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[jprocess]

Dear All:
API 521-2007 edition in part 5.20.1 states:
"A vapour-depressuring system should have adequate capacity to permit reduction of the vessel stress to a level at which stress rupture is not of immediate concern. For pool-fire exposure and with heat input calculated from Equations (6) or (7), this generally involves reducing the equipment pressure from initial conditions to a level equivalent to 50 % of the vessels design pressure within approximately 15 min."
And
"Depressuring to a gauge pressure of 690 kPa (100 psi) is commonly considered when the
depressuring system is designed to reduce the consequences from a vessel leak."
But in 1997 we read a combination of these 2 statements about final pressure that is 50% of vessel design pressure or 100 psig,whichever is lower.
I think this modification is right for plants with high design pressures for example above 20 barg and considering 100 psig as final pressure will result in conservative high depressuring loads to flare network.
I am interested of knowing your valuable comments about this modification.
Also I read in articles and some procedures that state according to API 521 the fire zone area is 2500-5000 ft2 and as I heard the upper range is more convenient in refineries.But I could find only the lower value in page 57:
"Although the size of the assumed fire zone can vary,
experience generally indicates that a fire that can be confined to approximately 232 m2 (2 500 ft2) of plot area will not affect the design of the main relief headers in processing areas where a depressuring flow discharges into the same relief header"
Your valuable comments are appreciated.

[JoeWong]

For paragraph related to depressuring...

API RP 521, 1997
"Background: The wording in Section 3.19.1 appears to make a differentiation between a fire depressuring scenario and a process upset depressuring scenario. This wording is repeated in the second paragraph with respect to depressuring thick-wall vessels to 50%. In the fifth paragraph, however, it appears that when a fire is controlling, API recommends the depressuring of all vessels (including thick wall vessels) to 100 lbf/in.2 gauge (690 kPa) of 50% of the vessel design pressure, whichever is lower, in 15 minutes.

Question: Does emergency depressuring only apply to systems greater than 250 lbf/in.2 (1724 kPa) with a vessel wall thickness greater than 1 in. (25 mm), or can it be used to depressure any system?"

API committee response :
Equipment operating below 250 lbf/in.2 (1724 kPa) may not necessarily need emergency depressuring since the consequences of equipment failure due to fire exposure would be less than that for larger high-pressure equipment.

The statement “where fire is controlling” is intended to separate those applications where there are no reactive hazards. If there is a reactive hazard that can cause loss of containment due to over-temperature, then emergency depressuring valves may be appropriate for equipment designed for any range of pressures or services.

For equipment below 250 lbf/in.2 (1724 kPa) in light hydrocarbon service, emergency depressuring may be provided. One of the original intents was to provide emergency depressuring LPG vessels to prevent BLEVE during fire exposure. Generally, systems in LPG service will have a design pressure of 250 lbf/in.2 (1724 kPa) or less. Consequently, the plate thickness is often less than one inch. As stated in 3.19.1, a greater depressuring rate may be required for vessels with wall thicknesses less than one inch. This implies that depressuring
to 100 psig is not a requirement for the higher pressure applications.

Probably above leads to revised statement in API STD 2007.