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Affinity Laws - Goulds Pumps - ITT Corporation http://www.gouldspumps.com/cpf_0010.html

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Affinity Laws

Section A -- Centrifugal Pump Fundamentals

A-8 Affinity Laws

The affinity laws express the mathematical relationship between the several variables involved

in pump performance. They apply to all types of centrifugal and axial flow pumps. They are as

follows:

With impeller diameter D held constant:

Where:

Q = Capacity, GPM

H = Total Head, Feet

BHP = Brake Horsepower

N = Pump Speed, RPM

1.

With speed N held constant:2.

When the performance (Q1, H1, & BHP1) is known at some particular speed (N1) or diameter (D1),

the formulas can be used to estimate the performance (Q2, H2, & BHP2) at some other speed (N2)

or diameter (D2). The efficiency remains nearly constant for speed changes and for small changes

in impeller diameter.

Example:

To illustrate the use of these laws, refer to Fig. 8 below. It shows the performance of a

particular pump at 1750 RPM with various impeller diameters. This performance data has been

determined by actual tests by the manufacturer. Now assume that you have a 13" maximum

diameter impeller, but you want to belt drive the pump at 2000 RPM.

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Affinity Laws - Goulds Pumps - ITT Corporation http://www.gouldspumps.com/cpf_0010.html

2 of 3 1/8/2012 8:52 PM

Affinity Laws - Goulds Pumps - ITT Corporation http://www.gouldspumps.com/cpf_0010.html

3 of 3 1/8/2012 8:52 PM