SUBJECT : The pump affinity laws 2-1
There are occasions when you might want to permanently change the amount of fluid you're pumping, or change the discharge head of a centrifugal pump. There are four ways you could do this:
Of the four methods the middle two are the only sensible ones. In the following paragraphs we'll learn what happens when we change either the pump speed or impeller diameter, and as you would guess, other characteristics of the pump are going to change along with speed or diameter.
To determine what's going to happen you begin by taking the new speed or impeller diameter and divide it by the old speed or impeller diameter. Since changing either one will have approximately the same affect I'll be referring to only the speed in this part of the discussion.
As an example:
The capacity, or amount of fluid you're pumping, varies directly with this number.
The head varies by the square of the number.
The horsepower required changes by the cube of the number.
The following relationships are not exact, but they give you an idea of how speed and impeller diameter affects other pump functions.
The net positive suction head required by the pump manufacturer (npshr) varies by the 1.5 power of the number at the BEP.
The amount of shaft run out ( deflection) varies by the square of the number
The amount of friction loss in the piping varies by about 90% of the square of the number. Fittings and accessories varies by almost the square of the number.
The wear rate of the components varies by the cube also
I started this discussion by stating that a change in impeller speed or a change in impeller diameter has approximately the same effect. This is true only if you decrease the impeller diameter to a maximum of 10% . As you cut down the impeller diameter, the housing is not coming down in size so the affinity laws do not remain accurate below this 10% maximum number.
The affinity laws remain accurate for speed changes and this is important to remember when we convert from jam packing to a balanced mechanical seal. We sometimes experience an increase in motor speed rather than a drop in amperage during these conversions and the affinity laws will help you to predict the final outcome of the change.
A couple of things to keep in mind as you use the affinity laws:
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