5 WAREHOUSE STORAGE
5.7 Cold storage warehouse fire protection
Cold storage warehouses present two special fire protection challenges. One challenge is the presence of foamed insulation wall and ceiling construction. The second challenge is the need for dry pipe sprinkler systems to protect large storage arrays of combustible items.
The May 3 1991 fire in a cold-storage warehouse complex in Madison, Wisconsin is indicative of the challenges posed by both the storage and insulation. As reported by Isner (1991), the fire originated in a 55 ft high building constructed with floor-to-ceiling storage racks. The roof assembly, which was supported by the rack structure, consisted of metal decking under foamed insulation covered by tar and gravel. Three separate storage areas, with storage temperatures ranging from −23◦C (−10◦F) to 1◦C (+34◦F), had walls of foam insulation between metal sheathing. The palletized rack storage commodities consisted of 13 million pounds of butter, and an assortment of other refrigerated foods including meat, poultry, cheese, cranberries, and various vegetables. The storage area was protected by a dry-pipe ceiling sprinkler system designed to discharge 0.15 gpm/sq-ft. There were no in-rack sprinklers.
When Madison firefighters responded to the alarm at the warehouse, they found the ceiling sprinklers operating over ‘an apparent pile of rubble engulfed in yellow flames’ in one of the freezer storage areas. There was also a separate roof fire, which spread from an initial area of about 150 ft2, to involve most of the 260× 170 ft roof in that section of the building. Concern for potential rack storage collapse, which in fact occurred 47 minutes after arrival, limited manual firefighting to hosestreams discharged from outside the storage area. Plate 4 is a photograph of the melting butter and flames after the collapse of the rack structure and an exterior wall. The fire eventually spread to two other storage buildings and caused a loss in excess of $100 million. The most important deficiency in the Madison, WI warehouse was the absence of dry-pipe in-rack sprinklers, and the inadequate ceiling sprinkler discharge density.
Design of a dry-pipe sprinkler system for a cold-storage warehouse presents several challenges besides installing in-rack sprinklers. The design numbers of flowing in-rack sprinklers and ceiling sprinklers must be increased to account for the delay time between sprinkler link actuation and the arrival of water at the open sprinklers. NFPA 13 allows a delay time of as long as 60 seconds, which can cause a rack storage fire to increase by a factor of two-to-four beyond the size at which a wet pipe system would begin discharging water. There are also serious reliability issues associated with the dry pipe valves, the normally dry piping in the refrigerated area, and the dry-pipe sprinkler heads. The dry pipe reliability issue is the mechanical complexity of many dry pipe valves that render them vulnerable to hang-up when not regularly maintained and tested. The reliability issues associated with the dry piping and sprinkler heads are their propensity to corrosive deterioration and to be occluded by condensation and freezing of moisture in the air-filled piping. The former issue can be mitigated by periodic sample testing of sprinkler heads (UL testing has indicated that more than half the heads did not discharge water at the minimum sprinkler operating pressure), and replacement of obstructed heads. The latter issue can be mitigated by using dry air with a dew-point substantially lower than the storage temperature.
The March 1992 Missouri cold-storage warehouse fire started when grease residue on an insulated wall was ignited during hot cutting operations. Workers used portable extinguishers on the fire, but it continued to spread up the wall and ceiling, which were insulated with steel-faced expanded polystyrene. There was no automatic sprinkler protection in the 25 ft high warehouse, which was fully loaded with palletized frozen foods. The fire eventually involved an 180,000 ft2 section of the warehouse between two firewalls (NFPA Journal, November/December 1993). The bursting of an ammonia refrigeration line during the fire jeopardized the manual firefighting operations. The loss was estimated to be $100,000.
The most tragic cold-storage warehouse fire occurred in November 1999 in Worcester, Mas-sachusetts. After being shutdown for several years, the warehouse was occupied by a pair of homeless people who inadvertently started a fire by knocking over a burning candle. Unfor-tunately, the homeless couple left the warehouse without reporting the fire. When Worcester firefighters responded, they entered the warehouse in search of any occupants. A sudden, rapid increase in fire size caused visibility to deteriorate while six firefighters were disoriented and trapped inside. The firefighters perished in the unsprinklered warehouse, which was insulated with an assortment of highly combustible wall/ceiling insulation.
A 1997 – 1998 series of fire tests in the FM 25 ft corner test facility has provided valuable information on the contribution of different types of insulation to cold-storage room fires. Insu-lation panels included steel-faced expanded polystyrene in thickness varying from 102 – 254 mm (4 – 10 in), and polyisocyanurate insulated steel deck roofing. The thick expanded polystyrene pan-els presented the greatest challenge, and required discharge densities well in excess of 0.2 gpm/ft2, even when wet pipe systems were used. One effective cold room sprinkler configuration described in the 1998 FMRC Update article (Vol. 12, No. 2) consisted of dry pendant heads at the bottom of short dry pipe extensions from a wet pipe system installed in a nearby wet system in a heated area of the warehouse. Readers are advised to consult FM Data Sheets and Approval Guides for updated specifications of recommended discharge densities and areas, as well as for listings of approved insulation assemblies not requiring this level of protection.
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