Rubber tire storage

Rubber tire and synthetic rubber storage present a special challenge in warehouse fire protection because the deep-seated fires produce copious volumes of smoke and are particularly difficult to extinguish. Flaming within the hollow torroidal inner surface of the tire is often shielded from

the sprinkler spray, and the steel belts in the tire remain hot enough to cause re-ignition after an initial apparent suppression. Automatic sprinklers of suitable water discharge density can and will control tire storage fires, but, there is a tendency for the tires to re-ignite if the sprinkler water flow is shut off too early or if airflow into the storage array is allowed to increase.

The NFPA 231D standard for storage of rubber tires (now incorporated into NFPA 13 and NFPA 230) describes four stages of a typical sprinklered tire storage fire. The characteristics of the four stages, called the incipient, active, critical, and overhaul stages are listed in Table 6.5.

Manual firefighting efforts are usually futile and dangerous during the active stage in which the sprinklers are trying to gain control of the fire.

The three most commonly used tire storage configurations are: on-side (horizontal) storage (Plate 6), on-tread (vertical) storage (Plate 7), and laced (oblique angle with alternating rows) storage (Plate 5). The tires are often loaded into open frame portable racks as illustrated in the photographs. Storage heights can range from a few feet for on-floor storage to over 25 ft for rack storage. On-tread tire storage usually represents a greater fire protection challenge than on-side storage because burning can continue within the horizontal flue formed by the wheel holes, and propagate extensively while being shielded from the sprinkler spray. However the pallets often used in on-side storage also shield the smaller vertical flues in on-side piles. Interlaced storage has proven to be the most challenging sprinkler protection storage mode because it seems to allow fire redevelopment even more than the other storage configurations.

Rubber tire protection guidelines originally stem from several series of sprinklered fire tests at the Factory Mutual Test Center and from a 1971 test series in France (Cleremont-Ferrand, 1973). The French tests involved 20 ft high caged tire portable rack storage in an aircraft hangar with an arched roof. The tests demonstrated that a sprinkler discharge density of 0.55 gpm/ft² could control the fire providing there was ample (almost unlimited) water supply for at least 34 sprinklers. The French tests also provided an excellent demonstration of the advantages of high expansion foam used in conjunction with ceiling sprinklers. Protection guidelines (NFPA 231D and FM Data Sheet 8-3) for open portable rack storage now provide the option of using a high

Table 6.5. Stages of a sprinklered tire warehouse fire Stage Initiating

Incipient Ignition 2–5 Decreasing Black Portable extinguisher;

hose line; remove burning tires from storage array.

Active Sprinkler Actuation 60 – 90 Virtually None Turning from black to gray

Overhaul Smoke Clearing 24× 60 Clear None Small hose streams while sprinklers still flowing;

ceiling sprinkler design density (0.60 gpm/ft²over 5000 ft² and 0.90 gpm/ft²over 3000 ft²for up to 25 ft high on-side storage) without high expansion foam or a much lower density (usually 50%

reduction) with foam.

A 1970 Factory Mutual tire fire test inadvertently demonstrated the relationship between building ventilation and sprinkler control. Sprinkler control of 18 ft high on-tread storage was established with a discharge density of 0.60 gpm/ft²until the test building was ventilated to reduce the heavy smoke concentration. When the building was ventilated, an intense fire began to spread through the array and could not be controlled even with 95 flowing sprinklers (50 opened after the building was ventilated) over an area of 4750 ft². Sprinkler control was eventually achieved by reducing the building ventilation.

Figure 6.7 illustrates the tendency of tire fires to redevelop even when subjected to significant water spray densities. The heat release rates plotted in Figure 6.7 were obtained from 20 ft high Required Delivered Density tests conducted with a delivered density of 0.55 gpm/ft² under the Fire Products Collector. The prototype Group A plastic commodity is suppressed with this density as is evident from its heat release rate curve in Figure 6.7. On the other hand, the on-tread tires showed a very different behavior. The fire remained in an incipient stage for the first four minutes after ignition. Sudden fire development at four minutes triggered the actuation of the water spray to the top of the tire array at that time. The water did immediately reduce the fire intensity and maintained the heat release rate under 30,000 btu/min (780 kW) for the next 31/2 minutes, but the heat release rate eventually grew back up to 150,000 btu/min (2600 kW) when the test had to be terminated.

Tire

FMRC standard plastic commodity

00 30000 60000 90000 120000 150000 180000 210000 240000 270000 300000

1 2 3 4 5 6 7

Time from ignition (min)

Convective heat release rate (Btu/min)

8 9 10 11 12 13 14 15

Figure 6.7. Comparison of on-tread tire array and plastic commodity (20 ft high) heat release rates with 0.55 gpm/ft²delivered density._2002 Factory Mutual Insurance Company, with permission

As with many other commodities, recent tests have demonstrated the advantages of larger orifice sprinklers for protecting high-piled rubber tires. ESFR sprinklers and Large Drop sprinklers have successfully protected 25 ft high tire storage in a series of fire tests conducted by FM (Dean, 1996) and shown in Plates 5 – 7. The Large Drop sprinklers discharging 100 gpm over each 100 ft² of coverage (density of 1.0 gpm/ft²) protected both on-side and on-tread storage. The ESFR sprinkler, which had previously been shown to protect 25 ft high on-tread storage at a water flow rate of 100 gpm per sprinkler, needed a flow rate of 125 gpm per sprinkler to protect 25 ft high interlaced storage. The interlaced storage test opened 14 ESFR sprinklers, and the NFPA and FM standards require designing for 20 flowing ESFR sprinklers since this is a fire control application rather than an early suppression application.

One special consideration in dealing with the sprinkler and hosestream runoff from a tire storage fire is the presence of oil generated during the pyrolysis of the tires. Based on reports of the oil recovered from large fires of scrap tires, there have been between 0.1 and 1.0 liter of oil produced for every tire burned. This oil should be separated and recovered from the runoff before allowing it to reach groundwater or nearby lakes or rivers. Fire-fighting guidelines for outdoor scrap tire fires are contained in Appendix C of NFPA 231D, which is based on the report of the International Association of Fire Chiefs (1995).