Pressure Washer GPM

Posted by MLD on November 3, 2008, 9:49 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.

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. 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

Posted by on November 3, 2008, 3:01 pm

>> 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.

Posted by MLD on November 4, 2008, 9:19 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.

>>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

Posted by on November 4, 2008, 10:51 am

>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

If you put a pressure gauge one foot away from the end of a
water supply that is plugged you will see 50 psi. Gradually open
a gate valve to create flow and what was once 50psi will start to
drop immediately. Once the flow is completely unrestricted you will
see 0psi on the same gauge. At that time you will have your
maximum free flow rate.

If you put a pump inlet at that same water supply, and that pump can
develop a vacuum, you should be able to draw slightly more flow, but
your gauge will be below atmospheric if this occurs.

The pump manufacturer suggest to have a free flow supply of 5gpm
for their 3.9GPM pump. This means they don't want negative pressures
at their pumps inlet. Cavitation can result. Some pumps have ceramic
plungers that wear great but are easy targets for cavitation damage.

As you have stated when the relief valve is activated the actual
flow at the inlet is reduced so that a less than adequate supply may
suddenly become adequate, however is your relief valve always
activated? It is not, and hence the manufacturers recommendations.

Posted by MLD on November 4, 2008, 2:21 pm

> If you put a pressure gauge one foot away from the end of a
> water supply that is plugged you will see 50 psi. Gradually open
> a gate valve to create flow and what was once 50psi will start to
> drop immediately. Once the flow is completely unrestricted you will
> see 0psi on the same gauge. At that time you will have your
> maximum free flow rate.
> If you put a pump inlet at that same water supply, and that pump can
> develop a vacuum, you should be able to draw slightly more flow, but
> your gauge will be below atmospheric if this occurs.
> The pump manufacturer suggest to have a free flow supply of 5gpm
> for their 3.9GPM pump. This means they don't want negative pressures
> at their pumps inlet. Cavitation can result. Some pumps have ceramic
> plungers that wear great but are easy targets for cavitation damage.
> As you have stated when the relief valve is activated the actual
> flow at the inlet is reduced so that a less than adequate supply may
> suddenly become adequate, however is your relief valve always
> activated? It is not, and hence the manufacturers recommendations.

So what are you disagreeing with?? Pump manufacturers don't want to run
their pumps under conditions that will result in cavitation-very bad, will
ruin a pump. In most cases they will specify the inlet conditions (pressure
& temperature) under which they (pumps) should be operated. Specifying a
free flow supply is just a loose way of defining the pump inlet
requirements.
Don't understand your comment-"when the relief valve is activated the actual
flow at the inlet is reduced so that a less than adequate supply may
suddenly become adequate".
When you are on the relief valve just about all the flow is being
recalculated via the valve. Almost nothing gets out so in the end, an
adequate supply drops to basically no supply. In answer to your
question--No, relief valves are not normally activated but be glad when they
do--dead ending a positive displacement pump, without relief, will destroy
it.
MLD

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Pressure Washer GPM - Page 3