System Head Curve Question

[mae1133]

I have a question regarding development of a system head curve. I've attached a basic example to help explain my question. If I have a duplex wet well and given a pump curve, I'm trying to determine the single and combined pump flow. My issue is in what system head flow do I use to determine this. In the attached simple example, I have it split into two pumping scenarios.

One Pump On: In this scenario, I've developed my system head using increments of 1x pump flow from pump, all the way through to destination. The blue curves represent my single pump curve and two pumps in parallel curve. My single pump flow is at point "A" and my combined flow is at "B".

Two Pumps On: In this scenario, I've developed my system head using 1x pump flow from "A" to "B" and 2x pump flow from "B" to "C". As with the scenario above, the blue curves represent my single pump curve and two pumps in parallel curve. My combined flow is at "D", but what would "C" represent where two pumps are on?

Which method is correct in determining my multiple pump flow?

I am assuming the x-axis represents the total system head (as opposed to the single pump flow)?

I've searched to the end of the internet and most examples are all based on my first scenario above, but they don't really explain the development of the system head curve if two pumps are on.

 

[77JQX]

If I understand correctly, the two system curves were developed (1) with flow through one pump branch to the manifold and then in a single pipe to the tank, and (2) through the two pump branches (in parallel) to the manifold and then in the single pipe to the tank. With the parallel flow found in system curve 2, you get lower frictional losses for the same flow. If this is correct, with single pump flow the system will operate at point A, 300 gpm, and in dual pump mode the system will operate at point D, 400 gpm. Points B and C will not happen in operation, since they represent frictional losses calculated in impossible operational situations (i.e., point B is two pump flow with single pump friction, while C is single pump flow with two pump friction).

 

[LittleInch]

I agree with 77JQX.

Also unless distance B to C is close to A to B, normally you wouldn't bother calculating A to B.

Using the same letters doesn't help in explaining things but you only need one graph with a single system curve and two pump curve lines, one with one pump one with two pumps. If distance BC is similar to AB then you might need two curves but I doubt it.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[mae1133]

Thanks, sorry about the letter designations. I realized that may be confusing after I posted it.