Lawrence Webster Forrest (LWF), Fire Engineering and Fire Risk Management Consultants
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Fire Engineering Design and Risk Assessment - Water Mist System Components - Part 15

Posted by LWF: 08/08/2018 9:55

In LWF’s Fire Engineering blog series for Architects and others involved in building design, we have been looking at methods of fire suppression and most recently, water mist systems. In part 14, the basis for design was discussed and in part 15, those components which go into making a water mist system are outlined. 

Water mist nozzles are primarily designed to generate the very fine droplets of water necessary and to provide momentum for delivery into the fire plume. The mist itself is produced by one of four means:

- An orifice plate
- A spinner or internal swirl chamber
- Gas atomisation
- A deflector

Distribution of water mist over a prescribed floor area is not one of the features of water mist system design or performance.

In order for a water mist system to maintain the ultra-fine mist required, it is important that the tiny holes in the nozzles do not become blocked or partially impeded by debris of any kind. To this end, corrosion-resistant piping is used, most commonly stainless steel or copper. 

Some systems for low-pressure use may use larger orifice nozzles and may be found to use galvanised steel piping, although it is important that consideration of the necessity for filtration is considered in such cases.

All water mist systems must have control valves which enable the flow of water to be initiated and monitored.

The cylinders used for storage of water for the water mist system must be made of corrosion-resistant materials, as per the piping. Commonly, these are made of stainless steel or lined and at atmospheric pressure. Stored pressure with lined cylinders can lead to gas permeation behind the lining which would subsequently lead to system failure.

The pumps for a water mist system must be capable of meeting the maximum design flow and pressure and the capabilities must be assured by the provision of a dedicated power source which is reliable. A duplicate power supply may be necessary for critical hazards. 

In the case of pumps, design criteria similar to sprinkler systems may be applied, although type approved equipment is not available. For higher-pressure pumped systems, positive displacement pumps are necessary.

At installation, the pipework throughout must be thoroughly cleared of all debris and scale and a flush undertaken after installation. Testing must be undertaken to hydraulically check the pipework to 1.5 times the maximum operating pressure. The operation of the water supplies and control valves must also be functionally tested.

Open nozzle systems should be discharge tested where possible and all nozzles and strainers checked for cleanliness after.

Given the necessity for the system to remain free-flowing and unblocked, it is essential that regular inspection and maintenance be carried out on the system by individuals trained and experienced in water mist systems.

The next blog in the series will move on from fire suppression systems to look at Compartmentation. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.

Lawrence Webster Forrest has been working with their clients for over 25 years to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact Peter Gyere on 020 8668 8663.

While care has been taken to ensure that information contained in LWF's publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.


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