Fire protection challenges in storage spaces
Fire sprinklers for high-piled storage continues to evolve as challenges arise and technologies are developed, tested and implemented, driving forward the levels of protection
- Understand the history of and challenges associated with fire sprinkler systems.
- Recognize the limits of fire protection for high-piled storage.
- Learn about current fire sprinkler research.
Research on fire sprinkler obstruction criteria is ongoing by the National Fire Protection Association’s Fire Protection Research Foundation to evaluate the effects of obstructions on the fire sprinkler spray pattern development. With the completion of Phase 4 in August 2020, a tool was developed that can be used to provide reliable analysis of the impact of obstructions that can eventually be implemented by the NFPA 13: Standard for the Installation of Sprinkler Systems Technical Committees for new guidance and requirements.
Using actual delivered density testing to determine water distribution within the area of protection and measured over time, the effects of different obstructions (primarily structural) were measured to determine the extent of disruption.
Buildings continue to get taller and new fire suppression solutions are sought. A limitation of high-piled storage fire protection and research up to this point has been the assumption that facilities have a flat roof. This is generally defined as having a slope of less than 2 inches in a 12-inch run (16.7% slope). A new challenge emerged because former industrial and manufacturing facilities were looking for a new lease on life, but older construction types frequently have varying types of sloped roofs. Some of the various types of ceilings range from sawtooth, m-type, clerestory or even arched with varying degrees of slope, generally in excess of the current limitations.
While the number of manufacturing buildings may have declined in the United States, the amount of storage space has increased dramatically, with an estimated increase of approximately 20% in the number of facilities in the past decade to nearly 14 billion square feet.
Fire sprinkler research
As the demand for storage space continues to increase, a long-term research project was initiated by the NFPA FPRF. Its task was to evaluate the problem, research potential solutions, put together a theoretical framework for testing scenarios in increasing scale and make recommendations for applicable fire sprinkler criteria to be incorporated into NFPA 13.
Phase 1 of the project was undertaken and published in early 2016. To gain a better understanding of the need, a survey of known roof types, storage types and commodities, along with other determining factors, was conducted to help refine the focus. Computational fluid dynamics modeling was conducted to understand the effects of a sloped roof on sprinkler activation times based on their locations. Additionally, the effects of sprinkler spray patterns were investigated to understand the impacts of sprinkler orientation being parallel to either the ceiling (traditional) or the floor. This phase laid the groundwork for subsequent research in phase 2.
Phase 2 was broken into two separate parts, both published in late 2017, to look at developing a Full Scale Test Matrix and Measurement of Sprinkler Spray Patterns and Impingement Near Sloped Ceilings. This phase continued the research on sprinkler spray patterns to move beyond smooth ceilings and look at obstructions from ridges, purlins and other common structural elements.
This phase also looked at three of the most commonly used sprinklers: the K16.8 early suppression, fast response; K14.0 ESFR; and K11.2 extra-large orifice upright. Another focus for this phase was to evaluate both sprinkler activation time and water delivery performance. Small-scale laboratory experiments were also implemented to help validate modeling and initial assumptions. Spray measurements were obtained under sloped ceilings and found to be valuable for numerical modeling, validation and establishing trends for the impact of ceiling slope.
Finally, heat flux was measured to evaluate the effects of impingement on the spray pattern for each ceiling type and standoff distance, which was then compared to expected performance beneath a flat ceiling configuration.
Ultimately, impingement was estimated to be less than 5% for ceiling slopes less than 4:12. Using phase 1 and early parts of phase 2, a matrix was developed to guide large-scale fire suppression tests. The expense of undertaking large-scale and eventual full-scale testing made the value of the initial research apparent. This resulted in a recommendation of testing under a 60- by 60-foot ceiling at angles up to 4:12 while further considering obstructed ceiling types with depths up to 18 inches.
The fire source used was FM Global’s cartoned unexpanded plastic commodity. The testing matrix included a baseline under a horizontal ceiling followed by a series of tests with varying slope and obstructions. The goal was to perform an adequate number of tests to derive protection recommendations, with additional testing proposed for other considerations such as commodity to ceiling clearance and response type of the sprinklers. Acceptable performance was based on the number of activated sprinklers, fire spread and temperature of steel at the ceiling as compared to baseline results.
Phase 3 was an undertaking of large-scale testing with subsequent guidance released in September 2020. Seven tests were conducted at the large burn laboratory in the FM Global research campus in Rhode Island with four-tier racks and storage of the CUP commodity under a sloped ceiling with varying degrees of obstruction.
FM Global also released a report on the modeling and large-scale tests that supplement the details of phase 3. Ideally, the number of sprinklers activated, size of fire and steel temperature should not exceed baseline. In performing these seven tests, using the 16.8K ESFR, the sprinkler arrangement was found to be sufficient and generally satisfy the pass/fail criteria laid out. It was also validated that as the slope of the ceiling increased, so did the number of activated sprinklers. It was found that a parallel-to-the-floor deflector orientation was favorable, particularly for increasing slopes to avoid a significant reduction in water flux to the ignition region.
Additionally, this phase continued to perform theoretical modeling and play the “What if?” game with respect to activation times and particular obstructions that are likely to be present in storage facilities. It was found that when a fire was under a girder, activation times were not significantly increased compared to when a fire originated between girders; on average, they were less than 10 seconds longer than nonsloped ceiling configurations.
Lastly, the girder width had only a minor effect on the activation times. This large-scale testing will undoubtedly be discussed by the NFPA 13 Technical Committee for possible scenarios to include criteria in upcoming editions. These tests will likely lead to further testing and research, including eventual full-scale fire testing, to help cement criteria in place. This will then lead to the development of new products to solve this problem.
Alternative fire protection systems
In other parts of the world, where water is not as readily available as a commodity for fighting fires, an alternative approach has been studied to provide a Review of Oxygen Reduction Systems for Warehouse Storage Applications with a follow-up Phase 2. While not known to be used in the United States, it is a growing alternative that relies on reducing the available oxygen in combination with other active fire prevention and protection measures.
Existing standards and methods are not based on full- or real-scale testing with multiple types of ignition sources; a definitive oxygen concentration has not been validated that substantially extinguishes fire. One of the limitations for these types of systems is that they do not lend themselves to tenable conditions for human occupancy and should only be implemented in areas that are not normally occupied by personnel.
The lack of research and baseline information is pushing this type of protection scheme into more theoretical modeling and into the realm of using performance-based design analysis to qualify protection design criteria. Where water is readily available, this type of protection is not likely to become widespread, but could find a niche in special environments such as large commercial freezer operations or automated warehouses with narrow aisles. These systems have higher operational costs compared to water-based systems with more meticulous environmental maintenance required.
Throughout the evolution of protection criteria and products specifically designed for high-piled storage, some of the aim (in addition to storing higher) was also to eliminate or minimize the need for in-rack protection to be used to supplement overhead fire sprinklers.
However, in 2011, FM Global took a renewed interest in further developing criteria specifically for in-rack protection with a goal to reduce initial installation costs while retaining maximum protection. Largely abandoned previously for alternatives that eliminated the need for in-rack sprinkler protection schemes, FM Global decided to continue the research and go beyond prior established norms to look at using larger orifice sprinklers for in-racks where previously K8.0 or smaller were used.
FM Global took an incremental approach and performed computer modeling to formulate a method for performing intermediate- and full-scale testing to validate:
- Use of larger orifice sprinklers.
- Increased distance between levels of in-rack sprinklers.
- Increased distance between sprinklers.
- Eliminating face-sprinklers to simplify protection schemes.
- Exploring potentials for allowing in-rack systems to be hydraulically calculated independently of the overhead sprinkler systems.
Research had not advanced for in-rack protection since the 1970s and FM Global wanted to explore this further. The development of a virtual floor or what could be considered a horizontal barrier was explored. This was done by validating the use of higher K-factor in-rack sprinklers that can suppress any fire that starts beneath this virtual floor. This approach allows for an increase in protection above the highest level of in-rack protection by allowing the areas to be separated hydraulically. NFPA 13 requires that both the overhead and in-rack systems be calculated simultaneously and balanced at the point of connection.
On the other hand, FM Global testing proved that in-rack and ceiling systems would not be needed simultaneously. This allows each system to be calculated independent of one another, optimizing the required amount of water and size of a fire pump if necessary. This virtual floor first developed by FM Global and implemented into its datasheet 8-9 for storage of class 1, 2, 3, 4 and plastic commodities, has now been implemented into NFPA 13-2019. Once again, raising the bar in protection for high-piled storage and in-rack protection schemes while lowering upfront costs.
Fire suppression challenges
What will the next challenges be for fire protection of high-piled storage? Will it be finding protection for ever narrower aisles as warehouses become fully automated? Will it be going even taller? Will a higher challenge fire scenario than the group A plastic be invented? Will solid shelving be examined? Or, will it be a little bit of all the above?
Whatever the next challenge is, the technology and research to appropriately protect lives, buildings and contents from the devastation of fire will be right on its heel.