Problem of starting and stopping multiple pumps in series
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As is well known, centrifugal pumps need to be protected from air locking and cavitation during startup and shutdown. When multiple pumps are connected in series, for example two centrifugal pumps A and B in series, pump A operates normally first, after which pump B is started; the return line of pump B is closed, allowing the liquid discharged by pump A to flow directly through the inlet and outlet pipelines of pump B. In this case, is it still necessary to close the outlet valve to build up pressure when starting pump B? If both pumps are started and then Pump B is stopped first, is it okay not to close the outlet valve when stopping the pump? Also, how does connecting multiple pumps in series prevent the pipelines before and after from overpressuring and damaging the mechanical seal?
In my opinion, by using two pumps in series, a return line can be installed on pump A; this return line is opened when pump A is started, thereby preventing the seal from leaking due to overpressure that could occur if the outlet valve of pump B were closed. Also, the outlet valve of pump B needs to be closed, as this results in the lowest starting current, which serves as a form of protection for the pump.