In order to make a selection of the pump required for a specific installation it is necessary to first determine the desired flow rate or head. The NPSH available should be determined, and if a centrifugal selection is possible, a system head-flow-rate curve should be developed.
Generally, positive displacement pumps are better suited for high head and low flow-rate applications. Figures 10-13 and 10-14 can be used as guides for the type of pump that will probably be found to be most economical. Naturally, the economics and thus the choice of pump type will vary from installation to installation. Figures 10-13 and 10-14 should be used merely to provide general guidance and not to justify any one specific decision.
In choosing a pump type it is necessary to consider the physical constraints of the system. Space availability or weight limitations may dictate a centrifugal or rotary pump; the need to pump solids may dictate a centrifugal or rotary pump; the need to pump solids may dictate a centrifugal or diaphragm pump; and the need to run dry may dictate a diaphragm pump. Where discharge pressures vary over a large range, but flow must remain constant, a positive displacement pump is probably a good choice.
In many high-head applications requiring a reciprocating pump the NPSH available may not be sufficient for a straightforward pump choice.In these cases low NPSH centrifugal pumps are sometimes used as “charge” pumps to feed the suction of the reciprocating pump.
The number of pumps required for a given installation depends on a balance of capital cost and operating flexibility. For most small installations a choice must be made whether the cost of a standby pump is justified by the potential foregone income if the pump must be shut down for maintenance. On larger installations several other alternatives should be investigated. These could include:
1. One pump rated at 100% throughput.
2. Two pumps rated at 100% throughput each (one pump is standby).
3. Two pumps rated at 50% throughput each (if one pump is down, throughput is decreased to 50% of design).
4. Three pumps rated at 50% throughput each (two operate, one is standby).
5. Three pumps rated at 33% throughput each.
In making the selection it is important to consider the actual throughput requirements over the life of the installation and not merely the peak design throughputs. Finally, the choice should consider the availability of spare parts and service at the location and the preferences of operating personnel.