The LWF Blog

Fire Engineering Design and Risk Assessment – Firefighting & Rising Water Mains – Part 34

April 1, 2019 1:43 pm

In LWFs 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 33, we began to look at dry mains in terms of what they consist of, as well as what standards BS 5041 recommends for the purpose. In part 35, we will continue looking at dry mains from the point of view of NFPA 1 before discussing hybrid mains and wet mains.


NFPA 1: Fire Code refers to dry mains as standpipe systems within the document and splits them into three categories  dependent upon the size of outlets recommended for different applications. The document concurs with BS 5041 in regards to the diameters of 100 mm or 150 mm for the mains. While a maximum of 150 mm is a usual recommendation, it can be useful to discuss requirements with the Fire Service in the context of the particular building in question, it may be that their recommendation is for a greater diameter, particularly in the case of taller buildings.


In the US, NFPA 1 indicates that the door to the inlet box of the rising dry main should be indicated by the word ‘Standpipe’, however, in the UK, it would be more usual for it to be labelled ‘Dry Riser Inlet’.


Hybrid dry and wet rising mains


Although both dry and charged dry mains are commonly used in the UK, NFPA 1 discusses hybrid dry and wet systems called ‘automatic dry standpipe system’ and ‘semi-automatic dry standpipe system’. Both systems work on the same principle, which is that dry main is filled with pressurised air which means that when the landing valve is opened, water is pulled into the system pipework. As with all rising main systems, the water supply available must be sufficient for the system demands.


Wet rising mains


The main difference between a dry rising main and a wet rising main is that instead of an inlet breeching at the lowest level, a wet rising main is connected to a permanent water supply capable of supplying the pipework directly. In the US, wet rising mains are referred to as ‘automatic wet standpipe systems’.  On occasion, a wet rising main is connected to the water company main where such a connection is approved and of sufficient capacity. More commonly, the system will be connected to a water tank along with a pump or gravity feed, or even both.


In part 35 of this series, LWF will continue looking at wet rising mains including the circumstances they are most commonly used in and water pressure control. 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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