Let Sprinklers Run at Fires In Plastics, FM Suggests

Let Sprinklers Run at Fires In Plastics, FM Suggests

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A novel approach to the fire fighting problem presented by a plastics storage fire is being suggested by Factory Mutual.

When the burning plastics are in an adequately sprinklered building, Factory Mutual urges fire fighters to button up the building as tightly as possible to minimize combustion air supply and “let the sprinklers run, for hours if necessary, until the fire is contained enough so that it can be effectively extinguished and mopped up by conventional means.”

In a discussion of the problem in the November-December issue of its magazine, Record, Factory Mutual cautions that this different approach can work only “in an adequately sprinklered building which can draw on good water supplies.”

The objective of this tactic is to avoid what Factory Mutual terms “the tremendous risk of shutting off sprinklers to find the fire” and the chance of the fire expanding out of control. Factory Mutual regards the extra water damage from extended operation as “far preferable” to total loss of the building.

Plastics fires different

The reasoning behind the advice is that plastics fires defy the long-established technique of venting a smoky fire to locate the seat of the blaze and then making an effective attack with hand lines. Factory Mutual points out that this simply does not work in a plastics storage fire because the sprinkler discharge drives the heavy plastics smoke down, further obscuring the area. When such a situation exists, opening roof vents, Factory Mutual explains, “simply will not clear out the smoke.”

The alternative of shutting down automatic sprinklers to let the smoke rise and vent is rejected as “extremely dangerous in a plastics fire because high combustibility of the materials means the blaze can flash out of control in no time at all.” Without sprinklers operating, Factory Mutual warns, temperatures can rapidly rise high enough “to cause structural damage and precipitate roof collapse.” Also, fire spread can be so speedy that the resultant fusing of a large number of sprinkler heads overtaxes the water supply. When this happens, the building heads for total destruction.

Two problems

The advice to depend on the sprinkler system to control a plastics storage fire is based on 65 full-scale, realistic fire tests with plastics made by Factory Mutual in addition to the experiences of large-loss plastics fires.

Factory Mutual warns that fire fighters responding to a plastics fire have two things going against them. A plastics fire “will probably spread faster than other warehouse-type fires” and “a plastics fire presents a substantially greater threat to the fire fighter’s safety than a ‘routine’ storage fire.”

Because plastics burn hotter than other combustibles, Factory Mutual explains, there is an increased chance that the building will collapse. Furthermore, the faster spreading flames of plastics can more readily trap fire fighters, who are more likely to lose their bearings and become separated in the thicker smoke. Fire fighters also face the “greatest chance” that piled stocks of plastics or the building itself will collapse on them or block their escape.

Fire fighters also face a health threat in a plastics fire because many plastics emit toxic fumes, including “carbon monoxide, hydrogen chloride, hydrogen fluoride, hydrogen cyanide, ammonia, oxides of nitrogen and numerous other less well known but highly toxic gases.” Polyvinyl chloride, when burning, gives off hydrogen chloride, which can form hydrochloric acid when it mixes with water. Also, some burning plastics yield corrosive gases.

Major characteristics

Fire fighters responding to a fire in a plastics warehouse can expect to encounter three basic characteristics that are vastly different from conditions encountered at a fire involving ordinary cellulosic combustibles such as wood and paper.

1. There will be extremely high temperatures because the heat of combustion of plastics is “roughly 2 1/2 times that of other ‘ordinary’ combustibles.” This will intensify the usual radiant heat, structural damage and exposure problems.

Button-Up System for Plastic Fires

The steps in the button-up system of handling plastics storage fires follow:

  1. Clear the building of all personnel.
  2. Button up the building as tightly as possible to limit the air supply available to the fire.
  3. Have an engine pump into the fire department sprinkler Siamese.
  4. Keep sprinkler control valves wide open and let sprinklers operate for an hour or more until the fire has essentially been extinguished and can be attacked with hand lines.
  5. If the equipment is available, try to mechanically exhaust smoke from the building during the final stages of sprinkler operation and before beginning an attack with hose lines. This will help avert the possibility of combustible gases that may have built up in the oxygen-starved atmosphere from flashing or exploding when the building is opened up. Also, this action will facilitate extinguishment by hand lines.
  6. Try to evaluate the fire severity before shutting down sprinklers. This can be done by reconnaisance, using men with life lines and self-contained breathing apparatus, or by opening up the roof or a wall if the seat of the fire can be ascertained by questioning employees.

Now shut down sprinklers, open up the building to vent smoke and attack the remaining fire with hand lines. Keep a fire fighter at the sprinkler valve throughout this operation so he can turn on the sprinklers if the fire threatens to flare up. This man should have a walkie-talkie.

2. A high burning rate will be encountered because “the flammability of plastics is much higher than that of other combustibles.”

3. “Especially thick, dense smoke” will be given off by a plastics fire. The dense smoke will hide the seat of the blaze from fire fighters.

Major types of plastics

Factory Mutual reports that four types of plastics—polystyrene, polyethylene, polypropylene and polyvinyl chloride—account for about 70 percent of all plastics production in the United States.

In using these four types of plastics in its series of tests, Factory Mutual found that three factors were especially important to designing protection. They were open vs. close-pile-array storage, sprinkler discharge density and the size of sprinkler orifices.

As might be expected, the Factory Mutual tests showed that open-pilearray storage with 12-inch flue spaces between stacks, simulating stock removal by lift trucks, “will burn more fiercely than a close-pile-array with 6-inch spaces between stacks. The greater air access in the open-pilearray promotes both fire growth and heat radiation between stacks.

Sprinkler density

Although the common 0.3 gpm per square foot sprinkler discharge density was generally effective in fire tests of close-pile-arrays of plastics, it was generally not effective for open-pilearrays of burning plastics. However, doubling the sprinkler discharge density to 0.6 gpm “resulted in dramatically improved fire control” in both types of storage. Factory Mutual concluded that the “relative ineffectiveness” of the lower sprinkler discharge density was the result of two charac, teristics of plastics fires. First, the higher heat release in plastics fires results in stronger fire updrafts that repel sprinkler water to a greater extent than usual. Secondly, plastics have a physical characteristic of shedding water, thereby reducing the cooling effectiveness of normal sprinkler protection.

The third factor in the tests was the size of the sprinkler orifices. The standard 1/2-inch orifice sprinkler head showed inconsistent ability to control plastics fires. But when a 17/32-inch orifice was used, there was substantial improvement in fire control. The increase of only 1/32-inch in orifice diameter results in a sprinkler head that will deliver 40 percent more water than the 1/2-inch orifice head under the same pressure.

Factory Mutual acknowledges that in some situations, “conventional fire fighting tactics can be effective in controlling a plastics fire, and may in fact be the only effective methods. But there are also situations where access to a fire and venting conditions may be poor, where a building is properly sprinklered and protected by good water supplies.” Under these conditions, Factory Mutual recommends buttoning up the building and letting the sprinklers do their job.

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