How to Use Affinity Laws in Mag Drive Pump Operations

Posted: 12/24/2014

Affinity laws in mag drive operation are used to "express the relationship between variables involved in pump or fan performance such as head, flow rate," etc. These laws help mag drive pump operators to more closely predict the head discharge when delivered at different speeds or from different sized impellers. 

According to The Engineering Toolbox, there are 2 sets of laws

1) Laws for a specific centrifugal pump, which approximates the head, capacity and power curves for different speeds and diameters, and;

2) Laws for a family of similar pumps, to approximate the same types of measurements

These laws are sets of formulae that users can add their specific device's measurements into and then arrive at a solution. Some examples are below:


Most pump manufacturers provide charts with the flow/head/power of the impeller at different diameters. They don't always have curves at different speeds because there are so many options. This is where Affinity Laws become most valuable. These laws allow users to calculate the affect of increased velocity on the operation. A pump calculator can be used to determine the new flow/head/power at speeds. 

It is not recommended to go above the normal speed of the pump (in our case, 3450RPM) because the magnets (if a magnetic drive pump) are not strong enough and will cause the pump to decouple. Also, it will overload the motor. Slowing down the motor reduces power more than trimming the impeller or closing a valve restricting the flow. There is, however, a larger initial cost to buy the necessary equipment for variable speed.

It should also be noted that drastically changing the speed changes all three elements of performance differently. Please refer to the example below:


In conclusion, affinity laws can help a mag drive operator predict what effects may occur when adjusting an element in an operation. You can learn about this an more by contacting us today!

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