I have gas turbine power plant frame 9. sometimes i found trouble on bleed valve actuation system. almost in each inspection i found dirt and corrosion on actuator bleed valve and also on solenoid bleed valve 20CB-1. it's very dangerous when the unit on running condition, because bleed valve is used to protect compressor from surging pressure on 11th stage of compressor's blade. actually, the operator can shut off the bleed valve manually using manual butterfly valve. but there are two bleed valve located on extremely dangerous area which is located in narrow space and high temperature area. so it would be so difficult for operator to the protection manuver.

What could be the consideration if we change the pipeline position of these two bleed valve below the gratings??

I,m hoping some suggestion from who has experience and knowledge about this. Thank a lot

 

This is one of the most frequent complaints I hear. And when I'm on site and able to investigate the systems, I usually find very inadequate maintenance procedures (procedures like, if it ain't broke, don't maintain it; or, if it's not listed specifically in the Service Manual it's not required).

You should be able to look at the Piping Schematic Drawings and see some components and notes that should help greatly with maintaining, or even modifying the system.

First, let's talk about the air pressure supply for the bleed valve actuators. Generally, GE-design heavy duty gas turbines use an extraction from the axial compressor discharge as the air pressure source for the bleed valve actuator. By doing so, they do not have to provide a separate air compressor (usually electric motor-driven, reciprocating compressor, requiring power, controls, a tank, and more maintenance like oil, filters, driers, oil removers, etc.). The axial compressor discharge is a ready source of compressed air.

Many times, this axial compressor extraction air is run through a "poro-stone" filter and a pressure regulator (sometimes, there's not even a regulator).

The problem with using axial compressor discharge air is that, depending on the location and ambient environment and operating conditions, the discharge air can have a lot of moisture in it. Pneumatic actuators and control components don't like moist or wet air.

Also compounding the problem is the system is basically a static system, meaning that once 20CB-1 is energized and air pressure is allowed to flow to and operate the actuator, there is no more air flow; pressure is used to keep the bleed valves closed until 20CB-1 is de-energized and the pressure from the actuator is vented to atmosphere.

If the ambient is humid and/or if the unit is equipped with evaporative cooling, and/or if the unit has on-line water washing and it's used, the axial compressor discharge air will have increased levels of humidity on occasion. Off-line water washing can also cause water to be accumulated in the piping, even if proper isolation "procedures" are followed.

The tubing which supplies air to the bleed valve actuators *MUST* have low-point air bleeds in *each and every low-point* in the tubing run, all the way from the extraction port to the actuator. The poro-stone filter must also have a low-point bleed in the drain of the filter.

These low-point bleeds are usually nothing more than very small holes (orifices) drilled in ferrous, steel pipe. Ferrous, steel pipe rusts. The small holes get plugged. The ones that don't get plugged by rust usually get removed by operations or maintenance personnel because they mistakenly believe them to be "air leaks". The orifices of the low point drains will be continually blowing air under pressure through the holes and seem like "leaks", when in fact they are important and critical orifices to allow moisture in the low points of the tubing to be blown out of the piping continuously.

The poro-stone filter canister is also made of ferrous material that rusts and contributes to the problem if not properly maintained. (When was the last time you opened and inspected the poro-stone filter? (Be honest!)

Many of the new F-class turbines, because they are typically installed in combined cycle power plants with plant instrument air systems, use dry plant instrument air for the pressure source of the compressor bleed valve actuators.

If you are in a power plant with a reliable source of dry instrument air (usually multiple redundant compressors, with well-maintained filters and dryers and oil-removers, etc.) then you are absolutely free to use dry plant instrument air as the pressure source for the bleed valve actuators. Problem very quickly and simply solved.

If you don't have such a reliable source of dry instrument air and you don't wish to install one, then you need to very carefully review the tubing and components installed in the actuator pressure supply. Use only stainless steel tubing and fittings. Ensure that *EVERY* low point in the tubing to all bleed valve actuators has a continuously venting orifice to allow any entrained moisture from the axial compressor discharge air to be blown out of the tubing. Re-route the tubing to eliminate as many low points as possible to reduce the number of low point drains requires.

Make sure the poro-stone filter canister drain is also properly venting through an orifice and, if possible, is located in a low point of the tubing.

Make the inspection and cleaning of the low point drains a part of regular maintenance outages, especially if the site has evaporative coolers and/or on-line water washing systems. Make sure the compressor bleed valve actuator isolation valve is *closed* during off-line water washing, and make sure that any moisture which will make its way into the tubing during off-line water washing is drained before restarting the turbine. (The extraction port is usually located very low on the compressor discharge or combustion wrapper casing, and there should be an isolation valve close to the case/wrapper.)

GE used axial compressor discharge air as the pressure source for the bleed valve actuators because it simplified the manufacture and installation of the units. This didn't require any off-base air compressor(s) and controls, but they do have their own maintenance and construction requirements, which weren't usually very well documented, and are almost never considered when building the units or performing maintenance.

By analyzing and reviewing the system and its configuration and how it works, you can see that there is a lot which can be done to improve the reliability without moving the valves outside the turbine compartment.

They could probably be located outside the compartment, but they should be insulated (for personnel protection) and any piping changes should be closely and carefully considered so as not to introduce a lot of back-pressure on the system because of the addition of lots of elbows and extra piping length.

It would seem that it would be a lot easier to clean and dry the air supply than relocating the piping, and if you used the same air pressure source without any bleeds or drains, you would most likely still have the same problem!

Best of luck; and let us know what you decide.

 

during shut down try to test the actuators bleed valve by giving air pressure through solenoid 20-CB and check if there any leakage in feed line also actuators performance during opening and closing if you do this you will be able to fix your problem

in my site i face many problems related to bleed valve trouble during start up and shut down and no one help me to rectify the problems

i fix this problem by install another actuators after some modification on it this actuators it can work for alone time also there is kit available for it know this modification done in 2 machines until know i am monitoring this actuators during start up and shut down.
nooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo problems

also taking care of this compressor bleed valve its necessary to protect the compressor from surge do the right maintenance for it .

sorry for my english

Anwar
KSA NAJRAN

 

Dear CSA,

we have some times during loading open one bleed valve.

 

Dear Maint,

you some times have a problem during loading.

You didn't tell us if the compressor bleed valve opens while on line (when loaded) or fails to close when it should during starting. You also didn't tell us what you do to resolve the problem, which could be helpful in understanding what the problem might be.

Most times the two compressor bleed valves (most GE-design heavy duty gas turbines have two, or more, compressor bleed valves) are supplied from a single output of the 20CB-1 solenoid. Presuming that a compressor bleed valve opens after the unit is on line (loaded), the usual cause for bleed valves opening during loaded operation is either a failure of 20CB-1 (which doesn't seem to be the case from the limited information you provided) or a ruptured air supply line to one or both compressor bleed valve actuators. Since the compressor bleed valve actuators for are air-operated if the air line to one actuator ruptures then both bleed valves usually open. So, it's hard to understand why one of two would open if the air supply line to either actuator ruptured (which is the usual cause of bleed valve opening during operation).

So, it's probably not a ruptured air supply line. If not it could be a failed valve stem locking nut, allowing the valve to open even though the actuator is in the closed position.

Most of the time problems with compressor bleed valves can be traced to one or both of two issues: lack of maintenance, and rust caused by moisture in the air supply to the actuator. Most GE-design heavy duty gas turbines use axial compressor discharge pressure through a filter and 20CB-1 to the compressor bleed valve actuators. The air in many parts of the world is humid, and so the axial compressor discharge pressure has moisture in it. There are supposed to be continuous blow-downs in the low points of the piping, but many people don't understand the purpose of them (to drain moisture) and so remove them. Or, rust particles block the very tiny orifices of the continuous blow-downs.

But, proper and diligent maintenance of the entire system providing the air supply to the compressor bleed valve actuators, as well as the bleed valves (which is usually about the only part that gets any attention) is really critical to reliable operation of the compressor bleed valves.

A lot of sites with a reliable plant instrument air system use air from that system to supply the compressor bleed valve actuators through 20CB-1. The reason that GE-design heavy duty gas turbines use axial compressor discharge pressure for air-operated solenoids is that it reduces the need for a separate compressed air system, compressor, receiver, driers, etc., which reduces the cost of the unit and also reduces the spare parts inventory and maintenance costs for supporting a separate compressed air system. To be fully reliable, a compressed air system for air-operated solenoids, it would have to have redundant compressors, which would further increase the costs of purchase/maintenance and ownership. Because there is already a source of compressed air (the gas turbine axial compressor), it is used as the source of compressed air for air-operated solenoids.

But, it does require maintenance and proper operation.

That's about all I can think of to help with the limited information you provided.

Don't forget to write back and let us know what you find when you resolve your problem. Feedback is very important to this forum, and one of the things that makes it so useful for others who use the 'Search' feature when researching problems they might want to post a question about.