Lawrence Webster Forrest (LWF), Fire Engineering and Fire Risk Management Consultants
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Fire Engineering Design and Risk Assessment - Water Mist Systems - Part 14

Posted by LWF: 31/07/2018 12:11

In LWF’s Fire Engineering blog series for Architects and others in the building design business, we have been looking at Water Mist Systems. In part 13, the basic principles behind the use of very fine water droplets to fight fire were considered. In part 14, system configurations and the different types of water mist system will be discussed before moving onto system design.

 

Water Mist System Configurations

A water mist system can share certain characteristics with a sprinkler system. For example, a water mist system can be configured to use frangible bulb elements (which fracture when a certain temperature is reached. This allows for only those release nozzles in close proximity to a fire to activate. 

Alternatively, a system may be designed with all of its nozzles open, so that water mist is discharged from every head when the system becomes active.
A water mist system can be used to protect an enclosed space, in the same way that a gaseous system might be used, or to provide protection on a local level to a particular hazard within a larger space.

The water supply for a water mist system can be taken from mains water, where appropriate, but can also be taken from pumped water supplies, such as potable, sea-water or cylinders containing water which purge under gas pressure.


Types of Water Mist System

A water mist system will be classified as high, intermediate or low pressure, according to their operating pressures. An operating pressure can indicate various differences in system type, such as the method of generating water droplets, the droplet size ranges and the mechanism used to give momentum to the water droplets.

High-pressure systems operate at pressures above 80bar, intermediate systems at pressures between 15bar and 80bar and low-pressure systems operate up to 15bar.


System Design

Water mist system designs are based upon the full-scale fire testing of fuels and hazard configurations which are similar to the environment needing protection.
Each manufacturer of water mist systems will produce a unique design for a particular hazard which is specific to that manufacturer’s fire test results. The performance produced will be a functional combination of the unique nozzle operating parameters, the enclosure or area, the type of fire and the ventilation available.

In cases where water mist systems are requested for conditions where relevant test data cannot be provided, fire tests must be carried out to provide the design basis and prove validation of the approach.

The design and installation of water mist systems should only be permitted by those organisations with direct access to and understanding of fire test data and a proven knowledge of the technology.


In part 15 of this series, LWF will look at the components that comprise a water mist system. 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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