Fire extinguishants

There are a wide range of well-established fire suppression systems, including inert gas, water, water mist, dry chemical agents, halons, and HFCs.

Halons were originally developed as very effective fire extinguishants. Their effectiveness is because they contain bromine, which is active at supressing flame propagation. However because they contain bromine, they have a high ozone depletion potential and are regulated substances with no new production allowed.

The EU banned all non-critical uses of halons in 2003. Critical uses are listed in the current Annex VI to Regulation (EC) No. 1005/2009. All current on-board uses of halons in aviation are included on the critical use list under the EC regulation. Annex VI was revised in 2010 as per Commission Regulation (EU) No 744/2010 of 18 August 2010 which contains “cut-off dates” for the use of halons in new designs of equipment or facilities and “end dates” when all halon systems or extinguishers in a particular application must be decommissioned (i.e. ‘retrofit’)

Some hydrofluorocarbons are very good fire extinguishants and have low toxicity allowing their use in occupied spaces. However they are not ‘active’ fire extinguishants as they do not contain bromine.

F-Gas Regulation 517/2014 has measures that control the use of HFCs as fire extinguishants, including leak checking and recovery from fire protection systems. The use of HFC -23 was banned from 1 January 2016, due to its very high GWP and the availability of alternative fire suppression systems.

The most widely used HFC fire extinguishant is HFC 227ea [European Commission. Clima polices]

The benefits of using HFC’s in fire protection are:

  • HFC’s at extinguishing concentrations are safe and can therefore be used in normally occupied spaces. They are used at well-below-toxic
  • HFC systems require only a small amount of gas and achieve design concentrations in under 10 seconds. In many cases speed is critical to preserving life, property and the environment and maintaining continuity of operations.
  • HFC systems are compact with little storage space required for cylinders. This means they offer the most effective solutions in terms of space/weight considerations.
  • HFC’s are non-conductive, clean agents and therefore leave no residue.

HFC fire extinguishing systems are needed for fire protection in a limited but vital number of cases where speed, space and safety are critical. Typical applications are telecommunication facilities, computer rooms, process control centres.

HFC-125 has been used successfully as an alternative to halon for engine fire protection on US military aircraft developed since the early 1990s. In addition, HFC-125 is currently being developed for use on a military derivative of a large commercial aircraft (Boeing 767; military derivative KC-46). HFC-125 has increased space and weight characteristics that present installation concerns.

Recently new fluorocarbon and fluorocarbon ketone extinguishants are being developed and some are implemented. One of the objectives is to develop active replacements for halons that are suitable for use in aircraft in particular. Active fire extinguishants contain bromine or iodine and these are much more effective HFCs, but have higher toxicity (much lower exposure limits compared to HFCs) and lower stability. Active extinguishants have lower space and weight requirements which makes them particularly suitable for use on aircraft.

A new fluoro-olefin, HBFO-1233xfB, (CF3CBr=CH2 or 2-bromo-3,3,3-trifluoropropene, shortened as 2-BTP) has been proposed by the US EPA under SNAP as a drop-in replacement for Halon 1211 Fire extinguishing agent on aircrafts, based on its technical performance, a GWP of 0.23-0.26. Although it contains bromine, due to its very short atmospheric lifetime (7 days) it has an insignificant ozone depletion potential (0.0028).

The EPA is proposing 2-BTP on aircraft, acceptable as a total flooding agent as a streaming agent for use in handheld extinguishers. Other additional potential fire suppression applications for 2-BTP could be envisaged. There continues to be interest in CF3I which is an effective active extinguishant, but has stability issues and a very low exposure limit due to its toxicity.