Page 17 - Confined Space Training - Student Manual 2021

A flammable or explosive atmosphere may contain methane, acetylene, solvents, fuel vapors or
grain dusts.

In these atmospheres a spark from a tool or a discharge of static electricity may be all that is
needed to create an explosion.

SDS information should be always available on any substances found in confined spaces. The
SDS will help identify the hazards of these substances.

Upper and lower explosive levels

It is important to understand that flammable or explosive chemicals ignite only when the
concentration of the flammable material is in the explosive concentration range. That range is
bounded by the upper explosive level (UEL) and the lower explosive level (LEL). These levels
differ from material to material. If the gas-air mixture or vapor-air concentration is below the LEL,
ignition cannot occur because the mixture is too “lean” to burn.

Ignition also will not occur if the gas-air or vapor-air mixture concentration is above the UEL
because the mixture is then too “rich”. When a combustible has concentration rises above the
UEL, the atmosphere should still not be considered safe. A high gas concentration can be diluted
rapidly into the combustible range by the introduction of air from outside the confined space.

The limit for flammable/explosive atmospheres in permit-required confined space operations is
10% of the LEL. Fortunately, all instruments designed to detect flammable atmospheres are
calibrated to alarm to 10% of the lower explosive level. This provides a safety factor to minimize
the likelihood that an explosive atmosphere will be encountered.

Confined Space Training for Entrants, Attendants & Supervisors 17