In order to put liquid from a "A" tank to a "B" tank, parallel operating centrifugal pumps can be used. Money and time can be saved. Indeed, 2 little pumps can be less expensive than 1 big centrifugal pumps. Other points can obviously be taken into account, like NPSH, power, spare pumps.
Which flow for such a system ?
We assumed 2 pumps i parallel : pump 1 and pump 2. For that explaination, we will take the example of 2 pumps which characteristic curves are not the same. In order to avoid recycling flow
it is recommanded to operate with the same pumps, to have the same friction loss in pipes and to add anti-recycling flow valve at discharge of each pump.
To treat a general case, we assume that pumps don't have the same characteristic curves.
pump 1 and pump 2 operate in parallel. Resultant curve (Q3) is the sum of pump 1 flow (Q1) and pump 2 flow (Q2) at a given head. For a given head, we simply sum the two flowrate. In the low flow area where pump 1 head is higher than pump 2 head the resultant characteristic curve is the same than the pump 1. It's in that area that pump 1 discharge flow can go into pump 1 discharge flow because pump 1 head is higher than pump 2 head. An anti-recycling valve shall be used.
perating point for that system ?
That system operating point (5) is at intersection of resistive characteristic curve of pipes with the resultant characteristic curve. Pump 1 flow is Q1 (point 1), pump 2 flow is Q2 (point 2) and we have got Q1+Q2=Q3. The resistive characteristic curve of pipes is the pipe friction losses. It is composedof a static part (high between both A & B tank) and a dynamic part which is proportional to flow square.
What hapen if one pump failed ?
We take into account the following assumption : resistive pipe characteristic is unchanged, operating pump driver power is high enough to drive it.
If pump 1 failed, system go to point (5)
If pump 2 failed, system go to point (4)
We see on the chart what could happen. The pump which operate alone go to the high flow area. Its required NPSH (plot on the chart) increases. If available NPSH is low there is a risk of cavitation.