The steam turbine unit has Electro-Hydraulic Oil which controls the Actuation of the main stop valve and the control valves.
The EH Oil system keeps the Lube oil System pressurized (i.e, when the pressure of EH Oil is obtained, Lube Oil system can be taken into service). Therefore, header pressure of EH Oil should be normal in order to run the Lube oil system of the Steam Turbine.
During startup, when the EH oil pumps are started, the system doesn't make enough pressure and a low pressure alarm is persistent. Re-circulation valve (normally open) is then closed in order to develop pressure in the system. When the Re-circulation valve is closed, system starts to develop pressure and header pressure is back to normal. After the pressure is normal, the re-circulation valve is opened again. No further low pressure alarms are obtained.
This shows that the pumps are delivering the required pressure & there are no leakages present in the system as after successfully starting the system once, no low pressure alarms are obtained.
My question is, what can be the cause for this low pressure alarm only during the startup? Why do we need to close the re-circulation valve in order to develop initial header pressure?
Can you briefly describe the arrangement of the system pumps? Are they in a one in service/one in standby arrangement? Are there check valves on the discharge of each? What are the drivers of these pumps? Motors/turbines? When was the last time that ARCs were serviced? Are there any control valves on the system?
It seems that the ARC is giving a problem with re-seating. With the turbine offline and the control oil introduced to pressurize the system, set up the conditions as you were doing such that the ARC is closed, then once system pressure comes up, you can re-open.
Verify your system pressures are healthy. Slowly close the discharge of the pumps to force flow through to the ARC. Once the ARC opens to facilitate the full flow for some time, try re-opening the discharge valve of the pump to identify if the ARC is re-seating or if it remains open. If you still cannot get the forward flow, the ARC needs to be serviced/replaced.