Gases - Explosion and Flammability Concentration Limits Flame and explosion limits for gases - propane, methane, butane, acetylene and more

The Flammable Range (Explosive Range) is the concentration range of a gas or vapor that will burn (or explode) if an ignition source is introduced.
Three basic requirements must be met for explosion to take place:

1. flammable substance - fuel
2. oxidizer - oxygen or air
3. source of ignition - spark or high heat

Below the explosive or flammable range the mixture is too lean to burn and above the upper explosive or flammable limit the mixture is too rich to burn. The limits are commonly called the "Lower Explosive or Flammable Limit" (LEL/LFL) and the "Upper Explosive or Flammable Limit" (UEL/UFL).
The lower and upper explosion concentration limits for some commonly used gases are indicated in the table below. Some of the gases are commonly used as fuel in combustion processes.
Note! The limits indicated are for gas and air at 20^oC and atmospheric pressure.
It is important that areas that store flammable gases are well ventilated. When designing ventilation systems be aware of the specific gravity of the actual gas. The gas mixture from a leakage will not be homogeneous and lighter gases concentrates along the ceiling. Heavy gases concentrates along the floor.
Ventilation, natural or mechanical, must be sufficient to limit the concentration of flammable gases or vapors to a maximum level of 25% of their "Lower Explosive or Flammable Limit" (LEL/LFL).

• Minimum ventilation required: 1 cfm/ft² (20 m³/h m²)
• Recommended ventilation: 2 cfm/ft² (40 m³/h m²) or 12 air changes per hour - half the air supplied and exhausted near the ceiling and half the air supplied and exhausted near the floor