3 replies to this topic

[theleftcoast]

Hello everyone,

I am sizing a pump and ran into a question of how to apply the affinity laws and interpret their results. If a centrifugal pump is driven by an AC driver, then it should operate at a fixed rate (RPMs) dictated by the frequency of the supplied power. The maximum break horsepower (BHP) would then be determined using the driver's efficiency as well as the max current it can draw without damage. Assuming the impeller diameter and driver rpm are held constant, how would the break horsepower requirement change as the specific gravity of the fluid is varied? From my understanding of pump affinity laws, the relationship would look like this:

(BHP1/BHP2)=(SG1/SG2)

Assuming the driver is at steady state and drawing the max current it can, would an increase in specific gravity of the pumped fluid destroy the driver? For example, if the SG changed from 1 to 1.5, the motor would have to supply 50% more power than its maximum rating. I also understand that the flow rate and head of a centrifugal pump is independent of the specific gravity of the pumped fluid. This would mean that even though the specific gravity goes up, the flow will remain constant. Do these statements sound correct? Please correct me if I am wrong. Another question, how are the affinity laws derived?

Thanks in advance for the replies.

-JT

[Zauberberg]

Normally the pump will be protected from overamping and the motor should trip due to excessive current rather than to move into a self-destroying condition.

As Norm Lieberman says in his book, pumping heavier fluid requires more power simply like lifting a heavier stone from the ground requires more power from human's hands: (E = m*g*H) or, the potential energy of heavier fluid taken to the same height will be higher than the energy of lighter fluid at the same height, proportionally to the ratio of their specific gravities (actual densities).

As for the derivation look into a few pages from here (locate "Affinity laws" paragraph and read it through): http://books.google....epage&q&f=false

and also at the article attached below.

Attached Files

•  AFFINITY LAWS & SPECIFIC SPEED.pdf   193.44KB   52 downloads

[Art Montemayor]

Lefty:

For detailed explanations and examples on the theory and application of pumps (both centrifugal and PDs), go to: http://www.lightmypump.com/

Jacques Chaurette offers a free, downloadable book on the subject of pumps and his website also offers other interesting help and software to make your life easier. I highly recommend you go there and study all he has to offer - for free.

In fact, I recommend anyone reading this thread to go there if you are not familiar with Jacques and his background.

When you download his free book, go to Chapter 4 and you will find the derivation of the affinity laws as they apply to centrifugal pumps.

[theleftcoast]

Thanks for the replies. I found the lightmypump.com link particularly helpful and recommend that everyone read it. I also have Chemical Engineering Fluid Mechanics by Ron Darby and the resources given in lightmypump.com were great supplements to Darby's chapter "Pumps and Compressors".

-JT