Dry risers are the vertical mains fitted into staircase enclosures or other appropriate positions, with outlet valves on every floor and inlets fitted at ground level to enable the fire brigade to connect supply of water in case of fire in the building. The only function of the dry risers is that they provide a ready made means of distributing substantial amount of water to stub out or prevent the spread of fire.
Where dry riser should be installed?
Riser should be installed in the building where any floor is higher than 18 meters above ground level. On the other hand, buildings where there are floors higher than 60 meters above ground level, wet risers are used.
The number and disposition of dry rising mains should be such that:
1. One is provided for every 900 square meters of floor area at each level other than ground floor of the building
2. Rising mains are not more than 60 meters apart in a horizontal direction of the building
3. No part of any floor area is more than 60 meters from a landing valve measure along a route appropriate for hose lines.
Design Consideration for Dry Riser:
1. In dry riser pipe work and fittings should be of Galvani wrought steel and pipe work should be joined by screwing
2. Changes in pipe’s direction should be made with standard bends rather than elbows
3. Dry riser should have a nominal bore of 100 millimeter where only one outlet is provided on each floor. If two outlets are provided on any level on any one riser the main should have a nominal bore of 150 millimeter
4. Each riser should be provided with a 25 millimeter drain valve at the lowest point of the pipe work. When drain valve is not in use, it should be strapped and padlocked
5. An appropriate automatic air release valve should be fitted at the highest point in each rising main to allow the pressurised air in the pipe to discharge when water is pumped in.
Dry riser should be electrically earthed and bond those joints which do not provide electrical continuity. These considerations are very important and need proper concentration at the time of installation. There are many professional companies in UK which are providing quality services for dry riser servicing, installation and maintenance.