To control fires and explosions, you need to understand how they happen. They are not random. They need specific ingredients, and taking away any one of them prevents the event.

This module explains the fire triangle, the extra conditions needed for an explosion, the flammable range, dust explosions, and the many sources of ignition found at work.

A fire needs three things at the same time:

• Fuel: something that will burn, such as a flammable liquid, gas, dust, wood, paper or plastic.
• Oxygen: usually from the air, but also from oxidising substances.
• A source of ignition: heat, a flame or a spark.

Together, these three are known as the fire triangle.

Remove any one of the three sides, and a fire cannot start, or cannot continue. This is the basis of all fire prevention:

• Limit the fuel, by keeping only small quantities of flammable materials, and good housekeeping.
• Limit the oxygen, for example by keeping oxidisers apart from fuels, or by filling equipment with an inert gas such as nitrogen.
• Control ignition sources, by keeping heat, flames and sparks away from fuel.

An explosion needs the three sides of the fire triangle, plus two more conditions:

• The fuel must be dispersed in the air, at the right concentration.
• The cloud is often confined, for example inside a room, a vessel, a duct or a silo.

These five factors together are sometimes called the explosion pentagon. When they are all present, ignition releases energy almost instantly, producing a damaging pressure wave.

When an explosive atmosphere ignites in a confined space, the pressure builds so quickly that walls, roofs and equipment can be blown apart. People are hurt by the blast, flying debris, burns and collapsing structures.

In the open, the same cloud may burn as a flash fire: a sudden sheet of flame that passes through the whole cloud in seconds, causing severe burns to anyone inside it, and setting light to other materials.

A gas or vapour will only ignite when its concentration in air is between two limits:

• The lower explosive limit, or LEL. Below it, the mixture is too lean to burn.
• The upper explosive limit, or UEL. Above it, the mixture is too rich to burn.

Between these limits is the flammable range, also called the explosive range. This is the danger zone. A mixture that is too rich can still become dangerous as fresh air mixes in.

The main way to prevent an explosive atmosphere is to keep the concentration of gas or vapour well below the LEL.

Good ventilation, either natural or mechanical, dilutes and removes vapours and gases. Where there could be a leak, fixed gas detectors can sound an alarm, or shut down equipment, long before the LEL is reached. Portable gas detectors are used to test the air before work such as hot work, or entering a tank.

Many ordinary materials, such as flour, sugar, grain, wood, metal and plastic, can explode violently when their fine dust is suspended in air and ignited.

A dust explosion needs five things: the dust as fuel, oxygen, an ignition source, dispersion of the dust into a cloud, and confinement. The finer and drier the dust, the more easily it ignites.

Dust explosions are especially dangerous because of secondary explosions. A first, small explosion shakes loose dust that has settled on beams, ledges and machinery, and throws it into the air. This creates a much larger cloud, which then ignites in a second, far bigger explosion that can destroy buildings and kill people.

This is why dust must never be allowed to build up, and why it should be removed with suitable vacuum cleaners, not by blowing or dry sweeping.

Ignition can come from many sources, including:

• Naked flames, hot work such as welding and cutting, and smoking.
• Hot surfaces, such as engines, exhausts, heaters and dryers.
• Electrical sparks from switches, motors and faulty equipment.
• Static electricity.
• Mechanical sparks and friction from grinding, impacts and worn bearings.
• Lightning, and self-heating of some materials, such as oily rags.

Static electricity builds up when liquids flow through pipes and hoses, when powders are poured or blown, and when people move about. A static spark can easily ignite a flammable vapour or a dust cloud.

Static is controlled by earthing and bonding: connecting containers, pipework and equipment to each other and to earth, so that the charge drains away safely. Antistatic footwear and clothing, conductive hoses, and slower flow rates also help.

Fuel and oxygen are often hard to remove completely, so controlling ignition sources is a key defence. This means:

• Using only equipment designed and certified for the hazardous area.
• Controlling static, by earthing and bonding.
• Banning smoking and naked flames, and controlling hot work with permits.
• Maintaining equipment, so that it does not spark or overheat.
• Using non-sparking tools where required.