The LWF Blog

Fire Engineering Design and Risk Assessment – Firefighting & Rising Water Mains – Part 35

April 9, 2019 10:50 am

In LWF’s Fire Engineering blog series for Architects and others in the building design business, we have been looking at those provisions which can be made to assist firefighting in case of a fire. In part 34, we commenced looking at wet rising mains and noted that while they can be permanently connected to water mains, it is more common to see the systems connected to a water tank with a pump or gravity feed, or both. In part 36, we will continue discussing wet rising mains.

While wet rising mains are suitable for builds of all heights, they are most necessary in buildings where the highest floor is more than 50 metres above fire service access level. If a dry rising main system were used in a very tall building, the water pressure necessary and the time taken to pump water to the highest level would be prohibitive. 

As sufficient pressure is required in the upper levels of a wet riser system, it can mean that excessive pressure is present in the lower portion. In order to avoid the potential for overly high pressure in firefighting hoses used in the lower levels, it can be necessary to limit the delivery pressure. The running pressure from each landing valve should be between 4 and 5 bar when in use. The hose pressure, when the nozzle is in the off position, should be no more than 7 bar.

The method for controlling pressure is the inclusion of a pressure relief connection, built into the delivery side of the landing valve which is permanently connected to a waste pipe. Ideal pressure is achieved by calibrating each pressure relief connector at each landing valve location, allowing for the pressure differential to be adjusted to suit.

An alternative to pressure relief connections is to use a dead shut-off pressure reducing valve which doe not require a drain connection.

As fires can be approached by the Fire Service in a three-pronged attack – from below, from above and on the floor of fire origin, it is necessary that any wet rising main is capable of delivering sufficient water to the landing valve locations of any three consecutive floors of the building simultaneously. This means that the water supply to the system must be sufficient and reliable. In practical terms, the system will require 1500 litres/minute with a minimum running pressure of 4 bar at the highest possible landing valve.

In part 36, LWF will continue to look at the provision of sufficient water to wet riser systems. 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 0800 410 1130.


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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