The centrifugal pumps in series work accordindg to the following laws,
Total head is Ht=h1+h2+h3+....hn
Total flow is Qt= q1=q2=q3=....qn
In parallel the centrifugal pumps work according to Ht=h1=h2=h3=.....hn
Qt=q1+q2+q3+.....qn
Series pumping" can be successful, but there are a few things to consider before you do this:
1. The doubling of pumping head could cause both pumps to run far to the right of BEP where the NPSH required can get very high. This, in turn, could result in cavitation at the first pump unless you have ample NSPSH available. To know for sure, you'll need to compare the pipe-system hydraulic characteristics to the composite pump curve.
2. Reliability and firm capacity of the pump station (i.e. capacity with the largest unit out of service) will be much lower than originally intended.
3. A better approach, at this point, is to conduct a flow test that allows you to accurately characterize the system curve (pumping head vs. flow). That will take the guess work out of the hydraulics so you don't have to design using trial and error. Results from the flow test will tell you if there will be a problem with running pumps in series.
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