Hydraulic System Pressure Not Released after GT Shutdown


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In our GT9E It was observed that the hydraulic pressure hold after unit shutdown and the aux pump stopped at 1200 rpm. Hydraulic pressure still 103 bar until the speed reach 300 rpm. Then the pressure start to decrease slowly until reached 10 bar. At this time the turning gear running and the hydraulic pressure back to increasing to settle 20 bar.
In the normal shutdown the hydraulic pressure released to 2 bar in two minute.

The hydraulic transmitter was be check and operate well. what is the reason of this issue?

It would be necessary to see the Hydraulic System P&ID to be absolutely certain of what's going on, but most people don't realize that the hydraulic systems of GE-design heavy duty gas turbines are really pretty static systems. What that means is that pressure is present during normal operation, but the only time there is flow is when device position needs to change, like fuel control valves or IGVs. When the devices are in a steady-state condition (such as when the unit is operating at a steady load) there is very little flow.

I believe if you look at the Hydraulic System P&ID you will see that there is nothing to relieve the hydraulic system pressure during a shutdown. There are hydraulic dump valves for the Trip Oil system, but nothing for the hydraulic system--at least on GE-design Frame 9Es packaged by GE USA before GE Belfort got involved in manufacturing and packaging Frame 9Es and decided they had a "better way" to do just about everything (which involved needlessly over-complicating just about everything).

Typically, there is some small flow through the servo-valves (which is just about the only place for hydraulic flow to go!) during shutdown and because the Aux. Hyd. Pump runs so low in the shutdown process there is usually a lot of residual pressure. And with only a very small orifice in the servo to flow through during shutdown, well, it's going to take a while for hydraulic pressure to decay.

<b>AND,</b> if the hydraulic accumulators are properly charged and properly valved-in they will also serve to maintain hydraulic pressure for long periods of time after the Aux. Hyd. Pump shuts down. (Long periods of time can be 5-10 minutes.)

Most sites don't realize the hydraulic system pressure takes so long to decay after the Aux. Hyd. Pump shuts down--because most sites don't have pressure transducers monitoring hydraulic system pressure, and those that do, well, the operators think once flame is out, well, their job is done. As long as the unit goes on cooldown, they can go back to surfing the Web, eating their biryani, and talking about Formula One and futbol, and ignoring alarms (because the unit's not running, so all alarms must be erroneous, nuisance alarms, right?).

It's only been in the last decade or so that packagers of GE-design heavy duty gas turbines began providing pressure transducers for hydraulic system pressure and L.O. system pressure, and in a very few cases, Trip Oil system pressure. (Difficult to believe, I know--but in "the old days" operators actually had to go out to the Accessory Gage Cabinet and manually record pressures--archaic, I know, but it serves a very good purpose to have operators walking around the unit, sensing leaks with their eyes and ears and sense of touch that electronic sensors can't easily detect. But, operators don't want to leave the Control Room these days.)

I would question the data you are using to say that this shutdown was different from any other shutdown. Is it anecdotal (meaning, one of the operators said, "It's never done THAT before!") or have you gone back to check historical data/trends from previous shutdowns (because the operators usually don't do that unless they are prodded and poked to do so--they just rely on their (not so) keen sense of what happens during start-ups and shutdowns, which don't usually happen very often, and most operators don't know what's supposed to happen and not happen anyway. A good start-up for most all operators is one that results in synchronization and loading, and a good shutdown is one that results in the unit going on cooldown--and all other alarms and sequences be damned. As long as the unit synch's and loads, and as long as the unit goes on cooldown, all the operator has to do is silence alarms (if they haven't already disabled the audible alarm indication). As long as the unit doesn't trip starting-up or loading or shutting down, well, their job is complete as long as they keep the audible alarm indication as quiet as possible.

Sorry; I've just been trying very patiently to work with some operators--and their supervision and management--that have just the beliefs and practices stated above. They never received good training, and all of their knowledge comes from what they remember--and what they remember isn't very detailed, and what they think should or shouldn't be happening is very flawed. But, they're certain it's done this or that, or not done this or that, MANY times before, and it's just recently started "mis-behaving." And, this particular perceived problem with hydraulic pressure is just one of several recurring incidents over many years. And, because there's usually no recorded data from past events to refer to, I have nothing to bolster my thirty-plus years of experience and knowledge--and their turbine is DIFFERENT from any other turbine (not).

Anyway, have a look at past shutdown data (if there is any), and keep an eye on future shutdowns to see if the problem is intermittent or persistent. I believe you will find it's very normal, and in fact, it doesn't hurt anything. The fuel control valve reference positions when L4 is dropped out are usually -25% to -40%, and the IGV reference position is usually 30-32 DGA, and they're already "closed" anyway, so they can't be closed any further, so even if there was Trip Oil pressure AND a positive position reference the residual hydraulic pressure isn't going to open the valves/IGVs, and if it did, well, the residual pressure won't last long as the accumulator pressure will quickly decay trying to open them without any pressure from the Aux. Hyd. Pump.

Hope this helps!
I failed to mention that the Accessory Gear-Driven Main Hydraulic Pump is still providing flow and some pressure even after the Aux. Hyd. Pump is shut down, and along with the Hydraulic Accumulator will also work to keep the hydraulic system pressure from decaying quickly on a shutdown.
I believe that the operators have the same behavior around the world.

Sir, we have 8 gas turbine each one started around 250 time two years ago. The normal shutdown was mentioned Previously which the hydraulic pressure is 2.5 bar at cooldown, and this pressure built up by main hydraulic pump rotating. The strange thing when the pressure hold 20 bar at the cooldown that mean the valve which responsible to drain the prssure after the unit shutdown failed but i don't any one.

I know this issue will not affect on the operation but i wanted to know the explanation of this abnormal shutdown.

Thanks sir

If you would put your Hydraulic System P&ID up on, say, DropBox or OneDrive or Google Drive, and then post the link to the drawing in this post we might be able to make a more reasoned assessment. In my experience with GE-design heavy duty gas turbines--prior to Belfort getting involved--I have never seen any solenoid or device that was energized to relieve hydraulic pressure during a shutdown. Having said that, many units have a 20TV-1 solenoid that ports hydraulic pressure to a double-acting cylinder that moves the IGVs. I could envision a condition whereby when 20TV-1 was de-energized it would port a significant amount of hydraulic oil to the actuator to cause it move the IGVs to the mechanical closed stop and act to reduce the pressure in the hydraulic system (from the Main Hydraulic Pump and the Hydraulic Accumulators).

But even that isn't a really significant amount of oil and in my experience doesn't significantly reduce the hydraulic system pressure at the time of 20TV-1 de-energization.

So, unless GE-Belfort have re-invented the wheel, it's not clear what you are referring to. Only by reviewing the Hydraulic System P&ID from your site could any further comment be made about the perceived issue you raise in this thread.