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Pressure Of An Isolated Flow In Pipe
#1
Posted 30 January 2025 - 02:49 AM
In the past few days, I have been thinking about the following situation and I would be grateful if you could help me to figure the answer out:
Assume a multiphase fluid is flowing through a pipe.
We decide to close the isolation valves at the begining and end of the pipe (Confining the fluid inside to pipe) and open a drain line connected to the pipe.
Is the fluid still pressurized? Or it will flow to the drain line by gravity?
I am of the opinion that although the fluid is stagnant at the pipe, it still is in equilibrium and ,therefore, has its equilibrium pressure. Of course, if it finds a way to atmosphere, like by opening a vent valve, it then will be atmospheric and flow by gravity.
Am I right?
But what about a single phase flow? If it stops flowing a pipe by isolation valves being closed, will it still have pressure?
I really thank you in advance for your assistance.
#2
Posted 30 January 2025 - 03:24 AM
Hi,
Consider using the flexible hose in your garden to visualize the phenomena.
Yes, it stays under pressure until you release the pressure.
Breizh
#3
Posted 30 January 2025 - 04:40 AM
We decide to close the isolation valves at the begining and end of the pipe (Confining the fluid inside to pipe) and open a drain line connected to the pipe.
Is the fluid still pressurized? Or it will flow to the drain line by gravity?
The details and timing of closing the isolation valves is critical. Is supply valve closed first, then destination valve? Vice versa? Simultaneously? Are valves manual or automated? What is the separation distance between the valves? Is there one man or two?
#4
Posted 30 January 2025 - 05:00 PM
I believe that a multiphase flow isolated as indicated will act just like gas trapped in the pipe. The pressure will exist until the gas phase bleeds down to atmospheric pressure. Until that time both liquid and gas will flow by pressure in pipe. Once gas pressure reaches atmospheric then liquid will flow by gravity.
A pure liquid will flow by gravity. As soon as the drain valve is open, just a minute amount of liquid will cause the pressure in the pipe to drop to atmospheric due to the incompressibility of the liquid. The amount of liquid volume exiting the pipe it takes to drop the pressure is due to the volume of compression of the liquid under pressure and the expansion of the volume of the pipe under pressure.
Edited by snickster, 30 January 2025 - 05:04 PM.
#5
Posted 05 February 2025 - 03:36 AM
First of all, I apologize for my delayed response and appreciate your replies and guidance.
Latexman's reply got me thinking deeply. I see now. If we firstly close the upstream valve and give it enough time, the fluid will still flow in its direction until it loses its pressure and stops flowing. I guess at that point it will be equal to the destination pressure rather than 0 barg, right?
breizh's and snickster's replies are the case when we close the isolation valves simultaneously, I think.
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