When two pumps are operated in series, as shown in Figure 9, the net pump curve can be constructed by adding the head for each pump at several flow rates and plotting the resulting head curve. Pumps in series may generate too much discharge pressure for the system design pressure. The control and relief system must be designed for safe operation. One remedy is to install a safety valve at the discharge of the second pump as shown below.
When two pumps are operated in parallel, the net pump curve can be constructed by adding the flow rates at several heads and plotting the resulting curve (Figure 10).
A problem that can occur with pumps operating in parallel is shown in Figure 10. Two pumps are never exactly like. If two pumps are installed in parallel, one pump may take more than half of the total flow and the other pump less than half. The pump with the lower flow rate may be operating below its minimum acceptable flow rate. The head produced by the two pumps will be identical because they are connected to the same process. If the head curve produced by pump B is lower than the head curve produced by pump A, the situation shown in Figure 10 will occur. Pump B will decrease its flow rate until it can produce the same head as Pump A.
This situation is most dangerous when one pump is driven by a motor and the other by a
turbine. It is impossible to set the two speeds exactly equal, and the difference in speed will
cause a difference in head produced.
If two pumps are nominally identical and both driven by motors, the two head curves can be assumed to be within 3% of each other. If so, you can make the worst assumption, that is, the head of pump B is 3% lower than the head of pump A. Then, using the system operating conditions, plot the flow through both pumps. Make sure that the lowest flow rate is not below the pump minimum allowable flow rate.