Types of Fire Suppression Systems

A fire suppression system is the part of fire protection that acts on its own, or at the push of a button, to put out or hold back a fire before firefighters reach it. There is no single best system. Each type uses a different agent and suits a different hazard, and some of them are dangerous to anyone left inside the room. Here is how the main types work and where each one fits.

Water-based systems

Ordinary sprinklers open one head at a time, only where the heat reaches them. Two other water systems work differently.

• Deluge systems. The nozzles are open all the time and connected to the water supply through pipework. When the system is triggered, every nozzle discharges at once and the whole area is soaked. Deluge is used for high-hazard areas such as chemical stores and aircraft hangars.
• Water mist systems. These spray very fine droplets. The mist cools the fire, pushes oxygen away from the flame and shields nearby surfaces from the heat. Mist works on a wide range of fires and is often chosen for sensitive places such as museums, data centres and hospitals.

Gas systems

Gas systems flood a closed room with an agent. The room has to stay sealed for the gas to hold the fire down, and anyone still inside is at risk. Both types below need to be treated with respect.

• Inert gas systems (for example Argonite or Inergen). They bring the oxygen in the room down to a level where burning cannot continue, and they leave no residue, so they are used in control rooms, server rooms and archives. That same drop in oxygen is an asphyxiation risk for people. For rooms where people normally work, the system must be designed within the oxygen and exposure-time limits of NFPA 2001 (a US standard) and the Indian Standards: IS 15493 for general requirements, and agent standards such as IS 15501 for IG-541. It needs a pre-discharge alarm and a time delay. Everyone must leave before the gas discharges and stay out until the room has been ventilated and declared safe.
• Carbon dioxide (CO2) systems. CO2 pushes out the oxygen and smothers the fire. It is common in industry, for example in paint booths, printing presses and electrical substations, where water could damage equipment or carry a risk of electric shock. At the concentration needed to put out a fire, CO2 is lethal to people and can kill within seconds. Use it only in spaces that are normally unoccupied, with a lockout valve for maintenance and a pre-discharge alarm and time delay, so that everyone is out before it discharges. In India, IS 15528 covers CO2 systems.

Foam systems

Foam spreads a blanket over burning liquid, holds down the vapour and stops the fire flaring up again. The foam has to match the fuel.

• Protein foams. These are made from natural proteins. They work on fires in hydrocarbon fuels such as petrol, diesel and oil, and are widely used in petrochemical plants, the oil and gas industry and on ships.
• Synthetic foams. These are made from synthetic surfactants and form a stable blanket over the fuel surface that keeps vapour down and prevents re-ignition. Many synthetic foams are used on petrol and oil fires. For polar solvents such as alcohol, ketones and esters you need an alcohol-resistant (AR) grade, because ordinary foam breaks down on these liquids. Check that the foam you buy is listed for the fuel you actually store.

Dry chemical systems

• Class D dry powder systems. These are meant for fires in combustible metals such as magnesium, titanium and lithium. The special dry powder forms a crust over the burning metal and stops it burning further. Water can make a metal fire worse.
• Vehicle systems. Fitted in buses, mining machines and military vehicles, these detect a fire in the engine compartment or another high-risk area and put it out automatically.

Pre-engineered and special hazard systems

• Special hazard systems. These are built around one specific risk or a high-value asset, such as dust collectors in factories, transformer vaults and telecom facilities.
• Clean agent systems. They use agents that do not conduct electricity, do not corrode and leave no residue, which is why they protect sensitive electronic equipment and archives.

Choosing and looking after a system

Start with two questions: what can burn in this room, and do people work here? Then have a qualified fire engineer choose the agent and design the system, and get it approved by your fire authority. Buying on price or brand alone is how a server room ends up with the wrong gas, or a solvent store with the wrong foam.

A system that has never gone off still needs regular inspection, testing and servicing. Cylinders lose pressure, foam stock gets old and nozzles get blocked or painted over. Keep a service record and stick to it.

If you work in a room protected by gas, ask this week what its pre-discharge alarm sounds like. When you hear it, you leave and shut the door behind you. You do not stop to save the file.