Centrifugal pumps parallel operation

Two identical centrifugal pumps are operating in parallel and serving a single distribution loop. Which of the following will occur if one of the pumps is deenergized ... 

Figure 3-35. Operation curves of two duplicate centrifugal pumps in series and parallel.

Diskind (1959) determined the operating characteristics for centrifugal pumps in parallel and in series using a simple graphical method. 

The piping and valves may be arranged to enable two centrifugal pumps to be operated either in series or in parallel. For two identical pumps, series operation gives a total head of 2Ah at a capacity Q and parallel operation gives a capacity of 2Q at a total head Ah. The efficiency of either the series or parallel combination is practically the same as for a single pump. 

Figure 7.17. Operating points of centrifugal pumps under a variety of conditions, (a) Operating points with a particular pump characteristic and system curves corresponding to various amounts of flow throttling with a control valve, (b) Operating point with two identical pumps in parallel each pump delivers one-half the flow and each has the same head, (c) Operating point with two identical pumps in series each pump delivers one-half the head and each has the same flow.

---