by Dr. Lev Nelik, P.E., APICS
December 17, 2011

Dear Readers:

In the next three issues, we will present three Pump Puzzles  to solve. The first one-an impeller cut problem-is an application of the Affinity Laws to reduce the motor amps and solve a motor tripping problem. The second one will cover a practical question of saving money with the application of a VFD instead of valve control. Finally, the third one will challenge you to evaluate a pump's efficiency given the flow, head and speed, and compare two pumps running at different speeds, but producing the same flow and head performance.

Here is Pump Puzzle #1. Start up your calculators!

Statement of Problem: A vertical pump has a 400 hp motor. As can be seen in the pump performance curve, the pump motor gets overloaded at certain flow ranges. It is decided to trim the impeller diameter to ensure the motor is never forced to deliver more than 400 hp.

Given: Pump manufacturer original performance curve for the installed impeller with 19.625 in diameter

Question 1: What will be the trimmed diameter of this impeller to keep the pump motor under 400 hp?

Question 2: What will be the new BEP point for the pump?

Hint: Applying the Affinity Laws, determine the new (reduced diameter) pump head curve and motor curve, and find out where the new BEP point is.

pump-performance-curve

Pump Performance Curve

The solution-and Pump Puzzle #2-will be in the next P&S issue.