Reconsidering the idea of interconnected risers.
NFPA Journal®, March/April 2009
A model fire protection ordinance recently developed in Germany for high-rise buildings includes a feature that some in the United States will remember from the past: interconnected risers. The ordinance requires sprinkler protection for all new buildings more than 197 feet (60 meters) high and for existing buildings more than 72 feet (22 meters) high unless they are compartmentalized into areas not exceeding 2,000 feet2 (200 meters2) and provided with additional protection to reduce the chance of exterior flame spread. The sprinkler requirement includes the provision that the system be served by “two rising main tubes [or risers] which shall be installed in different shafts.” Should one riser fail, the second will ensure the water supply. On each riser, standpipe outlets must be provided on each floor for the fire brigade’s use.
This concept originally appeared in U.S. model building codes in 1978 to address concerns about pipe breakage during earthquakes. The requirement for interconnecting combined sprinkler/standpipe risers was accompanied by a requirement for check valves at each connection to the sprinkler system from the risers. The theory was that the sprinkler system would still be provided with water should a riser rupture.
However, the Committee on Automatic Sprinklers disagreed and added an appendix section to the 1983 edition of NFPA 13, Installation of Sprinkler Systems, stating that “combined automatic sprinkler and standpipe systems should not be interconnected by sprinkler system piping.” This caution remained in the standard for several decades.
NFPA 14, Installation of Standpipe Systems, has traditionally required standpipes to be interconnected at their bases. When preparing the 2003 edition of NFPA 14, however, the Committee on Standpipes deleted the phrase “at the bottom” from the requirement for interconnection. This recognized that, for standpipes supplied by tanks at the top of a building or zone, the standard also required risers to be interconnected at the top, with check valves installed at the base of each standpipe to prevent circulation. The committee also required that all connections from a riser to a sprinkler system in a combined system have a check valve to avoid needing to drain all floor piping in the event of work on the system, without closing floor control valves.
During the development of the 2007 edition of NFPA 13, the discouragement of interconnected risers was deleted, with the substantiation that “NFPA 14…now allows standpipes to be interconnected by sprinkler system piping.”
Section 126.96.36.199 of NFPA 13 still sets rules for the use of combined risers. Each connection to the sprinkler system must have a control valve and check valve the size of the connection. Each combined riser must have a riser control valve to isolate the riser without interrupting the supply to other risers using the same source. The requirement for a check valve is new to the 2007 edition of NFPA 13.
The committee didn’t believe interconnected risers would resolve the problems associated with earthquake damage. Unless the appropriate riser control valve is closed, the loss in water pressure could still compromise system performance. The committee was also concerned that flow to an open sprinkler could be balanced among multiple risers and stay below the threshold needed to activate any single water flow switch. Nor did the committee want to create the possibility of hydraulic calculations for a “vertical grid” of piping that would allow pipe size reductions based on the expectation of water being supplied through multiple risers.
The idea of two supply risers to improve reliability is tempting. However, a fire department’s ability to take advantage of redundant piping during a fire requires extensive prefire planning. That’s just one good reason to think twice before interconnecting risers.
Russ Fleming, P.E., is the executive vice-president of the National Fire Sprinkler Association and a member of the NFPA Technical Correlating Committee on Automatic Sprinklers.