The height needs to be considered against pool fires

API RP 521 (2020) 

25ft (7.6m)

To determine vapor generation, it is necessary to recognize only that portion of the vessel that is wetted by its

internal  liquid and is equal to or less than 7.6 m (25 ft) above the source of flame.

 

NOTE Hydrocarbon fires can exceed 7.6 m (25 ft) in height;

however, experience has shown that it is necessary only to size relief  devices on the basis of the averaged heat input

up to a height of 7.6 m (25 ft) above the base of a pool fire.

 

The term “base of a pool fire” usually refers to ground level but could be at any level at which a substantial spill or
pool fire could be sustained. Various classes of vessels are operated only partially full. Table 4 gives recommended
portions of liquid inventory for use in calculations. Wetted surfaces higher than 7.6 m (25 ft) are normally excluded
because pool fire flames are not likely to impinge for long durations above this height. Also, vessel heads protected
by support skirts with limited ventilation are normally not included when determining wetted area. The user shall
specify whether to include the wetted surface area of connected piping in the wetted-area calculation.
The wetted area for spheres includes all area up to the maximum diameter. Table 4 recommends the wetted surface
area for spheres be based on “the maximum horizontal diameter or up to the height of 7.6 m (25 ft), whichever is
greater.” Hence, as a minimum, the wetted surface area of the entire bottom hemisphere shall be used even when
the sphere “equator” exceeds 7.6 m (25 ft) in height. The criterion is supported by previous incidents and tests that
have shown that pool fire flames can follow the underside profile of spheres resulting in the entire bottom
hemisphere being exposed to a high fire heat load.

 

API RP 2030(2014)

Vertical vessels and towers may need to be protected only to a height of 40 ft (12.2 m)  above the level at which a liquid pool fire could form.

 

API RP 2218 (2020) 

20 ft to 40 ft (6 m to 12 m) vertically from the source of liquid fuel. 

 

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