Introduction

There are five key properties of every substance that must be reviewed during a hazard assessment; Flash Point, Open Cup Flash Point, Lower Explosive Limit, Auto Ignition Temperature and Upper Explosive Limit. Understanding these properties as they pertain to combustible gases and substances will help you in selecting gas and fire protection instruments. Below is an introduction to these properties.

Flash Point

The lowest temperature at which a liquid provides sufficient surface vapor to form a flammable or an explosive mixture is the flash point. There are many hazardous liquids which have flash points at or below room temperature. These substances are covered by a layer of flammable vapors that will ignite immediately if exposed to a source of ignition. The hazard is elevated as the temperature rises or when vaporization occurs.

Open Cup Flash Point

Open Cup Flash Point is the minimum temperature at which a liquid hazard provides enough vapor to form an ignitable mixture with air. Although less accurate than closed cup testing, open cup testing is a method of determining this flash point and is often necessary for certain substances that are used in open environments.

Lower Explosive Limit (LEL)

The minimum concentration of a flammable gas or its vapors that will fuel a flame when exposed to a source of ignition is known as the lower explosive limit (LEL). A mixture below the LEL is considered too diluted to fuel a fire. An increase in temperature or pressure will decrease the LEL of a gas or its vapor.

Auto Ignition Temperature

Auto ignition temperature is the minimum temperature at which a gas or vapor can provide self-sustained combustion independent of a source of flame. The auto ignition temperature is also referred to as the spontaneous ignition temperature (SIT). The Auto ignition temperature is commonly well above the open-cup flash point.

Upper Explosive Limit (UEL)

The upper explosive limit (UEL) is the maximum concentration of gas in air that will combust. Any mixture with a higher vapor or gas content is too rich to combust when introduced to a  source of ignition.

Visit our Gas Detection Knowledge Database for a table of common combustible gas properties. If you are designing a gas detection system to protect your facility from combustible gas hazards our team of application professionals will be glad to assist, click here to contact us.