Flammability diagram - "Explained"

Please read the below definitions before proceeding to understand the flammability diagram.

Definition:

• LEL (Lower Explosive Limit) / LFL (Lower Flammable Limit)

            The concentration of a Hydrocarbon gas in air below which there is insufficient Hydrocarbon (fuel) to support and develop combustion.

• UEL (Upper Explosive Limit) / UFL (Upper Flammable Limit)

            The concentration of a Hydrocarbon gas in air above which there is insufficient oxygen to support and develop combustion.

• Flammable range

            Range of Hydrocarbon gas concentrations in oxygen between the LEL and UEL  (LFL and UFL). 

            

            The composition of this mixture has to lie within a range of proportions, and this range is called the FLAMMABLE RANGE. 

• Too lean

            A tank atmosphere made incapable of burning by the deliberate reduction of the hydrocarbon content (fuel content) below the LFL  (i.e < 1%)

• Too rich

            A tank atmosphere made incapable of burning by the deliberately maintaining  the hydrocarbon content (fuel content) over the UFL (i.e > 10%)

Limits of flammability:

• Flammable mixture (HC+O2) mixtures will ignite and burn only over a well-specified range of compositions.
• The mixture will not burn when the composition is lower than the lower flammable limit (LFL); the mixture is too lean for combustion.
• The mixture is also not combustible when the composition is too rich; i.e, when it is above the upper flammable limit (UFL).
• In this diagram, the following figures are used for HC gas 'C' LFL as 1% of vol and point 'D' UFL as 10% vol (LFL 1% and UFL 10% is only for understanding the purpose of the diagram, for each cargo LFL and UFL varies, for that refer MSDS of that cargo.

• LFL (Lower flammable limit): 1% gas to 99% air.
• UFL (Upper flammable limit): 10% gas to 90% air.
• Therefore the FLAMMABLE RANGE IS 9%
• In inerted condition:
• The Oxygen level in the tank if below 11% will not support combustion.
• Maximum permissible allowance is 8%.

Flammability diagram:

Flammability diagram is generic and for ambient temperature.

A diagram illustrating the relationship between composition and flammability of mixtures of cargo vapours, oxygen and inert gas.

Purpose of these diagrams is to enable procedures to be developed for avoiding flammable mixtures in the cargo system at all times.

Every point on the diagram represents a mixture of oxygen, flammable vapours and inert gas-specified in terms of its flammable vapours and oxygen content by % volume.

Let us learn and understand the flammability range diagram.

Let us consider the tank atmosphere at point 'F' on the graph.

The atmosphere will contain 3% oxygen and 11% hydrocarbons as by volume, where the atmosphere will be just too rich in hydrocarbon gas, and too lean in oxygen, to support combustion.

If we introduce fresh air into the tank the happening inside the tank is shown in the graph by drawing a dotted line 'FA'.

While introducing fresh air will slowly increase the oxygen level and will not displace enough hydrocarbons. At one point the line 'FA' will enter into flammable range.

Thus, the dilution with air from point 'F' is considered as unsafe.

In order to avoid this unsafe operation, we need to purge the tank with inert gas and displacing HC and O2 and coming to point 'L'.

Now we will consider the tank atmosphere at point 'L' on the graph

The atmosphere is approximately 3% oxygen and 4% hydrocarbons gas drawn on the graph as point 'L'. Where the tank atmosphere will have enough HC gas to support combustion but will be too lean in oxygen. So, if we maintain this atmosphere too lean in oxygen, combustion cannot take place even with an ignition source.

Now we introduce some fresh air into the tank

If we introduce fresh air into the tank the happening inside the tank is shown in the graph by drawing a dotted line 'GA'. 

While introducing fresh air into the tank will increase the oxygen and also displace some hydrocarbon, and the 'GA' line will pass very close to the flammable envelope. As the line passing very close to the flammable envelope, diluting with air at point L is not considered as safe.

So, diluting with air at point 'L' is called Critical dilution with air.

Now we purge the tank further and displacing the HC and O2 and coming to point 'H'.

Consider the tank atmosphere at point 'H' on the graph.

The Tank atmosphere at point 'H' is approximately 2.5% oxygen and 2.5% hydrocarbon gas. Both hydrocarbon and oxygen are too lean to support combustion.

If we introduce fresh air into the tank the happening inside the tank is shown in the graph by drawing a dotted line 'HA'.

As this line passing clear from flammable range, the dilution with air is considered safe at point 'H'.

Why the AB line is tilted?

It's because of the composition of air. As the HC increases in concentration, the value of oxygen decreases.

At point 'C' HC concentration = 1% and air = 99%

∴concentration of oxygen will be (99 x 21) / 100 = 20.79%

At point 'D' HC concentration = 10% and air = 90%

∴concentration of oxygen will be (90 x 21) / 100 = 18.90%

 

I hope u have understood the flammability diagram. If you have any questions please leave a comment below.

Thanks😊