Pressure Washer GPM

Posted by ransley on November 2, 2008, 8:28 am

On Nov 1, 10:42=A0am, nos...@nospam.com wrote:

> I understand that GPM can be as important or more important than PSI
> for "getting work done". =A0So, I'd like as high a GPM as possible,
> obviously. =A0I'd be using the pressure washer for occasional home use
> and occasional semi-pro use for handyman/odd jobs.
> But I'm concerned about source water flow. =A0I'm on city water, but I
> just checked an inside line and an outside hose, and I'm getting 4 GPM
> max, maybe 3.5 GPM. =A0Does that definitely rule out a 4 GPM machine?
> I'm concerned about cavitation in the pump.

No that rating is the max the machine accepts, a 6 gpm machine works
usualy on 4 gpm for example, find minimums needed to run pump you want

Posted by MLD on November 2, 2008, 10:05 am

>I understand that GPM can be as important or more important than PSI
> for "getting work done". So, I'd like as high a GPM as possible,
> obviously. I'd be using the pressure washer for occasional home use
> and occasional semi-pro use for handyman/odd jobs.
> But I'm concerned about source water flow. I'm on city water, but I
> just checked an inside line and an outside hose, and I'm getting 4 GPM
> max, maybe 3.5 GPM. Does that definitely rule out a 4 GPM machine?
> I'm concerned about cavitation in the pump.

The chacteristics of a centrifugal pump (pressure washer) is pressure rise
(Delta P) vs Flow at constant speed.
The output of the pump is a function of the system line loss. As the
restriction in line increases---length of hose, diameter of hose, valves
etc. there will be a resulting loss of flow (vs max pump capability)--small
at first but can be significant at higher line losses. As long as the inlet
to the pump is not severely restricted the inlet pressure will not get low
enough to cause any problem for the pump (cavitation). Most centrifugal
pumps can work OK with inlet pressures well below atmospheric. Checking
flow from a faucet or outside hose is not the same a what will come out of
the pressure washer pump. What that tells you, (knowing the inlet pressure)
is Delta P and flow. From that you can determine the resistance (line loss
vs flow) of the house piping.
MLD

Posted by on November 2, 2008, 10:39 am

>>I understand that GPM can be as important or more important than PSI
>> for "getting work done". So, I'd like as high a GPM as possible,
>> obviously. I'd be using the pressure washer for occasional home use
>> and occasional semi-pro use for handyman/odd jobs.
>> But I'm concerned about source water flow. I'm on city water, but I
>> just checked an inside line and an outside hose, and I'm getting 4 GPM
>> max, maybe 3.5 GPM. Does that definitely rule out a 4 GPM machine?
>> I'm concerned about cavitation in the pump.

>The chacteristics of a centrifugal pump (pressure washer) is pressure rise
>(Delta P) vs Flow at constant speed.

A pressure washer is not a centrifugal pump. The ability to
create such high pressures can only be accomplished by a
positive displacement pump. All pressure washers that I am
aware of are piston pumps.

>The output of the pump is a function of the system line loss. As the
>restriction in line increases---length of hose, diameter of hose, valves
>etc. there will be a resulting loss of flow (vs max pump capability)--small
>at first but can be significant at higher line losses. As long as the inlet
>to the pump is not severely restricted the inlet pressure will not get low
>enough to cause any problem for the pump (cavitation). Most centrifugal
>pumps can work OK with inlet pressures well below atmospheric. Checking
>flow from a faucet or outside hose is not the same a what will come out of
>the pressure washer pump. What that tells you, (knowing the inlet pressure)
>is Delta P and flow. From that you can determine the resistance (line loss
>vs flow) of the house piping.
>MLD

Posted by MLD on November 2, 2008, 3:02 pm

>>>I understand that GPM can be as important or more important than PSI
>>> for "getting work done". So, I'd like as high a GPM as possible,
>>> obviously. I'd be using the pressure washer for occasional home use
>>> and occasional semi-pro use for handyman/odd jobs.
>>> But I'm concerned about source water flow. I'm on city water, but I
>>> just checked an inside line and an outside hose, and I'm getting 4 GPM
>>> max, maybe 3.5 GPM. Does that definitely rule out a 4 GPM machine?
>>> I'm concerned about cavitation in the pump.

>>The chacteristics of a centrifugal pump (pressure washer) is pressure rise
>>(Delta P) vs Flow at constant speed.

> A pressure washer is not a centrifugal pump. The ability to
> create such high pressures can only be accomplished by a
> positive displacement pump. All pressure washers that I am
> aware of are piston pumps.
SNIP>

In reflection, you're correct--the washers do have positive displacement
pumps. That being the case, as long as the downstream restriction doesn't
cause the pump relief valve to open the flow output should be very close to
pump spec. This is not to say, however, that centrifugal pumps are not
capable of generating high pressures.
MLD

Posted by on November 3, 2008, 12:10 am

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.

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Pressure Washer GPM - Page 2