Combustible storage

NFPA Journal, Nov/Dec 2001 by Comeau, Ed, Duval, Robert

Improperly stored oxidizers can pose a fire risk in warehouse occupancies.

On August 2,2000, Phoenix, Arizona, experienced one of the largest fires in its history in a multi-tenanted building containing Central Garden and Pet Supply and Cardinal Distributors. By the time fire crews extinguished the blaze the next day, the building was a smoldering ruin. A number of civilians had to be evacuated from the surrounding neighborhood, and the loss is estimated to be in the hundreds of millions of dollars.

What caused the fire to become so large? NFPA sent Senior Fire Investigator Robert Duval to Phoenix to find out. Investigation into the cause of the fire continues as of this writing.

The fire department

The 20-year-old building in which the fire started contained two occupancies separated by a concrete panel wall that extended from the floor of the building to the underside of its roof deck. On one side of the wall, with multiple fire door penetrations, was a home and garden supply warehouse containing a variety of products and commodities. On the other was a pharmaceutical supply warehouse.

The main portion of the building measured 400 feet (122 meters) by 208 feet (63 meters). The section occupied by the home and garden supply operation measured 220 feet (67 meters) by 208 feet (63.4 meters), and the pharmaceutical distribution area was 180 feet (55 meters) by 208 feet (63.4 meters). The building, which would be classified as a storage occupancy by the 2000 edition of NFPA 101(R), Life Safety Code(R), also had an office wing 105 feet (32 meters) long and 30 feet (9 meters) wide.

The exterior walls of the warehouse had no windows, though the office wing did. Overhead doors through which stock was loaded and unloaded were on the east side of the building.

The structure's roof assembly consisted of a 1/2-inch (1.3-centimeter) plywood deck covered with many layers of mopped asphalt and supported by 10-by-4-inch (25-by-10-- centimeter) purlins supported by engineered laminated beams. The beams were supported by steel columns 5 inches (13 centimeters) in diameter on a 40-by-23-foot (12.2-by-7-- meter) spacing. In the warehouse, the ceiling was 30 feet (9 meters) high.

The exterior walls were tilt-up concrete panels, 6 inches (15 centimeters) thick, attached to the concrete floor slab with threaded connections.

The building was equipped with an automatic fire sprinkler system. The pharmaceutical warehouse and home and garden supply warehouse had ceiling-level systems with .495 gpm/feet^sup 2^ over 2,000 square feet (186 square meters) with sprinklers rated at 286 deg F (141 deg C) with orifices 17/32 inch. There were no in-rack sprinklers.

Building contents

Because the warehouse supplied area home improvement stores, the products offered varied significantly, from bird feeders and tools to redwood products and batteries. Also among the materials were fertilizers, insecticides, and pesticides. They were stored in different configurations, on double and single-row racks, and in solid piles 10 to 20 feet (3 to 6 meters) high. The solid- and open-shelf configurations were 20 feet (6 meters) high, separated by aisles 8 to 10 feet (2 to 3 meters) wide.

Many items were stored in corrugated cartons scattered throughout the warehouse wherever space allowed. Some pallets were encapsulated in plastic sheathing.

Chlorinated pool chemicals, which are oxidizers, and are addressed in NFPA 430, Storage o(Liquid and Solid Oxidizers, were also stored in the warehouse. While not combustible themselves, oxidizers can significantly increase the intensity with which other products burn. Furthermore, they're incompatible with other products that may be stored in home and garden occupancies.

Oxidizers are divided into four classes based on the inherent hazards they present. Class 1 oxidizers increase the burning rate of other products slightly. Class 2 oxidizers moderately increase the burning rate and can cause spontaneous ignition. Class 3 oxidizers severely increase the burning rate of other products, can cause spontaneous combustion, and will undergo self-sustained decomposition. And Class 4 oxidizers severely increase the burning rate of other products, can cause spontaneous combustion, will undergo self-- sustained decomposition, and will cause an explosive reaction.

When contaminated with incompatible products, oxidizers can cause spontaneous combustion. If the heat of combustion is confined, nearby combustibles may ignite. To suppress fires involving oxidizers, large amounts of water may be required. Small amounts of water can cause oxidizers to react and release chlorine gas.

The fire

On the day of the fire, employees reported an unusually strong chlorine odor in the home and garden supply side of the warehouse. Chlorine could often be smelled in that area because many of the products stored there contained the chemical, but this was worse than usual. To identify the source, the employees moved pallets of the pool chemicals outside and opened several large overhead doors to help ventilate the building. At 11:30 a.m., the chemicals were moved back into the building, but the chlorine odor became strong again, so all of the pace tri-chlorine in 35-pound (16-kilogram) plastic buckets and some pace tri-chlorine in 75-pound (34-- kilogram) fiber drums were taken outside to the east side of the building. None of the cal-hypo or pool shock was removed from the building. The work day ended before workers discovered the source of the odor.