How to interpret "pressure" in incompressible fluids pipes

[adrich91]

Hi all!

I would like you to help me to understand the concept of pressure in pipes where there is an incomprehensible fluid.

I imagine a pump with a pipe full of water and a valve at the outlet.
I understand the curves of the system and of the pump, to see which pump satisfies the conditions of my system (head, pressure drop, etc.).
What I find difficult to understand is the concept of pressure (in gases I don't have such a problem as they are volume dependent and can be compressed).
Imagine that for the attached system, I need 5 bar of pressure at the discharge for a flow of 30 L/s at the outlet. The pressure gauge at the pump discharge will show 5 bar, but what does that imply? That the water pipe full of water is exerting a force of 5 kg per cm2? If I throttle the valve, the pressure will rise to 8 bar for example. In both cases, the pipe is completely filled with water, isn't it? So what is the basic principle of the manometer to know that it is 5 or 8 bar? Is it because a column of water is displaced?

I don't know if I'm making myself clear. In short, my doubt is:
1. to understand the concept of pressure in a pipe, given that if the pipe is always full of water regardless of the flow rate, what does it mean that a manometer reads 5 bar or 8.
2. If fluids are incompressible, what happens if the pump keeps pumping fluid against a valve that is closing? I don't understand how ‘more water gets in’ if the pipe is completely full.

Thanks a lot! Sorry I am not explainning correctly, it is also hard for me how to put an example

 

[katmar]

If the valve at the end of the line is open then the 5 bar that you read at the pump discharge is the pressure drop due to friction caused by the flow from the pump to the end of the pipe.

If you throttle the valve at the end of the line then the pressure drop will increase because you have introduced additional frictional resistance. If you are using a centrifugal pump the flow will decrease as the pump's delivery pressure increases. You can see this on the pump curve.

I find it helpful not to think of a pump as generating pressure but rather think of it as creating kinetic and potential energy. If a centrifugal pump pumps against a closed valve then you see it as pressure on the gauge but you can also think of the pressure as potential energy. Consider Bernoulli's Principle and the conversion between kinetic energy, potential energy and pressure.

It is not correct to say that the pipe is always full of water regardless of the flow rate. If the flow is small enough the liquid will run along the bottom of the pipe and it will not be full.

It is also not strictly accurate to say that liquids are incompressible. The volume does decrease as the pressure increases, but by a very small amount. If you pressurise a line with a liquid and then close the valves at both ends then you can have the pressure remain high, but if you crack open one of the valves and allow just a few drops of liquid to leak out the pressure will drop dramatically as the liquid expands by this very small amount. This ratio between the stress (pressure) and strain (expansion) is known as Young's modulus.

The pressure that you would read on a gauge at the pump discharge while the liquid is flowing has virtually nothing to do with the expansion or compression of the liquid - it is solely pressure drop due to friction. The specific volume at the point where the 5 bar is measured would be infinitesimally smaller than the specific volume at the pipe end. This change in volume is nothing compared with the volume of flow that would normally be caused by the pressure drop overcoming the friction.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[3DDave]

Imagine you apply pressure on the end of a one square centimeter steel rod. How much pressure does that apply to the other end? That's what an incompressible fluid is like.

If the valve is closed and it is a positive displacement pump, such as a piston pump, the pump motor will stall or the pump will break or the seals will fail. For something like a centrifugal pump, the water in the pump will get hot.

 

[FMJalink]

Hi adrich91,
As far as I know, there are no really inompressible liquids. As the molecules in a liquid are very near to each other, the pressure increase is high with a small volume discrease.This normally allows liquids to be co sidered to be incompressible, allowing easier formulas/mafhematics to be used. Hope this helps with the understanding.

 

[LittleInch]

You are much better thinking of this in terms of "fluids", not gas or liquid as separate issues.

So gaseous fluids are less dense and have more compressibility, but as noted above all fluids, including liquids, have some level of compressibility, it is just very small compared to gas.

So e.g.. if you get say 1000m of fully filled liquid pipe at 0 barg, you can inject a liquid into this to raise pressure in the same way a pressure test works. The liquid fluid compresses a small amount ( see Bulk Modulus) plus the pipe itself expands a very small amount, but together you could get maybe 0.1 % increase in volume, or maybe 0.01%, but it is not zero, with an increase in pressure.

Like a lot pf people I think you've got this "absolutely incompressible" thing into your head when it should be "virtually incompressible"?

Does that help your thinking?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Exactly. If it helps you understand what's going on, consider the liquid as compressible. Virtually everything is compressible at least to some small extent anyway. Water is actually compressible. We sometimes say it is not compressible, because we usually work at such low pressures that we can hardly see or measure the tiny change in volume our small pressure produces and it makes life simpler to ignore what you cannot easily see or measure.

Even diamonds are compressible, but they brittle and prefer to shatter rather than show us their microscopic volume change.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[adrich91]

Sorry for the delay in replying, I was travelling.

I wanted to thank you all for the explanations you have given me.
I will think about it a bit more and if I have any doubts, I will comment.

Thanks again!