Compressor Bleed Valve Failure: Causes and Recommendation

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benjie

My power station in Nigeria often experience problems with the compressor bleed valve of Hitachi H25 gas turbine units. we need solution to these problems. As a new intake, (Graduate Trainee), i have been asked to come up with different causes of compressor bleed valve failures, and give recommendations. I have been following threads in this site and believe your responses will help me in proffering solutions to this problem.

Thanks
My email address: [email protected]
 
benjie,

When you're analyzing a problem like this it's important to look at all aspects of the problem. In this case, that includes the valve itself (ambient temperature rating of the valve; rating of the valve components/seals; pressures against which the valve has to operate and the rating of the valve for operating pressures; etc.), the valve actuator (usually a pneumatic actuator), the source of motive power for the actuator (often, via an extraction from the gas turbine axial compressor), valve placement (inside or outside the turbine compartment), and most importantly, what the failure mode(s) of the valve(s) have been at the site. There have been a LOT of problems attributed (incorrectly) to valves when it was poor design of the brackets to which the limit switches sensing valve position (open/closed) were attached.

A lot of failures of valves have been caused by moist air used for the actuator motive power. Coming from the axial compressor discharge, especially in humid environments or when inlet cooling (evaporative coolers or chillers) the air can have a lot of moisture which, if not properly drained from low points in the piping/tubing, can be pushed into the actuator causing rust and problems with the actuator--not the valve, itself, but the actuator.

Another large heavy duty gas turbine manufacturer uses continuous blow-down orifices in the low-points of the piping tubing. These are really nothing more than very small holes drilled in short pipe nipples with a chamfer applied to the outside of the hole. These are purposely left open and will blow high-pressure air out of the hole--but that's necessary to ensure condensate gets blown out (removed) from the piping. Many sites fail to understand the purpose of these continuous blow-down orifices, and consider them to be unnecessary and the source of "leaks" in and out of the compartments, and remove them--with poor results. Also, they can easily become plugged over time with rust and dirt particles--and they are never cleaned during maintenance outages.

This same manufacturer uses a cast-iron filter canister with a porous stone filter element. Moisture can, and does, condense in this canister and results in rust. There is supposed to be a continuous blow-down orifice on a drain of the canister, but it usually gets plugged with rust particles before it gets removed because it's thought to be a "leak" or unnecessary.

Some valve actuators require a slightly lower pressure than full axial compressor discharge pressure, so there are air pressure regulators in the air supply line. These regulators can and do get plugged with rust particles and dirt and are rarely, if ever, checked for proper setting and regulation.

You should be able to find all of these elements--if present on the unit(s) at your site--on the P&IDs supplied by the OEM/packager of the turbine(s).

Some sites that have a source of clean, dry instrument air have switched the air supply for the compressor bleed valves from axial compressor discharge extraction to clean, dry instrument air with very excellent results (much fewer actuator/"valve" problems).

You even need to look at valve/actuator mounting to be sure there is as little angular misalignment as possible. Poorly installed or remounted (after maintenance outages) valves have caused lots of "failures."

So, the definition of "failure" needs to be fully understood. When the failures occur needs to be fully understood. All aspects of valve operation, including the valve, the actuator, the motive source for the actuator, etc., all need to be understood.

Hope this helps. Again, don't just look at the valves themselves; look at all aspects of the valve and it's application--the actuator, the source of the motive force for the actuator, the temperatures the valve and actuator are located in, and so on. Any formal root cause analysis must consider all aspects lest something be overlooked which turns out to be the ultimate root cause after much investigation, time, effort and money is spent without considering all aspects of the application.

You can even ask valve vendors and their representatives for recommendations for the application to compare valves and actuators against one another. Good vendors and their representatives will be happy to help with the analysis if they believe they will sell valves/services as a result of the process.

Please write back to let us know how you fare. We try to discourage taking discussions like this off-line (via private email messages) so that others who read these threads--and there are lots of people who do read these threads--can learn from the experience of others.
 
I have seen many failures of CBVs as a result of just these causes. My experience is primarily with GE 7B and 7FA. For the 7Bs, it was almost always the failure to maintain the Porostone filters and keep the blowdown orifices clean. We had some success in mitigating this by using monthly Kroil treatments into the actuator.

The 7FAs benefited from the offsite instrument air being used for the actuators instead of CPD air.
 
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