Re: [APD] Re: Closing valves on pumps -- or - Putting the squeeze on water

["S. Hieber" <shieber at yahoo_com>]

Most of the electrical energy going into a pump if shed as
heat. Water is usually used to provide some or all of the
cooling. The water is usually used also to provide
lubrication for the moving part of the pump.

If the pump is centrifugal (has an impeller rather than a
piston or diaphragm), then you can restrict the output up
to 100% and all that happens is the impeller spins arounds
in the water trapped in the pump without moving any of it
out of the pump. Since it's jsut spinning in the water and
not moving much or any water through the pump, the motor
actuallys ees less resistance to turning and uses less
electricity. It might seem unintuitive but restricting the
flow of  water (on the poutput) ouput on a centrifugal pump
eases resistance for the motor and saves energy, at least
up to a point. However, one can go too far -- some (most)
external pumps rely on the waterflow for cooling the mump
and the manufacturer will usually warn you what minimum
flow is necessary to provide adequate cooling. Time is a
factor also so the warning should be something like, not
less than 40% continuously or not less than 20% and for
period no longer than xxx minutes. It's generally not an
issue with internal pumps sitting in a large tank of water
since the water provides adequate cooling. 

If you restrict the *input*, at some point the impeller
will cavitate -- start generating bubbles from the water as
it tries to keep pushing water out without enough new water
coming in. The gases in the impeller chamber then reducing
the coolant and lubricant effect of the water and the
impeller will wear much faster than normal. You can hear
when cavitation is occurring -- it's noisy. 

Piston and diaphragm pumps (positive displacement pumps)
are best controlled by controlling the motor speed or
setting up a bypass and controlling the amount of water
going through the bypass. It doesn't save any energy, but
if you need reduced flow in your system, it saves the pump.

Scott H.
> Laith Arif wrote:
> 
> >Or have you put some type of on/off valve on the hose? 
> If so, blocking the
> >flow of water with a valve doesn't harm the pump?
> >  


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