Cavitation while running 1 pump but not 2

[BronYrAur]

I have a hot water system pumping 140 degree F water. Pumps are variable speed to maintain a pressure differential out in the system. When running only 1 pump, VFD is around 50Hz to maintain the 10 psi differential. I hear cavitation at this condition. Suction pressure is 40 psig, and discharge is around 60 psig but the discharge needle bounces all over. When I turn on a second pump in parallel, everything quiets down. The needle stops bouncing. Hz is less but I don't know exactly where.

Is there an obvious reason for this?

 

[LittleInch]

Sounds like excessive flow through one lump but not evident detail here.

Inlet pressure is 40 psig is huge for a 140F systems, but what does the piping layout look like?

 

[EdStainless]

I (like LI) think that this is likely a piping layout issue.
Do you know your flow rate?
Do you have pump curves corrected for density and viscosity?
Are you actually near BEP?
As a guess (it depends on the system curve) two pumps would be running about 36Hz.

 

[Snickster]

It is not clear how your system is functioning. So you have two pumps in parallel and you are trying to maintain a constant differential pressure. So are the pumps controlled by flow at a constant flow value? What is the control philosophy.

So you have one pump running to produce a given flow. Say this requires 60 psig discharge pressure with 40 psig on the suction or 20 psi differential at full pump RPM. The only reason you would need to operate two pumps to get the same flow is if the suction pressure for some reason drops. So if the suction pressure drops to 30 psig then you need to kick in the other pump so both pumps produce the same flow as before but 1/2 flow each so the head developed is higher to make up for the drop in suction pressure. Could you better explain the process system and controls.

 

[goutam_freelance]

Suction pressure is 40 psig, and discharge is around 60 psig

Difference is 20 psig, but you wanted 10 psig. Is there a control issue?

Also is there any unknown obstruction between suction pressure gage and pump suction?

 

[LittleInch]

The 10 psi is somewhere else in the system.

Please quote flow and pipe sizes and a piping layout.