PSV for dense phase CO2 pipeline 1

[cjvanderhoef]

I am working on a high pressure dense phase CO2 pipeline.
I am a recent graduate in chemical engineering and have not sized a PSV before. My company has a sizing spreedsheet, but I wanted to make sure I am addressing everything that needs to be addressed. This is the reason for posting to this site. Here are my initial concerns: This PSV will be in an enclosed building. Therefore the PSV will vent outside the building. The operating pressure of the pipeline is 1,800 psig. The JT effect from a DP of 1,800 psig to atm will reduce the temperature to below -50F. How is this temperature drop handled? Do you use low temp steel or heat trace, or is there another way to handle it? Also, what am I missing or what other things do I need to consider when sizing this PSV.

Any comments are greatly appreciated,

Thanks,

Christian

 

[MortenA]

Normal carbon steel should be able to handle -46 deg C. This equals about -50 deg F (probably the same when rounding a couple of times .

But its not certain that the steel will acquire the same temperature as the fluid. A resent article in Hydro ´carbon processing addressed this subject and found that in many cases the temperature dint get this low.

Best regards

Morten

 

[dcasto]

OK, you are going to have to come up with some risk based compromised. Yes, your dischrage temperature will be below the the temperature for carbon steel, but, the stress' on the carbon steel will be reduced while at the low temperature. So the vent line from the relief valve can be made of carbon steel, the probability and consequences of a line with CO2 failing is a low risk.

The problem with a relief valve and its materials of construction will be deciding on its individual materials. Finding low temperature springs that have the allowable stress at 1800 psig is tough. So, again, analyze the risk of a fail using carbon steel and the appropriate material for the spring (or pilot operated parts). ou should be able to stick with a carbon steel valve too.

 

[MortenA]

dcasto:

Normal A333 6 SMLS CS for pipe could have a design temp as low as -46deg C/-50 degF. But since its CO2 it may be something more corrosion resitant?

Best regards

Morten

 

[Ashereng]

I am not sure what "normal" carbon steel is for you.

For me, normal carbon steel is ASTM A106 Gr B SMLS, which is good to -29 to 200 °C.

If you need to go below -50 °C with CO2, I am thinking 316SS?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[dcasto]

The limit of temperature is for the stress level at a temperature. As you lower the temperature on CO2, the equalibrium pressure and stress drop. And the reverse is true, as you drop the pressure, the temperature drops and the stress levels drop.

Are you going to make your entire pipeline out of 304SS? If you have a line rupture in the middle of nowhere, the whole line, including those portions running through people backyards will auto refrigerate to -100F. Now what, -100 F line made of carbon steel?

 

[gene2007]

Be careful about discharging high pressure CO2. Consider its phase diagram and be sure you don't end up with a solid that can plug the discharge line.

 

[MortenA]

gene2007

Good point!

Best regards

Morten

 

[dcasto]

The CO2 that turns solid will be swept out due to velocity, 1 90 bend. There will be no corrsion.