Hazardous Heat

NFPA Journal, Sep/Oct 2006 by Raj, Phani K

Review of the radiant heat flux hazard criterion used for establishing safety zones around LNG and other hydrocarbon fires

THE HAZARD FROM A FIRE, over a hydrocarbon liquid (pool fire) or the burning of dispersed vapor mixed with air (vapor fire) to people and objects outside the fire, arises from heat radiated by the fire ("thermal radiation").

Different levels of hazards result depending upon the level of the incident radiant heat flux or intensity (indicated in units of kW/m2 or Btu/hr ft2), the area of exposure, the duration of exposure, and in the case of humans, the percent of body covered by clothing and the evasive actions taken. Hazard to people manifests in the form of skin burn or, in extreme cases, fatality. Wooden structures exhibit charring, ignition, or complete burning when subjected to high levels of thermal radiation flux.

NFPA 59A, Production, Storage, and Handling of Liquefied Natural Gas (LNG), stipulates 5 kW/m^sup 2^ (1,600 Btu/hr ft^sup 2^) as a safe level of exposure at a property line that can be built upon next to a LNG storage facility. One of the comments submitted to the Committee challenges this value and recommends lowering this value to 2.5 kW/m^sup 2^ (800 Btu/hr ft^sup 2^).

This white paper addresses the scientific data that are available in the literature related to safe levels of exposure of people to heat radiation from fires (based on historical evidence) as well as from burn injury tests. To put the above values of the heat intensity in perspective, we have compiled in Table 1 different "heat exposures" to which human beings are subject in the course of daily life and the quantitative values of such exposures. Society, in general, is tolerant to these types of "hazardous heat" exposures. Table 2 shows the different levels set by various regulatory and standards setting agencies that are concerned with public safety. It is seen that public exposure hazard threshold is universally set at about 5 kW/m^sup 2^ level.

Burn Injury Criteria

A person's skin exposed to heat radiation reacts by perspiring and increasing blood flow to the "hot" area. Pain is felt when the [normal 37°C (98.4°F)] skin temperature rises to just above 44°C (111oF) over a depth of 0.1 millimeter. Pain and injury continue whilst the temperature remains above 44°C. The rate of injury increases by a factor of 3 for every degree above 44°C, such that at 50°C the injury rate is ~100 times that at 44°C. Burn injuries are reversible or non reversible depending upon the degree of burn (based on the exposure heat flux, heat dose or duration of exposure). The burn injury to a human skin can range from heat pain to first, second and third degree burns (see Table 3 for definitions of the degrees of burn).

Burn injury to a skin is the result of coagulation of the protein "collagen." The degree of necrosis (death) of skin and coagulation of protein depends upon the total amount of energy absorbed after the epidermis (the outer skin) reaches 44°C. In fact, the degree of burn is related to a modified dose quantity given by the product of 14/3 and t, where I is the intensity and "t" the time of exposure. Figure 1 shows the results of experimental data as well as data from accident investigations involving skin burns. These data are also correlated in Table 4 in terms of burn injuries vs. modified dosage values. Also indicated in this table are the exposure times to cause an injury at an intensity level of 5 kW/m^sup 2^.

It is noted from the results indicated in Figure 1 that second degree burns can be expected when a bare, unprotected (by clothing), skin is exposed to a thermal intensity of 5 kW/m^sup 2^ for 30 seconds. However, it is also known that when human beings are exposed to a heat episode they tend to take evasive action within 5 seconds of exposure. It is estimated that a person can ambulate at a speed of 4 m/s in an emergency. Therefore, within a 30 second exposure a person may be able to seek cover in a shadow behind a building, a tree or some other taller object. Such a shadow will provide significant protection from potential burn from the fire exposure. It is estimated that in an urban or an industrial area, it is highly likely that such a (shadow) protection would be available within, say, 20 meters, a distance that can easily be covered by ambulation in about 5 seconds. The maximum distance that a person may be able to run, away from the fire, to safety over the 30 second period is estimated to be 100 meters at which distance the radiant intensity will be less than that would cause a second-degree burn. In addition, the clothing on the person (normally a person is clothed over 85 percent of his/her body) provides additional protection not considered in the above assessment.

Discussions

It is seen from the information presented in this document that most regulations, worldwide, stipulate a level of 5 kW/m^sup 2^ as the acceptable level (for the purposes of facility design and location) for public exposure to thermal radiation hazards from a liquid hydrocarbon pool fire. In addition, the historical burn injury data and controlled experimental results on human burn injury indicate that an exposure over a duration of about 30 seconds to the above thermal radiation intensity level results in a second degree burn on a bare skin, unprotected by clothing. At least, one fire department (Phoenix, Arizona) is of the opinion that such a level and duration are acceptable since a second-degree burn is reversible if attended to promptly.