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AFFINITY LAWS -
CENTRIFUGAL PUMPS |
If the speed or impeller diameter of a pump change,
we can calculate the resulting performance change using:
Affinity laws:
- The flow changes in proportion to speed
ie: double the speed / double the flow
- The pressure changes by the square of the difference
ie: double the speed / multiply the pressure by 4
- The power changes by the cube of the difference
ie: double the speed / multiply the power by 8
Notes:
- These laws apply to operating points at the same efficiency.
- Variations in impeller diameter greater than 10% are hard to predict due to the change in relationship between the impeller and the casing.
I know you are thinking "what does this have to do with anything"?, but if you can understand these 'laws' then you can make rough estimates without having to find full information (which might not be available anyway)
ie:
Boss: "Hey Joe, put this new pulley on that pump"
Joe: "But that will speed the pump up by about 10 % which increases the power by a third, do you reckon the motor will handle it ?"
For rough calculations you can adjust a duty point or performance curve to suit a different speed.
NPSH (r) is affected by speed / impeller diameter change = DANGER !
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