When warehouse managers are told they need to install in-rack sprinklers, they usually react with trepidation and dismay. Who can blame them?
Although in-rack sprinklers have been successfully used to protect storage racks for more than five decades, protection guidelines have historically required the installation of a large number of sprinklers, making this type of protection very expensive compared to ceiling-level sprinklers. In addition, warehouse managers worry that in-rack sprinklers could cause water damage to stored products when accidentally struck by fork-lift trucks.
Given these drawbacks, it’s easy to understand why facility managers might be reluctant to supplement their ceiling-level sprinklers with in-rack sprinklers.
As available warehouse space in certain parts of the world becomes scarcer, and as new material-handling equipment capable of reaching higher storage heights becomes available, warehouse facilities today are trending toward storage and ceiling heights that are much taller than ever before. Because the natural path for fire growth is vertical, as storage and ceiling heights increase, so will the fire hazard associated with this height increase.
In an effort to improve the protection of storage racks and lower the overall cost of fire protection, FM Global embarked on an in-rack sprinkler research project in 2011. Although we have previously conducted thousands of loss-prevention projects, this one was different. Our research division performed small-, intermediate-, and full-scale fire tests; unlike previous research programs, though, the testing was strategically coupled with computer modeling to help identify potential protection solutions. This approach demonstrated that by using larger orifice sprinklers and higher water flow rates, the number of in-rack sprinklers needed for an installation could be greatly reduced. This could lower the cost of an in-rack sprinkler installation by an estimated 40 percent, as well as reduce the likelihood of damage to sprinklers and stored products.
These new in-rack sprinkler protection options are alternatives to existing guidelines rather than replacements for them, but they provide owners and designers with new possibilities for protecting buildings and goods that may be difficult, or impossible, to protect with ceiling-level sprinklers alone.
A new look at in-rack sprinkler potential
Since the invention of the first automatic sprinkler in 1874, scientific research and development efforts have improved ceiling-level sprinklers to the point where they can now protect storage under ceilings as high as 45 feet (13.8 meters). This has been achieved primarily due to improvements in the response time of the sprinklers, the size of the sprinkler orifice through which the water is discharged, and the design of the sprinkler deflector that directs the water flow downward toward the fire area. With future scientific research and development it is possible that ceiling heights can be further increased above 45 feet without requiring in-rack sprinklers. Even so, sprinkler manufacturers acknowledge that ceiling heights are quickly approaching the limit where ceiling-level sprinklers alone can be effective, necessitating the use of in-rack sprinklers.
Because of the potential drawbacks of in-rack sprinkler systems, sprinkler manufacturers have tried to find ways to produce ceiling-level sprinklers that can protect both higher storage and more hazardous commodities in open-frame storage racks without the need for in-rack sprinklers. While an abundance of scientific research and product testing has been conducted over the years to advance the performance of ceiling-level sprinklers, minimal work has occurred to advance sprinkler protection within storage racks. In-rack sprinklers are typically needed when the water penetration from ceiling-level sprinklers to the base of the storage array is impeded or requires a significant period of time, which is often the case for most high-storage configurations. Couple this with very fast vertical fire growth, and it’s easy to understand the overwhelming challenge that high-storage configurations present to ceiling-level sprinklers.
The protection guidelines used for most in-rack sprinkler designs and installations were developed through a test program initiated in the late 1960s by NFPA’s Rack Storage Fire Protection Committee for the purpose of providing design guidelines for NFPA. As a result of the sprinklers that were available on the market at the time, the test program used sprinklers that today are commonly installed for the protection of offices and other low-hazard occupancies. Very little advancement in the field of in-rack sprinkler protection has been made since this initial test program.
As a consequence, most in-rack sprinkler installations require a large number of sprinklers within the racks with relatively small horizontal and vertical spaces between them. A typical in-rack sprinkler arrangement for the protection of cartoned plastics would require that sprinklers be installed on an approximate 20-square-foot (1.9-square-meter) horizontal spacing every 15 feet (4.6 meters) vertically throughout the height of the storage rack. This not only drives up the cost of fire protection due to the amount of materials and labor required for the installation, but also increases the likelihood of a sprinkler being accidentally hit by something and discharging water onto stored items.
Additionally, even with all of those sprinklers installed, a fire in high-rack storage can still grow upward through a rack, opening both in-rack and ceiling-level sprinklers as it goes, making final extinguishment by the public fire service very challenging due to the height at which the fire is located above floor level. Ideally, a fire originating anywhere within a storage rack would be suppressed by the in-rack sprinklers, which would prevent the fire from growing vertically past the sprinklers, thus making it potentially easier for final extinguishment by the public fire service.
Finally, another important variable in protecting rack storage is the type of container used, which can have a significant impact on the sprinkler protection required for these spaces. Products that were once maintained within corrugated cardboard containers are now increasingly stored in plastic tote containers. While this trend may seem beneficial from the perspective of being able to reuse the containers more frequently, the fire hazard associated with the plastic containers greatly increases compared to cardboard containers. This is due to the higher heat release rate of plastic materials compared to cardboard, along with the benefit of water absorption that cardboard exhibits compared to the lack of water absorption by plastic materials. Closed containers that absorb water make it more challenging for fire to spread horizontally, which is a major factor in controlling a fire event by automatic sprinklers. With the concurrent trends toward higher storage and ceiling heights, as well as replacing cardboard containers with plastic totes, many warehouse facilities are no longer able to protect their storage in racks using a ceiling-only sprinkler system and must rely on supplementing the protection with in-rack sprinklers.
Testing and optimized designs
To get a better understanding of the nature of fire growth within open-frame storage racks and the corresponding heat release rate at the time of in-rack sprinkler activation, we conducted several intermediate-scale tests using different test commodities. For each test, the response time of both standard-response and quick-response in-rack sprinklers, installed at different locations within the rack, helped us determine which factors most influenced the operation of the sprinklers and which factors caused a delay in operation.
Water distribution tests were also conducted to better understand how quickly, and how much, water flowed across a commodity and down its sides to reach the base of the storage array. Many variables were assessed, including sprinkler type, sprinkler location relative to the top of the commodity, sprinkler location relative to the flue spaces, and flow rates. The goal was to determine the effect they had on the amount of water that made it from the sprinkler to the base of the storage array in a timely fashion. These data were used to validate the computer multi-phase flow model.
Additionally, intermediate-scale in-rack sprinkler suppression tests were conducted over a range of sprinkler conditions, including K-factor—the formula used to calculate the discharge rate from a nozzle—and pressure. Computer modeling was used to simulate the cold flow water distribution under those in-rack sprinkler conditions, yielding an indication of the amount of water necessary to control or suppress the specific fire. This optimized in-rack sprinkler water flow rate was then validated in large-scale in-rack sprinkler fire tests. Ultimately, the use of computer modeling helped us reduce the number of large-scale fire tests needed and shortened the time it took to create the new in-rack sprinkler protection options included in the June 2015 release of FM Global Property Loss Prevention Data Sheet (DS) 8-9, Storage of Class 1, 2, 3, 4, and Plastic Commodities (fmglobaldatasheets.com).
A key finding was that by using larger orifice sprinklers and higher water flow rates, the number of in-rack sprinklers needed for an installation could be greatly reduced. Instead of using the traditional K5.6 (K80) sprinkler at a flow of 22 to 30 gallons per minute (83 to 114 liters per minute), K14.0 (K200) and higher sprinklers were used at flow rates of 65 gallons per minute (246 liters per minute) and higher, based on results of the coupled experimental and modeling strategy.
This reduction in sprinklers is achieved by increasing the vertical space between in-rack sprinkler levels and by increasing the allowable storage height above the top level of in-rack sprinklers. Traditional installations use a vertical spacing between levels of in-rack sprinklers ranging from 10 to 15 feet (3 to 4.6 meters). This is due primarily to the smaller K-factor sprinklers and their relatively low design flow rates. FM Global testing helped establish greater vertical distances between sprinkler levels. The new guidelines in DS 8-9 allow in-rack sprinklers to be installed every 30 to 40 feet (9.1 to 12.2 meters) vertically, depending on the commodity hazard being protected. This larger vertical distance between sprinklers greatly reduces the number required compared to traditional in-rack arrangements.
Traditional in-rack sprinkler installations limit the storage height above the top level of sprinklers to 10 feet (3 meters), primarily due to the concern that fire growth within the storage rack will reach the top of the array and that the amount of heat from a fire may be too large for the ceiling sprinkler system to control. Research testing established that, when using the newly developed in-rack sprinkler system, the stored height of commodity above the top level of in-rack sprinklers can be much greater than previous restrictions. This was achieved by not only the type of in-rack sprinklers chosen and their discharge flow rates, but also by the location of the sprinklers horizontally within the footprint of the rack. The goal of the in-rack sprinkler horizontal arrangement was to prevent any fire spread above the in-rack sprinklers, which was achieved in all of the full-scale validation fire tests. By eliminating the ability of the fire to grow vertically above the in-rack sprinklers, the sprinklers essentially become a virtual floor on which the ceiling system can be designed. As a result, instead of limiting the storage height above the top in-rack sprinkler level, the new guidelines in DS 8-9 allow for storage above the top in-rack sprinkler level to be as high as 35 to 40 feet (10.7 to 12.2 meters), again depending on the commodity hazard being protected. This larger vertical distance above the top level of in-rack sprinklers greatly reduces the number of in-rack sprinklers required compared to traditional in-rack sprinkler arrangements.
A final factor to point out is that traditional in-rack sprinkler installations require that the water supply account for both the ceiling and in-rack sprinkler systems operating simultaneously, and also require them to be hydraulically balanced at their point of connection. This is because the existing in-rack sprinkler arrangements may allow the fire to grow to the top of storage and actuate not only in-rack sprinklers but ceiling sprinklers as well. The new in-rack sprinkler arrangements outlined in DS 8-9 prevented the fire from growing vertically past the in-rack sprinklers and thus never operated ceiling sprinklers. This performance is attributed to the timeliness of the in-rack sprinkler activations along with the amount of water that is discharged from them. As a result, instead of requiring the water supply to account for both the ceiling and in-rack sprinkler systems operating simultaneously, the new guidelines in DS 8-9 allow for the water supply to be sized based on the larger of the two system requirements, a feature that will help reduce the size of the water supply required for adequate protection.
What these findings mean is that better in-rack sprinkler protection is now available at a significantly lower cost compared to traditional in-rack sprinkler arrangements. Although these design criteria are currently only available in FM Global’s Data Sheet 8-9, efforts will be taken to incorporate them into codes and standards throughout the world, including NFPA 13, Installation of Sprinkler Systems, so that all warehouses where in-rack sprinkler protection is needed can take advantage of this important new information.
BY WESTON C. BAKER, JR.