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Compressor bleed valve
Application Questions and Problems topic
Posted by Pritpal Singh on 13 September, 2008 - 12:47 am
Dear All,

I work in a power plant, and are using GE Frame 6 Gas Turbine and had been in operation since 2006. For some months back we are having problems with the compressor bleed valve. Problems are arising on start up and shut down. The problems are bleed valve trouble signal on start up or bleed valve trouble trip on shut down. The maintenance contractor had advise that the bleed valve be replaced at each inspection. To replace this bleed valves are very expensive. Is it a common practice to replace the bleed valve at each inspection. Is there any other solutions to this problem.

Thank you for your advise.

Regards,

Pritpal


Posted by MIKEVI on 15 September, 2008 - 11:40 pm
Pritpal,

We have 2 frame 5 machines in peaking operation since 1985. We have never had a need to replace the compressor bleed valves. We test the bleed valves during each spring and fall outage by supplying compressed air to the compressor bleed valve solenoid and forcing the solenoid on-off. We have found leaks from the valve actuators due to failed o-rings and loose bolts. The valves were still operating well, but with the turbine running you could not hear the leaks. I would take a good look at the operation of both your valves with the turbine down. Are you having trouble with both valves or just one? These valves are indeed expensive, and lead time for replacements long, but they work well in the harsh environment they have to operate in. I have never heard anyone recommend yearly replacement of these valves.


Posted by CSA on 16 September, 2008 - 12:20 am
What have you done to troubleshoot or resolve the issue? It's really difficult for us to provide meaningful responses when we don't know what's been done. Many people complain about responses they get from forums such as this which suggest they try something which has already been tried. How would we know what has been done at your site unless you told us? If we tell you to do something which you've already done, how will you feel about the response you've received? Some people like it when they learn they were clever enough to consider this or that on their own; the majority of people take offense, "That's obvious! Everyone would have thought of that! Why did you tell me to do something so obvious? I wouldn't have written if I hadn't tried that!" But, again: How would we know what you've done and what you haven't, unless you told us?

Remember: We're *not* there at your site. We don't know all the
circumstances and everything that you've done (or haven't). If you want a quick and concise answer, give us the details and we can eliminate suggesting things you've already done. If, on the other hand, you haven't really started your troubleshooting yet and are looking for all suggestions, tell us that, too. Some of us spend a lot of time trying to propose meaningful solutions and we get very little feedback; fortunately, most of the feedback we receive is positive (thanks!) but if we've missed the mark or are completely off target with our responses, we'd like to know that, too. What we don't want to hear is, "We've already tried that."

So, you've spoken with the maintenance contractor who's made a recommendation. Have you spoken to the turbine packager/OEM to ask about what their experiences are in this area?

It appears from your description the valves are sticking and don't want to close in the allotted time or open in the allotted time. Have you done any analysis of the valves to try to determine why they are sticking? Are you certain the compressor bleed valve solenoid (20CB-1) valve is working properly?

Can you tell us who the packager of the turbine at your site is: BHEL, GE Energy Products Europe, ??? (GE-design heavy duty gas turbines are packaged by several other manufacturers, who are basically free to use the components of their choice they feel meet the requirements of the application.)

Can you tell us who manufactured the compressor bleed valves in use at your site?

Are the bleed valves at your site on top of the turbine compartment, exposed to ambient conditions (rain, sun, humidity)? Or are they inside the turbine compartment enclosure? If they are outside of the compartment, have you considered putting some kind of weather cover over them to protect them from the elements? If they are inside the turbine compartment, is the compartment adequately ventilated? Can you place some kind of "heat shield" between the valve and the turbine shell to try to deflect at least some radiant heat if they are located very close to the turbine shell?

GE, as a packager of their turbines, has had trouble with bleed valves in years past, but it's my understanding they have settled on a particular manufacturer's valves which seem to be well suited to the application. There was a period where they used a very thin "butterfly"-like valve (I can't recall the manufacturer), which would stick if it wasn't installed exactly perfectly between the pipe flanges (it had to be nearly perfectly centered between the flange bolts and the exact right type of gasket had to be used; it usually took two or three people to get the valves properly installed). But, again, I think they've since gone through a couple of different valves/manufacturers since then. But then we don't know who the packager of the unit at your site is, or what valve is being used.

Have you tried contacting the valve's manufacturer or their representative to see if they can offer any advice? Usually, if the smell an opportunity to sell something, they'll be at site in no time, and quite often they can be very helpful.

Have you ensured the limit switches used to indicate valve position are working and adjusted properly? Until recent years, there was only one switch per valve which would indicate when the valve was 100% open, but it seems in recent years they have also begun monitoring the closed position as well.

One last thing; usually the air for actuating the valves has to pass through a filter with a very small low-point drain orifice which is meant to be continually venting to remove any condensed moisture from the air which usually comes off the axial compressor discharge. Many people think the orifice should not be continually venting and actually remove the orifice or close the valve upstream of it. Also, this filter and orifice should be located in the lowest point of the piping, and if there is a lower point past the filter and bleed valve solenoid, another low-point drain orifice should be installed. One of the problems with sticking actuators was moisture getting to the actuators and rusting the cylinder walls.

So, you have several things to try if you haven't already. Please let us know how you fare in your troubleshooting and problem resolution. Again, one of things which makes these kinds of forums so useful is the feedback from people to let others know if their problem was resolved and what was actually done to resolve the issue. The 'search' function of control.com is excellent; if you haven't tried it, do so. We are building a large body of information on this site which is very accessible from the search function, which, unlike those of most other similar sites/forums, accepts multiple word search terms!

We look forward to hearing from you! Feedback is the most important contribution to control.com. If you can take the time to write and ask a question or ask for help, you can provide some feedback, too.


Posted by Simon on 17 September, 2008 - 1:23 am
Hi Pritpal,

I have had this problem on a 9E and it was caused by the solenoid valve 20CB-1 getting stuck in a mid position when de-energised. This caused the GT to trip on the way down and on subsequent restarts there was enough air pressure getting to the bleed valves at approx 50% speed to lift them of the limit switches and trip the unit.

Simon


Posted by A.BALI on 18 September, 2008 - 12:53 am
Hi,

In our site we have over than 50 turbines, all of them are equipped with LAWRENCE SOLENOID VALVES. Until now the valves were still operating well. Maybe by respecting periodical maintenance and testing.

In your case I think the causes are: your valves are placed in a warm coin, the holding and inrush current of your valves specified by your manufacturer do not conform.

Hope this helps.
A BIENTOT


Posted by Ero on 19 September, 2008 - 12:57 am
Hi,

We had some problem with CDP bleed valve last time. I checked servo controller and again seen calibration procedure. CDP bleed valve is important for turbine stall conditions. I think you can get mechanics calibration on the product company. Which company...


Posted by CSA on 20 September, 2008 - 12:22 am
Ero,

I'd be really interested to know what GE-design heavy duty gas turbine uses servo-controlled compressor bleed valves that require calibration.


Posted by francis on 2 October, 2008 - 1:31 pm
This problem is very present before the cleaning off line. You must drain the filter water/air increase to 2mm. When the bleed valves are open, 2OCB is not supply. This solenoid has a resistance of 13 ohms when the actuator is open or closed. We have change 20CB because open R=13 ohms, closed R=840 ohms, and the solenoid has not enough power to activate this system admission air power for bleed valves.

Sorry for my very bad English.


Posted by Iqbal on 13 December, 2008 - 5:34 am
We had a similar issue with bleed valves. Turbine trips always while normal stopping.

If the control air is from PCD, there is chance that this air gets condensed and causes the actuator piston hard to move. You can use plant air instead. this should solve the problem.


Posted by vk on 22 June, 2009 - 4:31 pm
Can i ask you whats the number of stage that bleed valve work at it?


Posted by GA on 23 June, 2009 - 1:05 am
We've got four 7FA's with four bleed valves on each machine and had a lot of problems with the bleed valves until we rerouted the CBV supply air to plant air, rather than using compressor discharge air.

We've also replaced all of the original bleed valves (not the SOV's) after about 4 years of service and since then we've only had one stick in the open position which caused trips as soon as the generator circuit breaker closed. We'll rebuild it and use it somewhere else.

More often than not our problems have been limit switch failures: on a few occasions water got into them and on other occasions screws loosened up or fell out due to the heavy shaking during frequent start ups.

GE suggests annual replacement of the bleed valves, but I think that's overkill. But we run them to failure which I think is underkill?? I think thorough annual maintenance would suffice, such as tightening all bolts and screws, stroking the valves preferably by forcing the outputs and observing proper SOV and bleed valve operation.

And be sure to keep one or two bleed valves and SOVs on hand for quick replacement once it becomes necessary. And it will, eventually, be necessary.

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