Page 56 - Know and Understand Centrifugal Pumps

The Affinity

Laws

Introduction

The Affinity Laws are a group of rules that govern your pumps.
Sometimes they’re called the Laws of Similarity. In years gone by, these
laws served to predict a pump’s operation when exported to a country
with different electricity. The electricity in the United States is 60 hertz
(Hz). This means that the electricity moves in waves with a frequency
of 60 cycles per second. Some other countries in the world have
electricity moving at a frequency of 50 Hz., or 50 cycles per second. A
change in frequency brings about a change in running speed (rpm).
Upon exporting a pump into a country and connecting it to an electric
motor running at different speeds, the pump has different operational
characteristics. Those characteristics change by the Affinity Laws. Now,
with the arrival of the variable speed electric motor, called Variable
Frequency Drive (VFD), the Affinity Laws will become increasingly
important with industrial pumps.

The Laws
~~
When a pump’s velocity changes, measured in revolutions per minute
(rpm), the operational characteristics also change. These changes can be
calculated using the Affinity Laws. Before continuing, let’s define some
terms we’ll be using:
1. Flow = Capacity = Q: Liquid volume measured in gallons per
minute (gpm) or liters per minute, cubic meters per hour, or some
other rate.
2. Head = H: Liquid force measured in feet of elevation. H can be
converted into pounds per square inch (psi). This is discussed in
Chapter 2.

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