Fire Case

Chapter from the Pressure Relief Design Guide.

EXTERNAL FIRE CASE RELIEF SIZING — API 521

Fire Case Heat Input Methodology

Total heat absorption from external fire per API 521:

ADEQUATE DRAINAGE (fire-proofed area with drainage slope ≥1%):
Q = 21,000 × F × A_ws^0.82 (BTU/hr)

INADEQUATE DRAINAGE:
Q = 34,500 × F × A_ws^0.82 (BTU/hr)

Where:
Q = total heat absorption (BTU/hr)
F = environmental factor (see table below)
A_ws = wetted surface area exposed to fire (ft²)

Wetted surface includes: vessel shell and heads up to 25 ft above grade (or up to fire-protected height). Does NOT include nozzles, support skirt, or surfaces above liquid level in vapor-only vessels.

Environmental Factor (F) Table

Insulation / Protection	F Factor	Basis	Notes	Common Application
Bare vessel (no insulation)	1.0	API 521 Table 4	Default — no credit	Most process vessels
Water spray (deluge)	1.0	API 521 Table 4	No credit per current API	Use drainage credit instead
Insulation (approved, secured)	See calc	API 521 Table 4	F based on insulation conductance	Insulated vessels — calculate F
1-inch mineral fiber insulation	0.3	Typical calculated value	k=0.5 BTU/hr·ft·°F	Common refinery insulation
2-inch mineral fiber insulation	0.15	Typical calculated value	Verify k at fire temperature	Cryogenic or high-value vessels
Fireproofing (concrete/cement)	0.3	API 521 Table 4	Must survive fire exposure	Support columns, skirts
Earth-covered (buried)	0.03	API 521 Table 4	Essentially no fire input	Underground storage
Depressuring system (credit)	See calc	API 521 7.3	Reduce relieving rate by depressuring	High-pressure vessels

Wetted Surface Area Calculation

Wetted surface = all surfaces in contact with liquid up to 25 ft above grade:

Vertical Vessel:
• Cylindrical shell from grade to min(liquid level, 25 ft)
• Bottom head (2:1 ellipsoidal: A = π × D² / 2 approx; hemispherical: A = π × D² / 2)
• Top head only if liquid level above tangent line

Horizontal Vessel:
• Use 50% of total surface area (assuming half-full) unless specific liquid level is known
• Both heads if vessel elevation puts them below 25 ft

Column/Tower:
• Liquid on trays counts — typically assume wetted surface = total shell area up to 25 ft
• Packed columns: consider liquid holdup in packing

Heat Exchangers:
• Shell side: use shell OD × length
• Tube side relief: use total tube surface area exposed to shell-side liquid

Source: Pressure_Relief_Design_Guide_v1.xlsx · sheet “Fire Case”

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