As warehouses become increasingly difficult for the fire service to protect, a panel of experts offers a trio of fixed fire protection schemes designed to achieve complete extinguishment
NFPA Journal®, July/August 2011
By Richard Gallagher | Illustrations by Peter and Maria Hoey
There is no question that automatic sprinklers play an essential role in controlling warehouse fires, but NFPA 13, Installation of Sprinkler Systems, recognizes that sprinklers alone are not intended to extinguish such fires. Sprinklers are intended to control or suppress a fire; NFPA 13 defines the concepts of fire control and fire suppression, and the NFPA Automatic Sprinkler Systems Handbook offers further insight. It is firefighters who are expected to go in and manually achieve final fire extinguishment.
That’s getting tougher to do, however, since the manual firefighting hazards associated with warehouses have steadily increased over the past 60 years. Today’s warehouses are larger, taller, filled with more storage, and contain larger amounts, and more variety, of hazardous commodities than they used to. It is not unusual to see industrial-park warehouses that exceed 10 or more football fields in area, with many reaching 30 football fields or more. These large warehouses also have 30- to 40-foot-high (9.1- to 12.1-meters-high) roofs; where automatic storage and retrieval systems are installed, storage can be placed on racks 100 feet (30.5 meters) or more above the floor. Warehouse economics and efficiencies drive the need to maximize storage height and reduce unused floor area, which means reducing both the number and the size of aisle ways. The greatest advantage can be achieved through the use of piled storage or multiple row racks. Additionally, stored products including plastics and aerosols introduce significantly greater fire challenge as opposed to ordinary combustibles.
In light of the increasing hazard, is it still reasonable to expect firefighters to enter a warehouse to extinguish a fire controlled by sprinklers? While there have been many developments in protective gear and manual firefighting equipment, the advances have not addressed the challenges of warehouse firefighting. Consider the responsibility left to the fire officer managing a warehouse fire controlled by sprinklers. The officer-in-charge will have a number of critical questions to address, but few resources that provide the precise data needed to reach informed decisions. In a smoke-charged building, how do you know sprinklers are controlling the storage fire? How do you know sprinklers are keeping all building steel cool and structurally sound? How do you know where stock may become unstable and collapse due to fire damage or wetting? How do you know when the fire is knocked down, or when it’s time to ventilate the building and turn the sprinklers off? How will firefighters safely access and handle tons of storage stacked or racked over their heads? Increasingly, fire officers who are left guessing at such critical questions are making the reasonable decision not to risk their personnel in an uncertain effort to save property that may already be lost.
While it is anticipated that the fire service can manage warehouse fires within certain reasonable limits, the boundaries of the reasonable limits need to be understood. For those warehouses that fall outside of reasonable limits, there is a need for imaginative solutions to not just control, but also extinguish fires in those structures without human intervention.
To initiate a discussion on possible solutions, the Fire Protection Research Foundation held a workshop on high-challenge warehouses during the Suppression and Detection Research and Applications Symposium, or SUPDET, held in February, 2010, in Orlando, Florida. The workshop case study focused on a hypothetical high-bay warehouse in a rural community — a common location for today’s mega-warehouses, in fact, due in part to availability of affordable land. The case study warehouse was 55 feet (16.7 meters) wide, 150 feet (45.7 meters) long, and 70 feet (21.3 meters) high, constructed of steel, and stored cartoned Group A plastics in a 13-level multiple rack array 65 (19.8 meters) feet high. Storage would be handled by an automatic storage and retrieval system operating in five-foot-wide aisles. The main rack would be four pallet loads wide. The local fire chief says firefighters would only enter the building in an effort to save savable lives.
Faced with a set of conditions that challenged conventional wisdom, a collection of presentations were delivered at the workshop that offered forward-looking visions of fixed fire protection approaches designed to extinguish the fire without fire service intervention. Among the presentations were three that utilized commercially available fire protection systems applied in unconventional ways. These methods are presented here only as hypothetical possibilities in conceptual form, not as detailed engineering realities that have been reviewed by the fire protection community or subjected to rigorous scientific analysis. The intent is to stimulate further discussion on a matter of critical importance to industry, insurers, the fire service, and standards development organizations such as NFPA, and to press for further work to create solutions that are both realistic and cost effective.
CONCEPT - This concept uses early-suppression, fast-response (ESFR) flue space sprinklers as primary suppression, supplemented with carbon dioxide — an agent that was already commercially available, proven in fire protection applications, and could be piped long distances using its own stored pressure. (NFPA 12, Carbon Dioxide Extinguishing Systems, provides guidance for applying carbon dioxide systems.)
Carbon dioxide is a fire-extinguishing agent of choice for flammable liquids, electrical hazards, and water-sensitive occupancies. When discharged, a carbon dioxide system leaves behind no agent residue. This means it does not contribute to the extent of contamination during a fire, and does not aggravate the clean-up effort after a fire. The concept divides the space into high and low zones; the high zones rely on ESFR sprinklers only, while the low zones are protected by ESFR sprinklers at the top of the zone, and supplemented by local application of carbon dioxide to reduce the oxygen content in the zone to the point where open burning is not possible. The low zones would be reserved for the most challenging storage hazards. While the model utilized carbon dioxide only in the lower zones, it would be interesting to consider its use in the high zones as well.
LAYOUT - Solid horizontal and vertical barriers are installed in the racks to separate the space into four protection zones, two low zones 25 feet (7.6 meters) high, and two high zones 45 feet (13.7) high (above). Zoning was proposed to provide protection barriers consistent with the maximum ceiling height limit for ESFR sprinklers and limit the amount of carbon dioxide that would be discharged in response to a fire. NFPA 13 provides guidance on barrier materials, including sheet metal and wood.
For primary protection, two levels of ESFR sprinklers are included, one at ceiling level designed to protect the top 40 feet (12.1 meters) of storage (above right), and another in-rack level to protect storage at 25 feet (7.6 meters) and below (center). The design is enhanced by selectively locating the ESFR sprinklers in the flue spaces at the top of the each zone. In-rack carbon dioxide nozzles are installed at the second, third, and fourth tiers of storage of the low zone only, between 15 feet (4.5 meters) and 25 feet (7.6 meters) above the floor, as indicated in the center illustration. Carbon dioxide is stored in a refrigerated tank, possibly located outside of the building, and the gas is piped to the nozzles in the rack storage array in the event of fire. Spot-type or linear heat detection is located within the zones.
HOW IT WORKS - Heat detection in an upper zone triggers the fire alarm, and waterflow from the ESFR system is initiated and continues until the fire is extinguished (at right, above). In the lower zones, heat detection triggers the fire alarm, waterflow from the ESFR system is initiated, and a CO2 system discharge time delay is initiated and coordinated with the ESFR system (lower left). This time delay is required by NFPA 12 to allow for personnel evacuation and to ensure that ESFR sprinklers have operated. The CO2 discharge time delay is often 30 seconds, but more time may be needed for complete evacuation in larger facilities. At lower right, as the ESFR system continues to operate, beginning fire suppression and keeping the building’s steel shell and rack steel cool, carbon dioxide is delivered to the nozzles in the rack storage array and is released in the zone where the fire is located, extinguishing the fire by reducing oxygen in the protected zone.
BENEFITS AND CHALLENGES - The model takes advantage of a known and proven technology in the form of ESFR sprinklers, but goes one step further by extending the ESFR technology using “ceilings” via the installation of solid barriers and then inserting the ESFR sprinkler into the flue spaces at only two levels of the 70-foot-tall (21.3 meters) structure. A conventional system of in-rack sprinklers at every level is avoided.
A particular benefit of carbon dioxide in a warehouse setting is its ability to handle fires involving high-hazard commodities such as flammable and combustible liquids. In the past, large warehouses have been lost when fire protection was overwhelmed by fires involving commodities that were higher-hazard than originally intended. Higher-hazard commodities can creep into storage over time, as when metal goods are gradually replaced with plastic goods. Higher-hazard commodities can also be intentionally allowed in storage due to business needs. An example might be an overflow of flammable liquids that will not fit in the normal flammable liquids storage area.
Personnel safety becomes a concern any time carbon dioxide is considered for protection. NFPA 12 provides requirements for evacuation alarms, evacuation discharge delays, and system lock-out/tag-out requirements to manage exposures associated with an unexpected release of this inert and suffocating gas. It is essential that an installation of a system utilizing CO2 fully comply with all NFPA 12 requirements.
Before carbon dioxide can become a mainstream warehouse protection solution, research, testing, and listing are necessary for two specific system features: a suitable in-rack fire detection layout to release the carbon dioxide system, and a carbon dioxide nozzle for use in a rack system. An additional challenge for this model is that at present no ESFR sprinklers are listed for in-rack use. A procedure for testing and listing in-rack ESFR sprinklers would be necessary to support this innovative and promising protection approach.
CONCEPT - The proposed use of high-expansion foam taps into a solution that has already been proven for protecting high-challenge fire hazards. High-expansion foam applied in accordance with NFPA 11, Low-, Medium-, and High-Expansion Foam, is a fire extinguishing agent commonly used to protect aircraft hangars, flammable liquids storage, rolled paper storage, and a number of other applications. The design principle is simple: use total flooding in a zoned area to a depth that submerges the fire. The concept utilized research on the use of high-expansion foam to extinguish fires in shipboard spaces involving both wood pallets and a pool of flammable liquids. Hughes Associates presented the findings of this research at the Fire Protection Research Foundation 2009 SUPDET conference.
LAYOUT - To reduce the total water supply requirement, solid or fabric vertical barriers are used to divide the space into four equal protection zones, shown below. Two methods of heat detection were considered: ceiling and in-rack spot heat detectors, and linear heat detection located within the racks, with lines alternating front-to-back and side-to-side at each level up the array. Flame detection was also proposed to cover open building areas and aisles. Video imaging detector (VID) systems were offered as an option that could detect either flaming or smoldering fires. The video cameras, indicated in brown, would be positioned in the upper corners of the zones for optimal coverage of the space. The cameras would also allow monitoring of the foam level within the warehouse following the initial foam submergence, and permit the management of foam depth using a manual foam system override.
Foam concentrate is stored in a heated space in the adjacent low-bay building, and picked up by water piped to the high-bay building in the event of fire. The solution is sent to large foam generators located at the ceiling; each quadrant includes at least two generators, located over the cross aisles at the front and back of the building. Each pair of 17,000-cfm-rated generators is capable of filling a protected zone within three to four minutes.
HOW IT WORKS - The detection system triggers the fire alarm and the foam system. Foam concentrate is delivered to a foam proportioning system; about three parts of concentrate are blended with 97 parts water to form a foam solution. The foam solution is then delivered to a high-expansion foam generator, where one part of foam solution is mixed with between 500 and 1,000 parts of air to form high-expansion foam. The foam is delivered by ceiling- or wall-mounted foam generators that would completely fill the protected zone and extinguish the fire.
USE OF ASRS - The model took two views of the automatic storage and retrieval system (ASRS): one view considered the system as a likely source of fire, and upon fire detection the ASRS would be interlocked to return to its home station and shut down. The other view assumed a hardened automatic storage and retrieval system intended for use during a fire.
Such a system could be considered for several uses. First, a pallet-mounted fire extinguishing system equipped with an infrared, wireless camera and a remote-controlled fire extinguisher could be transported to the fire area to locate and extinguish burning or smoldering material. The system could also be used to remove stock in and around the fire to reduce the fuel load. Additionally, fire-damaged stock in the immediate fire area could be removed following extinguishment, though this task would require means to handle damaged and unstable pallet loads.
BENEFITS AND CHALLENGES - A significant benefit of high-expansion foam is the four-minute submergence time, which would provide rapid fire control and limited fire spread. High-expansion foam also reduces the wetting of stock; after a fire, the pallet loads of stock are likely to remain stable, allowing easier removal by the automatic storage and retrieval system. With a supply of foam concentrate to allow a flow of foam for 30 minutes, the maximum use of water in this scenario would be limited to less than 18,000 gallons — less than 20 percent of the water requirement for automatic sprinklers — which means less runoff of contaminated water. The ability to divide the warehouse space into multiple zones would further reduce water demands, as well as stock damage, due to contact with foam. Unlike the more robust barriers needed with other fire extinguishing agents, barriers for high-expansion foam can include fabric curtains. In warehouse aisles, the curtains can be slit to allow the normal, unobstructed movement of the ASRS.
The zoning approach proposed here raises potential concerns, since a fire near a zone separation could lead to operation of foam protection in more than one zone. The development of a reliable zoning method would be necessary to address the concern of fires near zone separations. Although the total amount of water within the foam is relatively low, all commodity within the zone would be wetted by foam. Most unburned commodity should be recoverable.
Concept by RJA
CONCEPT - This model was one of the workshop’s most innovative approaches. The design principle employs a system of air dampers and exhaust fans to pull water mist through the rack array. Water mist is a recognized fire suppression system for a broad range of challenges, from light-hazard occupancies such as ballrooms to plastic clean-room wet benches where flammable liquids are in use. NFPA 750, Water Mist Fire Protection Systems, provides guidance for applying water mist systems where the system has been specifically listed for the hazard to be protected.
LAYOUT - Unlike carbon dioxide and high-expansion foam, the water mist approach does not involve zoning of the warehouse space with horizontal or vertical barriers. The system is designed to automatically operate in a zoned manner based upon the location of the detected fire. A linear heat detection system is installed at each tier of storage in the rack array as well as at the ceiling above the racks.
The warehouse wall shared by the low-bay space is a plenum, a wall that would contain open space with access to outside air. A series of make-up air louvers is positioned along this wall. An array of powerful exhaust fans is positioned along the opposite wall.
High-pressure water mist nozzles are positioned along the rack faces on the air louver side of the arrays. Nozzles run parallel with the aisles at every level up the rack arrays, and are zoned vertically from floor to ceiling (detail inset).
USE OF ASRS - The automatic storage and retrieval system was assumed to be hardened to allow operation during a fire. The system would carry a pallet-mounted, self-contained fire extinguishing system. The pallet-mounted extinguishing system would incorporate an infrared camera to locate burning or smoldering material. A monitor nozzle would apply up to 600 gallons of compressed air foam to achieve final fire extinguishment.
HOW IT WORKS - Activation of the linear heat detection system triggers the fire alarm and identifies the fire location within the rack array. Near right, the water mist nozzles in appropriate vertical zones are activated. The exhaust fans are also activated, drawing air across the width of the warehouse from the make-up air louvers on the opposite exterior wall. The discharged water mist is pulled through the array toward the exhaust fans, extinguishing the fire.
BENEFITS AND CHALLENGES - While no estimates of water demand were developed for this model, it is anticipated that water mist would use even less water than the high-expansion foam approach. For rural locations with limited water supplies, water mist could offer a cost-effective option to automatic sprinklers supported by fire pump and large water-storage tanks.
The concept of combining water mist and airflow is intended to reduce smoke generation and remove from the building a portion of the smoke that is generated. This delivers the benefit of improved visibility within the building, along with the potential for reduced smoke damage to stock. However, this combined approach is new and would require testing and the development of design guidelines.
In addition, the development of a listed water mist design for warehouse fire extinguishment would have to be pursued, since NFPA 750 requires such systems to be specifically listed for the hazard being protected.
Toward a new conventional wisdom: Perspectives and next steps
For 60 years, conventional wisdom has told us that sprinklers will control or suppress a warehouse fire and firefighters will achieve final extinguishment. It’s time to recognize that warehouses have changed, and that the tasks faced by firefighters can be far greater and more dangerous than they were when the conventional wisdom was created. The innovative thought leadership shown in the high-challenge warehouse workshop illustrates that there are existing technologies available that may offer cost-effective alternatives that can extinguish fires in our most challenging storage configurations.
This has obvious ramifications for a host of stakeholders, starting with firefighters. Fire service tactics have not been able to keep pace with warehouse size, which introduces a storage geometry that precludes the use of hose streams and ground ladders. Limits in air pack capacity introduce the additional threat of running out of air while being too far from an exit. The design of the suppression systems in modern warehouses must account for the limits of manual firefighting.
For risk managers, these warehouses play a critical role in the supply chain of any business. Whether owned, operated by a third party, or maintained by suppliers, almost every industry relies upon warehouses to maintain their business continuity. Risk managers often require warehouses in their supply chain to be protected in accordance with recognized fire protection standards. The removal of the fire service from the final-extinguishment equation means that the standards for fixed fire protection in these structures will need to change. As a result, standards development organizations like NFPA will be asked to adapt existing standards, or develop new ones, to meet the new requirements of warehouse protection. Fire protection designers will reference the new standards as they devise innovative and cost-effective solutions to protect these spaces.
Moving toward a new conventional wisdom will require a great deal of additional discussion and research to validate new approaches to warehouse fire extinguishment. In addition to the research needs mentioned for each of the approaches presented here, there is a general need to justify research and development expenses for a new generation of fire extinguishing systems. Additionally, there is a need to demonstrate that there are cost-effective options to the use of automatic sprinklers alone. Benefits may be realized through considerations such as reduced water storage, smaller fire pumps, and lower exposures to environmental impacts and cleanup. These benefits may also support green initiatives.
We must also take a closer look at the role of automatic storage and retrieval systems. At present, ASRS are not intended to operate reliably during a fire. ASRS of the future, however, could be enhanced to tolerate heat, water, humidity, and smoke, and could be used as a much more active and effective tool in the overall suppression effort. In addition, these systems could be used to remove fire-damaged, wet, and unstable pallet loads from the rack structure following a fire event.
Finally, there is a need to engage the fire service in additional dialog regarding warehouse fire extinguishment. Do they accept warehouse fire extinguishment as their responsibility, or do warehouse characteristics such as size, height, and hazard level effectively create boundaries beyond which fire service intervention is no longer reasonable — and does that introduce an unforeseen gap in warehouse fire protection? Understanding the evolving impact of the fire service risk-management model may allow building owners and insurers to recognize that a fire in a challenging warehouse may not be extinguished even if that building is equipped with the best fixed protection available.
The sooner we act, the sooner we can replace the expectation that firefighters will brave the dangers of warehouse fires with a more realistic approach: that they will support effective automatic fire extinguishing systems from a safe distance.
Leading Causes of Warehouse Structure Fires: 2003 - 2006
According to NFPA data for 2003 – 2006, the U.S. fire service responded to an average of 1,350 warehouse fires per year. Annually, on average, warehouse fires resulted in five civilian deaths, 21 civilian injuries, and $124 million in property damage.
The leading causes of warehouse fires were electrical equipment, intentional, and heating equipment. Sources of electrical fires in the tallest warehouses include lightning and automatic storage and retrieval systems.
Source: Warehouse Fires Excluding Cold Storage, by Marty Ahrens, NFPA, February 2009
Planning Help for the Fire Service
Fixed fire protection may one day extinguish warehouse fires, but for the immediate future that task is up to the fire service. NFPA 1620, Pre-Incident Planning, is the fire service’s primary document for planning responses to fires and other emergencies in an array of occupancies, including storage facilities like warehouses. The fire service also uses NFPA 13E, Fire Department Operations in Properties Protected by Sprinkler and Standpipe Systems.
NFPA recently formed a task group, made up of insurance industry and fire service representatives and NFPA staffers, to develop a communications strategy designed to increase the awareness of, usage of, and compliance with NFPA 1620 and NFPA 13E. That heightened awareness is through the implementation of “Fighting Fire in Sprinklered Buildings” (FFSB), a program developed by the insurer FM Global. The program is intended for pre-incident planners, first-arriving company officers, and incident commanders, and is designed to help them understand automatic sprinkler systems. FFSB instructs users on how to implement a pre-fire planning process in buildings with automatic sprinkler systems, and how to work with those systems on the fire ground.
Development of a new edition of FFSB will begin sometime around the end of the year. The task group will devise a communications strategy for the edition and will work with national fire organizations and state fire training academies to publicize the program, including NFPA 1620 and 13E. For more information, contact Gary Keith at NFPA (firstname.lastname@example.org) or Mike Spaziani at FM Global (Michael.Spaziani@fmglobal.com).