Standpipe, Hose, and Private Fire Service Main Systems
Classes and what they deliver
Class I standpipes provide 2½ in. outlets for fire department use; Class II provide 1½ in. hose for occupant use; Class III provide both. Systems may be automatic (wet), manual, or dry, and manual systems are supplied through a fire department connection. The sizing rule candidates are tested on: the system must deliver 250 gpm at a residual pressure of 100 psi at the hydraulically most remote 2½ in. outlet (lower residual pressures apply to 1½ in. outlets). Pressure reducing valves are used where the static pressure at an outlet would exceed safe hose pressure, and they must be listed and set for the flow expected.
Private fire service mains
Mains between the water supply and the building are governed by NFPA 24. Key items: burial depth below frost line, thrust blocks or restrained joints at bends and tees, gate valves with post indicator valves, a detector check valve assembly, and a fire department connection sized and located as the code requires. Hydrants, when part of the private main, need the specified valve and drainage arrangement.
Traps
- Mixing up Class I and Class II outlet sizes.
- Quoting the 100 psi residual for a 1½ in. outlet.
- Forgetting thrust restraint at direction changes.
- Assuming an FDC can make up for an undersized riser.
Worked example
A 60 ft tall building with a Class I wet standpipe: the topmost outlet must still show 100 psi residual at 250 gpm flow. If the supply leaves only 60 psi there, either the supply must be upgraded or a fire pump added - a pressure reducing valve at the outlet cannot create pressure, it can only limit it.