The general formula for the heat intensity q is :
q = (e)(Q)/(4 pi R^2), where
(e) is the emissivity of the flame taken equal to 0.0048 x(sqrt of MW of the vapor burnt);
(Q)is the heat generated by the flame, and
R is the distance from the flame.
Codes give different formulas to include factors such as flare stack diameter, vapour flow rates, flare height, etc.
As far as I know, the usual criterion is that the heat intensity at the base of the flare stack shouldn't exceed 1500 Btu/(h.sqft).
Tables show that a figure of 2000 results in blisters after 20 seconds, 5300 in 5 seconds. Values of 3000-4000 ignite wood and vegetation. Compare with solar radiation: 350 Btu/(h.sqft).
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You indicated that 'tables' show the results of the radiant ground level heat from a flare to produce certain results such as blister, ignite wood, etc. Could you please tell me where I may find a copy of these tables for reference on a job I am currently working.
a. The book titled Chemical Process Safety: Fundamentals with Applications 1990 by Crowl and Louvar, Prentice Hall, Inc. ISBN 0-13-129701-5, has a small table on the effects of radiation under the heading "Flares"/
b. "Standardized Numerical Risk Assessment" by Ebdon, appearing as a website by Google:
I was recently involved with evaluating a (sucessful)proposal to send people to the top platform of a live flare stack and thus had the opportunity to get involved with similar issues. Take a look at API 521 part 4.4.2.3.1 Table 7.
I'll take issue with one number presented in this document and also repeated earlier in this thread. My impression is that the 250-350 BTU/hr*sq.ft. value given for thermal radiation is a 24 hour average. In other words, solar radiation at night (zero) is averaged in to get this value. It is useful perhaps for solar energy calc's, but not so much for a relative comparison of relatively short exposures. Take a look at
and in particular at the "hourly data files". I would argue that an employee wearing dark coveralls (with an absorbtivity of ~1) would receive over 1000 BTU/hr*sq.ft between noon and 1:00 pm on a hot summer day in a warm climate (Alabama, California...).
If someone can authoritatively confirm or deny my impression, I'd appreciate it!
