What's the best practice for sizing and conducting radiation analysis for relief scenarios with high instantaneous flow but short duration due to limited inventory?

Several programs can predict this such as HyRAM or PHAST. The inputs are critical to a safe
answer.

This is not a simple answer due to the many types of flame lengths and flame orientations due to pressure and direction. NFPA 2 recommends that vent systems should be designed so that if the safety relief valve is relieving at capacity the radiative heat felt by an individual at grade does not exceed 500 Btu/hr/ft2 (5.68 MJ/hr/m2) (A.10.4.4).

NFPA 2, section E has a lot of good information on this subject.

As an example, let’s look at a gaseous H2 cylinder release through a rupture disc. The flow is straight up through a vent. If ignited, initially the radiation will be at it’s highest. As the pressure in the cylinder is reduced the flow rate is reduced, and thus so is the radiative area. At the same time there is a maximum time in which a person can be in the radiative heat flux. Distance to radiation heat flux level of 4732 W/m2 (1500 Btu/hr · ft2) with exposure to employees for a maximum of 3 minutes.

The heat flux location will define how transient flow vs radiation locations will be defined. The heat flux values in NFPA 2 include safety factors.

Auxiliary information

• 1,577 W/m2 (500 Btu/hr ft2) is defined by API 521 as the heat flux threshold where personnel with appropriate clothing may be continuously exposed. This value is close to, but actually less than what the Society of Fire Protection Engineers determined to be the “no-harm” heat flux threshold (540 Btu/hr ft2), that is, the maximum heat flux to which people can be exposed for prolonged periods of time without experiencing pain.
• 4,732 W/m2 (1,500 Btu/hr ft2) is defined by API 521 as the heat flux threshold in areas where emergency actions lasting several minutes may be required by personnel without shielding but with appropriate clothing. It is also defined by the International Fire Code
  as the threshold for exposure to employees for a maximum of 3 minutes. This value is close to the heat flux level used by other standards (e.g., NFPA 59A, EN 1473) as the threshold for public exposure (1,600 Btu/hr ft2).
• 20,000 W/m2 (6,340 Btu/hr ft2) is generally considered the minimum heat flux for the non-piloted ignition of combustible materials, such as wood.
• 25,237 W/m2 (8,000 Btu/hr ft2) is the threshold heat flux imposed by the International Fire Code for non-combustible materials. Other standards use somewhat larger values for heat flux damage to prevent damage to non-combustible construction.
  • API 521, Guide for Pressure Relieving Systems, 1997 Ed., Table 8, pg. 41
  • SFPE Engineering Guide, Predicting 1st and 2nd Degree Skin Burns from Thermal Radiation, March 2000, page 8
  • API 521, Guide for Pressure Relieving Systems, 1997 Ed., Table 8, pg. 41
  • 2003 International Fire Code, Sec. 2209.5.4.2(3)
  • According to literature, exposure to a heat flux of 1,600 BTU/ft2-hr will lead to 2nd degree burns over the exposed skin in approximately 30 seconds.
  • The Center for Chemical Process Safety )CCPS) book titled “Loss prevention in the process industry” lists 23,800 W/m2 as the minimum heat flux for unpiloted ignition of various kinds of wood
  • 2003 International Fire Code, Sec. 2209.5.4.2(3)
  • NFPA 59A sets a threshold of 10,000 Btu/ft2-hr for a property line that can be built upon.