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Monday, August 3, 2009

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Earlier post "Heat Radiation For Pain & Blistering Threshold" has briefing discussed about the exposure time for heat radiation level which lead pain and blister threshold. Equations have been proposed in order to link between exposure time for dedicated heat radiation. Example, with heat radiation of 6.3 kW/m2 (2 000 Btu/h·ft2), the pain threshold is reached in 8 s and blistering occurs in 20 s.

Maximum allowable personnel exposure time to particular heat radiation level proposed in API RP 521 (previous revision - 1997) and API Std 521 (latest revision - May 2008) are marginally different. Detail list out as follow

Heat
Radiation
(kw/m2)
API Rp 521
(March 1997)
API Std 521
(May 2008)
9.45
Exposure must be limited to a few (approx. six) seconds, sufficient for escape only. May consider tower or structure provide some degree of shielding.
Required urgent emergency action. Radiation shielding and/or special protective apparel (e.g. a fire approach suit) required
6.31Emergency actions lasting up to 1 minutes without shielding but with appropriate clothing.
Emergency actions lasting up to 30s without shielding but with appropriate clothing.
4.73
Emergency actions lasting up to several minutes without shielding but with appropriate clothing.Emergency actions lasting 2 min to 3 min without shielding but with appropriate clothing.
1.58personnel with appropriate clothing can be continuously exposedpersonnel with appropriate clothing can be continuously exposed

Above listing shows that latest API Std 521 has more stringent requirement compare to previous revision (1997), specifically for 6.31 and 4.73 kW/m2. The maximum allowable exposure time limit have been reduced. Continuous exposure limit remain unchanged.

Emergency Exposure
Maximum allowable exposure heat radiation of 6.31 and 4.73 kW/m2 (during emergency scenario) have been used as criteria in determining sterile area, an area around flare stack which no personnel shall be around without any personnel protective apparel. For conservative design and/or expect potential increase in flaring rate in future, a limit of 4.73 kW/m2 may be used. For common design, heat radiation limit of 6.31 kW/m2 is widely used. In general, maximum allowable heat radiation of 9.46 kW/m2 will be limited at the flare stack base where personnel may access to sterile area with proper personnel protective apparel.

Continuous Exposure
For continuous exposure, operator is assumed wearing appropriate clothing in a plant. Heat radiation at any location shall be limited to 1.58 kW/m2.

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posted by Webworm, 8:10 AM

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