Vessel Wall Temperature after Gas Depressurization trough an orifice
Vessel Wall Temperature after Gas Depressurization trough an orifice
(OP)
Dear All,
I need to determine the Vessel Wall Temperature after Gas depressurization trough an orifice ( API 521 ) and to verify the necessity of applying Low Temperature Carbon Steel. I know the possibility of using Hysys ( ASPEN Tech. )Depressurization Utility, however I have the following questions:
Assuming adiabatic process, for given initial conditions ( Gas Temperature, Pressure and composition), Vessel Volume and Final Pressure, will be the Final Temperature a function of the depressurization time ( or orifice size )? Is there an analytical relation to estimate the depressurization temperature? Any rule of thumb?
Many thanks and best regards,
Max
I need to determine the Vessel Wall Temperature after Gas depressurization trough an orifice ( API 521 ) and to verify the necessity of applying Low Temperature Carbon Steel. I know the possibility of using Hysys ( ASPEN Tech. )Depressurization Utility, however I have the following questions:
Assuming adiabatic process, for given initial conditions ( Gas Temperature, Pressure and composition), Vessel Volume and Final Pressure, will be the Final Temperature a function of the depressurization time ( or orifice size )? Is there an analytical relation to estimate the depressurization temperature? Any rule of thumb?
Many thanks and best regards,
Max





RE: Vessel Wall Temperature after Gas Depressurization trough an orifice
If no heat transfer to the fluid, the answer is simple.
If no heat transfer from the outside surroundings to the vessel, then a more sophistacaded model must be made?
RE: Vessel Wall Temperature after Gas Depressurization trough an orifice
I would be interested in knowing both the cases:
1) No Heat Trasfer to Fluid ( is there an analytical solution ? )
2) No heat transfer from the outside surroundings
thanks
RE: Vessel Wall Temperature after Gas Depressurization trough an orifice
The primary source of temperature change of a fluid being depressurized through an orifice is Joule-Tompson cooling. The formula for J-T cooling are very well documented and a search should give you several representations.
This cooling takes place across the orifice and I've often seen significant ice form on the piping downstream of a pressure drop, but the ability of that temperature drop to be reflected upstream is pretty limited. For example, when I see ice downstream of a pressure regulator, the regulator itself is generally the temperature of the upstream process fluid. I would expect in your case that the vessel wall will be the temperature of the upstream process fluid.
David
RE: Vessel Wall Temperature after Gas Depressurization trough an orifice
RE: Vessel Wall Temperature after Gas Depressurization trough an orifice
I've come across this problem a few times on natural gas compressor stations. Some of the above posts refer to JT cooling across the orifice, but this only helps you with the temperature downstream of the orifice. If I read your question right, you are only interested in the minimum temperature in the vessel itself so JT cooling is not relevant.
I've always used the Hysys Depressurisation utility for these calcs. I have an old Excel calc that does something similar but its probably not as accurate. For a single vessel, the assumption of adiabatic expansion is slightly conservative as in reality there will generally be some heat transfer from the surroundings. Slightly conservative isn't a bad thing though. If you blow down the vessel slowly enough, this heat transfer can make a difference, but it makes your calc more complicated. There is no easy rule of thumb that I've seen. Many people follow API 520 / 521's recommendations on emergency blow downs in 15 minutes. You will probably find these standards a good guide.
Good Luck
Clinton
RE: Vessel Wall Temperature after Gas Depressurization trough an orifice
I've work at companies that insisted that their high pressure gas compressor station must be blown down in under 1 minute. When I asked them why or under what standard, none could be produced other than some guy back in 1963 wrote up a manual. I've seen 8" ball valve open on compressor stations blown down from 1100 psi. I finally got that practice stopped when the compressor lube oil in the discharge header was swept out and the farmer next door won a several hundred dollar lawsuit over having his farm sprayed with 30wt oil. The fix was a 1 in orifice downstream of the 8" ball valve.