2 pumps in parallel with one recycle line 2

[BENGY]

can anyone tell me what will happen at low flow if 2x100% centrifugal pumps are operated in parallel but with a single recycle line (that is sized for single pump recycle capacity)at the common discharge line? could one pump cavitate while the other operates satisfactory?

 

[TBP]

Many re-circ lines are grossly oversized, but each pump should have it's own line back to the tank. If the re-circ line is on the common header, then it's after the isolation valves for each individual pump. It should be connected before the isolation valves.

It's not really a cavitation issue, it's to prevent the pump from overheating. I've seen a fire pump handling cold lake water actually blister the paint, it got so hot. There have been cases of pumps exploding from operating without flow for short periods of time.

 

[tstead]

Be careful when operating pumps in parallel without dedicated min flow lines on each pump.

Imagine two pumps in parallel, each one operating back to the left of the curve. Now, no two pumps will have the same performance curve and, thus, one of the two pumps will have a slightly higher head than the other. In this case, if you have check valves at the discharge of each pump, the pump with the higher head could close the check valve of the other pump causing it to deadhead.

Best practice is to put the min flow recirc line IMMEDIATELY after the pump (before any valves). You can combine the min flow lines downstream of that point if you want and terminate that line in your DA tank.