Void Formation due to Pressure Drop 1

[vpl]

Recognize this is a pretty basic question, but I'm away from all my reference books by 1000+ miles and it's not an area I normally deal with!

I have a closed volume at average temperature of 370°F and pressure of 1100 psia. I cycle a valve opening the system to another volume at 230° and 21.5 psia. There is no driving force to exchange fluids between the two volumes (other than the different pressures and temperatures).

Questions:

1) Would the pressure equalize across the two volumes and if so at what value? (I have one person saying it would equalize at saturation pressure for the 370°; I'm thinking the pressure would drop to the 21.5 psia as the second volume communicates to a reservoir.

2) In either case, I will have temperatures either at or above saturation. Can I figure the fraction that will flash to steam by multiplying the specific volume by the volume?

Thanks in advance

Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[BronYrAur]

I am thinking about this, but I have a couple of questions:

1. Are the 2 volumes the same?
2. Are we talking about water here or steam? your first condition of 370 deg F and 1100 psia would not be steam. Saturated steam at 1100 psia is 556 deg F. Your second condition looks like it is close to saturation.

 

[vpl]

Answers:

1) The volumes are sections of a branch pipe that is normally isolated by the closed valve and a check valve. The first volume mentioned is between the two valves and is about 3 cubic feet. The other is open back to a reservoir and contains a pump) and is around 150 cubic feet. However, adjacent to the isolated valve, the pipe goes from horizontal to vertical. The volume of the horizontal portion is roughly the same as that isolated between the valves.

2) Both should be completely filled with water. The longer segment is vented routinely (every two weeks right now) to maintain it full of water. And it is very close to saturation. The smaller volume is separated from the main pipe by a check valve. Due to the temperature and pressure in the small volume, it's suspected there is some small back leakage through the check valve; enough that the pressure and temperature are close to that in the main line.



Patricia Lougheed

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[BigInch]

Did I miss something? The small volume will effectively do nothing. You will stabilize at WAG of 21.5 psia and 230ºF

Let me know if it turns out different.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[vpl]

Thanks BigInch. That's what I was thinking. I was being told that the pressures would stabilize at the saturation pressure of the higher temperature fluid. I needed a sanity check.

Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[BigInch]

If the volumes were the other way around and both happened to be vapor, I might agree with them, but... not this time.

Careful. I am not one that should be used for the purpose of gauging somebody's sanity.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[quark]

There are two to three aspects in this. First thing is that there seems to be enough volume for the expansion to take place. The volume ratio is about 50 and that of pressure is 51.2. There are chances that the pressure equilibrium may be at 21.5 psia. (actually this is applicable to gases in isothermal condition)

But, the liquid at 21.5 psia is close to saturation(231.79F) and thus chances are that 21.5 psia will be equilibrium pressure.

If you see the enthalpies, 344.184 at 1100 psia and 370F and 198.374 btu/lb at 21.5 psia and 230F, there is a difference of 145.81 btu/lb. This causes flashing in the system. The latent heat at 21.5 psia is about 959.22 btu/lb (1157.6-198.374). So, there will be 145.81/959.22 = 15.2% flashing.

The density of water at 1100 psia and 370F is about 55.12 lb/cu.ft and so mass of water is 55.12*3 = 165.36 lb. If 15% of this flashes into steam, then there is about 25.13 lbs of steam. Steam specific volume at 21.5 psia is 18.77cu.ft/lb, so total volume occupied by steam is about 471.69 cu.ft and that of water is 2.36 cu.ft. There should be a reduction of steam volume by 471.69/(150-2.36) = 3.194 times.

In an unvented condition, this may lead to a balanced pressure of 68.67 psia approximately. This requires iteration to get exact value.