Pressure calculation of 2 phase expansion

[sixpaq]

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. and 45 deg C (boiling liquid), what will be the new pressure in the expanded volume (V1 + Vp) and what will be the fraction of chlorine that is vaporize?

 

[Montemayor]

sixpaq:

Level with us. It's not that I've never seen nor heard of a potential thermal expansion being handled the way you describe it, but what you propose is not practical.

First of all, adding on an expansion chamber the way you describe it only contaminates your chlorine pipe with the contained air.

Secondly, after the initial expansion and disc rupture, the pipe contents will continue to heat up and subsequently expand.

The two options for thermal expansion in a blocked-in pipe used industrially are: a safety pressure relief valve and expansion chambers (with diaphragms).

The amount of liquid chlorine necessary to burst your disc is very miniscule (fraction of a cubic centimeter) so the initial air in the chamber will be compressed (the part not absorbed by the liquid chlorine) by the expanding liquid chlorine. You can make those calculations.

 

[mbeychok]

sixpaq:

I agree with Art that your planned system needs re-thinking. Is there some valid reason for wanting to use a rupture disc?

The usual method of protecting liquid lines from overpressure caused by thermal expansion is to use thermal relief valves that open to release the very small amount of liquid necessary to relieve the overpressure ... and then reseat themselves. A rupture disc will not "re-close" itself.

If you are concerned about releasing very small amounts of chlorine, you could have the thermal relief valves exhaust into the bottom of a small vented vessel containing a solution of sodium hydroxide.

Milton Beychok
(Contact me at www.air-dispersion.com)

 

[EGT01]

Actually, the arrangement that Sixpaq sounds like he is referring to is a common practice for chlorine pipelines. Without going into a lot of detail, the arrangement is designed for the expansion chamber to accommodate a certain volume of expanding liquid and also allows the formation of a vapor pocket. The system then is no longer liquid fuill and the pressure becomes a function of the chlorine vapor pressure.

I would suggest you purchase a copy of the Chlorine Institute's Pamphlet 60, Chlorine Pipelines. In there you will find the details for the standard designs (Type A and Type B). The Type A design is a two chamber arrangement with a primary chamber always intended to operate with some vapor space. The Type B design is a rupture disc straight off the pipeline to allow liquid to expand and flash into a single chamber.

You can download from the Bookstore section of their site

http://www.chlorineinstitute.org/

The pamphlet gives guidance for sizing and operation of the expansion chamber that may answer your questions. If not, post again.