Vaccum pump capacity drop

[Boiler1]

Hi,
I have got a skid with 4No vacuum pumps connected in parallel operating on duty stand-by basis. All the pumps are being rotated between duty & stand by. Under the normal atmospheric pressure all the pumps operate at more or less same RPM’s, however at the lower atmospheric pressure one of the pumps ramps up 20% more than the other 3 when on duty.
The pumps are identical, the system load is constant, the only variable appears to be the atmospheric pressure. The piping is nearly identical, bar the slight differences in lengths which doesn’t affect the other pumps
I’ve been thinking a leak very local to the pump in question could be the cause, but would that not be apparent regardless of the atmospheric pressure? Any thoughts?
This was noticed well into the pump’s lifecycle, not at the start.
Regards

 

[TugboatEng]

Has this gotten worse over time? I would first suspect wear of the impeller/casing assuming this is a liquid ring pump. If it has always been present it could also be a clearance issue in the pump. How do the sealing water temps compare?

 

[Boiler1]

Hi. Many thanks for your response.
Apologies for delayed follow up. It actually is a liquid ring pump. We only get the high level alarm on the seal temperature and that appears to be OK. Just looked at the impeller. The pump was overdosed with chemicals in the past and received abnormal levels of nitrates etc. It could be that the impeller or casing simply corroded more so the pump is pulling more power for the same head than the others.
Will probably try to measure the actual pump curve ( as oppose to nominal) just to get a baseline and monitor the regression.
Regards