two-phase relief

[barai19]

Dear Friend,
I have done many two-phase relief calculations by using HEM method. But still I am not clear about two-phase relief. I would like to know when and at what condition two-phase (vapor-liquid) relief can occur. I remember that when both vapor and liquid phase reaches its thermodynamic equilibrium, two-phase relief occurs. If it is true, at what condition a liquid filled vessel can reach its thermodynamic equilibrium. How can I determine there is a two-phase relief? I usually determine by flashing the liquid at relief condition through Pro II simulation software. Is there anyway I can quickly predict? I would appreciate it if anyone could answer my question.
Thanks
Barani

 

[monaco8774]

There are three cases for two phase relief

1) A manifold or flowline relief with blocked outlet - clearly both the liquid and gas have to be relieved (also aplies for a seperator with inadequate high level trips)

2) Relief from a seperator where at the relieving pressure the gas being relieved is above the cricondenbar (usually 100 bar or more for hydrocarbons) and not at high temperature so that as it relieves the gas goes into the two phase region

3) A liquid relief case where the liquid is at or around its bubble point so that gas eveolves as the pressure drops

Since you are talking about being at equilibrium, I assume you are talking about the third case. - this will happen any time the relief is on the liquid outlet of a seperator and there is no prior cooling or pressure increase through a pump.