Gaseous fire suppression systems

Sprinkler systems are a very effective means of extinguishing fires and minimising fire and smoke damage to property. However, since water can cause serious damage to electrical or IT equipment, alternative extinguishing systems are specified to minimise the risk of potential damage to mission critical equipment in communications rooms and other technical spaces.

This cost model examines the pros and cons of currently available gaseous suppression systems. It includes a detailed cost breakdown of FM200 and Argonite based-systems, together with indicative costs for a range of alternative fire suppression systems.

Localised water mist
Water mist systems are a variant of a conventional sprinkler system, directing a spray of fine water droplets at the source of the fire. Mist droplets have a high surface area to volume ratio and the mist readily vapourises on contact with a heat source. The energy required at vapourisation is absorbed from the surroundings, causing a reduction in temperature, resulting in the quelling of the fire.

Mist systems are directional, and spray nozzles need to be aimed at the fire source to ensure that it is extinguished. However, it is not possible to direct mist systems into individual equipment cabinets. Consequently, fires within cabinets might not be put out, and 100% suppression cannot be guaranteed.

Local mist systems are appropriate for applications in confined spaces such as generator rooms, but are not normally specified for general areas. However, in some instances, mist systems are also specified for technical areas in co-location centres, where the volumes of technical space involved are simply too large for it to be economical to install gaseous systems.

Once discharged, water used to extinguish the fire will condense, potentially causing damage to equipment, and requiring a local drainage installation.

Gas systems
Gas systems provide fire protection solutions which can deal with fires inside equipment cabinets, and which, in the event of fire, will not result in water damage to electrical equipment. There are substantial builders' work requirements associated with gas systems which include:

• requirements for airtightness affecting door, window and partition design
• requirements for pressure rated construction and pressure relief valves in order to resist the air pressures which occur upon the release of the gas
• dedicated extract and make-up air supply installations for systems which use toxic suppressants
• dedicated smoke discharge systems for all self-contained rooms without direct access to atmosphere

Gas systems are installed and maintained by specialist contractors, who often have access to a limited range of proprietary suppressant gases. If a procurement strategy based on a competitive tender is adopted, specifying a single gas may limit real competition between specialists.

Accordingly, it is good practice to use a performance specification, allowing specialists limited flexibility in the final selection of the suppressant agent.

Carbon dioxide
By excluding oxygen CO₂ suffocates a fire. When used in automatic fire suppression systems, CO₂ poses a life threatening risk to personnel, which must be mitigated by provision of door interlocks and other systems that prevent the release of gas while the protected space is occupied. Due to the risks involved, CO₂ systems are rarely specified and it can be difficult to find contractors to maintain or modify them. However, as CO₂ is a cheaper gas than other, less toxic suppressants, it is sometimes specified for use in unoccupied spaces.

FM200
This gas is a hfc. Although it is ozone friendly it is classified as having global warming potential, and there is some concern that the gas will ultimately be banned under more stringent environmental legislation. The use of FM200 is prohibited in some Scandinavian countries but there are no current plans to prohibit its use in the UK. Accordingly, it is reasonable to assume that FM200 will be available for the next 15-20 years, the typical plant life of fire suppression systems.

FM200 extinguishes fires by a combination of cooling and chemical reaction. Due to the efficiency of the process, relatively small quantities of gas are required, and fires are extinguished quickly, minimising smoke damage. However, small quantities of a highly toxic acid, hydrogen fluoride, which corrodes printed circuit boards, potentially damaging IT equipment, are produced as a by-product of the chemical reaction. Due to the fact that FM200 is a toxic gas, rooms require dedicated gas extract systems, make-up air and room sealing.

Argonite and Inergen are inert gases comprising a mix of nitrogen and argon. These gases are environmentally friendly products, with very low global warming potential. Unlike other products such as FM200, these gases produce no toxic by-products and represent no risks to general health.

Argonite and Inergen extinguish fires by reducing oxygen content below the level that is required to support combustion. There is no health risk, as oxygen levels are maintained above the levels that are required for human respiration.

Use of these gases will arrest the growth of a fire within 10 seconds and will extinguish it over a period of up to one minute. As the time taken to extinguish a fire is longer than for alternative gases such as FM200, there is a greater potential for smoke damage. When specifying Argonite or Inergen, this risk needs to be balanced against the benefits of the non-toxicity of the gas itself.

In contrast to most fire suppression gases, which are stored in a liquefied state, Argonite and Inergen are stored as a high-pressure gas. As a result, plant space requirements for bottle storage are approximately four times greater than for other gas systems, and local floor strengthening will be required due to the weight of the gas bottles. Similarly, because of the very high storage pressures involved, pressure relief dampers and dedicated pressure relief paths to atmosphere are also required to protect the structure.

Some of the disadvantages associated with the use of Argonite and Inergen can be mitigated through the rationalisation of gas storage and distribution, to provide a single bottle store, sized to serve the largest space. Centralisation has cost and space take-up benefits. However, disadvantages of this approach include:

• the system can only deal with one fire at a time
• following a discharge, full protection is not available until the gas supply has been replenished
• the controls associated with the centralised system are complex and represent a potential, albeit remote, point of failure

The decision to adopt a centralised strategy will be based on plant space availability, initial capital cost, and the perception of the risk associated with the short-term loss of protection in the event of system discharge. The adoption of a centralised strategy can result in an 85% reduction in the total space requirement for gas bottle storage.

Centralised storage solutions are not possible with liquefied products such as FM200 because of the change of phase which occurs upon release.

High pressure gases are typically stored at 200 bar. It is technically possible to store these gases at pressures of up to 300 bar, reducing both initial costs and plant space requirements. Certification of storage at 300 bar is scheduled for certification by late 2001.