Click here for a printable version of this article.

February 2009: Protection for Storage

There are several factors that go into determining sprinkler system adequacy. The main steps are to determine what is being stored, the storage arrangement present, and the distance between this storage and sprinkler protection.

The first step is to determine the overall storage commodity classification. This is a difficult task, since there can be thousands of products stored in a warehouse. Items such as metal typically do not burn, while paper will readily burn. Plastics usually are harder to ignite than wood, but they burn much hotter than wood. For example, the heat of combustion for items such as wood and paper generally ranges between 6,000 and 8,000 Btu/lb. The heat of combustion for plastics generally ranges from 12,000 to 20,000 Btu/lb. Plastics not only burn hotter than ordinary combustibles, but they also burn several times faster. Plastics are further subdivided into three groups. Group A plastics are plastics that have a high heat of combustion and a rapid burn rate. Group B plastics have heat release rates higher than ordinary combustibles, but have a slower burning rate than Group A plastics. Finally, Group C plastics have heat release rates and a burning rate similar to those of ordinary combustibles. As the commodity classification increases the hazard, the more the sprinkler design must increase to control a fire. The hotter and faster something burns, the more water is needed to control a fire involving this storage.

The materials that are in the storage arrangement are grouped into commodity classifications. Class 1 commodities are items that typically do not burn such as metal on wood pallets, placed in single-layer corrugated cartons, or shrink-wrapped. Class 2 commodities are mainly non-combustible items in slatted wooden crates, solid wood boxes, multiple-layered corrugated cartons, etc.

The introduction of plastics makes classifying storage more challenging. Class 3 commodities are items that are combustible such as wood, paper, natural fibers, Group C plastics, etc. A Class 3 commodity is also permitted to contain 5% by weight or volume of plastics or less. Class 4 commodities are typically combustible items with limited amounts of plastic composition. This could be Group B plastics, free-flowing Group A plastics (such as plastic pellets) or any item containing 5 to 15% by weight or 5 to 25% by volume of plastics. Unexpanded plastic commodities are Group A plastics such as ABS or polyurethane. Finally, expanded plastic commodities are items such as foam and some waxes.

Other items are considered special hazards and have specific storage requirements. These items include, but are not limited to: rolled paper storage, idle pallets, rubber tires, flammable liquids, aerosols, etc.

Once the commodity is classified, the storage arrangement must be determined. Storage can be stored stacked on the floor or on pallets, in bin-boxes, on shelves, or even in racking. For floor or palletized storage, the overall height of the storage is determined along with the height of the building. The height of the storage is important as this is directly related to the amount of storage and the resulting fire intensity. The higher the storage, the higher the sprinkler density needed to protect the storage.

The height of the building is important because this determines how far away the sprinklers are from the fire. The further the sprinklers are from the fire, the longer they take to operate. Prompt sprinkler operation is necessary for proper fire control. The same height of storage needs increasing sprinkler designs as the height of the building increases.

Shelf storage and rack storage may look similar, but the sprinkler protection needed for these two storage arrangements can be very different. Thus, it must be determined if this storage is actually on shelves or in racks. Shelf storage is referred to as storage on solid shelves that are less that 30 in. deep and aisles are usually 2 ft. or less. Rack storage is storage on open or solid shelves that exceed 30 in. deep.

If the storage is in racks, then further evaluation is necessary. The types of racking must be determined. If there is a row of racking - not exceeding 4 ft. deep - with aisles on either side, then it is a single-row rack. If there is a row of racking, then a longitudinal flue space, then another row of racking with aisles on either side, then this is a double-row rack. If more than two rows of racking are present or the overall depth of the racking exceeds 12 ft., then it is considered a multiple-row rack. The difference between an aisle and flue space is that flue spaces are a vertical opening less than 2 ft. wide and an aisle is over 2 ft. wide.

The width of the aisles between racks is also measured. The smaller the aisles, the harder it is for ceiling sprinklers to control a fire. This is due to fact that the total amount of storage is a lot higher when the racks are placed closer together.

It is also important to determine the presence of flue spaces. Please note that flue spaces are only needed in rack storage. Flue spaces allow the heat from a fire to reach the ceiling for sprinkler operation and they also allow water from sprinklers to reach the fire. Longitudinal flue spaces run the length of the racking. These flue spaces are not always needed but should have a minimum 3 in. net opening along the entire length of the racking when they are provided.

Transverse flues run perpendicular to the longitudinal flues and should have a net opening of at least 3 in. Transverse flues are always needed in rack storage. The spacing of these flues will vary based upon the storage and sprinkler system design present. A transverse flue should be present per each pallet load width or about every 4 to 5 ft. Wider transverse flue spacing results in considering the storage to be on solid-shelves. The presence of solid shelves increases the sprinkler design needed to protect the storage since water cannot as readily reach the fire.

Thus, the shelving material used in the racks is also important. Typically, the type of shelving used is a wire mesh, plywood, or wooden slats. A shelf must be at least 50% open to be considered and open shelf. The use of an open shelf gives the ceiling sprinklers their best chance to control a fire.

The overall storage area is necessary in determining loss expectancies, especially when the current sprinkler protection is not fully adequate. The number of tiers is also collected. A tier is a level of storage and this helps to determine the arrangement for in-rack sprinklers when they are needed.

All this information is then used to determine which table to look at in the associated NFPA standard or FM Global Data Sheet to determine the sprinkler system design needed. These sprinkler design tables have been developed over the years as a result of various tests performed and some engineering judgment.

The design of your sprinkler system is then compared to the design needed from these tables. If your sprinkler system can provide the needed sprinkler design, then it would be expected that a fire involving this storage would be controlled. If your sprinkler system cannot provide this design, then you need to determine how close your system is to the needed design. If it is fairly close, then your system is considered nearly adequate. If your system is not very close to being able to provide the necessary sprinkler design, then it is considered inadequate. An inadequate sprinkler system is not expected to be able to control a fire, and Risk Logic will recommend that a correction of this deficiency is made.

Risk Logic, Inc. can help you determine the adequacy of your sprinkler system and if any improvements may be needed.