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...
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.


A couple of comments--you said
"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."

If you are using your house supply than as I mentioned above--you have an
infinite supply of (city) water. You only lose inlet pressure as a function
of the inlet line loss. Assuming that the house inlet pressure is somewhere
in the order of 50 psi than the inlet line loss has to be greater than 50
before it will start to have any impact on pump performance. Most positive
displacement pumps can operate without any flow loss with inlet pressures
as low as 5-6 psi below atmosphere. So, again, as noted above: "as long as
you have a positive inlet supply pressure to the pump it will put
out the specified flow." As a side note: cavitation is caused when
entrained air comes out of solution resulting in a mixture of air and liquid
being drawn into the pump inlet. This happens when the pressure (due to
line loss etc.) falls below the vapor pressure of the fluid. Have you ever
heard of V/L ratio (vapor to liquid)? Depending on the application, V/L
capability is typically included as part of a pump requirement.
With respect to operating on the pump relief valve. True, the pump is still
pumping it's spec flow--but where is it going?? NO Where! Just about all the
pump flow is doing nothing but going around in a circle from output to input
via the relief valve and all it's accomplishing is to generate a lot of
heat---what little isn't going through the relief valve is getting out to
wash whatever you're washing. Since- flow in equals flow out- inlet line
loss and inlet pressure is of little consequence. If you are operating on a
relief valve then there is a significant restriction downstream of the pump
discharge.
MLD