Pressure Washer GPM

Posted by on November 4, 2008, 2:47 pm

>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.

The manufacturer makes suggestion so that your pressure washer will
not be run in a way that may damage it. They want pressure at the
inlet. They know that their pressure washer isn't the only user of
your water supply. Showers, washing machines ect. will further reduce
the free flow rate.

>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".

Example: If the pump can create 5GPM of flow and you have the
washer in rinse so that the relief valve is not activated then your
4GPM supply is not adequate for the rinsing stage. If the same 5GPM
pump is dumping most of the volume over the relief as it is when you
are power washing then you are only drawing about 1GPM from a 4GPM
supply so that during this power washing stage the supply suddenly
becomes 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

Posted by MLD on November 4, 2008, 5:15 pm

>>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.

> The manufacturer makes suggestion so that your pressure washer will
> not be run in a way that may damage it. They want pressure at the
> inlet. They know that their pressure washer isn't the only user of
> your water supply. Showers, washing machines ect. will further reduce
> the free flow rate.

>>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".

> Example: If the pump can create 5GPM of flow and you have the
> washer in rinse so that the relief valve is not activated then your
> 4GPM supply is not adequate for the rinsing stage. If the same 5GPM
> pump is dumping most of the volume over the relief as it is when you
> are power washing then you are only drawing about 1GPM from a 4GPM
> supply so that during this power washing stage the supply suddenly
> becomes adequate.
<SNIP>

You have a funny way of looking at things--Let me change your example around
just a bit. You have a free flow of 5 GPM and a pump that can put out 4
GPM. No problem it works just fine. Now increase the discharge resistance
such that the pump operates on its relief valve. The flow out of the pump
will drop to almost zero or slightly above zero.
In your example the pump will most certainly be in cavitation when not on
the relief valve, producing no flow, but when on the RV produces an output
of about 1 GPM. From your point of view, this suddenly qualifies as adequate
supply. I guess 1 GPM is better than nothing.
Why in cavitation--assuming a 50 psi supply pressure and a free flow of 4
GPM--- AT 5 GPM (pump demand) the inlet line pressure drop would be
approximately 75 psi (25% increase in flow requires 50% increase in pressure
drop). The pump will not work, it'll just screech itself to death.
I have a power washer, as you must also have, and it works just fine, so
obviously, my house supply more than meets the flow requirements of the
washer.
MLD

Posted by on November 4, 2008, 8:33 pm

>> Example: If the pump can create 5GPM of flow and you have the
>> washer in rinse so that the relief valve is not activated then your
>> 4GPM supply is not adequate for the rinsing stage. If the same 5GPM
>> pump is dumping most of the volume over the relief as it is when you
>> are power washing then you are only drawing about 1GPM from a 4GPM
>> supply so that during this power washing stage the supply suddenly
>> becomes adequate.

>You have a funny way of looking at things--Let me change your example around
>just a bit. You have a free flow of 5 GPM and a pump that can put out 4
>GPM. No problem it works just fine. Now increase the discharge resistance
>such that the pump operates on its relief valve. The flow out of the pump
>will drop to almost zero or slightly above zero.

I agree.

> In your example the pump will most certainly be in cavitation when not on
>the relief valve, producing no flow

I disagree.The pump will cavitate but will still pump water.

>, but when on the RV produces an output
>of about 1 GPM. From your point of view, this suddenly qualifies as adequate
>supply. I guess 1 GPM is better than nothing.

The 1GPM is not the adequate supply. When the power washer outputs
just 1GPM then the 4GPM supply is adequate during this time period.

>Why in cavitation--assuming a 50 psi supply pressure and a free flow of 4
>GPM--- AT 5 GPM (pump demand) the inlet line pressure drop would be
>approximately 75 psi (25% increase in flow requires 50% increase in pressure
>drop).

It doesn't work that way. When the water is flowing into the pump you
do not have 50psi. Furthermore you can't subtract 50psi by75psi. That
would give you a negative 25psi. There is no such thing.

The only force that puts water into the inlet of the pump is the
atmospheric pressure. At sea level you have about 14.7 psi maximum
to work with but this number will never be nullified by any consumer
power washer pump. At best you may be able to reduce the 14.7 psi to
9 or 10 so that you would have an effective atmospheric force of 5psi
that would create flow into the pump.

Note: The above method of creating flow is not the recommended way
for power washers.

> The pump will not work, it'll just screech itself to death.
>I have a power washer, as you must also have, and it works just fine, so
>obviously, my house supply more than meets the flow requirements of the
>washer.
>MLD

Even when a pump is screeching (cavitating) it will still pump.

Posted by MLD on November 5, 2008, 2:12 pm

>>GPM--- AT 5 GPM (pump demand) the inlet line pressure drop would be
>>approximately 75 psi (25% increase in flow requires 50% increase in
>>pressure
>>drop).

> It doesn't work that way. When the water is flowing into the pump you
> do not have 50psi. Furthermore you can't subtract 50psi by75psi. That
> would give you a negative 25psi. There is no such thing.
> The only force that puts water into the inlet of the pump is the
> atmospheric pressure. At sea level you have about 14.7 psi maximum
> to work with but this number will never be nullified by any consumer
> power washer pump. At best you may be able to reduce the 14.7 psi to
> 9 or 10 so that you would have an effective atmospheric force of 5psi
> that would create flow into the pump.
> Note: The above method of creating flow is not the recommended way
> for power washers.

>> The pump will not work, it'll just screech itself to death.
>>I have a power washer, as you must also have, and it works just fine, so
>>obviously, my house supply more than meets the flow requirements of the
>>washer.
>>MLD

> Even when a pump is screeching (cavitating) it will still pump.

Obviously, we're well beyond the discussion of a pressure washer. With your
most recent answer, you finally got in over your head (no disrespect
intended).
Where to begin-"The only force that puts water into the inlet of the pump is
the atmospheric pressure".
This statement makes absolutely no sense. The pump inlet is City Supply
pressure (approx 50) minus any line loss. If the line loss exceeds the
City supply pressure (in this case more than 50 psi) than the pump inlet
pressure will go negative. Positive displacement pumps will operate at a
negative pressure as they produce a suction effect (like drawing fluid up a
straw). The capability to operated at negative inlet pressures, that is,
below atmospheric, is limited to the pump's capability to do so. As I
mentioned in a previous post, most pumps, at least those that must meet
military specs, will function at Vapor/Liquid ratios of 0.45. That means
that the pump can handle a mixture of 55% fluid and 45% air without
significant cavitation. This will usually occur when the inlet pressure is
at or near 10 psi below atmosphere (not too sure of the exact value).
"Even when a pump is screeching (cavitating) it will still pump." Don't know
what your definition of "it will still pump" is but the screeching noise you
hear is the pump on it's way to destroying itself---scary to be near one in
that mode.
It's obvious you have limited, if any meaningful, experience in this area.
Have you ever been around a pump that was operating under severe cavitating
conditions? Have you ever measured, witnessed the wild flow and discharge
pressure pulsations when a pump was under such severe distress? Have you
ever seen the adverse effect a cavitating pump had on a system that was
receiving it's flow and pressure? All academic questions----that is unless
you care to answer them.
MLD

Posted by on November 5, 2008, 3:38 pm

>Obviously, we're well beyond the discussion of a pressure washer. With your
>most recent answer, you finally got in over your head (no disrespect
>intended).
>Where to begin-"The only force that puts water into the inlet of the pump is
>the atmospheric pressure".

This is over your head. The topic was if you could not supply the
volume that the pump pumps. At that point atmospheric pressure is
all you have to influence flow into the pump.

>This statement makes absolutely no sense. The pump inlet is City Supply
>pressure (approx 50) minus any line loss. If the line loss exceeds the
>City supply pressure (in this case more than 50 psi) than the pump inlet
>pressure will go negative.

Ok...and then what influences flow into the pump.? Answer: atmospheric
pressure. (pumps do not suck! they can only reduce the atmospheric
pressure at the inlet)

>Positive displacement pumps will operate at a
>negative pressure as they produce a suction effect (like drawing fluid up a
>straw). The capability to operated at negative inlet pressures, that is,
>below atmospheric, is limited to the pump's capability to do so. As I
>mentioned in a previous post, most pumps, at least those that must meet
>military specs, will function at Vapor/Liquid ratios of 0.45. That means
>that the pump can handle a mixture of 55% fluid and 45% air without
>significant cavitation. This will usually occur when the inlet pressure is
>at or near 10 psi below atmosphere (not too sure of the exact value).
>"Even when a pump is screeching (cavitating) it will still pump." Don't know
>what your definition of "it will still pump" is but the screeching noise you
>hear is the pump on it's way to destroying itself---scary to be near one in
>that mode.

I don't disagree with your generic statement but the manufacturer
suggest to not operate their pump in this manner, so why are you
using the explanation above? Is it to nullify the manufacturers
recommendation ?

> It's obvious you have limited, if any meaningful, experience in this area.

Wrong

> Have you ever been around a pump that was operating under severe cavitating
>conditions?

Many times

>Have you ever measured, witnessed the wild flow and discharge
>pressure pulsations when a pump was under such severe distress?

I have witnessed a cavitating pump pump many times.

>Have you
>ever seen the adverse effect a cavitating pump had on a system that was
>receiving it's flow and pressure?

Many times.

>All academic questions----that is unless
>you care to answer them.
>MLD

I've been doing this for over thirty years. I've answered your
questions.

*****************************
Time for me to question you.

If you are so sure of your explanation and or theory of pumps why
would you say that a power washer is a centrifugal pump?

Have you ever rebuilt pumps?

If so, how many?

If you didn't know the difference between a centrifugal pump and a
plunger/piston pump what kind of success rate do you have upon
rebuilding a pump that you have trouble identifying?

What type of pumps have you rebuilt and what were their applications?

What devices are used to seal various areas of a pump? Name the
devices and the areas of concern on different types of pumps.

What makes you more of an expert on pumps than CAT?

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Pressure Washer GPM - Page 4