Closed Loop Piping-Vacuum Issue

[DurgaprasadM]

The lube oil present in the shaft bearing sump has to be cooled externally using a heat exchanger. A gear pump is being used to circulate the lube oil from sump to the heat exchanger. This is a closed system. During the initial startup, the vent valve will be used to purge all the air in the circuit. The moment gear pump is switched off, it is expected that there will be vacuum in the system above the lube oil level (Magenta line). During the next start up of gear pump, is there any problem with this vacuum? If yes, what is the solution.

Note: I cannot able to provide a header tank as there is restriction of lube oil level in the sump of shaft bearing.

 

[Doug Hunter]

To answer your question directly, I don't think that the vacuum would cause a problem in the gear pump start up, unless it caused a start-up torque that exceeded your motor capability (unlikely). However, I'd also like to elaborate on your premise: unless your gear pump has some unusual sealing mechanisms, the suction vacuum will decay over time after shutdown (probably over only a few seconds) and the system pressures will equalize.

 

[LittleInch]

There might be low pressure, but unless your oil is close to its vapour pressure or the height of the line to the top of the HX is more than 8 or 10m, this should not result in any actual "vacuum" only a lower pressure due to the static head.

I have to say this does not look like a classic "closed loop system" to me with the variable being what the flow is in the HX and this volume in the bearing. There needs to be some allowance for fill, drain and pressure relief / regulation.

Does that help?

 

[GD2]

The lube oil present in the shaft bearing sump has to be cooled externally using a heat exchanger. A gear pump is being used to circulate the lube oil from sump to the heat exchanger. This is a closed system. During the initial startup, the vent valve will be used to purge all the air in the circuit. The moment gear pump is switched off, it is expected that there will be vacuum in the system above the lube oil level (Magenta line). During the next start up of gear pump, is there any problem with this vacuum? If yes, what is the solution.

Note: I cannot able to provide a header tank as there is restriction of lube oil level in the sump of shaft bearing.

Is there a reason the pump inlet has to be from the top of the sump? If you flip, the lines (pump draws from the bottom and cooled oil delivered at the top), the pump will not only avoid any vacuum condition but will also have reduced work done (backpressure will be reduced).