case studies by chohan sir

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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 GCV Liquid 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 2Q 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 resulted in clogging of filter. 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 9709E. 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.Seria l No Unit ID Cleanliness Level 5 15 Maximum Particles/100 ml. 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 conducted the Patch Test.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.

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

The NAS 1638 ComponentsClass Maximum particles/100 ml in specified size range (Qm) 5 15 00 0 1 2 3 4 5 6 7 8 9 10 11 12 125 250 500 1000 2000 4000 8000 16000 32000 64000 128000 256000 512000 1024000 25 25 22 44 89 178 356 712 1425 2850 5700 11400 22800 45600 91200 182400 25 50 4 8 16 32 63 126 253 506 1012 2025 4050 8100 16200 32400 50 100 1 2 3 6 11 22 45 90 180 360 720 1440 2880 5760 >100 0 0 1 1 2 4 8 16 32 64 128 256 512 1024

Lube Oil filtration schemeLube oil Return Header

GT Lub Oil Tank Capacity 12 KL NAS < 5.Drain Point

12 Absolute PALL part code HC 8900 FSK 16 H. (Rated with F >= 200) 06 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 ( 475 Deg C Pressure Diff. < 2.5 bar

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 boilerIndicating higher pressure upstream of ST control valve But the pressure observed was normal

The Variables :HP steam flow of incoming boilerIt 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

Unloading of Steam Turbine Just after coupling of 2nd Boiler

Unloading of Steam Turbine Just after coupling of 2nd 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 reduce When 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

Unloading of Steam Turbine Just after coupling of 2nd 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 command The HP bypass will close to 40-50 % to increase pressure to equalization The boiler will couple with flow of 80-120 t/hr Transition 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 boiler and hence Steam Turbine just after decoupling of first boiler

nd 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