Control Mode

Big changes in the world of storage sprinklers.

NFPA Journal®, July/August 2009

Twenty years ago, the development of the early suppression fast response (ESFR) sprinkler was greeted with optimism that went beyond the availability of a new option for storage protection. It promised a new era of improved sprinkler performance and a way to measure and predict that performance. Measurement of the sprinkler characteristics for required and actual delivered density over a range of heat release rates, along with a prediction capability for sprinkler response, meant that fire suppression, defined as reducing the heat release rate of a fire and preventing its regrowth by applying water through the fire plume to the burning fuel surface, could be achieved.

Today, substantial changes are taking place in storage sprinklers. As the 2010 edition of NFPA 13, Installation of Sprinkler Systems, was prepared, the technical committees decided to merge large drop sprinklers and specific application control mode sprinklers into a new category of control mode specific application (CMSA) sprinklers. Systems using these sprinklers are designed on the basis of a minimum number of sprinklers capable of operating at a specific minimum pressure appropriate to the hazard. The sprinklers are control mode, not suppression, sprinklers, since they do not guarantee the penetration of the fire plume or the prompt heat release rate reduction expected with ESFR sprinklers. But they do promise an extremely economical approach to fire control, with relatively few sprinklers expected to operate. CMSA sprinklers are already listed for lower operating pressures than ESFR sprinklers, and, in some cases, for larger protection areas per sprinkler and lower total water demands. It is conceivable that ESFR could largely be replaced by the new CMSA sprinklers.

Although Underwriters Laboratories listings follow the minimum design areas and hose stream allowances specified in NFPA 13, some of the special economies available for the CMSA sprinklers come through FM Global listings. NFPA 13 allows the use of such listings, provided the installer follows all of the criteria stipulated with the listing. FM Global is moving even further away from an absolute distinction between suppression and control sprinklers, announcing future plans to simply categorize sprinklers approved for use in storage applications as “storage sprinklers,” with all design criteria based on the number of sprinklers and minimum pressures, although minimum design areas will address the situation of close-spaced sprinklers.

Perhaps the future of storage sprinklers was shown in the inability of the quick response early suppression (QRES) sprinkler to materialize. This was the term reserved in the 1991 edition of NFPA 13 for the non-storage version of the ESFR sprinkler. The Fire Protection Research Foundation’s original program of the late 1980s that validated the ESFR had a second phase intended to develop this suppression-type sprinkler for light and ordinary hazard occupancies. Testing conducted at Underwriters Laboratories showed that it was possible, and a tentative design method was developed. But the effort stopped when it became apparent there would be no economic incentive for the new sprinkler. The use of control mode sprinklers in light and ordinary hazard occupancies was simply too efficient and economical.

The fact that no one is currently pursuing a QRES sprinkler design statement is a testament to the improved performance expectations for control mode sprinklers, which have been made possible by the wide range of sprinkler orifice sizes now available. One way in which NFPA 13 is acknowledging improved performance expectations is with the truncation of the density/area curves. Beginning with the 2010 edition of the standard, the curves will not show or allow design areas exceeding 3,000 square feet (279 square meters). Gone are the days when it was considered acceptable to open ring after ring of ceiling sprinklers. While the new performance objective may be something short of early suppression, it does involve early, and economical, fire control.