Fire torch warning... Pressure created by resistance to flow?? 11

[akkamaan]

For a hydraulic guy to say that pressure is created by resistance to flow is like an electrician would say that the resistor creates the voltage...Right!?

Listen to
Jim Pytel, Columbia Gorge Community College
in his Youtube lesson

LunchBoxSession

Insane Hydraulics

 

[hydtools]

Does the Darcy-Weisbach equation for pressure loss due to flow friction mean anything here? It was derived based on momentum principle. Newton may be okay with that.

Ted

 

[akkamaan]

Here is a link that explains Darcy-Weisbach equation

[URL unfurl="true"]https://en.wikipedia.org/wiki/Darcy%E2%80%93Weisbach_equation[/url]

 

[PNachtwey]

Does the Darcy-Weisbach equation for pressure loss due to flow friction mean anything here? It was derived based on momentum principle. Newton may be okay with that.

Yes, it is useful to computer pressure drops but it does compute pressure. This is the distinction that most people miss. Also, there is no pressure drop if there isn't a difference in energy potential. The pipe by it self does not create energy.

BTW, I wish they had specifications for pressure drop for pipes and hoses. Also capacitance. There should be a specification for "resistance to flow"/meter and capacitance/meter. Both values I could use in a simulator. However, I have yet to see an equation that calculates and absolute pressure except by integration.




Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

 

[EngineerTex]

The pipe by it self does not create energy.

Neither does the pump. That is the whole point here. All of the parts of the system convert energy.

I will not be condescending and act like you don’t know that energy is neither created nor destroyed. I will, however, point out that you conveniently and, I can only assume, purposely ignore it in the attempt to make your point.

Whether we talk about a simple energy conversion device like a resistor or an antenna, or we talk about more complex energy conversion devices, such as steam engines or desk fans, the energy is only being converted from one form to another. You are correct to point out that pressure is an indirect measure of a part of total internal system energy.

With that in mind, what does a bridge abutment do to a speeding truck? Its kinetic energy is converted into noise, heat and metal deformation by the abutment. The truck did not make this energy conversion. The wall did.

Now consider a nozzle, yet another energy conversion device. This particular one converts potential energy into kinetic energy. A nozzle fitted to the plumbing at the bottom of a water tower will accelerate water based on the shape of the nozzle. When comparing dozens of shapes of nozzles, you will find the flow characteristics to be determined by the shape of the nozzle and the plumbing, as opposed to the water tower. The energy conversion device is the nozzle, not the water tower.

What about an injection pump, which, with no moving parts, uses the steam from a boiler to pull water from an outside tank into the boiler that is producing the steam to begin with? It seems impossible, but by carefully noting where the energy is and how it is converted back and forth from potential to kinetic and then back again, it is seen that pressure and flow are changing based on the confinement and release of potential and kinetic energies.

How about a resistor? How about an LED? What about a throttle? Where is the energy going in each of those systems? At the same time that resistors create heat, motors make rotation and light bulbs make light, none of these things are creating energy, which brings us to the heart of the issue. Neither a throttle nor a nozzle nor a motor nor a pump creates or destroys fluid energy, whether that energy is potential or kinetic. Your entire argument rests on the notion that because motive energy is imparted by the pump, that the pump is the origination of the pressure. And this is wrong, wrong, wrong. The pressure arises due to the overall system — whether that system is a plug or an accumulator stuck into the pump’s outlet port or any configuration of hoses, pipes and valves — converting kinetic energy into potential energy, heat energy or work outside of the fluid system. Kinetic energy is converted to a static pressure by the containment of the fluid that would otherwise continue with its imparted motion from the pump. It is this resistance to flow that creates pressure. The system itself converts electrical energy to mechanical energy and from mechanical (rotating) energy to another form of mechanical energy (fluid) and without a containment system existing in some form or another, the kinetic energy is never converted to potential energy and pressure would never exist. The resistance to flow creates pressure.

To drive home the point, how is pressure created in systems that do not have pumps or motive force applied to the fluid, i.e., when the fluid itself is expanding or contracting? What occurs inside of a wax motor? Why is force only exerted at the end of a wax motor’s shaft when there is a rigid shell to resist flow in all other directions? What happens when water is contained in a rigid vessel and then frozen? Does the lack of a pump to apply motion mean that there is no pressure? Would there be any pressure at all if there were no resistance to movement? Finally, what happens to a pump when you run it without its case? You would get all kinds of fluid motion inside of your tank, but it does nothing useful because there is nothing to resist flow in any direction and no pressure is developed anywhere. This is the same as inducing eddy currents in a copper plate. It does nothing useful and no potential is created.

TL;DR: The containment of fluid, which converts kinetic energy to potential energy (pressure) is as much of an energy conversion device as a resistor, a motor or a pump. Ignoring that does not mean that a pump creates pressure.

Engineering is not the science behind building. It is the science behind not building.

 

[PNachtwey]

Neither does the pump. That is the whole point here. All of the parts of the system convert energy.

Yes! but that doesn't meant "pressure is created by resistance to flow" or what I hear more often is "pressure is resistance to flow".
Now if someone were to say "a pressure drop is created by resistance to flow", that is good.
This does not imply that resistance to flow creates energy nor to is imply that one knows an absolute pressure, just the pressure drop.


Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

 

[IRstuff]

In the case of an ideal voltage source, the voltage is completely independent of the resistance attached.

TTFN (ta ta for now)
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[hydtools]

That is because the ideal voltage source has unlimited current capacity. It is not a real life source.

Ted

 

[IRstuff]

And yet, there are lots of sources for which is this sufficiently valid that one can design with it.

TTFN (ta ta for now)
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[hydtools]

Yes. Yes it is, within limits.

Ted

 

[akkamaan]

This boils down to energy. Yes, energy can't be created, only transferred and converted to different other forms of energy. A certain form of energy has a local source. We can mechanically transfer energy from point A to point B. During the transfer of energy we have losses, pressure drop, voltage drop, volumetric leakage etc, usually into heat. We can transfer mechanical energy with a drive belt, bicycle chain, a piston rod in an engine etc. "Flow" can't transfer energy but "oil" can. Flow is just a measurement of how fast we transfer energy. Flow only tells us that energy transfer is happening. Point A is the hydraulic pump and point B is somewhere downstream of the pump ie the actuator. Resistance to motion of energy transfer, which the flow is an indicator of, causes the losses and part of transferred energy are converted into "heat" or eventually another energy form. The force or pressurizing the oil that is behind the energy transfer cannot come from an energyless resistance. That means the pump is the source of pressure behind the transfer of energy and the motion of an actuator. The actuator only causes the pressure drop which means the actuator is just converting incoming pressure energy into mechanical work which results in other energy forms like potential energy when lifting a mass, friction, and heat when moving frictious loads like when a dozer push dirt on the flat ground.

 

[Bumblyari]

In a dusty old bookshop in a back street of Cambridge (UK), an old manuscript has just come to light which is believed to be a previously unknown work of Sir Isaac Newton and entitled Principia Mathematica Part 2.

In it appears a corollary to his first law of motion which states:

‘Non quiescit quiescat urgeatur objecti manet volumen ex humore quodam tempore introitum hidrauliskās actus i.e. facit fluxus est ire. (Ut puto habeo hoc primum nefas) – I. N. (Sir)’.

Latin scholars have translated this as:

‘An object at rest stays at rest unless acted upon by the result of a volume of hydraulic fluid in a certain period of time entering an actuator, i.e. flow makes it go. (I think I may have got it wrong the first time) – I. N. (Sir)’.

It is likely that engineering textbooks around the world will now have to be recalled for corrections to be made to long-held beliefs.

For more fairy stories, check out the Brothers Grimm.

 

[EngineerTex]

Ok. Now we have two different views stating why

pressure is created by resistance to flow

is incorrect.

If I can boil down the first school of thought, which I believe is held by akkamaan, it would be the following: Because the energy imparted to the fluid originates inside of the pump, both flow and pressure are created by the pump. * If that summation is not correct, then please correct it and perhaps I will see something that I have not seen so far.

However, if that distillation is a mostly-accurate representation of the argument, then I remain unconvinced. For my final example for why I do not believe this to be the case, I will raise the phenomenon of the simple water hammer in household plumbing. When a faucet is suddenly closed, there is a large pressure spike throughout the plumbing inside the house. The origination of the pressure spike is at the valve and if you had a number of fast-acting pressure transducers located all through the plumbing, it would be seen that the pressure spike through the fluid moves backwards in relation to the flow, i.e., outward from the valve. That is to say that the pressure increase begins at the valve, not the source of flow.

Now, to the second school of thought on why the quote above is argued to be incorrect. I do not see enough difference between:

a pressure drop is created by resistance to flow

and

pressure is created by resistance to flow

to see where the problem lies. In a static system under pressure, is there not a pressure drop across a pipe wall from its inside to its outside? I will need further explanation before I can get on board. Again, I will read an outside article explaining it, if you have one to link.

Also:

This does not...imply that one knows an absolute pressure, just the pressure drop.

I do not follow what you are saying here.

*In this summation, I am not accusing anyone of claiming that energy is created or destroyed.

Engineering is not the science behind building. It is the science behind not building.

 

[hydtools]

Your water hammer example relates to an instance of energy conversion. The kinetic energy of the flowing water was converted to potential energy when the valve closed. The valve's sudden resistance to flow caused a sudden rise in pressure which was reflected back into the water system as following elements of flowing water were each brought to zero velocity and resulting rise in static pressure.

Ted

 

[zeusfaber]

At risk of reopening a shouting match....

Whether or not the "Pressure is caused by the load, not the pump" simplification is always (ever?) completely true, there are some people - myself included - who find it useful. Here's why.

I was brought up a sparky. Like most of my mob, I mis-spent my youth designing, building and setting fire to circuits that worked off (roughly) constant-voltage power supplies, and got myself thoroughly into a mindset that analysed things by starting with supply volts, then seeing what that would drive through the various impedances I'd stuck across it.

For a horrible first term at university, they made us do stuff like Norton's theorem, where we had to adapt our heads to the idea of constant-current sources - but then we dropped swiftly back into that comfortable constant-voltage headspace.

As a sparky moving into hydraulics, it was really useful to have that simple warning early on that you shouldn't try to draw too many analogies between batteries and hydraulic pumps - at a first approximation, pumps behave more like a constant-flow device than a constant-pressure device (so dead-heading them can be exciting in a way that open-circuiting a battery won't be).

I know the constant-flow model isn't universally true either - and actually have a fair number of systems that have constant pressure hydraulic ring-mains supported by big accumulators.

The aim of education isn't to tell you how it is - it's to help you to think for yourself.

A.

 

[akkamaan]

A somewhat fair analogy between an electric DC system and a hydraulic system only work with a hydraulic CP (Constant Pressure) system with an accumulator. Constant voltage alternator=constant pressure hydraulic pump and battery=accumulator. It has to be a constant pressure system and a DC constant voltage vehicle system (alternator and battery) if you want the make a fair analogy between the two.
This is one of my favorite electric vs hydraulic analogies...

 

[PNachtwey]

This topic has given me ideas about an article for H&P magazine.
BTW, the next article will be about valve flow constants.
I have queue of articles.
I will address the pressure is resistance to flow sometime later this year.

If only people would say a pressure DROP is caused by resistance flow.






Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/