Someone just had to ask.

[fluidpower1]

"The energy transfer is in proportion to flow * pressure, the ratio of the two being dictated by system resistance."

So 10 GPM * 1,000 PSI = 10,000 WHAT??????

[TWControls]

Maybe I'm thinking to simplistically for this conversation, but doesn't...

Fluid Power Horsepower (hp) = Pressure (psi) x pump flow (gpm) / 1,714

[CroCop]

Originally Posted by fluidpower1

"The energy transfer is in proportion to flow * pressure, the ratio of the two being dictated by system resistance."

So 10 GPM * 1,000 PSI = 10,000 WHAT??????

10,000 / 1714 Horsepower

[Peter Nachtwey]

[quote=fluidpower1]
So, Inreasing Pressure Drop across an ORIFCE will INCREASE FLOW and INCREASED FLOW will make aCylinder move faster.
[quote]
The actuator doesn't move in this case. The volume is fixed.

However, It was not the INCREASED PRESSURE that made the cylinder go faster it was the fact that the INCREASED PRESSURE forced more FLUID through the RESTRICTION and that INCREASED AMOUNT OF FLUID FLOW made the cylinder go faster.

You are looking at it backwards. Force makes it go. Flow just equalizes pressure.

Seems so simple to this MATH CHALLENGED individual. I still strongly believe "Flow Makes it GO" is not just a Ditty to repeat over and over but a proven fact.

This is why there are so many screwed up hydraulic servo systems.

Tom, you need to see Newton's law of motion. He didn't say anything about flow. Flow just attempts to equalizes pressure differences.

Alaric is right.
TWControls has the equation for calculating the required HP correct and it backs up what Alaric said. No pressure no power.

However,

The energy transfer is in proportion to flow * pressure, the ratio of the two being dictated by system resistance.

Only until the limits of the power capacity of the pump. If load or resistance is infinite ( doesn't move ) then what happens?

[fluidpower1]

TWControls;

I missed the /1714 in your first post. The HP formula you give is a good one.

[ndzied1]

Originally Posted by Peter Nachtwey

If load or resistance is infinite ( doesn't move ) then what happens?

Note: pay no attention to this post.

The oil just gets compressed... Better look up that natural log equation from the old thread...don't want to be creating any localized black holes from compressing things infinitely... Leave that to the folks at CERN.

This sort of reminds me of the classic electrical engineering debate about whether electrons move from positive to negative or whether "electron holes" move from negative to positive...

At the basic science level I have to say Peter is right. There is F=m * a and it's pretty clear.

But living with a teacher, you learn that not everyone thinks the same. I believe people come up with their own way to make sense of all these phenomenon. So, while not strictly scientific, they can think in their head, flow equalizes pressure so there is no flow without pressure and they are linked...ergo we're really talking about the same thing. (Sort of chicken and egg here).

Or, we could go back further... What turns the pump? An electric motor. What makes the motor turn? An electric current (hmmm a flow of electrons). What made the electrons flow? A voltage drop. And back to the same argument... Or further still... what made the electricity? A generator. What turned the generator? Steam... was it the pressure of the steam or the flow of steam? Same Argument. What made the steam? heat. What made the heat? Fire. What made the fire? A chemical reaction.

Now you know why or don't know why you should have paid attention when I said to pay no attention to this post.

[kamenges]

So is it such a bad thing to believe the Earth is the center of the universe or that the Earth is flat? In normal, every day events it really doesn't matter, does it? I can drive my car to work and walk around just fine whether the Earth is flat or round. The sun shines on me during the day whether the sun moves around the Earth or whether the Earth rotates.

The problems start coming in when you try to use this knowledge to expand your understanding; that is, you come across something you haven't seen before and you need to use your previous accumulated knowledge to determine what is going on. This is where someone using "their own way to make sense of all these phenomenon" can and often will lead to an incorrect conclusion.

At the end of the day, correct is correct. If incorrect looks correct in a couple of specific cases, it still doesn't make incorrect correct, correct?

Peter isn't in the business of making wood splitters or depalletizing tables, where you can get away with just about any explanation of why a cylinder moves. But he is in the business of trying to make a machine work after the wood splitter guy designed the hydraulic system for the machine, incorrect assumptions and all. So he has to use the correct model to diagnose his way out of the issue. The funny thing is it doesn't take any more effort to analyse the system correctly. It just takes a shift in the way you look at the problem.

Keith

[Alaric]

Many of the systems I work on use SI units and measure pressure in bar or MPa.

Power = Flow (in cubic meters per second) * Pressure (in Pascals) = The units line up nicely and cancel out, leaving power in watts.

Pa = 1N * m^-2
So N * m^-2 * m³ * s^-1 = N * m * s^-1 = J * s^-1 = watts

pressure * flow = power.

1714 is not fundamental to the equation. It is the conversion factor that converts your units when working in gpm, psi, and hp. No need for the 1714 conversion factor when working in metric. If you measure pressure in atmospheres, volume in bbl, and energy in BTUs, the power equation is still the same, but your unit conversion factor will be something else. Anyone care to take a stab at it?

[Alaric]

Originally Posted by Peter Nachtwey

Only until the limits of the power capacity of the pump. If load or resistance is infinite ( doesn't move ) then what happens?

Then flow is zero, and energy transfer becomes zero - with respect to the transfer of useful energy to the actuator. Any energy input into the pump driver becomes heat.

If we want to get down to the nitty gritty then either the motor stalls and the electrical energy begins to heat the motor windings,(or the engine stalls) or hydraulic fluid vents over a relief. Either way the energy input is now converted to heat, but no energy is transfered by the pump to the actuator.

[Tom Jenkins]

Peter and Alaric, I am a goin' to stick to my guns on this one. Ya ain't lookin at the whole picture in an exactly keerect way, fellers.

First, do an experiment, or imagine one if you don't have a pump and reservoir handy. Put a pressure guage on the pump at the discharge flange. Run the pump with a free discharge - no tubing or valves or anything else. (Except the bits needed to attach the pressure guage, of course.)

Result, lots of flow, but zero pressure. Power output of the motor will be close to zero - just enough to overcome the internal friction losses in the pump. For the future in this discussion lets assume that this is negligible and ignore these friction effects. Let's also agree for the purposes of discussion to ignore internal leakage in the pump. These aren't important to the concept under discussion.

Now add a valve to the discharge of the pump. Close the valve to say 90% open. What happens is a function of the kind of pump you are using. Let's keep it simple and say that our pump is a gear pump running at constant rpm. This will serve to illustrate the concept I'm after.

With a gear pump or other fixed displacement pump there will be measurable pressure at the discharge of the pump, and flow will be unchanged. The pressure will be proportional to the square of the velocity through the valve and the Cv of the valve at its current position. This is based on the formula Q=Kv*sqrt(ΔP) in Peter's first post. Motor output power will be equal to gpm x psi / 1714.

Close the valve further. The pressure will go up, the power will go up, the flow from our ideal pump will be unchanged. The pump is producing flow, the system resistance to flow is producing more pressure. The power output of the motor will increase proportionately.

Close the valve completely. Because I neglected to install a relief valve in my imaginary system I will blow up the pump or the tubing or the valve.

Newton's laws have to be applied in all cases. In this case the force created on the fluid by the valve pressure drop has an equal and opposite force created on the valve. It has nothing to with F = m x a because no accelration, including gravity, is included in the analysis under consideration.

In a constant flow variable pressure system like our pump increasing the restriction in the valve results in a higher pressure drop at the same flow. In a constant pressure variable flow system, say a standpipe or a large accumulator, increasing the valve restriction can't change the pressure, but the flow will decrease until the pressure drop across the valve matches the pressure available per the previous equation. Note this is in no way contradictory to the statement that pumps make flow not pressure - a large accumulator is not a pump and does not act the way a pump acts.

And as for the concept daddy tole me about water seeking it's own level it does so because there isn't an equal opposing force if one column is higher than the other. When the heights of the columns are equal the forces are equal and opposite, per Newton's laws.

The first law of thermodynamics is in effect. Bournouli's Law is in effect. All of the laws of physics are in effect and the behaviour I describe is in complete conformance with them.

[ndzied1]

Originally Posted by Tom Jenkins

Newton's laws have to be applied in all cases. In this case the force created on the fluid by the valve pressure drop has an equal and opposite force created on the valve. It has nothing to with F = m x a because no accelration, including gravity, is included in the analysis under consideration.

You just proved the opposite of what I think you intended. You closed the valve all the way so there is no flow! The pump is making pressure until one of your afformentioned situations (tube break, motor o.l. etc) occures.

Originally Posted by Tom Jenkins

The first law of thermodynamics is in effect. Bournouli's Law is in effect. All of the laws of physics are in effect and the behaviour I describe is in complete conformance with them.

I don't think anyone ever said anything to the contrary.

[fluidpower1]

"You closed the valve all the way so there is no flow! The pump is making pressure until one of your afformentioned situations (tube break, motor o.l. etc) occures."

If there was No Flow, What causes the Aforementioned situations???????

How can No Flow cause something to BREAK????

[Electrically_Bound]

I'm siding with Newton on this one:

Force = mass X acceleration

If someone on this thread has discovered that something besides force causes acceleration, then they need to publish a paper and prepare to collect their Nobel Prize.

[Alaric]

Originally Posted by Tom Jenkins

...Power output of the motor will be close to zero

...Motor output power will be equal to gpm x psi / 1714.

...The power output of the motor will increase proportionately.

What exactly is putting power into the system again?

I'll say it again:the pump doesn't make anything, it transfers power.

Result, lots of flow, but zero pressure

Actually zero pressure would be impossible. Even assuming that you had ideal zero resistance piping and the pump discharge port was perfect, there must be some net pressure change between the inlet port and impeller/gear/pistons and discharge port of the pump, otherwise there would be no force to accelerate the fluid, which brings us right back to f=ma.

[ndzied1]

Originally Posted by Alaric

What exactly is putting power into the system again?

I think it was either a chemical or nuclear reaction when I traaced it back. Further than that I'm at a loss...