are you trying all pumps together irrespective of system demand?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

Head drops off by the square of the speed. You need to figure the minimum speed that will produce 7 GPM from the required TDH. Then you can figure the minimum hertz.

Well just follow the system curce and see where it crosses with your thick balck lines this iindicates the max flow avaialable with 1,2,3, or 4 pumps.

Hence to start with at 10gpm with one pump that seems to be about 47.5 htz where 10GPM crosses your system curve.

ONe pump then goes up to 60htz. Then the second pump kicks in and both pumps go to about 50.5 htz (where the system curve intersects the 60 hts 1 pump line,

Same again - three pumps all start at 51 htz,

4 pumps all start at 55 htz

This is all based on all pumps getting the same frequency. Nothing else works.

On the way down the same frequency limits apply but then from say 3 pumps to 2 you go from 3 @ 51htz to 2 @ 60htz

Using VFDs for such a usage is not a good way to control pumps. Having a high head which doesn't change much for the pump range is poor.

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Also: If you get a response it's polite to respond to it.

Why have 4 parallel pumps been condisered for this application. Wouldn't be one single VFD-pump sufficient?

I think all the four pumps are needed in order to read the curve.

hi

it looks like the first pump would need to start slightly above 50 hz for 10 gpm. (round up to 51 hz as you say)

speed of one pump increases to 60 hz for 34 - 35 gpm

two pumps together start at 52 - 53 hz for 34 - 35 gpm

speed of two pumps increases to 60 hz approx 68 gpm

three pumps start together at about 54 hz 68 gpm

speed of three pumps increases to 60 hz approx 104 gpm


etc

you would leave a bit of margin for startup points.