Choosing fire suppression systems

We were particularly concerned about a generalised suggestion that mist and sprinkler systems are interchangeable for the same applications. This is far from the truth and could give rise to serious problems should the article be used as “evidence” that mist might be appropriate for a particular application.

The association has previously expressed its concerns regarding such issues and has issued an information document that clarifies some of the restrictions to be considered when evaluating which water fire suppression to use. The information sheet welcomes the obvious benefits of mist, including lower water storage requirements, reduced amounts of water applied and faster suppression times.

A number of groups, including the BRE, Fire Protection Association, insurers and many of the larger fire engineering consultancies, such as Arup Fire, have expressed concern at the misunderstandings that exist in respect to mist and have advised caution in assuming that mist is a panacea.

There are also inaccuracies in the article in respect of proposed standards. The UK has firmly rejected the contents of the proposed European Standard pr EN14972 and has, instead, set up two working groups to draft new British Standards for mist systems, DD8458 Watermist fire suppression systems for residential and domestic occupancies – Code of practice, and DD8459-1 Watermist fire suppression systems for commercial and industrial premises – Code of practice. Both are well advanced and DD8459 may be published before the end of the year.

One interesting observation made by the author that also must be questioned is the matter of the space needed for mist systems being smaller than for typical sprinkler systems. This, of course, would only be true for sprinkler systems supplied by a tank and pumps. One recent study undertaken for Heathrow Terminal 5 emphasises this, making it clear that installing a watermist system for this large structure would not only have been more expensive but would also have demanded water storage space which was simply not available in T5. (Oldham J, Terminal Velocity, FP&EJ, October 2007).

As a practising fire consultant, I have been responsible for recommending and specifying many water mist systems and I am a strong supporter of this fire protection approach – but only for the correct applications. Mist is ideal for local area application, it is positively brilliant on lubricating and fuel oil systems in power stations, for the protection of engine and machinery spaces and for some applications in heritage protection. It is much less suited to the protection of large, open spaces such as warehouses, public buildings and the like and for any hazard which could result in slow, deep-seated fires.

We should welcome the availability of mist as another weapon in the fire engineering armoury but must resist the temptation to be blind to its limitations.

Stewart Kidd, secretary general, British Automatic Fire Sprinkler Association

Steven Cooper responds:

I would like to offer my sincerest apologies if I misled any readers into believing that water mist systems were a direct replacement for sprinkler systems.

Nothing could have been further from my mind, my intention being solely to inform your readership that there are other alternatives that should be considered. I would also be surprised, although secretly quite chuffed, were the article to be used as “evidence” that a water mist system would be appropriate for any particular application.

The article does state that mist systems may not be appropriate for deep-seated or low-energy fires, and the fact that each system must be designed specifically for the risk would hopefully prevent systems being installed where they would not be effective, especially as the design should be based on full-scale fire tests. Nonetheless, Mr Kidd’s point is well made.

It is good to hear that we are developing our own Standard, but it is doubtful that we will be able to ignore completely a European norm, and a system designed in accordance with the EN would have to be accepted in this country. So much for harmonisation. However, the fact that Mr Kidd refers to these being codes of practice rather than specifications does give some cause for hope.

The issue of space saving is an interesting one, and water mist systems will always win hands down where tanks and pumps are required. Mr Kidd, in his letter, is presumably alluding to sprinkler systems fed solely from

a towns mains, which obviously require no tanks, but the prudence of relying on a sprinkler system with a water supply over which you have little or no control over should be questioned.

It is possible to design a system fed entirely from a towns mains, but it must be recognised that water companies are not able to guarantee the reliability of such a supply. Where a client wants a degree of assurance that the system will be available at all times, it is likely that a tank and pump arrangement will be required, and it is our experience as practising engineers that this is usually the case.

In closing, I would reiterate that the intention of my article was to raise awareness generally and not to suggest that water mist systems could be regarded as a direct replacement for sprinklers, although in many cases this may actually be the case. I would like to thank Mr Kidd for giving me the opportunity to make this clarification.