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Join Date: Jul 2010
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In-compressible Liquid Flow

08/12/2011 9:08 AM

I'm trying to wrap my head around incompressible fluids, if you have a pump which is pumping against 2800kpa(a) you get a some flow x m3/h based on the pumps discharge curve.

If the line has a few valves install on it, flow control valve, check valve, isolation valve etc., these along with frictional losses in the pipe equate to a pressure drop of 2800kpa(a) at x m3/h.

My question is, as the fluid passes through these valves and the pressure reduces, does the velocity of the fluid increase? or remain constant?

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#1

Re: In-compressible Liquid Flow

08/12/2011 10:37 AM

The velocity remains constant, so long as the pipe size remains constant.

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#2

Re: In-compressible Liquid Flow

08/12/2011 12:58 PM

I think you are overcomplicating the question.
Consider a pipe 100metres long open at one end (opening onto a wide chanel) with a pump at the other. There will be a pressure gradient along the pipe, effectively zero pressure at the open end and max pressure at the pump.
if the open end is closed then the pressure will be more or less constant along the length of the pipe.

At any restriction there will be higher pressure upstream than down stream.
Anyhow to answer the question the flow remains the same (because the fluid is incompressible) therefore the velocity must change if there is a change in cross sectional area. The velocity is dependant on the cross section of the pipe (assuming ideal flow etc), so if the dipe diameter decreases the velocity must increase and if the diameter increases the velocity will decrease.
This is shown just past the open end of the pipe where the cross section becomes huge and the velocity slows to a crawl.
Del

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#3

Re: In-compressible Liquid Flow

08/12/2011 1:23 PM

GA Del, per usual.

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#4

Re: In-compressible Liquid Flow

08/12/2011 3:15 PM

If your question is understood correctly.

Your mistake seems to be that you consider the pump discharge rate to stay constant and therefore suspect compression to accomodate the difference.

No compression can occur - At any flow that flow will be present at any point on the line.

Consider your system is running with all valves open, mark your imaginary duty point on the curve (Q and H)

The pump has no knowledge of the system and only reacts to the back pressure. Higher back pressure = less flow and lower back pressure = more flow.

The back pressure at the pump consist of Static + working pressure + friction + obstruction (friction) in valve.

As the valves are closed progressively the back pressure at the pump will increase and the duty point point will move to the left on the pump curve delivering less water at a higher head.

The flow , velocity and friction in the pipe / system will be less.

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#9

Re: In-compressible Liquid Flow

08/13/2011 3:47 AM

GA. Nice reply by Hendrik & TrevorM.

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#5

Re: In-compressible Liquid Flow

08/12/2011 8:32 PM

Ok I understand that, thanks for clarifying. One thing im also trying to understand is pressure recovery.

If you measure pressure along a piece of pipe or across a restriction orifice etc. the upstream pressure will obviously be greater than the downstream pressure. This difference in pressure to me is the permanent pressure loss of the system. Why do they say there is some pressure recovery through a restriction orifice? ie. the pressure within the orifice is lower than the downstream pressure after the orifice plate?

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#8

Re: In-compressible Liquid Flow

08/12/2011 10:51 PM

The balance or your questions is answered by understanding the Bernoulli equation - which is basically that the total (physical) energy in a fluid stream is the sum of its pressure, head and kinetic energies.

Friction flow losses are reported as head or pressure losses and are a reduction of the total physical energy figure above - and are lost from the fluid stream as heat and noise. These are not recoverable.

The recoverable component that you have read about is where, for example, the kinetic energy of the fluid is converted back to a pressure (or a head) by a process that is not lossy - for example a slowly diverging restrictor (and so slower flow, lower kinetic energy and higher pressure) with a largely laminar flow at the input. In examples such as the above the head loss due to any height change is usually inconsequential, but should be accounted for in a complete analysis.

As you would appreciate the conversion from pressure energy to kinetic energy is "easy" and highly efficient. The process back the other way is more "difficult" and usually not so efficient.

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#6

Re: In-compressible Liquid Flow

08/12/2011 10:35 PM

You may get by fine with an electrical equivalent. It is exactly correct, if properly applied. Do your analogy, it is not rocket science. Use it to your advantage.

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#7

Re: In-compressible Liquid Flow

08/12/2011 10:44 PM

In your hypothesis of incompressible fluid flow, the velocity increases when the cross section area decreases.

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#10

Re: In-compressible Liquid Flow

08/13/2011 8:57 AM

There is no such thing as absolute incompressibility, although liquids come really, really close. When you put a liquid under pressure, it decreases in volume a very minute amount. Therefore, as the liquid passes the restrictions, since mass flow is constant, the velocity increases by a very minute amount. For any real world calculations, liquid can be considered incompressible without any significant error.

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#11

Re: In-compressible Liquid Flow

08/13/2011 11:42 AM

I've read all the answers, and still come away with the nagging idea that in the OP scenario, it is a trick question. There is no flow, or velocity, in a hydraulic line that supplies a pressurized vessel, unless the vessel volume changes (as in a cylinder/piston) Although there will be some very small compressibility in the liquid, and any air pockets provide opportunities for compression, the velocity will be nil very quickly, won't it?

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#12

Re: In-compressible Liquid Flow

08/13/2011 4:53 PM

Why you think somebody would gives the drop pressure of each component in the line like valves,elbows,joints,etc.? Of course is not needed to solve this problem consider at all any air pocket or compessiblity of the fluid!!As was said uphere each component is pressure drop,let's say V1,V2,..Vn,constant flow is "I",so the energy losses mentioned by Trevor should be (V1+V2+...+Vn)•I,now the rest of energy should be converted in kinetic energy plus pot.energy.Remember,the Bernouilli eq. is just a conservation law you should adapt the idea to solve your own problems.-

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#13

Re: In-compressible Liquid Flow

08/13/2011 8:11 PM

I do not know why, do you?

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#14

Re: In-compressible Liquid Flow

08/14/2011 3:10 AM

"Why you think somebody would gives the drop pressure of each component in the line like valves,elbows,joints,etc.?"

You firstly need the maximum total head at the desired flow to select the pump and driver.

The maximum pressures at each point in the line can be used to select a lighter class of pipe or different material to make a huge difference in the capital outlay and even running costs. (ID of lower class may be bigger or the other type may be smoother)

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#15

Re: In-compressible Liquid Flow

08/14/2011 9:12 AM

How does this apply to the OP?

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#16

Re: In-compressible Liquid Flow

08/14/2011 5:48 PM

thanks guys, you've solved my problem!

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#17

Re: In-compressible Liquid Flow

07/05/2012 9:36 AM

Dear Mr. Polerz,

The Velocity WILL REMAIN CONSTANT which is dependant on cross-sectional area only and if there is change in cross sectional area, the velocity will inversely vary/

DHAYANANDHAN.S