Smoke obscuration

A Fire Hazard Index. There are a number of situations where all that is needed is an indication of whether a change in a specific commercial product is beneficial or not. In these cases, one can presume that the people exposed to the fire and the building in which the fire exists are fixed. Moreover, it must be established that toxicity is the sole threat to escape, not smoke obscuration or heat. 

Fire hazard is a combination of several properties, including ignitability, flammability, flame spread, amount of heat released, rate of heat release, smoke obscuration and smoke toxicity. 

This secondary effect of materials is illustrated by the difficulties encountered, in a recent study [54], when attempts were made to correlate CO concentrations measured in small scale and full scale fire tests. The same small scale equipment (typically the cone calorimeter rate of heat release test) could predict adequately a number of very important full scale fire properties, including ignitability, rate of heat release, amount of heat release and smoke obscuration. It could not, however, be used to... 

Fire safety in a particular scenario is improved by decreasing the corresponding level of fire risk or of fire hazard. Technical studies will, more commonly, address fire hazard assessment. Fire hazard is the result of a combination of several fire properties, including ignitability, flammability, flame spread, amount of heat released, rate of heat release, smoke obscuration and smoke toxicity. 

Smoke obscuration is an essential parameter related to fire hazard, because it may cause visual impairment both of the occupants of a fire scenario and of the rescue team, creating a potential danger. 

The traditional way in which smoke obscuration has been measured is by determining the maximum smoke density (or the specific maximum smoke density) by means of a smoke density chamber developed by the National Bureau of Standards (NBS smoke chamber, ASTM E662). This instrument measures the obscuration inside a static 500 L chamber, after a sample has been exposed, vertically, to a 2.5 W/cm2 radiant source. 

The fact that it is not possible to weigh the sample during the test means that there is no continuous output of weight loss (and, in fact, normally none of smoke obscuration either, since the output tends to be a single final number). Thus, it is, generally, not possible to relate the sequence of events to the burning... 

The Cone calorimeter yields smoke results which have been shown to correlate with those from full scale fires [10, 15-18]. The concept of a combined heat and smoke release measurement variable for small scale tests has been put into mathematical terms for the cone calorimeter smoke parameter (SmkPar) [10]. It is the product of the maximum rate of heat release and the average specific extinction area (a measure of smoke obscuration). The correlation between this smoke parameter and the smoke obscuration in full scale tests has been found to be excellent [10]. The corresponding equation is ... 

The same hazard concept could, potentially, be used for full scale tests, multiplying the total heat released, per unit surface exposed, by the maximum smoke obscuration. This is the basis for the magnitude smoke hazard (Smoke Haz.), shown in Table II. It is of interest that smoke hazard results yield the same ranking as mass of soot formed. Cone calorimeter tests are being planned with the same materials used in the full scale tests to investigate the usefulness of this concept. 

Smoke obscuration, as measured in the NBS smoke chamber, does not correlate well with full-scale fire hazard nor with smoke as measured in heat release calorimeters. In particular, those materials which do not burn up completely in full-scale fires are being treated excessively harshly by this method. 

Smoke parameter (in the Cone calorimeter) and smoke factor (in both calorimeters) are combined properties of smoke obscuration and heat release which compensate for the incomplete burning of fire retardant samples and which should predict smoke hazard in real fires. 

Two important sources of elemental phosphorus in soil are the creation of slag piles during the production of white phosphorus, and the disposal of solid wastes containing elemental phosphorus in hazardous waste landfills (Berkowitz et al. 1981 Idler et al. 1981). The field use of WP/F and red phosphorus/butyl rubber smoke/obscurant releases elemental phosphorus into soil primarily as unburnt phosphorus in munitions (Berkowitz et al. 1981 ... 

ASTM E 1995 Standard Test Method for Measurement of Smoke Obscuration Using a Conical Radiant Source in a Single Closed Chamber, With the Test Specimen Oriented Horizontally. ASTM International, West Conshohocken, PA. 

Smoke obscuration (reduction in light transmitted) is a serious concern in a fire, because a decrease in visibility reduces the light available, thus hindering both escape from the fire and rescue by safety personnel. The main way in which a fire causes visibility decreases is through smoke emission. This decrease in visibility is the result of a combination of two factors how much material is burnt in the real fire (which will be less if the material has better fire performance) and how much smoke is released per unit material burnt. 

Despite the understanding that smoke obscuration ought to be measured in a large scale, or by a method which can predict large-scale smoke release, the most common small-scale test method for measuring smoke from burning products is the traditional smoke chamber in the vertical mode (ASTM E 662)39 (Figure 21.14). The test results are expressed in terms of a quantity called specific optical density, which is defined in the test standard. This test has now been shown to have some serious deficiencies. The most important problem is misrepresentation of the smoke... 

The cone calorimeter,71 which is a dynamic flow-through fire test, can also be used to assess smoke obscuration. The rankings tend to be quite different from those found with the static smoke chamber and are much more realistic. Several empirical parameters have been proposed to make this compensation for incomplete sample consumption, including one called the smoke factor (SmkFct), determined in small-scale RHR calorimeters.188 It combines the two aspects mentioned earlier the light obscuration (as the total smoke released) and the peak RHR. 

ASTM D 5424 Standard Test Method for Smoke Obscuration of Insulating Materials Contained in Electrical or Optical Fiber Cables When Burning in a Vertical Cable Tray Configuration... 

ASTM D 5537, Standard Test Method for Heat Release, Flame Spread, Smoke Obscuration, and Mass Foss Testing of Insulating Materials Contained in Electrical or Optical Fiber Cables When Burning in a Vertical Cable Tray Configuration, Annual Book ASTM Standards, American Society for Testing and Materials, West Conshohocken, PA. 

Hirschler, M.M., Smoke in fires Obscuration and toxicity, Plenary Fecture, Business Communications Company Conference on Recent Advances in Flame Retardancy of Polymeric Materials, May 15-17, Stamford, CT, Eds. G.S. Kirshenbaum and M. Lewin, pp. 70-82, Norwalk, CT, 1990 Hirschler, M.M., How to measure smoke obscuration in a manner relevant to fire hazard assessment Use of heat release calorimetry test equipment, J. Fire Sci., 9, 183-222 (1991). 

Hirschler, M.M. and Janssens, M.L., Smoke obscuration measurements in the NFPA 265 Room-Comer Test, Proceedings of the 6th Fire and Materials Conference, February 22-23, Ed. S.J. Grayson, San Antonio, TX Interscience Commun., London, U.K., pp. 179-198, 1999. 

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