how does pump efficiency curve change with pump age? 1

[mbaker]

I'm working on recommendations for an upgrade to controls for an unusual water distribution system (but not because of the pumps).

Part of the system includes six vertical turbine pumps, nominally 1000 gpm w/ 100 hp motors. There are two different types having two different book curves. The pumps were installed in 1963, 1974, and one in 1989.

When tracking down the curves for the 1974 pumps, the manufacturer's rep cautioned me against using the "book curve" . When doing simulations on the system, I had to seriously increase the minor losses in the suction lines to get a calibration.

As someone will probably want the new control system to help the pumps operate efficiently, I am wondering if re-built pumps become less efficient with age. I've seen the threads on testing for head vs. flow but am not sure if it is reasonable or feasible to test for efficiency.

Thanks

Mary

 

[macm]

the quick answer is yes. there will be a loss in efficency as the internal components wear, and clearances increase, allowing by-pass or recirculation. The wear rate will depend on the amount and type of solids in the liquid (presumably water). Extreme cases occur in the mining industry, pumping slurry, where impellers can wear out in a matter of a few weeks. With process water, it usualy takes longer. If you have had these pumps in for years without too much repair history, then the wear may not be a problem.
mac

 

[raj2001]

answer to your post is "cannot say". the efficiency of the pump - of course cannot be improved beyond the design point, but you cannot judge from its' age. it is how the pump was operated and maintained during its' life that matters the most. a review of its' history will give the idea. even if it was not operating at the best efficiency point it can be made to do so even now. it is not the life that matters.do not draw conclusions that if the pump is old it would have shifted towards the lower side of its' efficiency curve.

all the best.

raj

 

[mbaker]

Raj:

Thanks for the answer. But how do I construct a "true" efficiency curve? I can measure hydraulic horsepower, and can get amp readings. How do you measure the actual motor horsepower?

Mary

 

[TD2K]

A torque meter will give you the actual motor output. Torque, along with rpm, gives you Hp.

 

[PacificSteve]

Simple answer to your question: If you know Pressure in, Pressure out, GPM (not from pump curves, from ultrasonic or flow meter in pipe), and kW in, you can then calculate the Wire to Water efficiency. Use a true watt meter such as AEMC 3910. Use new gauges for the pressures, consider that you may have a vacuum depending on system. Check this link for some simple formulas to get you the rest of the way their.

http://www.zoeller.com/Zep/Techbrief/JF1article.htm

There is a rule of thumb that says 5% to 10% the cost of a pump is in the intial purchase price, the other 90% to 95% is in the lifetime energy use. On pumps this large, a full-on energy analysis, possibly including replacement with optimal new pumps is quite feasible. These may not be PE or EE motors, but probably should be per economics. Perhaps beyond the scope of your obligations though.

Pumps generally do better than their pump curves...no one wants to get caught short and get sued. Pump curves are rather dangerous in my book. For instance, how do you know for sure what size impellers are in those pumps? One test is to deadhead the pump and compare pressures to the curves for different size impellers to zero in on the one you have. Find someone with a good ultrasonic meter, rent it. As some else stated, recirculation problems can create some very unexpected results that could leave lots of egg on your face if you rely only on pump curves.

Hope this is some help, Pacific Steve

 

[tstead]

You *can* increase the efficiency of a pump above it's original design point. A simple way of doing this is polishing & backfiling the impeller.

Of course, if the original impeller was polished & backfiled, then....no dice.

Further, polishing and backfiling only temporarily increases the efficiency becuase as the impeller wears, the vanes are not as sharp and smooth.

Tim