To properly accomplish a good and thorough ratinghizing of a centrifugal pump, the plant system designer should at a minimum do the following.
1. Understand the fundamentals of performance of the pump itself.
2. Understand the mechanical details required for a pump to function properly in a system.
3. Calculate the friction and any other pressure losses for each “side” of the pump, suction, and discharge.
4. Determine the suction side and discharge side heads for the mechanical system connecting to the pump.
5. Determine the important available net positive suction head (NPSH,) for the pump suction side mechanical system, and compare this to the manufacturer’s required net positive suction head (NPSH,) by the pump itself. This requires that the designer makes a tentative actual pump selection of one or more manufacturers in order to use actual numbers.
6. Make allowable corrections to the pump’s required NPSH (using charts where applicable) and compare with the available NPSH. The available must always be several feet (mm) greater than the corrected required.
7. Make fluid viscosity corrections to the required performance if the fluid is more viscous than water.
8. Examine specific speed index, particularly if it can be anticipated that future changes in the system may be required.
9. If fluid being pumped is at elevated temperature (usually above 90o F (32.2o C )), check temperature rise in the pump and the minimum flow required through the pump.
10. Make pump brake horsepower corrections for fluids with a specific gravity different from water. Select actual driver (electric motor, usually) horsepower in order that horsepower losses between the driver and the pump shaft will still provide sufficient power to meet the pump’s input shaft requirements.
11. If the pump has some unique specialty service or requirements, recognize these in the final sizing and selection. Consult a reliable manufacturer that produces pumps for the type of service and applications and have them verify the analysis of your system’s application.