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Flammable and Combustible Materials

May 27, 2014

Flammable and Combustible Materials

The material in this post is extracted from Chapter 1 – Operations – of the book Plant Design and Operations.

This book contains many discussions to do with the control of flammable and combustible materials. The terminology used for this topic — which can be confusing — is explained below.

Flammable Range

Fires require the presence of fuel and air (oxygen) along with a source of ignition. These criteria are often referred to as the fire triangle.

The fuel has always to be in the form of a vapor (liquids and solids do not burn directly — the fire generates flammable vapors at their surface and it is the vapors that actually burn). Moreover, not all fuel vapor/oxygen mixtures will burn — the concentrations have to lie inside the flammable range, which have upper and lower limits for the concentrations of the fuel in the vapor space. The flammability limits vary according to many factors, of which some of the most important are: the pressure and temperature of the mixture and the presence of inert components such as steam, carbon dioxide or nitrogen.

Flammable Limits

The flammable range for a fuel is defined by the Lower Flammable Limit (LFL) and the Upper Flammable Limit (UFL). These terms are also referred to as the Upper and Lower Explosive Limits. Below the Lower Flammable Limit (LFL) there is insufficient flammable material for a fire to occur — the mixture is ‘too lean’. It is the lowest concentration of a flammable vapor in air capable of producing a fire in the presence of an ignition source.

The UFL is similar to the LFL except that there is too high a concentration of vapor for a fire to occur — the mixture is said to be ‘too rich’.

For most flammable hydrocarbons the LFL is around 2 – 5%. For simple alkanes, such as methane and ethane, the UFL is in the 10 – 15% range. Some chemicals, such as hydrogen, ethylene oxide and acetylene, have much higher values for UFL. Values for flammable limit ranges for many flammable materials are provided by NFPA 704 — Standard System for the Identification of the Hazards of Materials for Emergency Response.

Flash Point

The flash point of a flammable material is defined as the temperature at which a vapor that is inside its flammable range that can be ignited. An ignition source such as a flame or spark is needed to make the material actually burn. It is important to recognize that an ignition source is required. The flash point is not the same as the auto-ignition temperature.

The flash point is determined by heating the liquid in test equipment and measuring the temperature at which a flash will be obtained when a small flame is introduced in the vapor zone above the surface of the liquid.

Figure 1.1 illustrates the concepts of ignition temperatures and flashpoints and flammable limits

Figure 1.1
Flammability and Ignition Limits

Flammability Limits

Before a flammable mixture will burn its temperature must be at or above the flashpoint. If the temperature is below this point then the vapor mixture will not burn, even if a source of ignition exists. The left line in Figure 1.1 is the flashpoint line.

Even if the material is above its flashpoint, the ignition source must be of sufficiently high temperature and must contain sufficient energy to ignite the fuel. The minimum energy varies with type of gas and concentration; for hydrocarbon vapors it is low, for high flash point liquids, such as diesel and fuel oil, it is much higher — usually in the form of an existing fire. This is why low energy flashes (such as might be created by a mobile phone or a digital camera) may not ignite a flammable mixture.

If a flammable mixture is heated to a high enough temperature it will spontaneously ignite; an ignition source such as a flame or spark is not needed. Spontaneous ignition occurs at the auto-ignition temperature (AIT), which is also shown in Figure 1.1.

Combustible Liquids

A combustible liquid (see OSHA’s 29 CFR 1910.106 (Flammable and Combustible Liquids) is defined as having a flash point that is above 100ºF (37.8ºC). Combustible liquids are divided into one of the following two classes.

Class II Liquids

This class includes liquids with a flash point at or above 100ºF (37.8ºC) and below 140ºF (60ºC), except any mixture having components with flash points of 200ºF (93.3ºC) or higher, the volume of which make up 99 percent or more of the total volume of the mixture.

Class III Liquids

This class includes combustible liquids with a flash point at or above 140ºF (60ºC). Class III liquids are subdivided into two subclasses:

  • Class IIIA liquids have a flash point at or above 140ºF (60ºC) and below 200ºF (93.3ºC), except any mixture having components with flash points of 200ºF (93.3ºC), or higher, the total volume of which make up 99 percent or more of the total volume of the mixture.
  • Class IIIB liquids have a flash point at or above 200ºF (93.3ºC).

In general, the term “Class III liquid” means “Class IIIA” unless specified otherwise.

When a combustible liquid is heated to within 30ºF (16.7ºC) of its flash point, it shall be handled in accordance with the requirements for the next lower class of liquids.

  • Flammable liquid: any liquid having a flash point below 100ºF (37.8ºC), except any mixture having components with flashpoints of 100ºF (37.8ºC) or higher, the total of which make up 99 percent or more of the total volume of the mixture. Flammable liquids shall be known as Class I liquids. Class I liquids are divided into three classes as follows:
    • Class IA includes liquids having flash points below 73ºF (22.8ºC) and having a boiling point below 100ºF (37.8ºC).
    • Class IB includes liquids having flash points below 73ºF (22.8ºC) and having a boiling point at or above 100ºF (37.8ºC).
    • Class IC includes liquids having flash points at or above 73ºF (22.8ºC) and below 100ºF (37.8ºC).
  • Liquids having flash points below ambient storage temperatures generally display a rapid rate of flame spread over the surface of the liquid, since it is not necessary for the heat of the fire to expend its energy in heating the liquid to generate more vapor.

Flammable Liquids

Since it is the vapor of the liquid, not the liquid itself that burns, vapor generation becomes the primary factor in determining the fire hazard. Hence liquids that have a flash point below ambient temperature generally have a rapid rate of flame spread over the surface of the liquid because it is not necessary for the heat of the fire to expend its energy in heating the liquid to generate more vapor.

If ambient temperature is defined as being 100ºF (37.8ºC) then any liquid having a flash point below that value is said to be flammable, as distinct from just combustible. Flammable Liquids are known as Class I liquids. There are three sub-classes.

  1. Class IA includes liquids that have a flash point below 22.8ºC (73ºF) and a boiling point below 37.8ºC (100ºF).
  2. Class IB includes liquids that have a flash point below 22.8ºC (73ºF), and a boiling point at or above 37.8ºC (100ºF).
  3. Class IC includes liquids that have a flash point at or above 22.8ºC (73ºF), and below 37.8ºC (100ºF).

Given that the above definitions are quite complex and can be confusing, OSHA developed the sketch shown in Figure 1.2 to help clarify the terms used.

Figure 1.2
Flammable and Combustible Liquids

Combustible Liquids

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