Fire Suppression Technologies

In recent years several new fire suppression technologies have been developed to have less of an impact on the environment and human health.

The phase-out of halon fire suppressants for environmental reasons has resulted in a thrust to develop new technologies. The challenge has been to find substitutes for halon that have sufficient ability to suppress fires in various applications without harming the environment and human health. This article gives an overview of several newly developed fire suppression systems.

Gaseous Systems

Gaseous systems use nozzles, pipes and pressurized cylinders to discharge a gaseous suppressant into an enclosed space. There are two types of gaseous agents available: halocarbon agents and inert gases.

Halocarbon agents are chemicals similar to halon, but their molecular structure has been modified to reduce or eliminate the chlorine and bromine atoms that are responsible for ozone depletion. Halocarbon agents extinguish fires primarily by cooling. More than a dozen halocarbon chemicals have been tested recently. The tests examine not only the fire suppression capabilities of these agents, but also the concentrations of toxic by-products they produce during the fire suppression. The by-products can be an important factor in the safety of the building’s occupants, and also in global warming. The main component of the combustion by-products of halocarbon gaseous agents is hydrogen fluoride, which is toxic and very corrosive.

Inert gas agents extinguish fire by oxygen depletion. Inert gases are not subject to thermal decomposition when used in extinguishing fires, and hence, unlike halocarbon gases, they form no combustion by-products.

To suppress a fire, the inert gas needs to reduce the oxygen level to about 12.5%. This requires a concentration of approximately 40% of the inert gas in the enclosure.

There are three inert gas systems currently available:

• Argonite (IG-55) which is a mixture of 50% nitrogen and 50% argon.

• Inergen (IG-541) which is a mixture of 52% nitrogen, 40% argon and 8% carbon dioxide.

• Argon (IG-01) which is 100% argon.

These inert gases are clean and naturally occurring in the atmosphere. They have zero ozone depletion potential and no global warming potential.

Water Mist

Water mist is fine water spray in drops with diameters less than 1000 microns. The efficacy of water mist systems for fire protection has been demonstrated in a wide range of applications and by numerous experimental programs. Their effectiveness is influenced by the fire size, the degree of obstruction, ceiling height and the ventilation conditions in the fire compartment. Performance is also affected by the characteristics of the water mist, such as drop size distribution, flux density and spray momentum.

There are several water mist systems currently available. Some use high or intermediate water pressures through small orifices to produce water mist, whereas others use twin fluid nozzles (water and air). Research has demonstrated that the fire suppression performance of water mist can be further improved by using a cycling discharge, which signif- icantly reduces extinguishment times and water requirements.

Water mist fire suppression systems have a number of advantages such as good fire suppression capability, no environmental impact and no toxicity. As a result, the use of water mist as a fire suppression system has been considered for a wide range of applications such as shipboard machinery spaces, where open doors and hatches can reduce the effectiveness of gaseous agents. Water mist also has the potential for extinguishing electrical fires, and may result in less equipment damage than the damage caused by the thermal decomposition products of gaseous halon replacement agents.

Compressed Air Foam

For decades, fire-fighting foams delivered from fixed-pipe systems have provided effective fire suppression for the chemical and petroleum industries and military installations. The overall effectiveness of current fixed pipe foam systems can be significantly increased if compressed air is used for generating the foam. Compressed-air-foam uses a much lower quantity of water and foam concentrates, and it is better able to penetrate fire plumes and reach the seat of a fire. As well, compressed air foam can be used with Class A foam concentrate, which is environmentally friendly, rather than Class B foam concentrate, which is used in traditional foam systems.

An effective compressed-air-foam system produces foam consisting of similar-in-size bubbles, it delivers the foam to nozzles without changing the foam properties, and it provides a means of uniformly distributing foam over a prescribed area. Specialized piping and nozzles have been developed to ensure the foam quality and distribution over the target area.

Compressed-air-foam has good potential, and pre-engineered systems suitable for a variety of commercial applications have been developed.

Aerosol and Gas Generators

Fire extinguishing aerosols are usually delivered either directly (on contact with the fire) or via devices located out-side the protected area of a facility. Commercial aerosol systems are available for fire suppression in spaces where people are not normally present, such as flammable liquid storage spaces or engine compartments.

Canisters contain solid rocket fuel. When electrically or thermally ignited, they discharge dry chemical particles and gas products in aerosol form through end-plate delivery nozzles.

Based on automotive airbag technology, gas generators have been developed for fire suppression applications. Gas generators are very compact and can provide very fast discharge (in a few milliseconds). There are two types of gas generators available. One is a conventional gas generator, which contains propellant and an electrical initiator. Another is a hybrid gas generator, which consists of an electrical initiator, a solid propellant chamber and a suppression agent chamber.

New Standards

New fire suppression technologies require new standards to identify their limitations and proper use. Two standards have been developed recently for new fire suppression technologies by the National Fire Protection Association (NFPA):

• NFPA 2001 “Standard on Clean Agent Fire Extinguishing Systems,” and

• NFPA 750 “Standard on Water Mist Fire Protection Systems.”

NFPA 2001 deals with the design, installation, maintenance and operation of systems using the new clean agents that are being developed to replace halon, including halocarbon agents and inert gas systems. The purpose of NFPA 750 is to provide protection from fire by standardizing the design, installation, maintenance, and testing requirements of water mist fire suppression systems.

The development of new standards for the new technologies, means they can be reliably employed.

Dr. Andrew Kim, Ph. D. is a senior research officer in the Fire Research Program of the National Research Council Institute for Research in Construction in Ottawa. E-mail andrew.kim@nrc-cnrc.gc.ca