Horsepower

T

the_GUNN_man

New member
I had a question about horsepower. I have read that you take gpm times psi and divide by 1100 (for gas engines) and that gives you what horsepower you need. My qusetion is if whatever formula you use to come up with the horsepower you need is more then what you have what suffers? For example say I have a 6 gpm/3000 psi pump. According to the formula I need about 16.3 hp. What if I only have 13 hp. What would happen? Do I lose gpm, psi, or both?
 
It all depends.

When using a vcariable lance wand, it is obvious that the machine is not working nearly as hard under low pressure as it is under high pressure. The volume does not change, but the pressure output does.
If you are using a trash pump, it is usually a little 5 horse motor for 200 gallons a minute, but it does not technically have any pressure output.
I also know that you can get the maximum rated pressure from a lower horsepower motor, but it is not necessarily good for the motor. It causes excessive strain on the motor, and shortens the motors life cycle.
So, if you want to run a 6 GPM pump at 3000 psi, My guess is that in reality you will not be able to achieve both numbers at the same time. You will probably get aboiut 4.5 GPM and 3000 PSI, or 6 gpm and 2300 PSI. Make your choice.

Scott
 
Theoretically what Scott is saying is true for basic PSI/HP/GPM calculations.

Other factors that enter into this are:
- Pump efficiencies
- Motor torque curve (higher RPM = lower torque vs. lower RPM = higher torque).
- Belt drive or direct drive so motor can be run at a slower RPM to obtain higher torque to run larger pump capacity and designed PSI.

Each motor design has it's own specific torque curve. So if the highest torque level is used at a specific RPM (with a belt drive), the pump can be matched to that RPM and you could still obtain 6 GPM's/3000 PSI with a 13HP motor.

System designs are important to understand when you are interchanging components. Some motors can fail prematurely if
pumps/RPM's are not correctly matched to the torque curve.

I hope I explained clearly.

Regards,
 
I have a 12 yr old Landa Belt Driven Hot water Skid rated at 3000psi at 6gpm's. THis PW has a blowned 24 Hp Onan engine. I have another 3000psi@4gpm's direct drive PW with an 18hp Honda engine.

I keep thinking about using this 18hp Honda and putting it on the above hotwater Skid. I could save $$$$ by doing this because a new 24hp Onan cost at least $1500.

And now Paul B. talks about a torque curve. Maybe I should just sell this PW which is about 5 yrs old and buy an Onan Engine and take the $800 or so hit. Hmmmmm.:confused:
 
John,

The 18 HP Honda should do fine, I am assuming your old Landa has a
General TS2021 which will deliver a maximum of 5.6 GPM with a pressure fed
inlet, or around 5.0- 5.2 GPM pulling from your float tank.

18 HP is adequate to deliver 5 GPM @ 3000 PSI and pull a 2kw generator.

:) If you under power a pump you will lose pressure not volume (gpm)
providing you are turning the pump at its rated rpm.

You can find some handy charts a formulas here for nozzle sizes, pump rpm etc.
Handy Formulas & Charts From AR North America
 
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Thank you Mark. That is what I was wondering. I was wondering if you under powerd a pump would you lose gpm, psi, or both. You have answered my question. I have a 5.5 gpm 4000 psi pump. In order to get the max out of it I needed 20 hp. I don't want to get 4000 psi out of it. I figured if I got 2600 psi out of it and ran it off of a 13 hp engine it would be ok.
 
The formula GPM x PSI / 1100 is a general formula for basic sizing.
To get optimum performance/long life, you need to know what is the best RPM to run your motor, but you also need to know how much torque is needed to achieve best results. Knowing where your equipment performs on that torque curve can tell you about the design, efficiency and longevity.

Every manufacturer has safety factors and they vary, depending on the design of the components used (it can be 20% or as much as 50%).

;) If you are running under-power (for the rated capabilities) will your motor RPM drop?
If it doesn't drop you're not under-powering, you may just not be running very efficiently for your motor (you may be at or close to the limits). This would mean shorter motor life, use more fuel, etc.
If your motor RPM drops, so will your pump RPM and GPM's will also drop along with your PSI.

To me knowing about torque and the torque curve is more important than just knowing the HP. Knowing the torque allows me to know what performance I can expect from the equipment.
I can always change pulley combinations on a belt drive to maintain my pump speed but use a lower motor RPM which will also give me higher torque.

That's why I was trying to say that you have to understand your whole system not just one component of it.
I understand not everyone cares or understands, that's why they have general formulas.

I guess I didn't explain it real well initially. Sorry.

Regards,
 
While on the surface it seems really simple to put a pump and engine/motor together to arrive at your own pressure washer. It is not!

As this thread shows quite well there are many other factors to look at.

One example is the performance of a long versus short stroke belt drive pump. They will not perform equally on the same power source because of the different load characteristics of the pump. A TS pump may work where a T will not; same rating for flow and pressure but at different pump RPM.

Using the (Flow X Pressure)/1100 is at best an engineering THEORETICAL value derived from every component working EXACTLY to specification.

A more effective 'Rule of Thumb' is to use the (Flow X Pressure)/1460 = Electric Brake Horsepower (a more reliable number) then take the result and multiply by 1.5 to arrive at gasoline engine horsepower.

From there experience and R&D lead to the correct component selection.

By the way an engine supplies power from an internal fuel source; a motor derives it's power from an external one.

Please search for my post about 'Rules of Thumb' as it has explanations of pressure and flow and their differences and effects.

Hope this helps? Cheers.
 
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I know this is a lot more info than most people care about, but if anyone cares, read it all. If you only want the bottom line, then read only the last 3 (short) paragraphs:

Torque vs. Horsepower.
The word horsepower (as it relates to machines), was defined by James Watt, the inventor of the steam engine in about 1775. Watt learned that "a strong horse could lift 150 pounds a height of 220 feet in 1 minute." (A horse exerting one horsepower could raise 220 pounds of coal one foot every minute.)

One horsepower is also expressed as 550 pounds one foot in one second at 33,000 pounds one foot in one minute. (These are just different ways of saying the same thing.)
Another way horsepower could be directly measured would be to have a horse hitched to a plow. In the hitch is a spring scale (like a fish scale). The horse pulls the plow one foot every second and you see 550 pounds on the scale. That would be one horsepower.
So horsepower can be directly measured.

However, there is a problem directly measuring horsepower of engines because they produce rotary motion not linear motion. Unless the engine is geared down, the speed at which they do work (RPM) is too great for practical direct measurement of horsepower. The solution was to directly measure torque (rotational force expressed in pounds at one foot radius) and RPM (time and distance, i.e. distance in circumference at the one foot radius). From these we calculate horsepower. Torque and RPM are easily measured directly. Early dynamometers used a brake device to load the engine. A torque arm was attached to this brake's stator. The brake's rotor was coupled to the engine's crankshaft. A spring scale or other measuring device connected the torque arm to the stationary fixture holding the engine and brake. During a test the brake's application loaded the engine. Torque and engine rpm were observed and recorded.
On a modern day dynamometer horsepower is a calculated value. Remember, the dyno measures torque and rpm and then from this is calculate horsepower. On the dyno it takes more water flow to the water brake to increase the load on the engine being tested. As the test engine's torque rises more water flow is needed. As the test engine's torque drops less water flow is needed. The dyno's water brake does not respond to horsepower. Major adjustments to water flow are needed as an engine crosses its torque peak but none are needed as it crosses its horsepower peak. In other words the water flow to the brake during a dyno test follows the engines torque curve and not its horsepower curve. Torque is what twists the tire, prop, or pump. Horsepower helps us understand an amount or quantity of torque. (Torque + time).

(Bear with me, I'm almost done).

Torque is the only thing that a driver feels in a car, and horsepower is just sort of an esoteric measurement in that context. 300 foot pounds of torque will accelerate you just as hard at 2000 rpm as it would if you were making that torque at 4000 rpm in the same gear, yet, per the formula, the horsepower would be "DOUBLE" at 4000 rpm. Therefore, horsepower isn't particularly meaningful from a driver's perspective, and the two numbers only get friendly at 5252 rpm.

So then HP = Torque * RPM / 5252

If you can generate 40 lb/ft torque at 4000RPM that's 30 HP.
However, if you can generate 40 lb/ft torque at 2000RPM, then it's 15 HP.

The slower your engine RPM has to run, the longer your engine will last (if all else is equal). To me efficiency and longevity is why torque is more important than HP. Why should I buy a 30HP motor when a 15HP will do the job and last a lot longer.

Regards,
 
Between Mark, Micheal and Paul B., You guys can teach one hell of a class at the PWNA Convention. I used to race cars with my Dad years ago and you guys remind me of my dad when it comes to engine Intellect.

Give me a car and I'll Kick your ass or die trying(Before Kids) but I'll leave the Intellect to the smart guys.:)
 
I tinkered with a few cars in the days when you could see the ground around your engine.

I scraped a lot of knuckles living next door to two brothers that were into drag racing. Then my ex-father in-law taught me a lot also - he was involved with race cars (roundy-rounds) and speed boats. He built a motor home out of a Greyhound bus, that we used to go to races in. The good old days.

Now I go with the low-ballers for my oil changes.

Regards,
 
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