Regarding the backflow issue in multi-stage pumps and high-speed pumps
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Multi-stage pumps and high-speed pumps are generally equipped with a start-up line, which is a return line at the pump’s outlet, aimed at preventing the pump from being blocked during startup and shutdown. I would like to ask what level of return flow generally needs to be maintained? If the return line is opened only during the pump testing phase right after maintenance, could this lead to excessive vibration of the pump due to low flow rates?
I haven’t seen any specific regulations, but I think it should be greater than the pump’s minimum flow rate.
The magnitude of the backflow volume can only be determined by referring to the pump’s performance curve; it is generally 30~45% of the flow rate corresponding to the point of optimal efficiency. The specific operating conditions corresponding to having the return line fully open depend on the flow rate through the pump and the pressure difference across the pump; it is also possible that high vibration levels are caused by an excessive flow rate. Another possibility is that, since you mentioned high-speed pumps or multi-stage pumps, the pressure is usually relatively high; thus, the pressure difference before and after the check valve in the return line is significant. During the pressure reduction process, cavitation can occur, leading to vibrations in the pipelines. If the pressure reducing valve is located close to the pump, these vibrations can also be transmitted to the pump itself. At the same time, be mindful of the pressure and temperature at the pump inlet; with backflow in a high-pressure pump, the temperature can rise rapidly, leading to cavitation and vibration.
There is a post for reference: http://bbs.hcbbs.com/thread-1099889-1-1.html