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Understanding Flammability Limit and Explosive Limit


Have you heard about Flammability Limit and Explosive Limit? What does it mean? What the difference? These day I kept wondering if flammability limit and explosive limit is the same? Since I have to check vent stack data sheet from our mechanical engineer, I need to put low flammability limit criteria. What is that?

Explosion occur when three conditions are met:

  • Fuel
  • Oxygen (air)
  • Ignition source (spark or flame)

Explosive limit is a necessary particular concentration of gas or vapor to support its combustion with air. Its term is divided into two, Lower Explosive Limit (LEL) and Upper Explosive Limit (UEL). To understand the difference between those two, please see below illustration.

LEL vs UEL
LEL vs UEL

Based on illustration above we can see that Lower Explosive Limit (LEL) is minimum concentration of combustible gas that lead to burn. Below this concentration, gas is too lean too burn. On the other hand, Upper Explosive Limit (UEL) is maximum concentration of combustible gas that lead to burn. Above this concentration, gas is too rich too burn. Concentration between LEL and UEL is optimum explosive concentration.

HOW ABOUT FLAMMABILITY LIMIT?

As well as Explosive Limit, Flammability Limit is also differenciated into two categories, Lower Flammability Limit (LFL) and Upper Flammability Limit (UFL)LFL is the lowest concentration of combustible gas required to create flame, while UFL is the highest one.

IS THERE ANY DIFFERENCE BETWEEN LFL/UFL and LEL/LFL?

I read several sources in internet and concluded that no difference between LFL/UFL and LEL/LFL. People usually used in interchangeably.

INTERESTING FACT!

I found in this source value of LEL/UEL of several component and this is very interesting!

Acetylene have the longest range of optimum explosive limit.

LEL of acetylene is 2.5%-v while its UEL is 100%.

Turpentine have the lowest LEL

Concentration as low as 0.7%-v can start burn.

 

 

 

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