API12B Allowable Overpressure?

[KernOily]

Morning guys. Have any of you ever seen an allowable overpressure value (i.e. % pressure rise in excess of the tank MAWP) during tank venting for an API 12B tank? I have reviewed both 12B and API2000 and I don't find anything. Working on venting calcs and wondering about 10% overpressure. Thanks in advance!

 

[IFRs]

Given that you do not see it in the standards, why would you consider any overpressure when designing vents?

 

[JStephen]

I believe in API 12D and/or API 12F, this is addressed in an appendix at the rear of the standard. You might check if the latest version of 12B has a corresponding section.
"Why you would consider it"- because the standards are rather arbitrary as to the definitions of the various design pressures, and whether they are increased for venting overpressure or whether the overpressure is included in the design pressure; ditto for emergency venting conditions.

 

[EmmanuelTop]

This subject has been discussed at PVRV Set Points for Atmospheric Tank


Dejan IVANOVIC
Process Engineer, MSChE

 

[JStephen]

If, after research of the newest standards, it still simply isn't clear, it may be worthwhile submitting a request for interpretation to the API committee. That may take 6 months or may see nothing happen at all, but would hopefully clear it up for the future.

 

[KernOily]

Guys thanks for the replies; very much appreciated. Based on this I will advise the owner to re-set the set pressures below the tank MAWP and then I'll check the available relieving rates through the relief devices against the required API2000 relieving rates. Devices are currently set to the MAWP which will create an overpressure situation in excess of MAWP when the devices open. I think this will satisfy the intent of everything I've read thus far. Thanks again! Pete

 

[IFRs]

The pressure relief devices should have curves of flow vs pressure / vacuum. They usually flow relatively little at the set pressure unless they are pilot operated types. It would be conservative to allow some head room under the MAWP as a cushion over the pressure at which the flow satisfies your needs, but that may not be available. Designing your relief system within the MAWP keeps you, your boss, your risk management team, etc comfortable and going "by the book" keeps you off the hot seat or witness stand.

My opinions only, and I tend to be conservative...not always the best approach...