Pressure calculation of 2 phase expansion (2) 2

[sixpaq]

Thanks to Montemayor, MBeychok and EGTO1 for responding.

Indeed the expansion pipe arrangement (Type B) shown in Pamphlet 60 of the Chlorine Institute is the actual design installed. My task is to connect the expansion pipe to the Derakane vent header (1 bar abs) so the pipe can be emptied. I want to know if I need to install a restriction orifice at the outlet expansion pipe to reduce the pressure below that of the Derakane pipe spec. If the rupture disk bursts at 14.8 bar abs, liquid chloine will flash and I would like to know how to calculate the equilibrium pressure in the expanded volume (V1 + Vp) and what will be the fraction of chlorine that is vaporize?


By the way the reason a PSV was not used was we did not want to get liquid chlorine in the vent header.

---------------------original Thread-----------------------
Dear Helper,
I feel like a “fish out of water” after recently transferring from R&D to the Process Engineering department of the company, so thank you for taking the time to respond to my problem. Here it is:

There is a pipe (volume (V1) = 12 liters), which has the potential to be blocked in at both ends. The process fluid is liquid chlorine. In order to prevent the pipe from bursting from thermal expansion (solar heating to 45 deg C), there is a rupture disk installed in a branch connection of the line (set pressure is 14.8 bar abs.). The outlet of the rupture disk discharges into an enclosed expansion pipe (volume (Vp) = 2.3 liters) at 1 bar abs. (air). I would like to know, if the rupture disk bursts at 14.8 bar abs what will be the new pressure in the expanded volume (V1 + Vp) and what will be the fraction of chlorine that is vaporize?

 

[Latexman]

At 45 C, I estimate the vapor pressure of Cl2 to be 183 psia. The liquid density to vapor density ratio looks to be about 34. Ignoring the air or inert gas in the expansion pipe and at your present configuration (no orifice to bleed off the Cl2), 2.3 l of vapor Cl2 will require 0.068 l of liquid Cl2. You should end up with about 2.368 l of vapor Cl2 and 11.932 l of liquid Cl2. The pressure will still be about 183 psia since there is liquid Cl2 present. The amount vaporized, about 0.5% of the liquid), should have negligible effect on the temperature and vapor pressure. My data source was Perry's 5th Edition, page 3-167.

Good luck,
Latexman

 

[EGT01]

Sixpaq,

Without any external source of pressure and by ignoring any further liquid expansion, the only source of pressure in the system will be that from the vapor pressure of your liquid chlorine. And as Latexman's analysis indicates, you should end up with your expansion chamber essentially full of chlorine vapor and at a pressure equal to the vapor pressure of the liquid at temperature.

Now I would caution you about your comment that you want to install a restriction orifice to reduce the pressure below that of the Derakane pipe spec. Don't count on the RO as a protective device for overpressure, they only work when there is flow. A block valve closed in the derakane piping downstream of the RO will allow the derakane piping to equalize to the same pressure upstream of the RO. Call me picky, but the RO would be intended to limit the flow to not exceed the downstream capacity of the piping or your vapor disposal system.

In one respect, you should rarely if ever have to use your expansion chamber system and if you are having to use it often then you should be looking into the root cause of your pipeline overpressure events. That said, I would tend to avoid overdesigning the chamber vent system unless there is good reason otherwise. Think about how you handle routine line preps, I don't see this being any different.

Also, I notice you started a completely new thread, which is okay, but when I suggested that you post again if you needed additional information, it would have been okay just to continue in the same thread.;-)