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.
On Nov 1, 12:42 pm, firstname.lastname@example.org wrote:
I doubt if you will have a problem. First the numbers listed are
usually .... shall we say optimistic. Second the power washer will
be able to "pull" some additional water than the water flow you
measured. In theory it can pull a 32 foot column of water before it
quits, in practice it is less, but still significant.
On Sat, 1 Nov 2008 17:12:34 -0700 (PDT), email@example.com
I understand your point, but most of the ones with high GPM are fairly
expensive with quality pumps, so I'd tend to accept their numbers
more. Did not know about "pulling" water though. What does a 32 foot
column of water mean exactly?
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.
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.
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.
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.
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.
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
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.
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
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
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.
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
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
I disagree.The pump will cavitate but will still pump water.
The 1GPM is not the adequate supply. When the power washer outputs
just 1GPM then the 4GPM supply is adequate during this time period.
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.
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
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
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.
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.
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)
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
I have witnessed a cavitating pump pump many times.
I've been doing this for over thirty years. I've answered your
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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