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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,
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,