super critical relief

[frankieh]

hello all,

i want to size a relief valve for a fire case. this valve shall be installed on a vessel which contains a mixture of water and light hydrocarbons. The pressure in the vessel is above it's critical pressure sow there is no heat of vaporization. How can calculate the max relief rate?? In the API 521 they suggest to use a very conservative heat of vaporization sow i am wondering if there is a better way to do the job..

any help is more then welcome....

 

[djack77494]

If there's no chance of a phase change, then you needn't consider one. The heat input from a fire would go to thermal expansion, and not to vaporization.

 

[JoeWong88]

frankieh,

For a mixture of water and HC, it is very much subject to quantity of water-in-HC or HC-in-water... You described that the system is at supercritical region, i presumed your system only have traceable amount of water and has little contribution to the phase behaviour of this mixture.

If the water is excessive,there is a great chances of having phase separation between water & HC. Whenever external heat input, normally HC will leave the mixture first and contribute to overpressure of vessel. The HC system may gone in to supercritical region but the water may stay as water phase... Anyway you have to review carefully the system...

For sizing PRV for supercritical fluid, grab the "Rigorously Size Relief Valves for Supercritical Fluid" by Ryan Ouderkirk in the tabulation here (Click HERE). It may further enhance your understanding...