Pressure Relief at Propane Chiller

[wenchop]

Hello, I have to design a pressure relief valve (PSV) for fire situation for the shell of a propane chiller. In the shell, we have propane in evaporating service that removes heat from the produced gas stream in the tubes. At relieving conditions, the propane is above the critical pressure. How do I go about calculating the relief load for fire for this shell? I am doing basic engineering and do not need to dive into so much detail at this point.

 

[Latexman]

So far, all your posts here have been on relief design, specifically PSVs. You may get faster, more, and/or better responses at the "Safety Relief Valve engineering (PSV) forum".

Remember Forum Rule #4 - Please do not cross-post the same question to two or more fora. If you feel strongly that your post needs to reach the audiences of two different fora, post your question to one forum and then post a link to that thread in the other forum.


Good luck,
Latexman

 

[Latexman]

My company treats a fluid in the supercritical phase as a gas under conditions of fire exposure because the temperature of the fluid will continue to rise for the duration of the fire. These supercritical systems should use the same criteria as vessels containing gas.

Good luck,
Latexman

 

[Latexman]

My previous post assumes the propane is above its critical temperature at relieving conditions too. That information was not given. Please confirm or correct me.

Good luck,
Latexman

 

[wenchop]

Latexman:

Yes it is above the critical temperature.

thanks,

 

[TD2K]

With a supercritical gas you start off with a fixed volume and mass of gas. As you add the heat from a fire, the temperature rises as a function of the gas's heat capacity and the pressure also rises. With a supercritical gas, until the PSV starts to relieve, the mass in the system is fixed and therefore so is its density as the temperature and pressure rises.

Once you reach the PSV setpressure, gas has to be relieved at a flow rate such that the internal pressure remains constant. In other words, the rise in pressure that would have occured from the extra heat and increase in temperature over some time step is offset by reducing the mass in the chiller through the PSV.

My concern with a PSV in this service is that without liquid boiling to a vapor to keep the metal shell cool, the shell will reach temperatures which will compromise its ability to withstand the internal pressure and the pressure vessel fails. Water deluge or depressuring systems are sometimes used to prevent this.