Rupture disc calculation

[dannybagpipes]

I need to calculate if a 1 inch rupture disc at 15 psig will be satisfactory for a 150 gallon tank. The two scenarios include: negligible liquid and nearly all vapor or a nearly full tank with liquid. The nozzle on the vessel is 1/2" and a reducer is used to increase the pipe size to 1 inch prior to the rupture disk. The manufacturer of the rupture disk states that Kr=2.40. The rupture disk piping ties into a header which then goes to atmospheric pressure. Can someone shed some light on this issue?

 

[StoneCold]

Danny
The flow is going to be determined by the 1/2" nozzle on the vessel.
You need to calculate the relief load and then the required orfice area. If the area is larger than the 1/2" nozzle you are in trouble.
Look at the Crosby engineering handbook for the loading calcuations. Look at one of the rupturedisk sites for how to incorporate a Kr factor into your orfice calculation.

Regards
StoneCold

 

[dannybagpipes]

I wish I had the Crosby engineering handbook! My company won't want to pay for it, maybe I'll just have to suck up the cost myself.

Thanks for your help!

 

[Latexman]

Google "rupture disk" and search through all the vendors websites. You may also google "safety valve" and do the same thing. I'm pretty sure you can get free catalogs and software this way. Just be sure you understand the fundamentals before you plug and chug with the formulas and software.

Good luck,
Latexman

 

[dannybagpipes]

Hey Stonecold, what do you mean by relief load? Do you mean a flow rate? I am not sure of the flow rate, that's what I'm trying to figure out........if the nozzle/relief piping will allow sufficient flow.

Dan

 

[StoneCold]

Dan
The crosby book is free. Here is the link. I think if you read it it will help you a lot with this problem.

http://www.tycoflowcontrol-na.com/ld/CROMC-0296-US.pdf

If after reading the book you still have questions come back and I will try to help you more.

Regards
StoneCold

 

[EGT01]

dannybagpipes,

What you have identified in your original post as being "The two scenarios" is really only partial descriptions of the operating conditions of your tank. Generally speaking, a "scenario" would be in reference to an event that causes the pressure in your system to increase and if the event can exceed the maximum allowable pressure of any component within the system you are trying to protect, then it becomes an overpressure scenario for which you need to provide overpressure protection.

Determining the relief load requires you to have a good understanding of the processes involved that are affecting the system pressure. For example, if your tank is provided with a nitrogen purge system then failure or mis-operation of the nitrogen controls to the tank can cause the tank to overpressure. If so, then you would need to provide overpressure protection equal to the rate of nitrogen that was causing the overpessure. Overfilling the tank by pumping material into the tank is another common case to consider as is external fire resulting in heat input that could cause liquid vapor pressure to increase and eventually boil generating vapor that must be relieved.

This are to name only a few of the possible sources of overpressure. Since you are talking about a rupture disc set at 15 psig, I assume you have a pressure vessel so you should consider the code requirements for overpressure protection of the system (see this thread for additional info thread1203-157530).

The Crosby Handbook is a very good reference and it does include some discussion about how to determine a fire relief load. For more thorough coverage about how to determine your required relief rate, I would suggest you get a copy of API RP-521.

Also, here's a link to a good online resource discussing considerations for rupture discs...

http://www.cheresources.com/asiseeit.shtml

.

 

[UmeshMathur]

dannybagpipes:

In addition to the important issues raised in the previous posts, don't forget to study the issue of process dynamics (instantaneous loads can be a lot higher than the steady-state or design value). Believe it or not, I have seen cases where the dynamic load exceeded the "maximum" steady-state value by over 25%. Such systems can be analyzed readily using currently available dynamic simulation software and studying the effects of worst-case jumps in the independent variables.

 

[Montemayor]

Danny Boy:

For information on everything you can learn on Rupture Discs, go to:

http://www.cheresources.com/asiseeit.shtml

Phil Leckner puts on a show there that is top-notch in know-how and experience regarding safety devices - and it's all free.

 

[JimCasey]

15 psi is a pretty low setpoint for a 1" rupture disc...And for a forward-bursting disc the operating/burst ratio shouldn't be greater then 0.7. So if you operate the tank at greater than (15*.7) 10.5 psi, the disc will probably fail after a few cycles at substantially less than 15 psi.