High challenge: High-piled storage fire protection– Part 2
The accurate classification of commodities in high-piled storage arrays is essential to successful fire outcomes because the level of fire protection must be capable of protecting the fuel load. If the potential combustible load exceeds the automatic sprinklers’ fire suppression capability, a small fire almost certainly will grow into a total loss. On the other hand, properly designed and maintained fire sprinkler systems can suppress or hold fires in check until they are manually extinguished.
A sprinkler system’s fundamental role is to apply cooling water at a rate that exceeds burning materials’ heat of combustion or heat release rate. The sprinkler designer must take into account the combustibility of the stored products in addition to the storage method of piles, shelves or racks. When the commodity classification is obvious, the sprinkler design approaches are relatively easy. However, when the commodity changes, unreported alterations in contents or packaging may create a condition where the sprinkler system can no longer provide adequate protection.
Perhaps the biggest challenge fire protection professionals face is the pervasive use of plastics in product, packaging or even materials-handling components. Adding even a small amount of some plastic materials to a commodity may have a significant impact on the fire characteristics.
To appreciate plastic hazards, one must understand the difference between heat of combustion (latent heat) and heat release rate (HRR). Latent heat is the amount of energy released from a specific mass of an item when it is entirely consumed. For example, if one kilogram of lignite coal is consumed in a fire, it will emit about 15,000 kJ of energy (one pound of the same material will release about 8,000 Btus). The heat of combustion is a means for comparing the heating value of two or more materials. The heat release rate is the amount of energy released over a specific period of time. Items that have a greater HRR may be said to burn more ferociously.
Not all plastics exhibit the same significant latent heat potential. In fact some are almost as benign as ordinary wood combustibles. To address the differences, the plastics industry has classified plastics into three groups related to their relative combustibility. Table 2 provides a description of the plastic groups and some examples of each.
HRR is a measure of the energy released by a fire over a period of time. Figure 2 compares the HRR of four common materials. The vertical axis represents heat energy that is released, the horizontal axis is time. The path of the curves represents the increasing energy released by the burning material until it reaches “peak heat release” then diminishes toward eventual extinction. The materials with steep curves –generally the plasticized products — reach their maximum energy release earlier in a fire than those having a more gradual curve. The peak heat release rate is used by scientists and fire protection engineers as a convenient descriptor to compare the burning characteristics of one or more items. Since this is the point where the material is emitting the most energy, the remaining values are less important in the comparisons. (However, when a material exhibits a curve with two or more upward slopes [such as the Douglas fir particle board test in Figure 2] the person conducting the test must be prepared to explain if this a typical fire behavior, an anomaly of the specific test, a specimen problem or some other reason. A complete body of test results is needed to fully understand fire behavior).
Plastics also are categorized by their physical form: unexpanded or expanded. Unexpanded plastics are solid, dense materials such as those use to make plastic rubbish bins, computer monitors or even some auto parts. Expanded or foamed plastics are created by injecting air into the softened parent stock and are valued for their thermal and shock resistance. Styrofoam® coffee cups and plastic packing “peanuts” are examples of expanded plastics. Generally, because of their lower density, increased surface area and entrained air, expanded plastics tend to have a greater HRR than unexpanded plastics. (See Figure 1).
The combination of latent heat of combustion and heat release rate that enables experts to group commodities into various classes. Once the fire protection designer knows the anticipated commodity classification in the storage area, they can design a fire sprinkler system with enough water volume, pressure and sprinkler placement to control or suppress a fire.
For the purpose of establishing high-piled or rack storage fire protection requirements, the International Fire Code categorizes commodities (the combination of product, packaging and material handling method) into one of five categories: Class I-IV and high hazard. Generally, the fire hazard is lower in the lower-number classes. Class I and II commodities include such things as ceramic and metal products in cardboard or wooden boxes. Adding plastic materials – especially those in the Group A category – can have a significant impact on the overall commodity classification and the fire protection systems required to protect it. In some cases, the additional plastics may increase the commodity to high hazard.
While having this information at the outset makes the fire sprinkler designer’s job easier, packaging changes over time can create conditions where the fire sprinkler system may be inadequate to control a fire. Look at the motor vehicle oil filters in the cardboard box (Figure 3). In this configuration, the metal outer case with paper filter inside and small amounts of plastic wrap likely make this a relatively low-hazard, Class II commodity. A properly designed fire sprinkler system could easily protect a warehouse full of these commodities in solid pile, shelf or rack storage arrays.
If, however, additional plastic – especially expanded Group A – that might be added to protect the shipment could have a hidden effect by increasing the commodity’s combustibility. The warehouse operator, fire code official and sprinkler designer must be aware of these changes to adapt to the new, more hazardous environment.
Plastics have the potential for significantly adding fuel to commodities in high-piled or rack storage. Even small changes – depending on the plastic type – can have serious consequences. In the next article in this series, you will learn how to estimate the impact of adding expanded or unexpanded Group A plastics to a packaging configuration.
For more information, go to codes.iccsafe.org