Lawrence Webster Forrest (LWF), Fire Engineering and Fire Risk Management Consultants
Lawrence Webster Forrest (LWF), Fire Engineering and Fire Risk Management Consultants



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Lawrence Webster Forrest
Legion House
Lower Road
Kenley
Surrey
CR8 5NH

Tel: +44 (0)20 8668 8663 Fax: +44 (0)20 8668 8583
E-mail: fire@lwf.co.uk

Water Mist Fire Protection in the UK

Water Mist Fire Protection in the UK

The use of water mist systems in the UK has increased over the last decade due to a number of reasons, the advantages being:

  • Non toxic;
  • No asphyxiation risk;
  • Environmental friendly problems;
  • Limited damage to properties. 

However when it comes to the approval stage of a water mist system installation a common problem that both the approvals authority and the consulting engineer encounter, is that there is a definite lack of robust design and installation standards. 

Therefore the purpose of this bulletin is to discuss the considerations that should be given to the specification of water mist systems. 

What is Water Mist?

A water mist fire protection system is a fixed active fire protection system that discharges a fine spray of small water droplets. The water droplets discharged from a water mist system removes the heat from the fire (cooling of fire plume and wetting/cooling of the fuel surface), displaces oxygen and dilutes the fuel vapour from the fire, resulting in fire suppression or extinguishment. The term ‘water mist’ refers to fine water sprays in which the drop diameter, (DV0,90) is less than 1mm measured in a plane 1m from the nozzle at its minimum operating pressure. 

Sprinkler Systems Vs Water Mist Systems

The application of sprinkler systems is largely understood and there are historic performance records for buildings. Sprinkler systems have standards and approvals for the components and the design and installation. For these reasons sprinkler systems are accepted as automatic fire suppression systems in the current Building Regulations in the UK. 

A frequently asked question from building control professionals and fire safety officers is whether a water mist system can achieve an equivalent level of performance compared to a sprinkler system. Unfortunately, there is no straight answer to this, as water mist systems are not designed in the same way as sprinkler systems. The use of water mist systems for volume protection in buildings is largely unproven. To date there appears to be very few reported fire events in buildings with water mist systems. As the water droplets produced by a water mist system are smaller than for sprinklers it is considered that a significant amount of the water mist droplets can be ‘blown away’ under certain conditions such as open doors or windows. As a result, the water coverage flux density at the fire location is reduced by ventilation. However, there is also empirical test evidence available showing that within a small compartment, water mist performed well in a fully sealed enclosure. 

Ventilation and compartment sizes are merely two examples out of many critical design details that can affect the successful operation of a water mist system. In order to use water mist systems as an alternative to sprinkler systems it is a requirement within the Building Regulations that such systems have been designed and tested for use in buildings and are fit for their intended purpose. 

Current Guidance

Currently there is no published British Standard for water mist systems or water mist system components. Existing water mist standards such as the published National Fire Protection Association (NFPA) 750 Standard on Water Mist Fire Protection Systems or International Maritime Organisation (IMO) Standards are not directly applicable to UK land-based applications. A European water mist (Technical Specification CEN TS 14972) has been published which includes a fire performance test for various applications. However this document has not been adopted in the UK and its suitability for UK application is uncertain. 

Two new Drafts for Development have now been issued: 

  • DD 8489: Fixed fire protection systems – Industrial and commercial water mist systems ; &
  • DD 8458: Fixed fire protection systems – Residential and domestic water mist systems  

There is currently insufficient technical data available for them to be published as full British Standards. These documents try to define the testing protocols for water mist systems in various different applications, however, they do not cover all water mist scenarios. It is anticipated that tests for further applications will be developed in due course and published in additional sections, as the specification for fire performance tests is developed for other uses. 

Due to the limitation of guidance on water mist systems, extra care should be taken when choosing a water mist system for buildings in the UK. 

Acceptability of Water Mist Systems

In the absence of a generalised design and installation standard (such as BS 5306 Part 2: Fire extinguishing installations and equipment on premises – Specification for sprinkler systems or BS EN 12845: Fixed firefighting systems – Automatic sprinkler systems – Design, installation and maintenance), the acceptability of water mist system installations for buildings will depend on the demonstration of their effectiveness in a suitable fire performance test relevant to the real life application. To choose an acceptable water mist system, it is imperative that relevant supporting evidence be made available by the supplier or installer, demonstrating that test fires have been satisfactorily extinguished, suppressed or controlled (whichever is the desired result from the system) in rooms of the same size and same occupancy with an equivalent fire load as the intended application. In other words, water mist systems for buildings should be considered on a case-by-case basis to determine whether a proposed system has met the fire protection objectives. 

As water mist does not primarily extinguish by direct wetting of solid fuels, its impact on, for example, deep-seated fires will be slow. Other scenarios where water mist may not be able to extinguish fires include small fires and shielded fires. For this reason, the fire protection objectives for a water mist system need to be clearly stated by the supplier or installer with supporting evidence, so that the building control professionals and fire safety officers can make a judgement about the acceptability of water mist systems. The fire protection objective terms for a water mist system are defined as: 

Fire extinguishment: A sharp reduction in heat release rate leading to complete elimination of any flaming or smouldering fire; 
Fire suppression: A steady reduction in the heat release rate resulting in a lower controlled level of burning; 
Fire control: Limitation of fire growth and protection of structure (by cooling of the objects, fire gases and/or by pre-wetting adjacent combustibles)  

The following is a brief ‘Water Mist Checklist’ suggested by London Fire Brigade (LFB), which they use for reviewing the acceptability of a proposal when submitted to them for approval: 

  • Are design objectives clearly stated i.e. extinguishment / suppression / control – what trade off if any are being sought?
  • Is relevant fire test data available / presented? 
    System configuration compared to test data – note any deviations.
  • Water supply – type / duration, is this suitable given the stated design objective?
  • System reliability and maintainability – they currently refer to sprinkler codes for advice on resilience requirements for life safety systems.
  • Other considerations such as extent of system, zoning and control valve locations, cause and effect, interactions with other systems.
  • Are appropriately qualified and experienced people undertaking the manufacture/design/installation/maintenance of the system? (No third party certification schemes)  

Mathematical Modelling

As mentioned, supporting evidence should be made available to demonstrate that water mist systems are fit for purpose. To meet this requirement fire tests are generally used in the industry. As an alternative method to fire tests, using mathematical modelling e.g. Computational Fluid Dynamics (CFD) may also be utilised to demonstrate the performance of a water mist system subject to that a particular numerical model used has been adequately validated for the intended water mist applications. For instance, the numerical simulation of fire extinguishment is an extremely difficult task. None of the current CFD models has been extensively validated to predict fire extinguishment and this may be the case for some years to come. Therefore, the fire protection objectives for water mist systems should be clearly identified before using mathematical modelling to demonstrate the performance of this form of suppression. 

Conclusion

It is important that water mist systems are being designed and proved fit for purpose using suitable standards, fire demonstrations and performance tests. The use of mathematical modelling to demonstrate the performance of water mist systems is only acceptable subject to the validation of a numerical model used. Misunderstandings and lack of knowledge about water mist systems can lead to inappropriate application of these systems and therefore caution is strongly advised.

 

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