Temperature effects flammable atmospheres

In view of the above adverse effects a safety factor should be applied where flammability is assessed using flash point. For pure liquids in containers the vapor should be considered potentially flammable if the liquid temperature is upward of at least 5°C below the reported flash point. For mixtures whose composition is less certain, such as petroleum mixtures, the safety factor should be about 15°C relative to the flash point [55]. Where combinations of adverse effects are identified the safety factors should be increased accordingly. A simple but very conservative approach is to assume that all liquids having a flash point <141°F may produce a flammable atmosphere under some ambient conditions, even where no mist or froth production is involved. A more practical approach is to assume that liquids handled in air at least 5-15°C below their closed cup flash points will not present ignition risks unless... 

Failure of a vessel containing a liquid at a temperature above its atmospheric boiling point may produce a BLEVE, with resulting blast and fragment effects. If flammable material is involved, a BLEVE may also produce a fireball. 

A boiling-liquid expanding-vapor explosion (BLEVE) can occur if a container of liquid or liquified gas at temperatures above their atmospheric-pressure boiling points were to rupture. Sudden loss of containment and reduction in pressure can result in explosive vaporization of some of the liquid, and the sudden increase in volume can propel parts of the container to great distances and create blast effects (pressure and impulse). Eurther, if the superheated liquid or liquified gas is flammable, a fireball involving the ejected vapor, aerosol, and liquid can result, particularly if the cause of the overpressure and rupture is exposure to fire. 

The atmosphere in a plant is normally air, but if the oxygen content of the atmosphere is reduced below 21% the flammability limits narrow progressively. Below about 8% oxygen the mixture will not bum whatever the concentration of vapour or gas in the atmosphere (unless the material decomposes to release its own oxygen). The minimum oxygen concentration to ensure nonflammable conditions depends on the temperature of the mixture and this, together with the effect of temperature on flammability limits, is illustrated in Figure 7.2. 

A California statue requiring ha2ardous materials management was passed in 1985 (24), but guidance for compliance for industries covered by the act was not issued until 1988 (25). A revised standard, which became effective in January of 1994 (26), appHes to faciUties handling any of 128 toxic materials flammable Hquids and gases in quantities of 10,000 lb (4.54 t) or more, except where used as fuel or in atmospheric pressure, ambient temperature tanks and explosives. 

Extreme care must be exercised in designing potentially flammable systems to use reliable flammability limits data and to recognize the effects of pressure/temperature on the data and its implications to the safety of the system in question. Unless otherwise indicated, most published data is at atmospheric pressure and ambient temperature and should be corrected for other conditions. 

Figure 7-48. Effect of initial temperature on limits of flammability of a combustible vapor-inert-air system at atmospheric pressure. By permission, U.S. Bureau of Mines, Bulletin 627 [43].

The basic test apparatus consists of a chamber into which a known concentration of vapor (gas) in air is introduced. After thorough mixing, ignition is attempted with a spark or a hot wire. A series of different concentrations are tested to establish the upper and lower concentration limits for flammability. Although normally run with fuel-air mixtures at ambient conditions, other oxidizing atmospheres, diluent effects and temperature and pressure variations can be studied. 

Phosgene has been shown to have an effect on the flammability limits of hexane in air [1362]. At room temperature and atmospheric pressure, the upper and lower limits of... 

There are no reliable theoretical equations for the combined effects of pressure, temperature, oxidant type and concentration, and fuel mixture composition on the limits of flammability. However, chemical processes are often operated at elevated temperatures and pressures and at times in oxidant enriched atmospheres. Flammability limits should be measured at actual process conditions with adequate test methods. 

Fig. 32. Effect of temperature on the lower limit of flammability of hydrogen in air at atmospheric pressure.

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