Relief Headers

[bnovak]

I have 2 separate 5,000 gallon reactors each fitted with their own rupture disk (no PSV), but tie into the same relief header. They’re (the RDs) stamped with 82 psig @ 72 F. Doing a quick calculation (assuming only water in the reactor) gives me around a 2-2.5” required area and around 30,000 lb/hr capacity (?)(fire case). Both of these relief lines go 20’ out from the reactor with 2” lines, then connect at a “T” and continue another 100’ in a 2” line before emptying into an open-top tank. How can I go about determining whether or not the “T” is OK, or if I should break it and run 2 separate 2” lines out to the tank?

 

[pleckner]

It's time to plug my ariticles at

http://www.cheresources.com.

Go to the site and on the menu at the left select "Free Articles", "Pressure Relief Desing". You will find my series on rupture disks there.

You treat the rupture disk (and holder) like any other piping component. Start the calculation backwards since we usually know the terminal pressure (atmosphere, some seal pot, etc.) Then calculate the final pressure at the vessel. This must not exceed the design pressure (or MAWP if known) plus allowable accumulation.

I discuss this and other issues in my articles such as how one rupture disk can affect another one tied into the same relief header.

 

[StoneCold]

bnovak
Just a quick comment. You are sizing the relief based on water in the reactor. Do you actually make only compounds in the reactor that use water as the solvent?

Second comment. All of our reactors have the reliefs sized for run away reactions as these cases are much worse than the fire case FOR US.

If the reactor was at our location the rupture disk size would be in the 10" to 12" range.

Granted all those things are based on what my company does but it may give you a little pause as to how you are calculating your relief capacity.

Good luck this is a difficult area.

StoneCold

 

[bnovak]

Thanks for the info. And as far as the water comment, I guess you could consider the reactor more of a mixing tank under 30 pounds of pressure. I’m dealing with at least 80% water by volume. After reading through all of the info from pleckner I have decided to go ahead and run 2 separate relief lines regardless of whether or not the 2” line is big enough. I also happened to get the burst certificate from BS&B for the RD and it is stamped for 82 psig @ 72 F AND!! 73 psig @ 365 F, so I’m assuming I should have been using the latter for the calculation. So I know I have 2” RD, but I would like to make sure that it is big enough, but I’m having a problem determining the cp/cv, the latent heat, and the compressibility factor. I can’t decide if I should use superheated steam tables or what?

And I’m not worried about a runaway reaction since there’s not a real temperature dependent reaction going on. I’m more worried about a fire starting underneath the reactor and boiling all of the water out of the reactor.

 

[pleckner]

You use the burst condition that most closely follows your relieving conditions. At the time of relief, you should be saturated in the vessel.