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June 2006
 
  The Affinity Police
Break the Laws at Your Own Risk 		 
   
 

Larry Bachus
As a general rule, it’s best not to break the law. Likewise, in the pump world you don’t want to break one of the Affinity Laws.

The Affinity Laws govern the performance of your pumps. They become important when modifying the pump speed and certain dimensions in the pump.

Most standard pumps are coupled to a fixed-speed AC electric motor. Because centrifugal pumps work with centrifugal force, the speed of the motor and the diameter of the impeller determine the head the pump can produce. The formula for the speed of the electric motor is:

Speed = 2 x 60 x 60/# Poles

The “2” is for alternating current (AC electricity). The “60s” are for 60-Hertz electricity and 60-seconds per minute. The “# Poles” are the electromagnetic fields in the motor. Working the formula, a two-pole motor spins at 3,600 RPM. A four-pole motor spins at 1,800 RPM. An eight-pole motor spins at 900 RPM, etc.

The speed can be adjusted by multiples just by changing the poles on the motor. You may have noticed that electric motors don’t exactly spin at 1,800 or 3,600 RPM. The motor also has a designed “slip factor,” which further influences the final motor speed.

Traditionally, industry would employ transmissions or belts and pulleys if an application required an incremental speed modification. For variable-speed applications, industry would employ DC motors, steam turbines, internal combustion engines, or hydraulic motors.

About 30 years ago, the first commercial variable-speed AC electric motors appeared, called variable-frequency drives (VFD). By varying the motor’s frequency, the speed is also variable.
The change in speed brings about a corresponding change in the head, flow, and horsepower requirements of the pump, according to the Affinity Laws.

Stated simply, the Affinity Laws say:
1. The change in flow (GPM) varies proportionally with the change in speed. This means that twice the speed is twice the flow.
2. The change in pump head (pressure) varies with the square in the change in speed. Two times the speed is four times (22) the head generated. Eighty percent speed is 64 percent (.802) the head.
3. The change in power requirement (BHp or Kw) varies by the cube of the change in speed. Two times the speed requires eight times (23) the power.

The third Affinity Law confuses some people, so let me explain it this way: A standard four-door car has an engine with about 100 horsepower (BHp). Maximum velocity might be about 100 miles per hour. Now, let’s consider NASCAR racing. These cars travel at approximately 200-miles per hour and sport engines approaching 800 BHp. Their speed is two times a normal car’s maximum speed, and their engines have about eight times the horsepower. This is the third Affinity Law. Two times the speed requires eight times (23) the power to drive the pump.

Mathematically, we would write the equations as:
1. New Flow = Old Flow x (new speed/old speed)
2. New Head = Old Head x (new speed/old speed)2
3. New BHp = Old BHp x (new speed/old speed)3

Why are these laws important to your work with pumps? Production managers and equipment operators like variable-speed motors for the first Affinity Law. If they need 40 percent more production, they run up the motor speed by 40 percent, and they get the increase. However, increasing the pump speed by 40 percent would almost double the head or pressure generated. This could blowout the screens in a downstream filter or burst a mechanical seal. Have you had an increase in mysterious seal failure since installing a VFD?

A pump’s performance is composed of both a flow element and a head (pressure) element. This is why some people put VFDs on fans and blowers, but not on centrifugal pumps. A fan or blower is only a flow application. If you needed air pressure, you’d use a compressor.

You must consider all the Affinity Laws with pumps. VFDs with pumps are generally good on loops, like a chill-water loop or filtration loop. Variable-speed motors can be used with pumps in pressure applications, but with limitations. Remember, 50 percent speed would only be 25 percent pressure, head, or elevation generated.

When I’m lecturing, I always spend plenty of time on the Affinity Laws. Either you dominate them or they’ll dominate you.

Larry Bachus, founder of pump services firm Bachus Company Inc., is a regular contributor to Flow Control magazine. He is a pump consultant, lecturer, and inventor based in Nashville, Tenn. Mr. Bachus is a member of ASME and lectures in both English and Spanish. He can be reached at larry@bachusinc.com or 615 361-7295.

www.bachusinc.com

Get Hands-On Training at the
PUMP GUY SEMINAR
Oct. 21-23 - Houston, TX

REGISTER NOW!!!

 ATTENDEES RECEIVE: Complimentary Companion Text - Free Shuttle Service To & From Hobby Airport - Free Parking - Discounted Hotel Rooms - Food & Bev Service.
Larry Bachus (a.k.a. "Pump Guy") demonstrates the principles of NPSHr vs. NPSHa at his Aug. 18-20 Pump Guy Seminar in the Chicago area.

Larry Bachus (a.k.a. "Pump Guy"), a regular contributor to Flow Control magazine and a widely recognized expert on pumping technology, recently presented his Pump Guy Seminar in the Chicago area to an eager crowd of pump users. Here's what some of the attendees had to say about this training event:
  • “I attended your seminar this week in Chicago, and it’s already paying for
    itself. Your seminar
    taught me about concentric reducers on suction lines and horizontal elbows on split case pumps, which will be quite helpful on several plant system designs I’m currently working on.”
  • “Just a brief note to say ‘thank you’ with regards to the Pump Guy Seminar. I thought the seminar was very informative and entertaining as well. I wish to thank you for all of your efforts with the seminar on behalf of the attendees. A good job well done!"
  • “The course was everything I expected. I needed this information 30 years ago, but it’s never too late.”
  • “This course holds tremendous value for anyone involved in the design, operation, maintenance, or purchasing of pump systems.”
  • “The information I’ve learned from this seminar will most definitely help my understanding of pump issues at work.”
  • “For my line of work, this seminar was dead on. It met my needs fully. Best money my company has ever spent for a training course.”
  • “The monetary price for your shared knowledge and ability to bring pump design back to the basics was worth every penny. Thank you for making your seminar attendees look good with our colleagues.”
FOR MORE INFORMATION & TO REGISTER FOR THE PUMP GUY SEMINAR, CLICK HERE.

KEY SEMINAR TOPICS INCLUDE:
 • Basic Pump Principles
 • NPSH
 • Cavitation
 • The Affinity Laws
 • Work & Efficiency
 • Pump Classification
 • Pump Curves
 • System Curves
 • Shaft Deflection
 • Pump-Motor Alignment
 • Bearings
 • Pump Packing
 • Mechanical Seals
 • Pump Piping

For a sampling of Larry's latest "PUMP GUY" columns from Flow Control magazine, see:
 •   "Cheat Sheets: Energy, Work & Power"
 •  "Cheat Sheets: Unwritten Pump Rules"
•   "Cheat Sheets: The Affinity Laws"
FOR MORE INFORMATION & TO REGISTER FOR THE PUMP GUY SEMINAR, CLICK HERE.

 If you have any questions about the PUMP GUY SEMINAR or need help registering, please contact Matt Migliore at 610.828.1711 or Matt@GrandViewMedia.com.

  
     
   

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