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Fuel Gas Supply Line
Pressure drop in fuel gas supply line

In a fuel gas supply line i have the two shutoff valve (valve #1) Woodward gsov25, and PT that measures the down stream pressure of the shutoff valve #2.

The shutoff valve will open once the turbine ready to fire. But the down stream pressure of the valve #2 increases to 42 bar within 74msec.
Valve #1 & #2 is not commanded to open, before only the down stream pressure increases quickly from 0 to 42bar.

pine (Maybe Pipe?) line dia is 2". So i want to know that the small passing can generate this much pressure with msec of time? Or the pressure will achieve 42 bar within 74 msec on valve fully open condition only?

It seems what you are trying to describe is what's called a "double block and bleed valve" arrangement. This is used to provide two shut-off valves for redundancy. There is often a pressure switch or a pressure transmitter to Messer the pressure BETWEEN the two shot-off valves--to detect leakage of the first valve in the flow path. Often there is also a second pressure transmitter to detect pressure downstream of the second shut-off valve (upstream of the Stop-Ratio Valve).

Quite often there are valve leak tests performed during starting (before firing) and sometimes on shut down, also.

A 2-inch line is a pretty small line, and it's conceivable that with a "strong" gas fuel supply system and a larger pipe upstream of the area where you are detecting the high pressure that it could build up pretty fast as the shut-off valve(s) is(are) opened.

But the details are not clear, and it's not clear what the problem is--if there is any problem. 74msec is pretty short, but, again the details are unclear. A small diameter pipe and small pipe volume (meaning a short pipe length) could be pressurized very quickly if the shut-off valve is opened fully and the gas fuel supply system is large ("strong).

Hope this helps.

Problem is fail start due to excessive fuel flow. But while analyzing the trend, i found that the down stream pressure of the shutoff valve #2 increases before the opening of the valve. So it seems passing, I want to know the small passing can be reach 42 barA within 74 msec?

1 out of 1 members thought this post was helpful...

>I want to know the small passing can be reach 42 barA within 74 msec?

Yes. Depends on how "small" the passing is, and how large or small the area where the pressure transmitter is measuring the pressure. You mentioned a two-inch diameter pipe???

It doesn't seem like a small leak ("passing") could cause this pressure increase, but, again--a small diameter pipe and a short pipe length (small volume) could pressurize very quickly.

Passing could be anything from a pinhole leak to an unseated valve or a cracked valve seat. It means the valve is not preventing flow--small or large. Passing is not size-related; it's leak-related. And for a shut-off valve any leak above the value specified for the class of the valve and by the valve manufacturer is a leak. (Leak specifications for shut-off valves are usually specified in bubbles--yes, bubbles; how many bubbles can be counted downstream of a valve with a particular test pressure upstream of the valve. Most valves ARE NOT 100% leak-tight; most have some very tiny leakage (passing) rate, but not enough to cause 42 barg in 74 msec.

How long has the valve been in service?

Have you checked the pressure transmitter to see that it is properly valved in to the system, and that it is working correctly? Is there a gauge that can be watched during the start sequence to see if it, too, builds up to a very high pressure in a very short period of time.

Shut-off valves are not typically control valves. Shut-off valves are either open or closed; there's not usually anything in between such as for controlling pressure. So, when the first shut-off valve opens there would be full pressure against the seat of the second shut-off valve. If the second shut-off valve was not fully seated, or the valve seat or plug was threaded or cracked, then the pressure downstream of the second valve would increase, and depending on the size of the leak opening AND the size of the downstream volume with the pressure transmitter the pressure could build up very quickly.

Failure to start on excessive fuel flow sounds like a GE-design heavy duty gas turbine alarm message. That particular alarm doesn't even measure fuel flow during starting--it simply looks at the position of the Stop-Ratio Valve during firing and says, "The valve looks to be too far open for firing and ir it were too far open with normal supply pressure upstream of the SRV there would be excessive fuel flow and therefore trip the unit." It's noted you did not say which manufacturer's turbine or control system you are working on. And, many GE-design heavy duty gas turbines have or have been retrofitted with double-block-and-bleed valves and pressure transmitters.

Usually in a double-block-and-bleed valve arrangement both shut-off valves are opened--and closed--at the same time. Many have limit switches to indicate fully open and fully closed positions. Some have visible valve stems which could be monitored to see if they are fully open or fully closed. (Unfortunately, my experience with Woodward gas valves is very poor; their reliability is very low and they require frequency maintenance, and some of their valves can ONLY be properly serviced by a Woodward-authorized facility because of the way the valve and actuator is manufactured and assembled.) I would be very surprised to see double-block-and-bleed valves being opened one at a time during a start-up or closed one at a time during a shutdown--but it wouldn't be unheard of, just surprising and different (since double-block-and-bleed valves are redundant valves to shut off flow they wouldn't need to be opened or closed one at a time).

At any rate, the exact set-up of the valves at your site is unclear and we have answered the question of whether or not the pressure in an area downstream of a leaking ("passing") valve could build up to a very high pressure very quickly.

Here is a "sketch" of a typical double-block-and-bleed valve arrangement on a GE-design heavy duty gas turbine with a Stop-Ratio and Gas Control valve downstream of the shut-off valves:


Bleed Vent Valve and P2
(or Vent) Pressure
Valve and Gauge and
Pressure P2 Pressure
Transmitter Transmitter
| |
Gas |\ /| | |\ /| |\ /| | |\ /|
Fuel------| X |---| X |-------| X |---| X |----> Fuel Nozzles
Supply |/ \| |/ \| |/ \| |/ \|
Gas Shut- Gas Shut- Stop- Gas
Off Valve Off Valve Ratio Control
#1 #2 Valve Valve

In the sketch above, there is a pressure transmitter on the volume between the two shut-off valve, and a small valve used as a "bleed" (vent) valve. When the two shut-off valves are open, the bleed valve (vent valve) is to be closed, and the pressure between the two valves would be equal to gas fuel supply pressure. When the two shut-off valve are closed, the bleed valve (vent valve) is open, and the pressure between the two shut-off valves should be zero barg. The reason the bleed valve and associated tubing lines are small is so that if there is a leak of either shut-off valve when the unit is running, or of the #1 shut-off valve when the unit is not running, the pressure in the area between the shut-off valves would build up quickly if the leak (passing) was large and could be detected by the pressure transmitter.

But, when the two shut-off valves are opened during a start attempt (usually at the same instant in time) the bleed valve (vent valve) is also closed simultaneously and the pressure between the two shut-off valves builds up VERY quickly to match the gas fuel supply pressure. Then at some point on a GE-design heavy duty gas turbine, the vent valve between the two fuel control valves (the Stop-Ratio Valve (SRV) and Gas Control Valve (GCV)) will be closed and the Gas Control Valve Stop-Ratio Valve (SRV) will be opened. Very shortly after that the SRV is then opened to build up pressure between the SRV and GCV (called the Interstage Gas Valve area, or the P2 area). The pressure should only be around 2 barg approximately and the Gas Control Valve then controls the fuel flowing to the fuel nozzles in the combustors to limit the fuel flow to establish flame.

The GE Mark* turbine control system monitors the LVDTs (Linear Variable Differential Transformers--position feedback devices) to see what position the SRV is in during starting and firing. If the SRV is "too far open" the assumption is that there is too much fuel flowing through the SRV and the GCV and that would be dangerous for the turbine and exhaust and the unit is tripped. The alarm message reads, "START_UP FUEL FLOW EXCESSIVE"--even though the Mark* isn't monitoring fuel flow-rate for this alarm!!!

Anyway, hope this helps.

1 out of 1 members thought this post was helpful...

Have you replaced the valve recently? GSOV25 has a plug that has to be set for internal/external pilot pressure. If not installed, it creates a "bypass" to the shut-off valve that gives around 400 pph extra fuel. I’ve seen a couple times deflagration in the exhaust due to excess fueling caused by making that mistake.

1 out of 1 members thought this post was helpful...

It seems what you are trying to describe is what's called a "double block and bleed valve" arrangement. This is used to provide two shut-off valves for redundancy. There is often a pressure switch or a pressure transmitter to Messer the pressure BETWEEN the two shot-off valves--to detect leakage of the first valve in the flow path. Often there is also a second pressure transmitter to detect pressure downstream of the second shut-off valve (upstream of the Stop-Ratio Valve).

Quite often there are valve leak tests performed during starting (before firing) and sometimes on shut down, also.

A 2-inch line is a pretty small line, and it's conceivable that with a "strong" gas fuel supply system and a larger pipe upstream of the area where you are detecting the high pressure that it could build up pretty fast as the shut-off valve(s) is(are) opened.

But the details are not clear, and it's not clear what the problem is--if there is any problem. 74msec is pretty short, but, again the details are unclear. A small diameter pipe and small pipe volume (meaning a short pipe length) could be pressurized very quickly if the shut-off valve is opened fully and the gas fuel supply system is large ("strong).

Hope this helps.

Thanks for all your replies.

Once solve the issue let me inform you all what was the actual problem.