We are proposing to connect a 4" force main to an existing 14" force main.  Based on old design plans, the 14" force main is operating at 1,600 GPM @ 216' TDH (two submersible 130 HP pumps).  Our proposed lift station will operate at 100 GPM @ 50' TDH.  We are proposing to add two check valves on either side of the MJ at the force mains connection.  QUESTION:  Assuming the p.stations can both operate at the same time, will the check valves actually function/open up against the pressure of the opposing force main flow?  In other words, if the 14" force main is running at the aforementioned flow, will the 4" check valve open once the smaller proposed pump station is in operation?

You asking if it is possible for a pump operating at a lower pressure (50' TDH) to pump into a higher pressure (216' TDH) force main.

It should seem obvious that this will not work. The check valve will never open.

Not sure why the need for the higher pressure on the larger force main, but if you want to pump into it, you will have to have a greater pressure than the line that you are pumping into.

You have posed what seems like a simple question, but in fact, it is a complicated question:

1. You will have to evaluate the system head curve. When both pumps are operating, the system head curve will increase because of the total increase in flow. Therefore the discharge pressure of each pump should increase. If the pumps are not designed to pump against the higher head (from the combined flow), then the flow will drop off accordingly.  The magnitude of this effect depends on the system head curve.

2. The pump discharge pressure when both pumps are operating will be the common for both pumps. The pump operating point of each pump will move to the common discharge pressure point. Assuming your pumps are installed at the same elevation, the low pressure pump operating point will move over on the pump curve to the shut off head (and zero flow).

3. It is generally not good practice to rely on a check valve to hold back the pressure when you have multiple pumps. Check valves do fails and you will have a mess on your hands. Consider an automatic valve to close when the pump shuts off. There is little benefit to installing check valves in series either.

4. Consider using VFD's. The VFD's will decrease the peak flow and water hammer when the pumps start/stop.

5. The large increase in pressure when the high pressure pump operates may cause a significant water hammer and/or reverse flow that may damage a pump.

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