Submersible Pump Advice [RE-wrenches]

Windy Dankoff windydankoff at mac.com
Tue Dec 27 09:25:01 PST 2005


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Doug,

I never heard of a theoretical max efficiency for "any pump". There may 
be one for centrifugal pumps (as there is for turbines), but I don't 
see why there would be one for positive displacement pumps. If one 
could be free of leaks, with no friction, it would be 100% efficient 
(not counting the motor). Now for reality --

In well pumping, when you wish to pump at a lower rate than around 5-10 
GPM, you are in the range where the usual (multi-stage) centrifugal 
pump's efficiency drops way off. A positive displacement pump does not 
lose ground. Both Grundfos (SQ Flex only) and Conergy/Lorentz (formerly 
ETAPUMP) use helical rotors for these low flow ranges. They seal water 
in cavities and force it upward (positive displacement).

As you said, Grundfos SQF approaches 50% wire-to-water efficiency in 
parts of its range. ETAPUMP has a larger diameter motor that helps it 
to exceed 50% in parts of its range. The efficiency peaks don't 
coincide so you need to compare on a case-by-case basis. (These pumps 
can be made more efficient, but clearance must be tighter which makes 
them prone to hard starting and sticking under imperfect conditions.)

Grundfos SQ, and ALL traditional AC subs, offer only centrifugal 
mechanisms. Efficiency rarely exceeds 40%. In flow rates around 5 GPM 
and less, it gets really pathetic -- in the 20's and less. It's 
literally spinning its wheels. This is why low-power solar pumps so 
often use positive displacement mechanisms.

The helical rotor, introduced into mass-production in the year 2000, is 
a breakthrough for reliable low-power pumping. It has positive 
displacement without reciprocating parts, flapping valves, or 
super-critical seals, and is very durable.

Windy


Doug wrote:

> Hi Jay,
> I've got no experience with the SQ-series, so I can't compare. I do 
> know
> that the nice Grundfos sizing program you can get from distributors 
> will
> show a projected efficiency when you click on the pump curve chart. 
> With the
> helical pump heads they're hitting just under 50%, which, as best I
> understand, is about the theoretical maximum for any pump. (Correct me 
> if
> I'm misinformed guys...)

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