Pressure Washer GPM

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.
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On Nov 1, 12:42 pm, snipped-for-privacy@nospam.com 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.
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On Sat, 1 Nov 2008 17:12:34 -0700 (PDT), snipped-for-privacy@columbus.rr.com wrote:

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?
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On Sat, 1 Nov 2008 17:12:34 -0700 (PDT), snipped-for-privacy@columbus.rr.com wrote:

Although power washer may be able to pull some vacuum they are designed to be force fed. Any restriction of the supply is not a good thing.
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On Nov 2, 9:36 am, snipped-for-privacy@mucks.net wrote:

I agree, but the better models handle it a bit better.
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On Nov 1, 10:42 am, snipped-for-privacy@nospam.com wrote:

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
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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
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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.

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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
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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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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
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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.
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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
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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.
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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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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 becomes adequate.

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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
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I agree.

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.

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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
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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 recommendation ?

Wrong
Many times

I have witnessed a cavitating pump pump many times.

Many times.

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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