Parallel pumping question 1

[cry22]

We have two pumps at 50% each, 800 GPM each, parallel pumping arrangement.
A water balancing comes out like this:
each pump, when tested alone delivers 1100 GPM.
When both pumps are enabled with everything open, they only deliver 1350 GPM.
One would expect 1600 minimum, better yet, 2200 since each delivers actually 1100.

Anyone run through such a problem?

Thanks

 

[Zesti]

I guess these pumps are feeding the same pipe-system, right?

Doubling the flow through the pipes will quadruple the flow-resistance!

So, the combined flow of two pumps together will raise the resistance they are up against.

When sizing the pumps, each pump must be able to generate the PRESSUREDROP of the pipesystem at FULL FLOW. But each pump only has to displace 50% of the FLOW.

So, my guess is this is what you are observing.

 

[imok2]

Although parallel operation does increase the flow rate, it also causes greater fluid friction losses, results in a higher discharge pressure, reduces the flow rate
provided by each pump, and alters the efficiency of each pump. In addition, more energy is required to transfer a given fluid volume and you may increase flow rate by only 25% which is about what you have.

 

[willard3]

Bell and Gossett has good information on parallel pumping:

http://www.bellgossett.com/Press/BG-unparallel.asp

 

[Artisi]

Simply put, if the friction loss thu' your pipe system with one pump running at 1100 gpm is let's' say for the discussion is 50 psi, then the friction loss at 1350gpm (2 pumps operating) becomes (1350/1100)^2 x 50 = 75 psi.

In 2 pump operation the pumps will run at a point on their curve where the flow / head intersects with the friction in the pipe system.