FLAMMABILITY

Flammable Limits

A mixture of hydrocarbon gas and air cannot ignite, unless its composition lies within a range of gas-in-air concentrations known as the flammable range.

The lower limit of the range, known as the "lower flammable limit", is any hydrocarbon concentration below which there is insufficient hydrocarbon gas to support combustion.

The upper limit of the range, known as the "upper flammable limit", is any hydrocarbon concentration above which air is insufficient to support combustion.

The flammable limits vary somewhat for different pure HC and for the gas mixtures derived from different petroleum liquids.

However in practice the lower and upper flammable limits of oil cargoes carried in tankers is for general purposes taken to be 01and 10 % hydrocarbon by volume, respectively.

Flammable Range Diagram

Hydrocarbon gas/air/inert gas mixtures effect on flammability

1. Any point on the diagram represents a hydrocarbon gas/air/inert gas mixture, specified in terms of its hydrocarbon and oxygen content.

2. Hydrocarbon/air mixtures, without inert gas, lie on the line AB, the slope of which shows the reduction in oxygen content as the hydrocarbon content increases.

3. Points to the left of AB represent mixtures whose oxygen content is further reduced by the addition of inert gas.

The flammable range for HC gas is between 1 – 10%, and for O2 between 11.5 - 21%.

If 10% of CH gas is introduced in air, the proportion of oxygen reduces to 21x 90/100= 18.9% (Point D)

When O2 is minimum (11.5%), CH gas must be at least 1.5% (Point E). At 1% CH gas, the oxygen must be at least 20.8% for the mixture to be

flammable (Point C). If the concentration of CH gas and O2 is outside the flammable range, the

mixture will not burn.

Dilution of CH Gas

When we dilute a CH gas – air mixture with air (concentration as at Point F) the air will slowly replace the CH gas till the mixture becomes 100% air.

The concentrations of CH gas and O2 will take the path F A. This passes through the flammable range. At some time during the

dilution, the mixture could explode if a spark was applied. We must try to avoid passing through the flammable range during this

dilution with air. To do this the mixture is diluted with inert gas (along the path FH) till a

point H is reached which is reached below the critical dilution line. As inert gas is added to hydrocarbon/air mixtures, the flammable range progressively decreases, until the oxygen content reaches a level generally taken to be about 11 per cent by volume, at which no mixture can burn

This is checked by sampling and testing the atmosphere with gas measuring instruments. If now O2 (air) is let in freely, the dilution will take place along line HA.

This does not pass through the flammable range at any time causing no danger of fire or explosion.