Electro-Hydraulic Control (EHC)

This excellent Electro-Hydraulic Control (EHC) scheme picture below is made by my friend Fatchurrohman. I would like to explore this topic due to at least twice we got Unit trip caused from EHC.

EHC is as energy to generate and control Turbine Control valves, Turbine Stop valves, High pressure and Low Pressure by-pass valve.




That normal pressure is quiet high 110 Barg or 11 Mpa so when start the EHC system, by-pass valve should be open first.
This is the procedure to start the EHC system.
1. Check hydraulic oil tank level.
2. Check accumulator pressure.
3. Check hydraulic cooling system is available.
4. Check the flow path.
5. Open By-pass valve.
6. Start selected hydraulic pump.
7. Adjusting hydraulic pressure by throttling by-pass valve till desire pressure (110 Barg).

The procedure to automatic back pressure test by Pump stand-by cut in.
1. Close Signal low pressure transmitter from transmitter header line.
2. Open reduce pressure valve from Signal low pressure transmitter to tank.
3. Transmitter will get signal low pressure.
4. Stand-by pump will cut in for back up pressure.
5. Open reduce pressure valve from Signal low pressure transmitter to tank.
6. Close Signal low pressure transmitter from transmitter header line.

The Procedure to normal change over.
1. Check the hydraulic oil pressure (110Barg).
2. Check Stand-by hydraulic Pump flow path.
3. Start Stand-by hydraulic pump.
4. Check hydraulic oil pressure (>110Barg).
5. Stop the 1st hydraulic Pump.
6. Check the hydraulic oil pressure (110Barg).