Wet risers are a form of internal hydrant for the firemen to use and are always charged with water. They are only required for buildings where the topmost floor is higher than 30 metres above the fire appliance access level. Wet riser system comprises duty fire pump with standby pump discharging into a 150mm diameter riser pipe with landing valves at each floor and to which canvas hose with nozzles can be connected to direct the water jet at the fire. A jockey pump is usually provided to maintain system pressure. For high rise buildings, each stage of the wet riser should not exceed 70 metres.
Codes and Standards
Wet riser system shall be designed, tested, selected, installed and maintained in accordance to:
- MS 1489 : Part 2 – Fire Extinguishing Installations and Equipment:Wet and Dry Firefighting Mains
- MS 1210 : Part 1 – Landing Valves for Wet Risers.
- MS.1210 : Part 3 – Inlet Breeching for Riser Inlets.
- MS 1210 : Part 4 – Boxes for Landing Valves for Dry Risers.
- MS 1183 : Part 5 – Firefighting Stairways and Lifts
- Any relevant Standards
Wet Riser Landing Valve
Landing valves should be provided at each floor and should comply with M.S.1210 : Part 1. They are usually located within fire fighting access lobbies, protected staircases or other protected lobbies, and installed at not more than 0.75 metres above floor level. To protect the landing valves, boxes can be provided and these should comply with M.S.1210 : Part 4.
All spaces of the floor should be protected through 45 metres coverage from a landing valve, such that every room can be reached with 30 metres hose length plus 15 metres water stream. Wet riser system should be designed such that maximum system pressure serving a fire main outlet do not exceed 20 bar. The pressure at the landing valve should have a minimum 4 bars pressure and should not exceed 7 bars. This is to cater for inhouse firefighting before the arrival of firefighters. To achieve this, there are two types of landing valves i.e. pressure reducing type with or without relief outlet. Those with relief outlets require a wet riser return pipe.
With the fire brigade breeching inlets the firemen can pump water into the wet riser storage tank to make up for water used. The breeching inlet should be a 4-way type complying with M.S.1210: Part 3. Where the breeching inlet is enclosed within a box, the enclosure should comply with M.S.1210 Part 5 and labelled ‘Wet Riser Inlet’. A drain should be provided at the bottom of the riser to drain the system after use. Breeching inlets should be located no more than 18 metres from the fire appliance access road and not more than 30 metres from the nearest external hydrant.
Wet Riser Pipe
The wet riser mains are usually located within smoke free lobby or protected areas. Where more than one riser is required for each floor, the distance apart between the risers should be 60 metres or less apart. The distance between the lowest and topmost landing valve in any upper stage riser should not exceed 60 metres. The riser pipe diameter should be 150mm galvanized iron to BS EN 10255:2004 (heavy gauge) or Class C. Where a relief pipe is required, this return pipe shall be minimum 100mm diameter galvanized iron to BS EN 10255:2004 (medium gauge) or Class B, discharging back to the wet riser tank wherever possible. An air release valve should be installed at the top of the riser to relief air trapped in the system.
All wet riser pipes should be coated with primer and finished with red gloss paint. Alternatively, the pipe can be colour coded with red bands of 100mm width and the elbows and tees painted red. The riser pipe should be electrically earthed to achieve equipotential with the building.
Wet Riser Pumps
The wet riser pumps draw water from wet riser storage tank should consist of two sets of pumps, one on duty and the other on standby. The pump capacity is usually sized to deliver a flow rate of 1,500 litres/min for three separate landing valves simultaneously at a running pressure of minimum 4 bars at each fully opened landing valve. The standby wet riser pumpset should be supplied with power from the emergency generator if this is available. Otherwise, the standby pumpset should be diesel engine driven. Fuel supply should be adequate for minimum 2 hours of continuous operation. Electrical cabling to supply power to the wet riser pumps should be of MICC or fire rated type. Batteries for the diesel engine should be maintenance-free type.
The wet riser pumpsets should be protected from fire and away from locations likely to be flooded. Sump pumps shall be installed where the fire pump room is located in the basement below external drainage levels. It should also be ventilated by natural or mechanical means and provided with tile necessary signage.
Flow and Pressure Test
A flow and pressure test should be carried out ensuring a minimum flow rate of 500 litres/min at running pressure of 4 to 7 bars is maintained at each of the three most remote landing valves are fully opened.