We are facing one strange problem with our gas turbine. We have GE Frame 6B DLN-1 Type Gas Turbine. Its Inlet Bleed Heating Valve (IBH) starts opening by itself without any command. What could be the possible reason for it?
Sometimes, it opens so much that the Gas Turbine Gets tripped on "Bleed Heat System Not Operational".
When we check the valve, it works fine, strokes fine. No apparent leakage through any component also.
Can you guys please share your experience as why it could be happening?
Can you give us a bit more info? Did the IBH work OK in the past? Was any maintenance done on the machine recently? Can you give us the logic for the alarm "Bleed Heat system not operational" that kind of sounds like your problem.
Thinking this through, the Mark* (V? VI? VIe?) has a 4-20 mA output to a pneumatic control valve--an I/P positioner, which converts 4-20 mA of current from the Mark* (the "I" in I/P) to a 3-15 psi pressure (the "P" in I/P) to move the valve. The "loop" is closed in the I/P positioner--to make feedback equal to reference. If I recall correctly there is a 4-20 mA position output device on the valve which just basically tells the Mark* where the valve is on the open/closed continuum.
So, EITHER the 4-20 mA output is drifting high for "no reason" causing the valve to open, OR there is something amiss with the I/P positioner/calibration (it does have to be periodically calibrated, contrary to (false) belief--if only to check that with a 4 mA signal it goes to or remains at 0%, with an 8 mA signal it goes to 25%, and a 12 mA signal it goes to 50%, and with a 16 mA signal it goes to 75%, and a 20 mA signal it goes to 100%). It could even be a problem with the 4-20 mA positioner output signal drifting and causing the Mark* to change it's output then suddenly jumping back to a normal output.
There aren't too many things it could be. If it's the Mark V output signal that's drifting, it would be surprising there's not at least one relevant Diagnostic Alarm to go along with that. If the unit has a Mark* controller, it has some kind of short term trending capability which could be turned on to capture CSRIHOUT and the IBH control valve position feedback signal to see if either of those are changing.
Refer to the valve actuator manufacturer's troubleshooting instructions in the manual for the valve/actuator to see if that's the issue (my best bet--in the absence of Mark* Diagnostic Alarms). But, setting up a short term trend (for the Mark V that would involve using one of the VIEWn tools to capture data and then analyze that data, possibly using MS-Excel).
But can't be too many things. Some IBH air supply systems have solenoids and "quick-exhaust" valves. It's possible the solenoid is leaking over time, or the quick-exhaust valve(s) aren't working. But, again the list isn't too long. There's also usually some kind of air pressure regulator in the system; perhaps it's having issues over time.
Have a look at the System Description in the Operations & Service Manual, and the IBH P&ID, and go through the components of the system one by one. Many times troubleshooting is a logical process of elimination--until you arrive at the cause. Try to start with the easiest or simplest things first (to get them out of the way)--and use the short term trending feature of control system to look at data to further help narrow the search down. And have a read of the valve/actuator manufacturer's instruction book for more troubleshooting help.
You can even put an ammeter in series with the valve output signal from the Mark* to the I/P actuator, and with the valve position output signal from the valve to monitor currents/signals.
And, please write back to let us know how you fare in troubleshooting--and resolving--the problem!
Even a journey of a thousand miles begins with a single step--just take it one step (component; possibility) at a time.
Thanks for your response.
We are using MarkVI control system. The valve has an Electro-Pneumatic Positioner whose air is supplied through a SOV (VTH-1). This SOV is supplied with air through a Pressure Regulator. The positioner feedbacks the position back to MarkVI. Also there is a quick exhaust relay which is used to close the valve quickly.
There is no recent maintenance done on this IBH valve. Recently we faced the problem, that the IBH valve starts opening by itself from 0 to 15% over the period of 15-20 minutes. We observed that during this time there was no output command to this valve (CSRIHOUT). First we receive this alarm:
ALM 357.5ZA94316 INLET BLEED HEATING VLV POSIT.TROUBLE
Then after 5 minutes we received the following alarm:
ALM 357.5YA94316 IBH CONTROL VALVE DISCREPANCY
Then after 6 minutes we received the following alarm:
ALM 357.5TA94305/PA94308 COMP.OPER.LIMIT CONTROL FAULT
ALM BLEED HEAT SYSTEM NOT OPERATIONAL - TRIP
and then the turbine was tripped.
We checked the valve by stroking it, did not find any problem or any apparent leakage from any of the component. We restarted the turbine and it is working fine. The question is how the valve starts opening by itself if there is no command. If any pneumatic component has failed then was it a momentarily failed? As I mentioned that the valve ramps up from 0 to 15% over the span of around 15 minutes. This does not look like a serious pneumatic failure.
We have cases with other IBH valves where the positioners have failed but when they fail, they look like failed and they remained failed. Can you please share your experiences?
If a failure of any component results in a turbine trip that qualifies as a serious failure in my estimation and experience.
Have you tried shutting off the compressed air supply as soon as it's noticed the valve starts drifting open?
When you say CSRIHOUT isn't increasing, is that from observing CSRIHOUT or from measuring the current being supplied from the CSRIHOUT?
Best of luck resolving your unserious tripping problem.
Not every problem or failure is exactly the same as every other problem or failure--even of the same type of device (in this case a valve positioner).
Keep an open mind when troubleshooting, because a closed mind can cause a lot of wasted time. Re-examine all your data, and how it was gathered. Hard data, as opposed to anecdotal data, is always best.
I've seen a lot of failures and problems in more than three decades in "the business,' and I'm never surprised to see failures and problems manifest themselves in different ways. What I am surprised to see, though, is how often people presume that failures and problems always occur in the same fashion, or that a failure of a device or component can only occur in one way. In this case, there are several devices and components which would--seemingly--have to occur at the same time. And, while the likelihood of that happening is not very high--it DOES happen. (I'm referring to a failure of the solenoid valve and the valve positioner at the same time--because if I understand the system as you have described it, the solenoid has to be energized for instrument air pressure to get to the positioner. And, if the air pressure were high because the regulator were not maintaining pressure properly, well, then there could be problems.
Or if the solenoid is leaking (passing) and there is some intermittent problem with the valve positioner (say moisture and/or dirt in the I/P device), then that could lead to intermittent problems. And, possibly a slow-acting problem.
Please write back to let us know what you find!