The LWF Blog

Fire Engineering Design and Risk Assessment – Firefighting & External Water Supplies – Part 33

March 25, 2019 2:12 pm

In LWF’s Fire Engineering blog series for Architects and others in the building design business, we have been looking at firefighting and the associated provisions that need to be made for the Fire Service. In part 32, we looked at the standards for wet and dry mains given in Approved Document B and BS 9999. In part 33, we consider dry mains in more detail.

A dry rising main is comprised of pipework installed vertically through the floors of a building with an inlet breeching available at Fire Service vehicle access level, plus outlets with hand-controlled valves on each floor (except ground level). The system allows the Fire Service to connect hoses from a hydrant to the fire pump and then from the pump to the inlet and charge the main with water. 

The firefighters may then enter the building and attend the landing valve situated one or two floors below the floor of fire origin, where they can connect their hoses and from there, take the primed hoses to the fire.

BS 5041-1:1987 – Fire hydrant systems equipment. Specification for landing valves for wet risers states that the diameter of the rising main water pipe should be 100 mm or 150 mm. The breeching inlet for a dry riser system should be situated in an external wall as close to the riser position as possible. There should be vehicle access for a fire pumping appliance within 18 m of each fire main inlet.

The breeching inlet box should be positioned so it is between 400 mm and 600 mm above ground level from the outside perspective. Where a 100 mm diameter dry riser pipe is used, a two-way breeching inlet should be fitted and where the pipework is 150 mm, a four-way inlet should be provided. The breeching inlet box should be protected by a secured door and marked ‘Dry Riser Inlet’.

Where the installed system serves only floors at breeching inlet level and above, a drain valve should also be fitted into the breeching inlet to allow for water to be removed from the system. 

Additionally, an automatic air-release valve should be provided at the highest point of the dry riser to allow the system to be charged without the prior opening of any landing valves. Dry mains could be left charged with water ready for operation which has two advantages, the main one being that the system is ready to go when it is required with no delay for charging. The second advantage is that it is any leaks to the pipework or landing valves left open will become obvious very quickly, rather than being a hidden issue until a fire is in progress. However, due to the risks of standing water, for example legionnaires, this is very rarely the case.

In part 34, LWF will continue looking at dry rising mains before moving on to discuss hybrid mains. 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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