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:

1. Operation of Bleed SOV G20VG was verified and was found to be okay. However, this SOV was preventively replaced with a new one
2. Both Interstage Pressure Transmitters G96FG-1 and 2 were verified and calibrated and were found to be healthy
3. Gas presence in intervalve chamber downstream of SRV was verified physically through Pressure Transmitters vent points and SOV vents and gas presence was observed
4. 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:

1. 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?
2. 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?
3. 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.

 

Makster,

1. 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?

That's 100% not correct. During GT operation, as the GCV opens/closes to change load the intervalve pressure will change--and the SRV responds appropriately to keep the P2 pressure stable across the entire load range (100% TNH to Base Load and back to 100% TNH). Also, leaking of the SRV will not affect operation at maximum load, only when the SRV should be closed.

1. 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.

The only thing which may be affected is there will be a "hard light-off" because of the increased pressure and fuel trapped between the SRV and GCV that is released into the fuel piping, nozzles and combustors when the GCV is opened. The normal sequence of events during firing when it's time to admit fuel and ignite it is for the ignitors to be energized first, then the GCV opens (releasing any trapped fuel between it and the SRV) and then the SRV opens to bring the P2 pressure up to reference. The intervalve vent solenoid, 20VG-1 closes when the unit starts turning, and that's when--if the SRV is leaking--fuel and fuel pressure will begin to build up in the intervalve cavity. And, it will continue to build pressure all during the purging sequence and up until the GCV opens.... If the SRV is leaking when the unit is at rest and the pressure in the intervalve cavity is building up with 20VG-1 open, then it's a pretty significant leak. Closing 20VG-1 with a leaking SRV is going to allow pressure and fuel to build up even higher (with no vent to atmosphere through 20VG-1). This will, as was written above, most likely result in a "bang" of some magnitude when the fuel is ignited.

1. 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.

LOTS of sites around the world perform their own SRV/GCV maintenance. There are kits for it. I'm sure the OEM would LOVE to sell a complete SRG/GCV assembly, but, if you have a competent mechanical department or have a local shop that you have confidence in--it's not a difficult task. The most dangerous part is opening the "lid" of the assembly without launching the springs and hurting someone (and, then, of course, closing the lid and compressing the springs when reassembling). But, it's been done in the field hundreds of times, maybe even more--this combined SRV/GCV assembly has been used around the world for decades.

 

Hi Makster,
All of the SRV's I've worked with have a elastomer seat. If it does not seat properly, chances of further deterioration is also high. And soon after a shut down, you will be able to replace the seat within couple of hours while turbine in turning gear). In my opinion, it would be worthwhile to replace the seat rather dealing with specially over speed protection. Sice GCVs are class 4 leakage class, it is important to have a tight SRV.
Pls ignor if your SRV does not contain a elastomer seat.
regards,

 

Makster,

I wanted to also add that 20VG-1 vents through what's usually referred to as a "goose-neck vent." (It vents through some type of atmospheric vent, regardless of what it's called; just follow the tubing and you will find the atmospheric vent. 20VG-1 is a normally-open solenoid-operated valve, and it is de-energized when the unit is at zero speed. 20VG-1 is usually a pretty small solenoid valve, with either 3/8-inch or 1/2-inch tubing connections. So, it doesn't pass much fuel when it's opened.... It's just there mainly to relieve the pressure between the SRV and GCV so that during starting/firing there isn't fuel/pressure trapped between them which can cause a "hard" light-off (a bang of some magnitude; I have seen the sides of HRSGs bulge because of SRV leakage when firing!!! it's pretty scary!!!).

SO, if pressure is building up between the SRV and GCV when the unit is at zero speed that means the SRV is leaking so much that 20VG-1 can't alleviate the fuel/pressure fast enough to keep the pressure to near zero. Which means the leak is NOT good.

As was written above, usually, when the unit starts turning (breaks away from zero speed) during starting, 20VG-1 is energized to close it--and it stays closed during starting and accelerating and loaded operation. SO that means that is the SRV is leaking, and leaking "badly" that pressure/fuel will build up between the SRV and GCV during purging and prior to firing....

My suggestion is to check the atmospheric vent to try to determine how much fuel is being passed by 20VG-1 when the unit is at zero speed. You could put your hand over the opening of the atmospheric vent to see how much pressure builds up as you hold your hand over the opening. You should also try forcing 20VG-1 to a logic "1" (to close 20VG-1) while the unit is at zero speed and monitoring the pressure between the SRV and GCV. If it increases quickly you know the leak past the SRV is pretty serious. You should ALSO be watching the P2 pressure during the next start--because if it's increasing above even 1 barg that can lead to high exhaust temperatures at light-off and the "hard" light-off being referred to. Hard light-offs are not good for combustion hardware (liners; transition pieces; nozzles) or exhaust components (diffusers; transition ducts) or HRSGs (if the unit exhausts into a "boiler").

This is pretty serious stuff. I know of a site that was having such hard light-offs that the induced flow through the axial compressor was causing the inlet filter house implosion doors to be sucked open!!! It was a simple-cycle unit with a short exhaust stack so the "bang" wasn't as noticeable as with an HRSG but damage was found on some of the last stage blades (rotating and stationary) of the axial compressor. (This had been going on for years.... Good thing it was a peaker unit and didn't run very much. But, it was started once every month and people were aware of and watched for the opening of the implosion doors. (The springs on the implosion doors are VERY strong--and it takes a lot of pressure differential to cause them to open, and then when they close--they SLAM shut with a lot of noise). Hard to believe it was ignored for so long.... Frame 5s are BEASTS and can take a lot of abuse but after so long there is ultimately going to be damage. The rotor had to be removed and the bill was more than USD 1,000,000.000 for repairs to the unit after all was said and done.



Hope this helps!