Vessel Wall Temperature after Gas Depressurization trough an orifice

[Max1976]

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

 

[sailoday28]

You must define adiabatic. Is the process without heat transfer to the fluid or without heat transfer from the surroundings to the vessel?
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?

 

[Max1976]

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

 

[zdas04]

My thermo book defines adiabatic as a reversable process that has zero heat loss to the environment and zero heat gain from the environment. I've never heard of a half-adiabatic (i.e., if the fluid gains significant heat from the environment, but doesn't lose any then the process is not adiabatic).

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

 

[sailoday28]

J-T across an orifice is applicable if the process is adiabatic and there is negligible change in KE.

 

[CSmith]

Hi Max,

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

 

[dcasto]

unless the gas starts out at a temperature near the limits of the componets, say -20F, there is no way a gas can be depressed fast enough to be of concern. Even with supecriticle ethylen wher there requires a huge energy input to vaporize the "liquid" ethylene, we never worried about failure of carbon steel during blowdown, but we did limit the blowdown rate to under 25,000 pounds per hour.

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.