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API 521 Vapour depressuring requirements

API 521 Vapour depressuring requirements

API 521 Vapour depressuring requirements

(OP)
According to API 521, 4th edition (March 1997), section 3.19.1:

"[...] Where fire is controlling, it may be appropriate to limit the application of vapor depressuring to facilities that operate at 250 psi (1724 kPa(g)) and above, where the size of the equipment and volume of the contents are significant. [...]

Does anyone know the background of this? Or have used this guideline before?

RE: API 521 Vapour depressuring requirements

(OP)
The last couple of days I have been further studying the API RP521, as well as literature on this subject. Examples of this literature are:

"Review of the Response of Pressurised Process Vessels and Equipment to Fire Attack", HSE Offshore Technology Report OTO 2000 051, (2000).
"Size Depressurization and Relief Devices for Pressurized Segments Exposed to Fire", Chemical Engineering Progress, September 2002, p. 38-45.

(both can be downloaded from the Internet)

My conclusions are that the API RP 521 guidelines both on when to apply emergency depressuring systems (EDS), and what depressuring rate to use are inadequate. Use of these API guidelines may not prevent catastrophic vessel rupture, especially in confined areas such as offshore platforms. Main reason is that heat fluxes from local poolfires or jet fires can be significantly (e.g. 3.5 times) higher than heat flux from open pool fire (as assumed by API 521).

EDS should be applied when a risk analysis shows that system without EDS results in unacceptable risk. For this assessment, both likelihood and consequences of the vessel rupture due to fire have to be taken into account.

For determination of the depressuring rate, a detailed iterative calculation sequence has to be performed (as outlined in above mentioned literature). Problems are that currently no (validated) software tools are available, as well the required thermo-mechanical material properties. It may also be difficult to estimate the time dependent fire characteristics.

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