Figure 2.17 Flammable range for propane

All the liquefied gases, with the exception of chlorine, are flammable but the limits of the flammable range vary depending on the particular vapour. These are listed in Table 2.8. The flammable range of a vapour is broadened in the presence of oxygen in excess of that normally found in air. In such cases the lower flammable limit is

Table 2.8 Ignition properties for liquefied gases

Liquefied Gas	Flash Point (°C)	Flammable range (% by vol. in air)	Auto-ignition temperature (°C)
Methane	-175	5.3-14	595
Ethane	-125	3.0-12.5	510
Propane	-105	2.1-9.5	468
n-Butane	- 60	1.5-9.0	365
i-Butane	- 76	1.5-9.0	500
Ethylene	-150	3.0-34.0	453
Propylene	-108	2.0-11.1	453
a-Butylene	- 80	1.6-10	440
b-Butylene	- 72	1.6-10	465
Butadiene	- 60	1.1-12.5	418
Isoprene	- 50	1.5-9.7	220
Vinyl Chloride	- 78	4.0-33.0	472
Ethylene oxide	- 18	3.0-100	429
Propylene oxide	- 37	2.1-38.5	465
Ammonia	- 57	14-28	615
Chlorine	Non-flammable

 

Table 2.9 Flammability range in air and oxygen for some liquefied gases

	Flammable range (% by volume)
	(in air)	(in oxygen)
Propane	2.1-9.5	2.1-55.0
n-Butane	1.5-9.0	1.8-49.0
Vinyl Chloride	4.0-33.0	4.0-70.0

 

changed little but the upper flammable limit is considerably raised. Comparative flam­mable ranges in air and in oxygen are quoted in Table 2.9 for propane, n-butane and vinyl chloride. All flammable vapours exhibit this property and, as a result, oxygen should not normally be introduced into an atmosphere where flammable vapours exist.

Flash Point

The flash point of a liquid is the lowest temperature at which that liquid will evolve sufficient vapour to form a flammable mixture with air. High vapour pressure liquids

such as liquefied gases have extremely low flash points, as seen from Table 2.8. However, although liquefied gases are never carried at temperatures below their flash point, the vapour spaces above such cargoes are non-flammable since they are filled entirely with cargo vapour and are thus safely above the upper flammable limit.

Auto-ignition Temperature

The auto-ignition temperature of a substance is the temperature to which its vapour-in-air mixture must be heated to ignite spontaneously. The auto-ignition temperature is not related to the vapour pressure or to the flash point of the substance and, since the most likely ignition sources are external flames or sparks, it is the flash point rather than the auto-ignition temperature which is used for the flammability classification of hazardous materials. Nevertheless, when vapour escapes are considered in relation to adjacent steam pipes or other hot surfaces, the auto-ignition temperature is worthy of note. Accordingly, they are listed in Table 2.8.

Energy Required for Ignition

Accidental sources of ignition of a flammable vapour can be flames, thermal sparks (due to metal-to-metal impact) and electric arcs or sparks. The minimum ignition energy necessary to set fire to hydrocarbon vapours is very low, particularly when the vapour concentration is in the middle of the flammable range. Minimum ignition energies for flammable vapours in air are typically less than one milliJoule. This is an energy level substantially exceeded by any visible flame, by most electric circuit sparks or by electrostatic discharges down to the lowest level detectable by human contact. The presence of oxygen in excess of its normal proportion in air further lowers the minimum ignition energy.

Only the flammable mixtures of ammonia have minimum ignition energies lying outside this typical range. Ammonia requires energies some 600 times higher than the other gases for ignition. Nevertheless, the possibility of ignition of ammonia vapours cannot be completely discounted.

Flammability within Vapour Clouds

Should a liquefied gas be spilled in an open space, the liquid will rapidly evaporate to produce a vapour cloud (see also 2.10.2) which will gradually disperse downwind. The vapour cloud or plume is flammable only over part of its area. The situation is illustrated in Figure 2.18.

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