3 replies to this topic

[Linus Van Pelt]

I'm looking to size a low pressure rupture disc set at 0.49barg for a BS799 vessel with a design pressure of 0.5barg. Presumably API 2000 is the applicable code, but I am confused as to how to apply these equations to a rupture disk as they seem to be assuming that a "conservation vent" type relief valve will be used. I can determined the venting requirement from section 4.3 but only "Capacity testing of devices" is mentioned in Section 4.6 not sizing per se.
Any ideas would be appreciated.
Many Thanks,
Linus

[pleckner]

In the future, please post these type of quesitons in the Pressure Relief Devices Forum.

You are correct in that there is very little guidance on sizing rupture disks for low pressure service. You should actually size them as you would for a high pressure syste, i.e. assume they are nothing more than another piping component (the Resistance to Flow Method). You would then use the certified Kr value obtained from the vendors. However, the vendors I've spoken to don't have Kr values for the typical models required for low pressure service (at least they aren't published). So the recommended procedure is to come up with your relieving requirement and discuss the application with the vendor you intend to purchase from.

The alternative calculation method is the Coefficient of Discharge method in which the rupture disk is treated as a relief valve, i.e. uses the same sizing equation but with a specific coefficient of discharge. The problem here is that in low pressure systems, you are rarely at choked condition and thus the basic equations don't apply. There are means of estimating but it is somewhat complicated and just that, estimated.

The bottom line, talk to the vendor, they won't bite.

[Linus Van Pelt]

Thanks Phil,

Apologies for the "wrong forum" error - of course I realised this as soon as I had posted
What do you mean by "The problem here is that in low pressure systems, you are rarely at choked condition and thus the basic equations don't apply"? What about the API 520 sub-critical equations?

Linus

[pleckner]

Again, the proper and more accurately way to size a rupture disk is using the Resistance to Flow Method. If you are intent on using the Coefficient of Discharge Method, i.e. you meet the 8 & 5 Rule and thus can use the API formulas, you still need a coefficient of discharge. The recommended value of 0.62 is based on choked flow. I have yet to see a suggested value for the coefficient of discharge for subcritical flows.

There is a procedure given to estimate the size of miscellaneous low-pressure relief devices in "Guidelines for Pressure Relief and Effluent Handling Systems", CCPS of AIChE, 1998. (By the way, to all reading this post, this is a must have book if you are doing any safety relief system calculations more than once in your career.)

My best advise is to talk with the vendor.