On Mon, 3 Nov 2008 09:49:30 -0500, "MLD" wrote:


wrote in message
.. .
Well, I was looking at the DeWalt DPH38003800 model, which claims 4
GPM. It has a CAT 67DX, and the manual for that says 3.9 GPM. The
DeWalt manual says the water source should be at least 5 GPM. So it
basically wants a guarantee of always having more than 4 GPM coming
in. I sent an email to DeWalt support to see what they say, but I get
the impression the pumps really want plenty of water coming in.

What's missing in you comment is the pump inlet pressure requirements. You
have an infinite source of water (city supply) and it seems to me that as
long as you have a positive inlet supply pressure to the pump it will put
out the specified flow.

But as soon as your pump exceeds the GPM of your source you will lose
your inlet pressure. The source will become atmospheric or less.

You'll only lose flow output for one of the
following two reasons: if the inlet is restricted to the point that
cavitation occurs or if the output line line loss (restriction) results in a
pump discharge pressure high enough to activate the pressure relief valve.
There is no mistaken which of the two events is occurring---cavitation will
result in a pulsating or sporadic flow rate and most likely will be
accompanied by very loud noises from the pump. Cavitation, if allowed to
continue, will in many cases damage/destroy the pump. If you activate the
relief valve, the loss in flow can be any where from a small reduction to
almost nothing coming out of the system.
MLD

Regardless of relief valve activation your pump still has to pump a
specified amount. This amount happens to be 3.9 GPM. If the manual
states that a 3.9 GPM pump needs a minimum of 5 GPM at the source
then it is telling you that they do not want anything less than a
positive pressure at the inlet. In other words the pump should not
depend on its own drawing/vacuum capabilities. The pump should only
be run by being supercharged.