Bob Darrah wrote:

My point was centrifugal pumps can be stalled by too much back
pressure and cavitate> Paul Lamar

Paul, I need an education here.  I thought  a free-flowing pump was more apt
to cavitate and cavitation could be stopped by increased back pressure.
Help me out here.

Bob Darrah

My calculus teacher taught me to to always look at the limits :-)
Obviously if you block the outlet of centrifugal pump the the 
blades will stall just like a turbo charger compressor stalls 
when it encounters too much restriction. The water just goes 
around and around in the pump. Technically this may not be called 
cavitation but calling it cavitation makes sense to me.

I think cavitation is usually thought of when the centrifugal 
pump has too much head to pump the water up from the supply.
In that case then a slight pressure increase on the outlet 
side MIGHT help... up to a point. At least Tracy makes this
claim on his artesian well water pump. 

If it occurs in a closed system such as a cooling system
I suspect an increase in overall system pressure would accomplish 
the same thing without reducing the flow such as would be provided
by an artificial restriction on the output side. In fact any additional
restriction anywhere in the system will worsen the cavitation
as it is closed loop system.

In Bill's case he is operating at atmospheric pressure so he can measure
the flow with a stop watch and a five gallon bucket so the system
pressure increase is not an option.

Paul Lamar
 
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