Help on fluid pressure in a contianer
Help on fluid pressure in a contianer
(OP)
I have a few question on pressure in the head space.
The container is 256cc. If the container is filled with water
of 225cc,
1)what is the pressure in the head space at room temperature. If the temperature is increased to 250F what willbe the new pressure in the head space?
2) How many cc's must he bottle expand to net 5 psi in bottle at 250F?
thank you in advance
The container is 256cc. If the container is filled with water
of 225cc,
1)what is the pressure in the head space at room temperature. If the temperature is increased to 250F what willbe the new pressure in the head space?
2) How many cc's must he bottle expand to net 5 psi in bottle at 250F?
thank you in advance





RE: Help on fluid pressure in a contianer
To answer 1 and 2, you must know either the pressure at room temperature or at 250F, then you can calculate the other.
You will also need to know the wall thickness of the bottle and Young's modulus for the bottle's material in order to calculate the change in bottle volume from a change of pressure within.
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Help on fluid pressure in a contianer
Regarding the first question, can I calculate the pressure in the head space at room temperature, with given volume. The content inside is water. This is the condition without thermal expansion. What formual's to use is it PV=nRT?? tahnks,
RE: Help on fluid pressure in a contianer
Would you mind explaining a little more about the use of this container/bottle/tank? What codes are you building this to? What materials are being used?
Looking at your last question, I'm a little concerned that you expect your bottle to expand due to an increase in pressure -- does the system not have a relief valve on it? Are you sure the material will actually expand versus rupturing?
In regard to your question about figuring out the initial pressure, I don't believe you can just consider the air as an ideal gas. A lot would depend upon the conditions under which the bottle was filled as to how much water vapor would be in the air. Can you describe how the bottle will be filled, and what initial conditions will be present -- is this a field application, where things could rapidly change or is this a laboratory situation where things are very controlled? Is this a cover gas applied afterwards or an air pocket left during filling?
Patricia Lougheed
Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.
RE: Help on fluid pressure in a contianer
The process is similar to retort. The jar is filled with faucet water then closure/cap is applied. The jar is filled with 225 cc of water. The head space is 30 cc (air pocket left after filling). I would like to calculate the initial head space pressure and room temperature. The jar is made of Polypropylene but very rigid (assume rigid structure for inital pressure). The closure is also polypropylene.
If the temperature is increased to 250F, the bottle expands and wuold like to know the pressure at 250F.
Thanks.
RE: Help on fluid pressure in a contianer
Being somewhat blunt, this isn't a homework problem, is it?
Patricia Lougheed
Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.
RE: Help on fluid pressure in a contianer
RE: Help on fluid pressure in a contianer
---------------------------------------------
The air expands according to V = nRT/P
The water expands according to it's thermal expansion coefficient and compresses according to its bulk modulus property, to which the net volume change will be the algebraic sum of both.
The air volume plus the water volume will equal the bottle volume, as the bottle expands in accordance with its thermal expansion coefficient and its internal pressure (if the bottle is basically circular, an approximation can be made using a hoop stress formula and the tangential strain calculated by dividing hoop stress by its Young's modulus).
Develop an expression for the total change in volume of the water and air and set that equal to the volume of the bottle: solve for the pressure.
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Help on fluid pressure in a contianer
In going to 250°F, don't forget that the water could go through a phase change. The presence of an air pocket would provide room for some water to flash to steam. At that point, I would not use the ideal gas law for the steam/water mixture. FYI, for saturated steam, the saturation pressure at 250°F is about 15 psig. However, the actual pressure is likely to be somewhat below that as there isn't sufficient air space for all the water to turn to steam.
Patricia Lougheed
Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.
RE: Help on fluid pressure in a contianer
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Help on fluid pressure in a contianer
I guess the information provided by you people should help me approach the problem to some extent.
RE: Help on fluid pressure in a contianer
Note that if the container is polypropylene, the allowable stress is going to decrease, possibly quite dramatically (I didn't look it up). Anyway, you shouldn't need FEA to do this, just conventional pressure vessel formulas, and a knowledge of stress allowable for the material at this temperature.
To ensure your analysis is conservative, I'd suggest not considering how much the container might expand. That expansion is never taken into account by vessel codes.
RE: Help on fluid pressure in a contianer
Now if there is adequate space for an equilibrium between the water and the saturated steam going from 60F to 250F will give me about 15 psig.
I'm fine with all that. Now here is where I get confused. If there isn't enough space for the steam/water to reach equilibrium then what controls? I guess as long as there is some compressible fluid for the water to expand into it is compression arithmetic, but if the expansion compresses the gases beyond 15 psig then the steam will condense and the only thing that matters is the thermal expansion coeeficient of the water. What am I missing?
David
RE: Help on fluid pressure in a contianer
Assuming the container is rigid, the situation is similar to that of a pressure cooker.
Liquid water expands by ~6% from 60 to 250oF.
The vapor space would therefore contract by 0.06×225 =13.5 cc, becoming 17.5 cc.
Vapor pressure of water at 250oF is 2.031 ata.
The air pressure is calculated from PV=nRT considered an ideal gas, where V= 17.5 cc.
Both pressures are added, as iainuts indicated, following Dalton's law of pressure additions.
RE: Help on fluid pressure in a contianer
We are using 265 F instead of 250 F
I may approach this way, find the change in volume by using delta V = B* Vi* (delta T)
where B is thermal expansion coefficient at 50F (10C) which is 0.000088/C
Vi = 225ml and delta T = 265-50F or 130-10C
Take the pressure at 265 F from steam tables, which is 39 psi then use P1V1 (vapour) = P2V2 (air) and find pressure P2
Any suggestions.
RE: Help on fluid pressure in a contianer
kdsilv1
• Liquid water expansion coefficients change much between 50 anf 265oF. Therefore, I suggest you use the tabulated specific volumes for water.
• The formula P1V1 = P2V2 is only valid when V1 and V2 are specific volumes. Otherwise the correct equivalence is P1V1/n1 = P2V2/n2
RE: Help on fluid pressure in a contianer
If you need more precision, calculate the volume change of the water using the change in density from room temperature to 250°F. Same goes for the container using the coefficient of thermal expansion.