Pump minimum-flow bypass location query?

[knapee]

Dear Pro

I wonder why the single minimum-flow bypass (protect tow or more pumps in the same service) has to be installed at branched off upstream of the check valves instead of downstream of the check valves.

Some senior engineer told me the reason is if run both pumps at once, the speeds may differ enough so that the higher-speed pump will develop enough pressure to hold close the check valve of the lower speed pump. In such case, the lower-speed pump can then overheat rapidly and fail, since
the common bypass can't protect it.

But I still can't get it.

Could anyone please kindly explain more detail?

Thank you very much.
I'll appreciated for your help.

 

[Pumpsonly]

Your senior engineer has already given the main reason.
The other reason is if one of the pump is on stand by, and the discharge isolation valve is kept open. If you install the tap off after the CV, there will be be a pressure from the discharge head will cause a back flow via the by pass line.

 

[BigInch]

Another common situation, but perhaps not in your particular case, is that when pumps need to be started against a high static head that would make it impossible for the pump to open the check valve when it is operating at low speed and developing low head, precisely when the bypass is needed.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso