4 - The affinity laws

Abstract

This chapter explains the affinity laws—a group of rules that govern the pumps. They are also called Laws of Similarity. Mainly, these laws are used to predict the operation of pump when exported to a country with different electricity. Upon exporting a pump into a country and connecting it to an electric motor running at different speeds, the pump has different operational characteristics that change with the affinity laws. The affinity laws indicate: flow, Q, changes directly proportional to a change in velocity; head, H, changes directly proportional with the square of the change in velocity; and power, BHP, changes directly proportional with the cube of the change in velocity. This chapter also explains the relationship between velocity and horsepower requirement (BHp) and explains VFDs, and how they are effective at maximum speed. The practical implications of these laws are explained with few real-life situations such as the seasonal rises and falls. Manipulating flow and controlling pressure by varying the impeller diameter conserves kilowatts of energy, and this is the third affinity law in this group. In addition, this chapter also explains the reduction of impeller diameter and how it helps in conserving energy.