Pressure of a Fluid and Gas in a Closed Vessel

[Genie0501]

Hello,

I am trying to figure out if a fluid and a gas in the same closed vessel have the same pressure. Specifically, I have a water pipeline that I want to attach a pressure monitor to. The thought is to attach the monitor at an air release valve but we're wondering about whether any air pockets at that location would give us inaccurate pressure readings since they would prevent the water from reaching the monitor. We are are concerned with measuring the water pressure in the pipeline.

Thanks!

 

[bimr]

Yes, the pressures will be the same.

 

[BigInch]

But you may be measuring pipeline pressure at a relative high point. That will get you the lowest pressure in the region. Reading at a low point will get you the regional maximum pressure.

 

[IRstuff]

"any air pockets at that location would give us inaccurate pressure readings since they would prevent the water from reaching the monitor. We are are concerned with measuring the water pressure in the pipeline. "

The water pressure will cause any air pockets to shrink until the air pressure in the pocket equals the water pressure. The one thing that might be different would be water hammer, since the air will damp any fast pressure impulses.

TTFN
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[bimr]

Excellent point by BigInch

The total energy in the pipeline is the sum of the static, dynamic and hydrostatic pressures. The dynamic energy depends on the velocity head and will be relatively small. The static pressure is the pressure as measured moving with the fluid that can be measured with a pressure gauge.

However, the hydrostatic pressure will depend on the elevation. If the pipeline is relatively level, the hydrostatic pressure will be constant. If there is significant elevation difference, then there will be differences in pressure due to the elevation. For these applications, the elevation head will have to be added to the static pressure.

 

[Genie0501]

Thank you everyone!

 

[rconner]

One thing I would caution. I guess the argument that pressure is pressure whether by water or air, at least at one location, may at least in some cases have some merit/legs. However, most venerable standards I am aware of do instead direct you to fill the line and effectively vent/eliminate the air from any significant high points. The further point I would make is that IMO this standard practice requirement is not only for generally really good safety reasons, but also for the ability to estimate reasonably well e.g. via relative filled pipe elevations what the applied test pressure/head really is along the pipeline (during that test), at other locations in the pipeline that may happen to be at distant locations above and below that gauge location.
If you instead know you have, or happen to have, a whole lot of trapped air, e.g. resulting from slow filling from an often high elevation water source though without careful air evacuation at intermediate higher points or peaks in lengthy slugs of air and water "columns" (in peaks and valleys of the pipeline), I can tell you one will not really have the pressure one may think one has at other positions in the line. Water will not in these circumstances, "seek its own (the same) level," in the sense of say a normal "water level" from one end to the other!
In other words, when you have some lengthy air slugs at multiple higher points in between, the actual pressure/head at any low point will e.g. instead of the gauge pressure applied to a water-filled pipe summit at a high point plus the water elevation head at the immediately lower point of interest, in the case of applying the pressure to a huge air slug over a hill be only the sum of the pressure of the overhead air at the gauge that is essentially the same as the air pressure at the lower air/water interface plus only the height of the immediately overhead water column from the elevation of that air interface to the valley, wherever that (unseen) interface actually is?