A key part of fire safety in modern tall building design is the inclusion of dry and wet risers; fixed systems used by firefighters to provide a supply of water throughout a building in the event of a fire.
This article delves into both systems, helping you understand the benefits and suitability for each type. If you’d like to learn more, simply get in touch with us here at Therma-Mech.
Dry Riser Systems
Dry risers are a requirement of building regulations in occupied buildings over 18m tall. They can also be found in environments with limited access or with compartmentation issues such as multi-level basements or hospital corridors.
The system consists of a network of pipes that allow water to be delivered to the upper floors of a building in the event of fire. It comprises of an inlet at ground level that can be accessed by the fire brigade to pump water up into the building to outlets situated on the landings of each floor. This allows a firefighter to attach their hose to any one of these outlets and source pressurised water to set about extinguishing the fire.
The advantage of a riser system is that it is a fixed distribution system within the building and requires none of the fire service’s resources or equipment. It also helps, by design, to maintain the compartmentation of the building.
The three elements of the riser system, namely the external inlet, the pipework and the internal outlets should be designed to meet BS 5041 BS 5306, BS 9990 and Building Approval Regulations.
The external inlet which enables the connection of fire brigade water supplies is usually to be found in an external cupboard or enclosure signed “DRY RISER INLET”. Within the enclosure is located a collecting head which has at least two BS Instantaneous male couplings. The cupboard will also contain a drain down valve that enables the dry riser to be emptied of water after the fire service operation or after testing. The enclosures need to be vandal proof, but nevertheless, provide immediate access for firefighters. Doors usually have a breakable area to allow urgent fire service connection.
The pipework of dry riser distribution systems is kept free of water and should be of galvanised steel. The British Standards requirements for the pipe’s internal diameter is 100mm for buildings over 18 metres, which increases to 150mm in taller buildings. The pipework is usually located in fire resistant shafts or enclosures. At the top of the pipework an air valve is located which allows air in the riser to be expelled when the system is charged with water.
The internal outlets or “landing valves” are the connection points in the system that enable the fire brigade to attach and advance its hose lines throughout a building. The outlet consists of either a single or double BS instantaneous female outlet which is under the control of a gate valve.
Like the inlets, outlets are usually protected by enclosures which also have a breakable area in the door to facilitate urgent connection. Outlets are usually located in a protected lobby, stairway or cupboard. This location is usually one of the building’s fire escape staircases, enclosures or lobbies. Often one outlet is located at roof level to allow for a “testing” outlet.
One outlet should be provided for every 900m2 of floor area on each floor other than the ground floor and must not be more than 60m apart in a horizontal direction. Furthermore, there shouldn’t be any part of the floor area that is more than 60m from an outlet as measured along the route suitable for hose lines.
The local fire brigade should be familiar with the layout, function and operation of dry risers in buildings and if any of these buildings is considered a special or specific special risk (PRM/SSRI), detailed information on the size, location and performance limits of the riser needs to be included within any risk data.
Testing and maintenance of both dry and wet risers should take place annually and is the responsibility of either the property owners or managing agents. British Standard 5306: Part 1 1976 (Revised by BS9990) recommends that the system is not only tested and serviced every twelve months but is also visually inspected every six months to guarantee that the equipment is ready for immediate use in the event of an emergency. It is also recommended that a full wet test is carried out annually, involving a wet pressure test to 150 PSI or 10 Bar.
Wet Riser Systems
The wet riser system is similar in design to dry risers and can usually be found in buildings over 50m tall. Unlike the dry riser system, it is kept constantly charged with water, in order that it can provide the same level of protection against a fire in the upper reaches of taller properties as on the lower floors. The wet riser system is charged with water from a pressurised supply, which is often pumped from a storage tank.
It is necessary to have wet riser systems in buildings that extend to more than 50m as the pumping pressure required to charge the riser in these cases is higher than that which can be provided by a fire service appliance, and thus ensures an immediate supply of water to high levels.
In the event of an emergency, such as a storage tank running low during long operations, it is usually possible for the fire brigade’s pumping appliances to supplement the water supply to wet risers.
A wet riser supply system should generally be able to maintain a minimum running pressure at the roof top outlet of 4 bar at a flow rate of 22.7l/s. The maximum running pressure allowed with only one outlet in operation is 5 bar.
As with dry risers, wet riser systems should be installed within fire-fighting shafts, and where necessary in protected escape stairs. Wet riser outlets should be located within protected lobbies, stairs or enclosures wherever these are available.
Wet risers should be inspected and tested regularly to guarantee that equipment is operating correctly and ready for use. Clearly any problems could have the potential to be very serious in the event of a fire. The usual causes of system failures are vandalism, theft, blockages, or by pipework or connection failure or by outlets being open.
Dry and wet risers are now an integral part of modern high rise buildings and when properly maintained, allow fire fighters to advance up a building quickly knowing that there will be a source of appropriately pressured water onto which they can attach their hoses at all levels of the building.
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