forwarding pumps were started in the morning at @ 0900 hrs the Filtering skids DP went very high and the pump pressure increases beyond its limit value

Waxing in HSD circuitWaxing in HSD circuit


ObservationsThe 5 mic paper filter elements were found

to be chocked with waxThe lowest temperature during the last night

observed was @ 12 deg C

AnalysisHSD comes in two specification

For ambient conditions below 4 Deg CFor conditions above 4 Deg CDifference is the wax content

Waxing in HSD circuitWaxing in HSD circuitSolutions

When the temperature increases above 20 deg the wax melted and the pump could be started

As flushing operation was going on no effect of wax on GT

Filters were replacedThe lot was quantified and used and

consumed during high ambient tempSpecification modified and incorporated

4. NAPTHA LEAKAGES DUE TO EXPANSION AT HIGH AMBIENT

TEMPERATURE

Problem Experience:During high ambient temperatures the

entrapped naptha was expanding and pressurizing the circuits resulting in gaskets failure and uncontrolled spillage of Naptha


Observations : There are 4 main lines going into or coming

out of NGL TanksPipe line transferTanker unloadingForwarding lineDrain Recovery

Except for the tank in service and under reciept all the valves of other lines were kept closed

Naptha leakagesNaptha leakages

Analysis : There are 4 main lines going into or coming

out of NGL Tanks with motorized valves at the inlet /outlet of the tank

Pipe line transfer Tanker unloading Forwarding line Drain Recovery

Except for the tank in service and under reciept all the valves of other lines were kept closed so naptha was getting entrapped


SolutionsThere are 4 main lines going into or

coming out of NGL TanksPipe line transferTanker unloadingForwarding lineDrain Recovery


Solutions

One valve in each line is kept open to any one of the tank so that no entrapment of naptha takes place.

Also suitably relief valves were introduced in the lines to take care of pressurization


• PROBLEMS AND MODIFICATIONS

• GOOD PRACTICES

• MAJOR C&I SHUTDOWN ACTIVITIES

• AUTO FUEL CHANGEOVERS

• EXIT

VARIOUS PROBLEMS AND MODIFICATIONS• GAS FUEL PURGE SYSTEM MODIFICATION

• LIQUID FUEL CONTROL VALVE OPENING DURING HEAVY PURGING

• PURGE VALVES INTERMEDIATE GAS PRESSURE HIGH – AUTO TRANSFER TO LIQUID FUEL

• TURNING GEAR LOGIC MODIFICATION

• LOADING AND UNLOADING RAMP MODIFICATION

• GCV AND SRV HYDRAULIC SCHEME MODIFICATION

VARIOUS PROBLEMS AND MODIFICATIONS

• GENERATOR BREAKER SIGNAL MODIFICATION

• FUEL CHANGE OVER PROCEDURE

• HAZARDOUS GAS/LIQUID LEAK DETECTOR CABLE MODIFICATION

• TURBINE COMPARTMENT VENTILATION DAMPER CABLE MODIFICATION

• INLET AIR FILTER P HIGH SET-POINT MODIFICATION

VARIOUS PROBLEMS AND MODIFICATIONS

• ELECTRONIC WATER TRAP IN PULSE AIR CLEANING SYSTEM

• HEAVY PURGING PROCEDURE

GOOD PRACTICE• PERIODIC FUEL CHANGEOVERS

• FIRE SENSOR OPERATION CHECKING FROM FIELD

• ONLINE REPLACEMENT OF HP FILTER

• MAINTENANCE DURING OPPORTUNITY S/D

• SPECIAL CARE DURING REPLACEMENT AND INSTALLATION OF BEARING THERMOCOUPLES

GOOD PRACTICE

GAS FUEL PURGE SYSTEM MODIFICATION

• We have two purging system

Liquid fuel purge system + Gas fuel purge system.

• When unit operates on liquid fuel then gas purge system becomes operational and vice versa.

• In past, we have faced problems in “Gas fuel purge air system” due to the failure or malfunction of pressure regulators and purge solenoid valves.


• Impacts of above problems on the availability of machine due to safety measures.1. Unavailability of machine on gas fuel, if gas purge

valves do not close properly.2. Only alarm appears, when unit is operating on

liquid fuel and purge valves do not open fully. But for safety, we can not continue to run on liquid fuel and machine becomes unavailable on liquid fuel.

3. As these components were situated in high temperature zone below grating in turbine compartment, it was causing “Forced outage” of machine (at least for 02 hrs) for maintenance and rectification of problem.


• A continues blow down is provided in impulse line.

• As a part of modification, we have shifted all C&I gas fuel purging system components outside the GT acoustic package.

• The components include all pressure regulators, solenoid valves, cabling and impulse lines.

A single outage saving because of this problem leads to profit saving of Rs 80,000/-

LIQUID FUEL VALVE OPENING DURING HEAVY PURGING

• Heavy fuel purging is the procedure of flushing the liquid fuel line up to nozzles with “HSD” by opening stop valve and engaging fuel pump clutch.

• Before modification, the liquid fuel bypass valve opening reference (FSR1) was 22%, toward turbine during purging.

• Problems due Bypass valve opening1. Frequent breaking of gauge glasses in drain line. This was

causing minimum outage of 02 Hrs in case of tripping on naphtha fuel due to high temperature in turbine compartment.

2. Possibility of more fuel going inside turbine during heavy purging, if VCK-1 passing.

LIQUID FUEL VALVE OPENING DURING HEAVY PURGING

MODIFICATIONS

1. LIQUID FUEL VALVE DOES NOT CLOSE DURING HEAVY PURGING.

2. GAUGE GLASSES REMOVED FROM THE DRAIN LINE.

TURNING GEAR LOGIC MODIFICATION

INSTANCE:

<C> computer failed and unit tripped. During costing down turning gear did not engaged. However, the <C> computer normalized with in 15 Minutes.

AFTER EFFECTS:

When unit trips and if turning gear does not engage then rotor will reach to zero RPM. As per the OEM guideline in such conditions we can not start machine within 48 Hrs.

LOSS: Rs 20,00,000/-

TURNING GEAR LOGIC MODIFICATION

MODIFICATION:

For turning gear operation jacking oil system should be healthy. The turning gear operation logics are incorporated in <RST> computers. After analysis we found that jacking oil pressure low and jacking oil pump breaker close signals were connected to <C> computer. So, when <C> computer failed then status of these signal to <RST> computers becomes unknown and as per protection turning gear did not engaged.

EVEN AFTER MODIFICATION IT IS NOT COMPLETELY SUCCESSFUL.

LOADING AND UNLOADING RAMP MODIFICATION

• INSTANCE: Generator breaker open due to sudden variation in grid frequency.

• There are two mode of operation of gas turbine

1. Auto Mode: Base and Pre-select load mode.

2. Manual Mode: In this mode the load is controlled by station load controller, as per set-point fixed by operator.

• Earlier loading and unloading rates were 9 and 18 mW/Minute in auto and manual mode respectively.

LOADING AND UNLOADING RAMP MODIFICATION

• MODIFICATION AND OPERATION PRACTICE

1. THE LOADING AND UNLOADING RATE INCREASED TO 12 AND 24 MW/Minute.

2. OPERATING GAS TURBINE IN MANUAL MODE FROM CCR. FOR BASE LOAD OPERATION WE GIVE HIGHER LOAD SET POINT, SO UNIT GOES TO TEMPERATURE CONTROL MODE.

3. FOR SCHEDULE LOAD INCREASE AND DECREASE WE USE 12 MW/Minute RAMP.

GCV AND SRV HYDRAULIC SCHEME MODIFICATION

• INSTANCES: Number of outages due to, failure of Gas Control/Stop Ratio electro hydraulic servo valve and filter chocking.

• The servo valves are supplied by M/s Moog Ltd. Model is F771-K200.

• PROBLEMS:

1. We could not replace servo valve online because they are part of hydraulic system. We have same hydraulic system for Liquid fuel and IGV control.

2. The operating hydraulic pressure is 100 Bar.

3. No provision for isolation of servo and filter.

GCV AND SRV HYDRAULIC SCHEME MODIFICATION

MODIFICATION:

WE HAVE RECTIFIED THIS PROBLEM BY PROVIDING ONE ISOLATING VALVE IN GAS HYDRAULIC SYSTEM. WE INTRODUCE THIS VALVE BEFORE GCV, SRV SERVOS AND INLINE FILTER.

MACHINE OUTAGE SAVING OF 1.5 – 2 Hrs.

MONITORY SAVING = Rs 80,000/-

INTER-VALVE GAS PRESSURE HIGH AUTO TRANSFER TO

LIQUID FUEL When unit operates on liquid fuel, then gas fuel line

from “Gas control Valve” up to fuel nozzle is purged with air with the help of 02 pneumatic On/Off valves. During gas fuel operation these purge valves remain close. The leakage from purge valve is sensed by pressure switch which is mounted between two valves.

INITIAL LOGIC was to shutdown the machine if, unit is running on gas fuel and inter-valve gas pressure goes high.

MODIFICATION : AUTO CHANGEOVER TO LIQUID FUEL.

GENERATOR BREAKER SIGNAL MODIFICATION

INSTANCE: Unit tripping with <C> computer failure.

PROBLEM: The “generator breaker connected to grid” signal was

interfaced with <C> computer. So, when <C> computer failed then in <RST> computer the status of above signal becomes unknown and unit got tripped.

MODIFICATION: The generator breaker signal was shifted from <C>

computer to <RST> computer.

FUEL CHANGEOVER PROCEDURE

INSTANCE: UNIT TRIPPINGS during fuel changeover from Gas to

Liquid fuel on exhaust temperature high.

PROBLEM: Load increase on HSD fuel + Opening of gas fuel

purge valves which causes inrush of trap gas fuel.

PROCEDURE:1. Reduce load from base to part load [80 MW].2. Keep IGV control off for protecting boiler

parameters.

HAZARDOUS GAS/LIQUID LEAK DETECTOR CABLE

REPLACEMENTINSTANCE:

UNIT TRIPPINGS on spurious hazardous gas/liquid leak detection protection of turbine compartment.

PROBLEM:

Cable burned in turbine compartment due to hot gas leakages from bearing no 02.

MODIFICATION:

The original cable used in leak detector circuit was ordinary cable. Now we replaced it with 03 core high temperature [260 Deg C] Teflon coated cable. We procured this cable from M/s Draka, France

TURBINE COMPARTMENT VENTILATION DAMPER CABLE

MODIFICATIONINSTANCE:

UNIT TRIPPING due to earth fault from ventilation fan damper limit switch cable.

PROBLEM: Due of leakages from ventilation fan, the insulation of

“turbine compartment damper limit switch” cable burned.

MODIFICATION: We replaced original cable with high temperature

Teflon coated cable [for 88BT and 88VG switches].

INLET AIR FILTER DP HIGH SET-POINT MODIFICATION

• The initial setting of gas turbine normal shutdown protection, due to inlet air filter P high was 1500 Pascal.

• We felt that this setting was on lower side, so we take-up the matter with M/s EGT and they agreed to raise the shutdown set point to 2000 Pascal.

AUTO FUEL CHANGEOVERSUNIT RUNNING ON GAS FUEL

Gas pressure low (17.5 Bar Before Stop Ratio Valve).

Transfer to HSD

VA 13-1,2 inter valve gas pressure High (>3 Bar, Time delay 40 Sec)

Transfer to HSD, if not S/D after 45 Sec.

UNIT RUNNING ON HSD

HSD tank level low (2.0 Meters). Transfer to NGL/ARN

AUTO FUEL CHANGEOVERSUNIT RUNNING ON NAPHTHA FUEL

Naphtha fuel pressure low - <3.0 Bar

Naphtha tank level very low - <1.1 M

Naphtha fuel flow low – 2.5 Kg/Sec

Both high tech pumps not running Transfer

Hi-Tech tank level low – 10 Min Delay To

Purge manifold liquid leak detection HSD

Atomizing air compressor DP low - <1.06 Bar

88-BT ventilation fan trouble.

<C> computer failure

HEAVY PURGING PROCEDURE

• Open 14 drain valves before VCK-1, H.P. filter vent and drain valves, warren pump vent, drain valve in trench between stop valve and three way valve near NGL skid, vent valve in NGL line near flow meters. Also close NGL skid manual valve.

• Opening of False start drain valves VA-17 1, 2 & 5 at coasting down speed below 30% and opening of liquid fuel bypass valve.

• Crank GT for 15 minutes and note wheel space temperature downward trend, check air releases from the false start drain valve and smoke from bypass stack.


• Close liquid fuel drain valve in trench, H.P. filter drain valve after ensuring no increase in level of sump tank

• Put data liquid fuel flow (FQ), flow divider speed (FQ_PR), liquid fuel bypass control valve position (FSL) for monitoring during purging operation.

• Take sump tank level before and after purging.• Purging ON command to be given and ensure no

flame, clutch (20CF) disengaged, stop valve (VS1) opens and diesel valve (20FD) open.


• In case of higher fuel flow or higher flow divider speed or higher liquid fuel by-pass valve closing or flame sensed by flame scanner, purging sequence to be stopped immediately.

• Check fuel leakage in turbine and auxiliary compartment during heavy purging.

• Close HP filter drain valve, warren pump vent valve and liquid fuel line vent valves after complete air release.

• Closing of all 14 drain valves after heavy purging and open NGL manual valve.

PERIODIC FUEL CHANGEOVERS

• To ensure availability of machine on both fuels.

• To avoid seizing of flow divider.

• Identification of leakages, if any.

• Operation check of fuel line components and purge systems.

FIRE SENSORS CHECKING FROM FIELD

• We check all 14 fire sensors from the field, by putting short link across the sensor. This ensures healthiness of following components.

1. Fire sensors wirings from field to panel.

2. Fire hooter and flushing light.

3. Related logics in Speedtronic panel

4. Fire protection cards in fire panel.

5. Mechanical timers, solenoid valves and push buttons like reset, re-injection etc of local CO2 skid.

• Functioning check of emergency fire push buttons

MAINTENANCE DURING OPPORTUNITY S/D

• STROKING OF GAS AND LIQUID FUEL CONTROL VALES

• PURGE VALVE OPERATION CHECKING

• PROTECTION CHECKING, IF SHUT DOWN IS LONG

CARE DURING INSTALLATION OF BEARING THERMOCOUPLES

• Physical inspection and resistance measurement of thermocouples to ensure healthiness.

• Measurement of insertion length of thermocouples.• Inspection and depth measurement of bearing

thermocouple holes when the bearing is out. It is important for repaired bearing.

• Final checking by actual heating thermocouple with hot air blower.

• Proper dressing and spot welding of thermocouple, so that it does not damage during bearing box up.

• Oil leakage checking from thrust & journal bearings thermocouples.

MAJOR C&I SHUTDOWN ACTIVITIES

• Calibration of gas fuel control valve, stop ratio valve, liquid fuel bypass control valve and IGV.

• Calibration of torque converter.• Calibration of field instruments.• Servicing of all gas turbine solenoid valves.• Insulation resistance measurements of fire detectors.• Replacement of internal parts of spark plug.• Servicing of false start drain valves.• Replacement of gaskets and broken TB of all junction

boxes.• Purge valve and 3-way valve timing adjustment.• HSD/Naphtha passing from 3-way valve.

THANK YOU

Major Milestones

• First GT Synchronized March 92• Combined Cy. Commissioning March 93• Liquid fuel Commissioning 1997• ABT Regime started Aug 2002

Tot. Gas Fired Time (Avg/GT): 46500 hrsTot. Liq. Fired Time (Avg/GT): 19500 hrs

Experience Sharing

Experiences related toGas Fuel FiringLiquid Fuel FiringBoiler Coupling & DecouplingBoiler tube leakage

1. Tripping of the Gas Turbines during Fuel Transfer

Tripping of GT during Fuel TransferTripping of GT during Fuel Transfer

Problem Experience:

From GAS to HSDFlow Divider not rotatingControl valve not opening or sudden

openingMistimed Opening of the gas fuel nozzle

purging valve

From HSD to GASStop or control valve not openingSudden opening of control valve

• Analysis:• Problem of servo valve • Oil purity problem• Jamming of Flow divider and sudden

rotation• Air entrapment in Liquid fuel line due to

reverse flow from VCK1 • Un-metered HSD through Liquid fuel purge

line due to VCK2 passing


• Solutions:

To eliminate tripping on “Exh Temp High”

All fuel changeovers are now done at 70 MW load with IGV mode OFF

Reducing Exh. temp from 540 to 450 degC

Opening of liquid fuel purge valve is delayed by 30 sec after GT is transferred from HSD to Gas

Reduces un-metered hsd into the combustion chamber


• Solutions: cont..

All fuel changeovers are now done at 70 MW load with IGV mode OFF

Reducing Exh. temp from 540 to 450 degC

Opening of liquid fuel purge valve is delayed by 30 sec after GT is transferred from HSD to Gas


Fortnightly rotation of the flow dividerReduces jerks and thus sudden inflow of HSD


• Solutions:

To eliminate tripping on “Unloading of GT”

Opening of the Gas fuel stop valve or the control valve is monitored after giving a transfer order to gas and if the control valve is not opening , the load started to reduce . Immediately transfer back to HSD is given

Reduces unloading of the GT

Same for Transferring from Gas To HSD .The Flow Divider rotation and the Control valve positions are monitored .If any one of them failed to start , a transfer back command to gas is givenOpening of liquid fuel purge valve is delayed by 30 sec after GT is transferred from HSD to Gas



2. Unloading of Steam Turbine after 2nd Boiler Coupling

Problem Experience:The Steam Turbine is unloading

from initial load of 40-50 MW to 5-10 MW just after coupling

Coupling conditions:S/h Steam temp. > 475 Deg CPressure Diff. < 2.5 bar


Observations:Opening of HP bypass valves of both boiler

Indicating higher pressure upstream of ST control valve

But the pressure observed was normal

The Variables :HP steam flow of incoming boiler

It was observed that the incoming boiler can be coupled if its saturation temp (475 Deg C) and pressure equalization is achieved

o Irrespective of HP Steam flow of the incoming boiler



Analysis:It was observed that the coupling command is

given as soon as saturation temp criteria is achieved

During that time HP Bypass opening seen was @40-50%

On giving coupling command the HP Bypass starts closing (10-20%) to increase the boiler pressure for equalization with other boiler. (10-20 %)

Closing of HP bypass valve causes flow to reduceCheck ultimately coupling takes place the low

flow of this boiler shifts the pr set point to a lower value thus opening HP bypass of both the boiler

Solution:Allow the HP bypass of the incoming boiler to

open more than 80 % to achieve @ 120 t/hr of steam flow

Then give coupling commandThe HP bypass will close to 40-50 % to increase

pressure to equalizationThe boiler will couple with flow of 80-120 t/hrTransition of ST control valve set point from

from one boiler flow to double boiler flow will be smooth

No unloading of ST after coupling


3. Tripping of 2nd boiler and hence Steam Turbine just after

decoupling of first boiler

Problem Experience:

After Decoupling of one boiler it was observed that the HP & LP circulation pumps of the 2nd running boiler were tripping on low recirculation flow causing tripping of the 2nd boiler also resulting in tripping of the ST on “both HRSG trip”



Observations made:The Diverter damper closing command is given first

then the header valves are opened for the boiler to be taken out

The motorized header valve takes long time in closing and some time even get struck also

Happening when the GT of the boiler to be kept running is at 70 % load

The HP/LP re-circulation flow of the 2nd boiler comes down and remains continuously below “low flow limit”locking the startup of the standby pump.

If at all the stand by pump is started the HP drum level reduces to low level trip value

The 2nd boiler also trips either on “ no re-circulation p/p running” or “HP drum level low”



The Variables:

Load on the other GTClosing or jamming of Header valve of the boiler to be

closedFor the boiler to be taken out whether Diverter damper

is first closed and then Header valve is closed or vice versa

Results of analysis of variablesGT load remaining high the impact is lessIf the valves closes the impact is lessClosing the header v/v first again reduces the impact

All the three reasons checked seems not the root cause



Further Observ.

The ST control valves closes down from 100 & to single boiler condition i.e, 10 % during decoupling

This takes place only when the header valves are fully closed (close limit switch)

The header valve takes 1.5 min to close and the ST control valve takes another 1-2 min to comes down to 11 %



AnalysisAs the control valves starts closing only when

the closed limit switch of the header valves are made So till the time the header v/s are fully closed

the control valves remain full open whereas the normal opening should be 10-11 %

The 2nd boiler steam flow increases ,The drum level comes down, the pressure in the drum varies and the recirculation pump flow comes down to abnormal value



SolutionThe closing of the ST Control valves from two boiler

operation to single boiler operation is now given from “not open” condition of the header valve

This closes down the ST control v/v to 10 % and prevents the 2nd boiler steam flow to increase

First the Header valve is given a closing command then its diverter damper is closed

A person is kept ready to manually close the header valve is its motor fails to do so

The time delay after low flow limit of HP recirculation pumps are incresed to 20 sec



4. WAXING IN HSD CIRCUIT

Problem Experience:

During Flushing of HSD circuit in Nov-Dec 1996, it was observed that when HSD forwarding pumps were started in the morning at @ 0900 hrs the Filtering skids DP went very high and the pump pressure increases beyond its limit value



ObservationsThe 5 mic paper filter elements were found

to be chocked with waxThe lowest temperature during the last night

observed was @ 12 deg C

AnalysisHSD comes in two specification

For ambient conditions below 4 Deg CFor conditions above 4 Deg CDifference is the wax content

Waxing in HSD circuitWaxing in HSD circuitSolutions

When the temperature increases above 20 deg the wax melted and the pump could be started

As flushing operation was going on no effect of wax on GT

Filters were replacedThe lot was quantified and used and

consumed during high ambient tempSpecification modified and incorporated

5. NAPTHA LEAKAGESDUE TO EXPANSION AT HIGH AMBIENT TEMPERATURE

Problem Experience:During high ambient temperatures the

entrapped naptha was expanding and pressurizing the circuits resulting in gaskets failure and uncontrolled spillage of Naptha


Analysis : There are 4 main lines going into or coming

out of NGL Tanks with motorized valves at the inlet /outlet of the tank

Pipe line transfer Tanker unloading Forwarding line Drain Recovery

Except for the tank in service and under reciept all the valves of other lines were kept closed so naptha was getting entrapped


Solutions

One valve in each line is kept open to any one of the tank so that no entrapment of naptha takes place.

Also suitably relief valves were introduced in the lines to take care of pressurization


6. Economizer Tube Leakage

Problem : HP#2 Economizer inlet header bend tubes

leakages in all HRSG’s Tube material - CS Tube dimension-32x3.2

Economizer tube leakagesEconomizer tube leakages

R&D Analysis Report : Severe wall thinning Corrosion on the Straight and bends

R&D Suggestion Wall thickness measurement Control of water chemistry to reduce

dissolved oxygen and impurity levels in feed water

Replacement of existing CS tubes with T11


Remedial measures taken

Tube thickness survey : Below 2.5mm HRSG 1A -17 HRSG 1B -29 Replaced with T11 HRSG 2A -23 HRSG 2B -18


New Observations

At GT Load < 70 MW the HP Approach

temperature was observed as 0-2 Deg C

whereas the reference approach

temperature was 7.5 Deg C The water enters into the HP2 Economizer

from one side and the tube leakage density is more on that side


Analysis Approach temp if less than designed can cause

Steaming in the economizerWater hammer and vapor lockHigh local velocitiesTube thinning and leakage

HP economizer recirculation is provided for prevent steaming at low load conditions and thus it also controls approach temp

The HP Recirculation is closed at HP steam flow 120 t/hr and at 70 MW load the HP flow is 125-130 t/hr


Analysis It was observed that at 70 MW The HP Eco

flow is 125-130 t/hr and the HP Economiser recirculation stops at 120 t/hr

If HP Economizer recirculation is kept on till 70 MW the approach temperature increases and reaches nearer to its design value

Solutions The HP Economiser recirculation is kept on till

HP Eco flow is 150 t/hr


Download - Case Studies by Chohan Sir

Top Related