Hello Guys,
I wanted to share a recent experience of mine on our Hitachi H-25 Gas Turbine Unit (with Mark-VI Control System) and wanted your recommendation / expert guidance in devising the way forward. We had our unit startup scheduled prior to which I checked the system, verified the calibration / stroking of IGV, GCV and SRV and found no abnormality. However, when we initiated startup and lined up Natural Gas on the upstream of SRV, machine sent a trip "Gas Fuel Intervalve Pressure High Trouble". This is a pre-ignition trip which is actuated when intervalve P2 gas pressure is greater than 0.30 MPa (60 sec delay) prior to GT firing. Upon checking, we found that P2 pressure was 0.41MPa and this is why Mark-VI was sending the trip command.
Following checks were carried out to find the exact root cause of the issue:
Based on above checks, it was quite apparent that the issue was due to slight leaking of SRV valve. Valve operation was OK but it was not providing tight shut off at zero position. We were able to start the machine by raising the setpoint to 0.5MPa. Machine started normally and is operating fine till now but I have some basic queries which I'd like to share here:
I wanted to share a recent experience of mine on our Hitachi H-25 Gas Turbine Unit (with Mark-VI Control System) and wanted your recommendation / expert guidance in devising the way forward. We had our unit startup scheduled prior to which I checked the system, verified the calibration / stroking of IGV, GCV and SRV and found no abnormality. However, when we initiated startup and lined up Natural Gas on the upstream of SRV, machine sent a trip "Gas Fuel Intervalve Pressure High Trouble". This is a pre-ignition trip which is actuated when intervalve P2 gas pressure is greater than 0.30 MPa (60 sec delay) prior to GT firing. Upon checking, we found that P2 pressure was 0.41MPa and this is why Mark-VI was sending the trip command.
Following checks were carried out to find the exact root cause of the issue:
- Operation of Bleed SOV G20VG was verified and was found to be okay. However, this SOV was preventively replaced with a new one
- Both Interstage Pressure Transmitters G96FG-1 and 2 were verified and calibrated and were found to be healthy
- Gas presence in intervalve chamber downstream of SRV was verified physically through Pressure Transmitters vent points and SOV vents and gas presence was observed
- SRV position was physically verified and also through LVDT feedback and it was ensured that SRV was closed physically as well
Based on above checks, it was quite apparent that the issue was due to slight leaking of SRV valve. Valve operation was OK but it was not providing tight shut off at zero position. We were able to start the machine by raising the setpoint to 0.5MPa. Machine started normally and is operating fine till now but I have some basic queries which I'd like to share here:
- During GT operation, SRV position remains relatively constant to ensure constant pressure at GCV upstream so will this slight leaking in SRV affect turbine operation on maximum load?
- We did some internal hazard analysis and didn't think that raising the trip setpoint to 0.5MPa could have adverse effect on turbine mechanically. This trip is just to ensure that gas does not enter turbine manifold in offline condition and we've already catered that by first checking that GCV is not leaking and is holding pressure at upstream and second by ensuring an interlock which depressurizes gas circuit whenever GT is stopped or tripped. I'd like to know if there's something we've missed or if this modification can be harmful?
- It is quite evident that SRV needs mechanical overhauling but OEM (Hitachi) does not recommend on-site overhauling and advises to send existing GCV-SRV set to their factory and in the meanwhile, purchase a new set from them as a 'roll-in roll-out' concept. This is, of course, a very expensive solution. Any recommendation or way out regarding this will also be appreciated.