Tracy Crook wrote:

It is true that a pump does not "need" to be designed to have back pressure.
BUT, if it  *is* designed to work with back pressure (and there are lots of
reasons to do so) then operating the pump without that required back
pressure will result in cavitation, less flow, and pump damage.  This was
shown quite clearly on some of the information previously posted.    Anyone
who has worked on their own well pump is familiar with the "churning gravel"
sound of a cavitating pump when the pressure is too low.   If the water pump
output is visible (like in my lawn sprinkler system) the loss of flow is
directly visible.

I'm amazed Jeff.  Did you really mean that statement as unconditional?

Tracy Crook
rws@altavista.net
www.rotaryaviation.com

I think you are confusing the lack of supply with back pressure. If you
restrict the output then the input from the well can keep up.

Cavitation is probably a non-linear effect with hysteresis(s?).
In other words once it starts if you slow the pump or the output
it will then stop cavitating and the flow or RPM can be increased 
again. Probably a lot like detonation in an ICE.

In our closed loop radiator system, input or supply and output are always 
the same so restriction (back pressure) means less water flow velocity.
Look at the limits. If the input of the pump is directly connected
to the output, the flow velocity will be at a maximum. If the output
is blocked and the input is blocked the pump will cavitate.
It will also cavitate if the output or input alone is blocked. 

If you restrict the output from the block, yes you will increase slightly
the pressure in the block compared to the pressure in the rest of
the system. This will reduce the flow velocity. The same block pressure can be 
achieved with out reducing the flow velocity by increasing the overall 
system pressure with a higher pressure rad cap.

Paul Lamar
 
The Aircraft Rotary Engine Newsletter.              Powered by Linux.

http://home.earthlink.net/~rotaryeng/            http://www.linux.org