sizing a relief valve for spuer critical fluids

[frankieh]

Hi folks,

does somebody know if hysys 2006 (depressuring utility) is capable to estimate the size of a relief valve for a fire case (api 521) when the fluid in the vessel is above it's critical pressure. There is no heat of vaporization sow i'm wondering if this is possible?
We have hysys at our office but not the support.

Thanks for the help

 

[ggordil]

I am not sure if HYSYS has the ability to estimate the size of a relief valve for this particular case.

When sizing for fire and the fluid is near or at the critical point, I assume a heat of vaporization of 50 btu/lb. It is a pretty conservative assumption but I think that it works well.

 

[MortenA]

When in the dense phase size for unwetted in the fire case.

Dont be fooled by the HYSYS "vapour fraction". Its either 1 or 0 when in the dense phae. As far as i remember its just based on density so that when the density becomes high enough it changes from 1 to 0. No real phase change.

Best regards

Morten

 

[frankieh]

Moretena, can you explain why i have to size for unwetted fire case?

Thanks anyway for your help.

 

[ProSea]

Hi frankieh

I am not sure about HYSYS, but according to API521 the discharge area of the PSVs on vessels containing super critical fluids exposed to open fires can be estimated by usuing equation 8, sec 5.15.2.2.2 (API521, 5th ed, 2007). the normal operating conditions can be below the thermodynamic conditions but the if the relieving pressure is crtical you can use it.i will sugest to use a spread sheet instead of hysys for this particular case.

where as 50btu/lb latent heat is a conservative case when the fluid (hydrocarbons) is below crtical conditions, if they will be over crtical conditions, then you wont be able to find the other parameters to use in the calculations, like ratio of specific heats and compressibility factor. and if you think you have found it by any commercial simulation software then i think you shouldnt belive it. what you do is try to make a phase envelope of the stream you want to size the PSV for, then check the crtical conditions, if the stream CP and CT is lower than the reliveing conditions then at Releving conditions whatever you are looking into your model is not correct.
so in this case using 50btu/lb wont be a good idea and unacceptable from authorities.

hope this may cleared your concern.
kind regards.