GE Steam Turbine Hydraulic Oil Pressure Drops

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Thread Starter

Irfan Thalib

Hydraulic Oil pressure Dropping when CVs start to open. In our GE Steam Turbine with Mark V Control System, there are Two MSV/CV with Three LVDT for each CV and One LVDT for each MSV. MSV/CV were overhauled. LVDTs were removed.

Before removing LVDTs, As-Found Voltages are as follows.
With CV1 Position Fully CLOSED. Mark V Reading 0%
LVDT 1: Excitation R-QTBA-1/2 : 3.624Vac, Signal R-TBQC-1/2 : 0.905Vac
LVDT 2: Excitation S-QTBA-1/2 : 3.624Vac, Signal R-TBQC-3/4 : 0.880Vac
LVDT 3: Excitation T-QTBA-1/2 : 3.623Vac, Signal R-TBQC-31/32 : 0.806Vac

With CV2 Position Fully CLOSED. Mark V Reading 0%
LVDT 1: Excitation R-QTBA-3/4 : 3.624Vac, Signal R-TBQC-5/6 : 0.9Vac
LVDT 2: Excitation S-QTBA-3/4 : 3.635Vac, Signal R-TBQC-7/8 : 0.865Vac
LVDT 3: Excitation T-QTBA-3/4 : 3.623Vac, Signal R-TBQC-29/30 : 0.879Vac

After restoration of LVDT. Present Voltages are as follows.
With CV1 Position NEAR FULL OPEN: Mark V Reading 93%
LVDT 1: Excitation R-QTBA-1/2 : 3.648Vac, Signal R-TBQC-1/2 : 2.388Vac
LVDT 2: Excitation S-QTBA-1/2 : 3.648Vac, Signal R-TBQC-3/4 : 2.420Vac
LVDT 3: Excitation T-QTBA-1/2 : 3.648Vac, Signal R-TBQC-31/32 : 2.414Vac

With CV2 Position NEAR FULL OPEN: Mark V Reading 94%
LVDT 1: Excitation R-QTBA-3/4 : 3.648Vac, Signal R-TBQC-5/6 : 2.470Vac
LVDT 2: Excitation S-QTBA-3/4 : 3.656Vac, Signal R-TBQC-7/8 : 2.384Vac
LVDT 3: Excitation T-QTBA-3/4 : 3.647Vac, Signal R-TBQC-29/30 : 2.433Vac

Now, with these values, although the CV are opening and the local scale of CV/LVDT feedback on Mark V matches with Mark V OUT Command, the Hydraulic Oil Pressure Drops gradually to Unit tripping Level (i.e., from 110 bar to 70 bar).

The Hydraulic Pressure is Normal when MSVs are open, and it starts to drop only rolling starts (i.e., when CVs start to open).

Although there was complaint of Hydraulic Oil Pressure dropping before Overhaul, but Unit was running (the pressure stopped to drop at 100 bar).

Please advise if these LVDT voltages are normal. Because, I suspect the excitation voltage should be around 7Vac RMS. Also, please advise which MarkV card could cause this voltage drop.

Regards,
 
Are you using a True AC RMS voltmeter to read the LVDT voltages?

Have you looked at the LVDT feedback voltages with AutoCalibrate or DIAGC and compared them to the voltmeter readings?

Why are you equating LVDT voltages to hydraulic pressure?
 
M

Muhammad Saad

we are facing the same problem as the CV starts to operate the FRF pressure goes down. though we have inspected the servos.
we have two pump of vickers PVB 45 eries and the relief valve.

can anybody on this forum, advice us about what to do in this regards.
 
the LVDT linear limits is 0,7 (top) and 3,5 (bottom) and your voltage are in the linear limits. you need review the acalib.dat in de MK_V config. for view the information to de LVDT travel and the IO_CFG
WHAT IS THE HIDRAULIC PUMP MODEL AND TRADE MARK? Because in this power station was e similar problema with de oscilacion in hidraulic pressure and was solucion with the new control pressure regulation

regard
 
I dont see any problem with hydraulic pressure drop due to LVDTs.
Please check all your solenoid valves and check valves at the HPU. Did you tried to change over from one pump to another and observe the difference.

Most probably the problem is due to malfunction of solenoid valves, passing of the check valves and/ or the hydraulic accumulators. I previously had similar problem and it was due to sticky solenoid valves. Try to push the manual plunger at the solenoid valves (I believe its Vickers solenoid valves for GE HPU units) and see whether that problem reoccur.

Is the hydraulic level in the tank maintained? If NO, most probably there are some leakages in the system. Try to play around (with the STG in tripped position of course ;)) and see whether any changes is evident. You may also try to perform some valve stroking with the steam line isolated, drain valves opened and vacuum breaker opened. Please ensure that the pressure upstream the valve is zero or else you might end up blowing up your LP diaphragm ;)

Good luck and keep us posted!
 
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