Gas thermal expansion and PSV

[tickle]

Dear All

I have got myself confused.

I have a gas station that operates near to its design pressure. There is the shut-in situation that may then be subjected to solar radiation and temperature increase.

The gas is methane, the operating pressure 9,000 kPag and the design pressure 9,600 kPag. The gas may be at 25 degC and may be heated to 65 degC. If I assume that the pipe volume remains constant and there is no loss of gas (the mass stays the same). Therefore the density will be constant and for the temperature increase above, the pressure will increase to 10,900 kPag.

Allowing 10% over pressure for the thermal relief case, the maximum pressure would be 10,600 kPag and a PSV would be required.

I have read several posts that state somehting like, the gas is compressible, and over pressure is not possible in gases, only for liquids. In my simple mind, to compress the gas requires a force, and this will be from pressure on the pipe walls.

Am I missing something?

 

[zdas04]

First, density is a function of pressure, temperature, and gas content. If you raise temp at constant pressure, density will go down.

Second, PV=ZnRT. If you raise temp from 25C to 65C without adding mass and assuming that changes in Z are insignificant (first approximation) then your new pressure is 10208 kPa.

If you can defend a credible scenario that says that the temperature can never increase above 78C then maybe you don't need a PSV. I'd never sign off on it.

David

 

[Latexman]

(9000+101.325)x(65+273.15)/(25+273.15)-101.325 = 10221 kPag

10221/9600 = 1.065 6.5% accumulation

You need a PSV because it exceeds MAWP, but it'll never exceed the 10% accumulation allowance. If this is the only credible scenario, install the smallest PSV allowed by company policy. This is usually a 1/2" or 3/4" PSV. I would not even size it other than show it'll never exceed the 10% allowance, like above. Other opinions?

Good luck,
Latexman

 

[tickle]

Thank you David and Latexman. You have confirmed what I have done. Thanks to the internet in allowing me to have the option of checking with my peers.

I used real values for gas, therefore I get slightly different pressures.

 

[dcasto]

you can use a 1/2" check valve around the blocked in valve, it's safer, cleaner, and no green house gas releases.

 

[tickle]

Dcasto

I would not put a check valve around an actuated/shut down valve on a gas transmission pipeline.

It may be okay on a liquid transfer system back to the storage tank.

Tickle

 

[tickle]

Follow up

I have been a bit concerned regarding my original post and the impact of it on the design (adding numerous PSV's on to the pipework for possible shut-in conditions).

I am designing to one code that states "if the pressure can increase above MAOP then a pressure relief device is required to prevent the pressure getting above 115% of the MAOP".

B31.3 is more descriptive in that it states liquid thermal expansion the set pressure shall not exceed the lesser of the system set pressure or 120% of design pressure. (B31.3 also has other permissable over pressure situations requiring the owners permission with limits on duration, magnitude and pipe stress).

I could then say that for my case, if I do not exceed 115% of design pressure due to gas expansion then I do not need to install a PSV for thermal expansion of gas.

However, the stored energy of a liquid is much less than for gas, so do we need to install a PSV for this reason?

Further comments would be appreciated.

 

[djack77494]

tickle,
Going back to your original post, I think it very unlikely (though not impossible) that ambient heating would cause a 25C gas to heat to 65C. Is the 65C just a maximum metal temperature due to solar radiation? Starting at 9000 kPa[G] = 9100 kPa[A] such heating would cause the pressure to build to 9100 * {(273+65)/(273+25)} - 100 = 10,221 kPa[G]. I'm not sure as to the basis of your calculations, but you may be overly conservative in your assumptions.