The LWF Blog

Fire Engineering Design and Risk Assessment – Firefighting & Rising Water Mains – Part 36

April 15, 2019 2:15 pm

In LWF’s fire engineering blog series for Architects and others in the building design business, we have been talking about firefighting and rising water mains in buildings. In part 35, we began to discuss wet rising water mains and in part 36 we will continue looking at that subject before considering horizontal mains, also known as internal hydrants, which are seen more commonly in buildings with large floor area.

It was established in part 35 that the supply of water to wet risers should be sufficient to service three landing valves in operation at the same time – including the three most unfavourable hydraulic locations. This requires a minimum flow rate of 1500 litres/minute.

The provision of a 45000-litre water storage tank is the minimum required, provided it is fitted with an automatic water company infill connection which is capable of replenishing the tank with 500 litres/minute reliably. This would mean that the combination of the storage tank and infill could provide a water provision of 1500 litres/minute for 45 minutes.

The system requires back-up systems in case of failure at a critical time. Duplicate automatic pumps should be provided, one of which is a standby. Each pump should be powered by different independent power sources and should automatically become operational when a drop in pressure or water flow is detected. Both audio and visual alarms should be provided to alert to the situation when this occurs.

The system should also be provided with an emergency tank filling connection for any situation where the automatic infill is not operational. The provision of a breeching inlet and 100 mm pipe which discharges into the wet riser storage tank is sufficient.

The breeching inlets and tank filling connections for wet rising water mains should be consistent with the standards required for dry risers.

Horizontal mains/internal hydrants

Horizontal mains work on the same basic principle as rising mains, but are designed to cover a large horizontal area, rather than a tall vertical one. Such systems can be wet or dry and are subject to the same requirements and standards as rising mains. They are most commonly seen in large shopping centres or complexes but are suitable for any large building.

In part 36, LWF look at how placement of internal mains and landing valves should be considered and decided upon. 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.

Share this post