Parallel Operation

From : Mr. Rajesh Patel on 12 th January 2011

Please explain me why we cant achive double flow with two same capacity parallel connected pump.

Response :-

If one looks at combined characteristics of two identical pumps running in parallel, at a given head, the flow on the combined characteristics will be double of flow of single pump.

But in a given system (shown hypothetically to illustrate the point), the point of operation is the point of intersection of the system curve with the single pump curve or curve of two pumps in parallel or curve of 3 pumps in parallel.

As can be seen in the diagram, at 32m head flow from single pump is about 80 m3/hr. Flow from 2 pumps is about 160 m3/hr and from 3 pumps is about 240 m3/hr.

But if you see points of intersection with a given system curve the flow from single pump is about 115 m3/hr, from two pumps in parallel is about190 m3/hr (which is not double of 115 m3/hr) and from 3 pumps about 260 m3/hr which is neither 3 times of 115 m3/hr nor 1.5 times of 190 m3/hr.

So the difference happens in the way one looks at the diagram, whether against a given head or whether in a given system.