there is a frame-V gas turbine which runs in natural gas. This machine had a smooth performance history till date. recently, we have upgraded its control system from mark-v to mark-VIe. After up-gradation during commissioning tests it was ok. after loading, around 3/4 days later it was observed that hydraulic oil pressure is getting a jerk from 85kg/cm2 to 97kg/cm2 (~for 0.5 sec in every 10sec). The pressure surge was present only during MHOP, when AHOP was running before FSNl no hunting observed.
In addition to it, IGV LVDT feedback trouble alarm was also appearing.
To solve the MHOP pressure surge issue, we take down the machine to FSNL then, checked the release valves of AHOP & MHOP line (interchanged the valves MHOP & AHOP for sure), checked almost every part mechanically but no improvement.
So being failed to solve the MHOP pressure hunting by mechanical checks, we started machine again.
Again after 7 days, machine put shut down and LVDT of IGV was replaced. After that, when we started the machine the MHOP pressure hunting just vanished like a magic.
My question is What may be the reason
1. for MHOP pressure hunting?
2. How it vanished after only rectified the IGV LVDT?
So, first thing to know is most GE-design Frame 5 heavy duty gas turbines don't have hydraulic accumulators. So if there are high hydraulic flow situations there can be pressure fluctuations (spikes, dips, etc.). Especially if the high flow conditions are intermittent and somewhat quick in nature.
Second, most IGVs have two LVDTs, and the Mark VIe (just as the Mark V) selects the higher of the two LVDT feedback signals to compare against the IGV reference. And, 100 times per second--which is four times faster than the Mark VIe scan rate of 40 msec (25 Hz)--the servo regulator on the servo output card compares the feedback to the reference and adjusts the servo current to try to make the feedback equal to the reference.
If one of the LVDT feedback signals is intermittent--meaning it's stable, then drops low, or goes high (which is unusual for LVDTs, but it does happen occasionally), then the servo regulator is going to be changing the servo current to try to make the feedback equal to the reference.
And, the same thing can happen if there's only one LVDT--it only has one feedback signal, and if it's unstable then the servo regulator will still change the servo current trying to make the actual (feedback) equal to the reference. 100 times per second. Even if the reference is perfectly stable, the servo regulator is comparing the feedback (actual position) to the reference and changing the current to try to make the feedback equal to the reference.
If the feedback is not stable, then the servo current will not be stable. If the servo current is unstable, then the flow of high-pressure hydraulic oil through the servo-valve to/from the IGV hydraulic actuator will not be stable, and if there's no hydraulic accumulator to dampen pressure swings during flow changes the hydraulic pressure is going to be unstable--and so, likely, will the IGV position.
You didn't mention what the IGV feedback--or what they physically were doing--during the hydraulic pressure swings. But, it's a safe bet, that given the alarm you mentioned, that the actual was not following the reference, for whatever reason.
Imagine if the feedback signal changes (goes high or low) for a split second--just 0.5 seconds, for example. The servo regulator will change the current to try to make the feedback equal to the reference, and let's say the IGVs move--but then, at the end of that 0.5 second "blip" the feedback signal goes back to normal. The servo regulator will again see a change in position and will again make a change in servo current to make the IGVs move "back" to where they are supposed to be. And, if this happens "a lot" then there's going to be instability. The IGV hydraulic actuator is one of the larger hydraulic actuators on the turbine, and it also is a double-acting cylinder (as opposed to a single-acting cylinder), and so can require even larger flows if the servo current is changing very quickly.
A LOT of LVDTs get worn on the inside of the stationary armature by the rod (the "core") which moves up and down as the device it is attached to moves up and down. The rods get bent during maintenance outages and that's the usual cause. You said this was an upgrade from a Mark V to Mark VIe, so the LVDT could be a couple of decades old and could have had some considerable wear. This wear usually occurs at a particular spot--where the device the core rod is attached to usually operates. However, sometimes it happens at other places. The usual result is the output (which is the feedback to the turbine control system) can be very intermittent at the position where the armature is worn through by the core. (The manufacturer likes to say LVDTs are frictionless devices--but that's ONLY if they are properly installed and maintained--and that doesn't usually describe many turbine applications. However, LVDTs are still extremely reliable and do not generally drift over time once the feedback is scaled ("calibrated").)
So, based on the information provided, I would guess the problem with the fluctuating hydraulic oil pressure was related to an issue with the LVDT, which also caused the alarm you mentioned, because of fluctuating feedback. Replacing the LVDT resolved the intermittent feedback, which stabilized the hydraulic pressure, and stopped the alarm.
Also, when the Aux Hydraulic Oil Pump is running when the turbine is running (starting or shutting down), there is also pressure--and flow!--from the Main Hydraulic Oil Pump, which would "supplement" the MHOP output.
Finally, hydraulic oil pump pressure is adjusted from the pump's built-in pressure compensator--NOT the relief valves. Using the relief valves to set hydraulic pressure causes EXCESS flow through the hydraulic system which contributes to any other flow for device positioning and therefore increases the risk of hydraulic pressure fluctuations, especially when the system does not have hydraulic accumulator(s). There should be NO flow out of the pressure relief valves under normal circumstances. The pressure relief valves are there to protect the system if the built-in pump pressure compensator fails and causes high hydraulic pressure; it has no other purpose. (To set the hydraulic pressure relief valve, it is necessary to use the pump's built-in pressure compensator to increase the pump pressure until the relief valve setting is achieved, then the compensator must be used to return the pump output pressure to the normal hydraulic system pressure--at which point the relief valve should stop relieving.)
Hope this helps!
By the way, the Mark VIe has an excellent Trend Recorder function which can be very helpful in cases like this for trending servo current(s), reference, LVDT feedback(s). Most older GE-design heavy duty gas turbines don't have hydraulic pressure transmitters, but some were retrofitted to include one and if that unit has one the feedback from it could have been trended, also. (A word about "third-party" transmitters--so-called SMART transmitters, while relatively inexpensive and easy to configure, are NOT very fast so when there are large, quick swings in the measured parameter (hydraulic pressure in this case) sometimes the feedback can "lag" the actual pressure swings and even not show the actual highs and lows.)