Pressure fluctuation when stopping one pump

[Wkcoo]

Hi, i would like to see if anyone experience following weird observation before.

So it's about the lube oil pump of a blower, main one (shaft driven) and auxiliary one (separate motor driven). By logic, the aux pump needs to be running to get start permissive, and once the blower starts running, a timer start ticking and will cut out the aux pump after one minute, expecting the main pump to take over.

When the aux pump cuts out, i observed the pressure gauge on the common discharge line dip momentarily to zero before picking back up. Am wondering if this pressure dip is normal when one of the running parallel pump stops, or actually the gauge is faulty. Am concerned because if this pressure dip is indeed true my blower may get trip easily.

 

[Artisi]

A sketch of the 2 pumps / circuit might help.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[JJPellin]

Addition of a pulsation damper will mitigate this dip. There may be other things that can be done with the pressure control system which you did not describe. What controls the oil pressure?

Johnny Pellin

 

[LittleInch]

What type of pump?

These types of pumps are often gear with relief valves and return lines.

When the aux pump stops there could be some sort of loss of volume which results in your pressure dip which the pd pump can't make up.

Details of both pumps needed as well as the set pressure of the relief valves.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[electricpete]

It could be that the check valve or foot valve is slow to respond in the electric-driven pump going off-line. I can't say I've ever heard of that, but who knows.

It could be you have a system pressure regulator that is responding poorly to the transient. Also I think some pd pumps have an internal regulator/relief to limit discharge pressure that may be slow to respond.

I recall some strainers get auto-bypassed on high strainer DP, I wonder if there are any that un-bypass themselves on low-strainer dp which may happen when the electric pump kicks off.

Yeah, more details like requested by the knowledgeable folks above would help.

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(2B)+(2B)' ?

 

[Compositepro]

Perhaps during shutdown the oil line to the main pump drains out and it takes more time to fill it than the timer is set for. After all, why is there a timer at all. Just run the auxiliary pump a little longer.

 

[georgeverghese]

Is there a pressure regulator on the discharge of the main oil pump - it may be taking some time to open when the aux pump stops?

 

[TugboatEng]

Are these pumps lifting from a reservoir? If the main gear pump is not primed at startup it may not be able to prime against a pressurized system. I have seen this happen to a gear pump self priming against an 8 bar relief. Once the aux pump cuts out the pressure drops and the main pump quickly primed and restores pressure.

 

[Wkcoo]

Hi,

Yes- the pumps are gear pump with internal relief, and the pressure control is done via a pressure regulating valve. And yes, they are lifting from a reservoir.

Summarizing the inputs including my own, i guess it's not normal for such dip. It could be due to
1. Gauge is faulty
2. Presence of air pocket in the main lube oil pump and in a sense, similar to what TugBoatEng mention, it can be main pump immediately prime itself in that split second. However, based on my understanding, gear pump is self priming, so within that one minute of co_running with aux pump it should have primed itself by then. One observation is also the presence of a tiny vent hole at the gear pump body which i assume is to squeeze out any air for priming purpose (anyone know this tiny hole)?
3. The pressure regulating valve is responding poorly.

I have another chance to observe the blower for the next start soon, will update again.

 

[TugboatEng]

The gauge is not faulty!

The main pump can't prime while the aux pump is running. Gear pumps don't pump air well.

The pressure regulating vale is unlikely to be causing issue in this case.

There are no vent holes in the pump body. There may be a tattle tail hole in the pump shaft coupling area but this is separate and away from the pump cavity.

If you run the main pump and such pump for more than a minute you may find the main pump will overheat due to lack of lubrication.