Force Main Total Head

[tmarsh]

I have a unique situation. I'm designing a sewer system for a large neighborhood that will be served by 3 sanitary sewer pump stations. Each of the pump stations pumps to a common force main. The common force main then discharges into an interceptor force main operated by the local sewer authority. The existing head in the interceptor force main is 43' (peak daytime head), but I'm required to design against an HGL of 93'.

If there were just one pump station, the TDH calculations would be simple. Simply design the one pump station to pump against the 93' of head. However, I need to consider various conditions such as two of the new pump stations online and pumping against the 93', or even only one of the three pump stations online pumping against the 93' of head.
Not only are the pump stations pumping against the 93' of head in the interceptor force main, they are pumping against each other when there are two or more of the three pump stations online.

Does anyone know of a simple way to take into account all the various combinations of possible head (TDH) conditions. Is there a software package that can handle this type of scenario?

Thanks.

 

[FLwater]

You could buy WATERCAD software and run it, but this is quite simple. If large enough, yoor new force main would not cause the TDH to fluctuate more than 5' either side of the 93' design. Make two pressure curves, one with all three pumps running and one with only one (the one closest to the discharge point) Plot them on your pump curves and pick the right pump. Your pump supplier can help you if needed.

 

[BRIS]

FLwater's advice is correct but if the pressure in the interceptor main is really only 43' and not 93' then you will end up with pumps operating way off their duty curve.

You need to varify what the likely maximum and minimum pressures are in the interceptor and plot your maximum head curve with all three pump sations running and maximum pressure in the interceptor (93') and minimum cuves with one pump sation running and minimum pressure in the interceptor. You will need to do this for each pump station.

Brian