The Return of the In-Rack Sprinkler

By Steve Wolin, P.E. | Fire Protection Engineering

Why would anyone want in-rack sprinklers? The development of Early Suppression Fast Response (ESFR) sprinklers by Factory Mutual Research Corporation, now FM Global, more than 30 years ago^1,2,3 revolutionized storage fire protection. Various ESFR and Control Mode ceiling level sprinklers introduced since then have led to ceiling-only protection dominating the storage sprinkler market. But as storage buildings have evolved, the limitations of ceiling-only sprinkler protection have become apparent.

LIMITS OF CEILING-ONLY SPRINKLERS

Height. The original ESFR sprinkler had a Nominal K-factor of 14.0 gpm/psi^1/2 and was designed with a discharge pressure of 50 psi to protect Cartoned Unexpanded Group A plastics stored up to 25 ft. high under ceilings up to 30 ft. in height.¹ That same criterion still exists in the current edition of NFPA 13,⁴ but the typical height of a new storage building has generally increased since the introduction of the ESFR sprinkler.⁵

Currently, NFPA 13 limits K14.0 ESFR sprinklers to ceiling heights of 35 ft. when protecting rack storage, unless in-rack sprinklers are provided. Ceiling-only design criteria to protect rack storage under ceilings up to 45 ft. tall are available with larger orifice K22.4 and K25.2 ESFR sprinklers. Specific application criteria are also available for certain ESFR sprinklers under ceiling heights up to 48 ft. Those are the current limits of ceiling-only storage sprinkler protection. Eventually the challenges associated with activating and delivering water to fires under taller ceiling heights catches up to the ceiling sprinkler.

To be effective, an ESFR sprinkler must operate early in the development of a storage fire. Even with a very sensitive operating element, ceiling-only ESFR sprinklers are challenged to activate prior to a fire spreading across an aisle as ceiling heights increase beyond 45 ft. The design criteria available for ESFR sprinklers under a 48 ft. ceiling require minimum 6 ft. or 8 ft. aisles. For storage buildings taller than 48 ft., in-rack sprinklers are likely to be required. Modern distribution facilities using either multi-level work platforms (pick modules) or automated storage and retrieval systems often gain efficiency by increasing the building height beyond the limits of ceiling-only sprinkler systems.

Water Demand. Even if not required by height, the efficiency of ceiling sprinklers decreases with height. As ceiling heights increase, the water demand required for ceiling-only sprinkler systems also increases. The table below summarizes NFPA 13 or UL Specific Application ceiling-only sprinkler protection criteria for Cartoned Unexpanded Group A plastics that results in the lowest basic sprinkler water demand.

Table 1: Low Water Usage Ceiling-Only NFPA 13 Criteria for Cartoned Unexpanded Plastics

Table 1 illustrates how the efficiency of ceiling-only sprinkler protection decreases with ceiling height above 30 ft. Increasing the ceiling height by just 5 ft., 17%, from 30 ft. to 35 ft. increases the basic sprinkler water demand by more than 50%. Increasing the ceiling height by 60%, from 30 ft. to 48 ft., increases the basic sprinkler water demand by 140%. Thus, even in buildings that could be protected with ceiling sprinklers, ceiling-only sprinkler protection may not be the most efficient use of water.

Particularly in areas of the world where fresh water is not plentiful, the use of large amounts of potable water for inspection, testing, and maintenance (ITM) of fire protection systems is increasingly scrutinized. Sprinkler systems with high water demands and design pressures can result in increased pipe sizes and the need for a fire pump. Large pipes, which must be drained for service, and the use of a fire pump, which requires periodic testing, both typically increase the quantity of water needed for ITM. Thus, limiting the demand of a sprinkler system typically limits the amount of water needed for ITM.

More Hazardous Commodities. The discussion above has referenced protection criteria for Cartoned Unexpanded Group A plastics. More hazardous commodities such as tires, Exposed Group A plastics, and aerosols present an increased challenge to a sprinkler system. These commodities typically require in-rack sprinklers to be provided at lower storage heights than would be required for a Cartoned Unexpanded Group A plastic commodity.

For example, NFPA 13 limits storage of rubber tires on racks to 35 ft. of storage under a 40 ft. ceiling with ESFR sprinkler protection at the ceiling. The criterion requires more than 2,600 gpm of basic sprinkler water demand from K25.2 ESFR sprinklers.

A new design criterion, based on a project conducted by the Fire Protection Research Foundation, is designed to protect Exposed Expanded Group A plastics stored in racks up to 35 ft. tall under a 40 ft. ceiling. The criterion uses approximately 2,350 gpm of basic sprinkler water demand from K25.2 ESFR sprinklers and requires the installation of vertical barriers in the racks to assist the ceiling sprinkler system. This new criterion is currently proposed to be included in the next edition of NFPA 13.

NFPA 30B provides a clear illustration of the enhanced protection that in-rack sprinklers can provide. Rack storage of the most hazardous category of aerosols, Uncartoned Level 3, requires in-rack sprinklers. Rack storage of Cartoned Level 2 and Level 3 aerosols are limited to maximum storage heights of 20 ft. and 15 ft., respectively, with ceiling-only sprinkler protection, while unlimited storage heights are permitted where in-rack sprinklers are provided.

Thus, while ceiling-only criteria are available for storage of Cartoned Unexpanded Group A plastics under ceilings up to 48 ft. tall, other commodities have lower ceiling height limits for ceiling-only protection. Even when ceiling-only protection options are available, the water demand or other required features may make the installation of in-rack sprinklers preferable.

WHY IN-RACKS

Location matters. Being located closer to a rack storage fire gives in-rack sprinklers an advantage over ceiling sprinklers in terms of both activation and delivering water to the burning commodity. Thus, in-rack sprinklers have the potential to activate when a fire is smaller and requires less water to control.

A key feature of in-rack sprinklers is that they do not depend on the configuration of the building enclosure. The tallest current listing for an ESFR sprinkler without in-rack sprinklers is 48 ft. There are typically no limits on building height with in-rack sprinklers.

Two of the biggest challenges with ESFR sprinklers do not affect in-rack sprinklers: (1) sloped ceilings and (2) complicated ESFR obstruction rules. These issues have been identified by the Fire Protection Research Foundation^6,7 and in Fire Protection Engineering⁸ as requiring further research to develop improved guidance for ceiling sprinklers. These are, however, not issues for in-rack sprinklers.

The ceiling configuration does not impact in-rack sprinklers. For years, sprinkler designers have been challenged with roofs having slopes greater than 2 in 12, where ESFR sprinklers are not permitted. A sloped ceiling changes the dynamics of the fire gases traveling along the ceiling, impacting both the operation of ceiling sprinklers and their spray pattern. In-rack sprinklers are not impacted by ceiling slope.

NFPA 13’s obstruction rules for ceiling sprinklers, including more complicated rules for ESFR sprinklers, do not apply to in-rack sprinklers. In-rack sprinklers are intended specifically for areas that have obstructions. While there are specific rules for the placement of in-rack sprinklers, such as having the deflector at or below the load beam, the rules are typically simpler than for ceiling sprinklers.

NEW OPTION FOR IN-RACK SPRINKLERS IN 2016 EDITION OF NFPA 13

The 2016 edition of NFPA 13 will likely include design criteria for in-rack sprinklers and horizontal barriers that are intended to protect a wide variety of fire hazards. The new NFPA 13 criterion is similar to FM Global Property Loss Prevention Data Sheet 8-9 Scheme 8-9A and is permitted to protect Group A plastics (expanded, unexpanded, cartoned, or exposed) as well as Class I through IV commodity.

The new NFPA 13 design criterion uses K8.0 or K11.2 quick-response sprinklers in the rack. The in-rack sprinklers are located directly below a horizontal barrier constructed of nominal 3/8 in. plywood or 22 gauge metal. In-rack sprinklers and horizontal barriers are required to be located at maximum intervals of 12 ft. vertically in the rack. If solid shelving is provided, the in-rack sprinklers must be provided below every level of solid shelving.

The in-rack sprinklers are located between each pallet load in a single-row rack. In double-row racks, the in-rack sprinklers are located between each pallet load at the face and between every other pallet load in the longitudinal flue. Multiple-row racks require in-rack sprinklers between each pallet load at the face, and alternating rows of in-rack sprinklers between every other pallet load and between every pallet load provided within the rack.

Although potentially requiring a substantial number of in-rack sprinklers, the new design criterion has a relatively low hydraulic demand. The in-rack sprinklers are hydraulically calculated to discharge 60 gpm. The hydraulic calculations are required to include six flowing in-rack sprinklers for single-row racks and eight flowing in-rack sprinklers for double-and multiple-row racks. An important advantage of this new in-rack criterion is that the in-rack sprinkler demand is not added to the ceiling sprinkler demand.

The new in-rack protection scheme can be a useful option for existing storage buildings where the ceiling protection is not adequate. The new in-rack system has a basic sprinkler water demand of less than 500 gpm. Because the in-rack system demand is separate from the ceiling sprinkler demand, the in-rack sprinkler system can be used in many existing storage buildings where the ceiling sprinkler system is not adequate, often without upgrading the water supply infrastructure.

The protection scheme can be used throughout a storage area, but it can also be used where a higher hazard commodity is stored only in designated racks. In that configuration, the in-rack sprinkler system and horizontal barriers must be extended one pallet position beyond the higher hazard commodity or a vertical barrier must be provided between the lower and higher hazard commodity.

While providing a potentially useful sprinkler protection option, the new in-rack design criterion has at least two significant drawbacks:

• The installation can be costly due to the number of in-rack sprinklers and the amount of piping required; and
• For double- and multiple-row racks, in-rack sprinklers are located between each pallet load at the face of the aisle where they can be vulnerable to damage.

These two concerns are common with conventional in-rack sprinkler technology. The next generation of in-rack sprinklers offers substantial improvements in both areas.

THE FUTURE OF IN-RACK SPRINKLERS

Further developments in in-rack sprinkler technology are intended to provide the advantages of the new in-rack design criterion described above, while reducing the system cost and the potential for damage. While not yet included in model sprinkler system installation standards, research is being conducted to develop the patent-pending concept of extended coverage in-rack sprinklers. Extended coverage in-rack sprinkler systems are currently being installed to protect high-hazard commodities and tall storage arrangements, including automated storage and retrieval systems, based on performance-based design analysis using full-scale fire test data.

The extended coverage in-rack sprinkler system uses a sprinkler with a nominal k-factor of 25.2 gpm/ psi^1/2coupled with a pendent deflector that is designed to spray within the relatively shallow space available in a rack. The sprinklers are designed to allow an increased spacing, both horizontally and vertically, compared with conventional in-rack sprinklers.

Conventional in-rack sprinklers are designed to surround a fire and typically have a maximum k-factor of 11.2 gpm/psi^1/2. This results in conventional in-rack sprinklers being closely spaced, typically approximately 5 ft. or 10 ft. apart horizontally. In addition, the relatively small k-factor of conventional in-rack sprinklers results in droplet distributions, which are not ideal for penetrating through a fire plume over a significant height. This limits the distance between levels of conventional in-rack sprinklers.

A series of full-scale fire tests has been conducted to investigate the extended coverage in-rack sprinkler system. Tests have included the following commodities:

• Cartoned Unexpanded Group A plastics
• Exposed Expanded Group A plastics
• Combustible Liquids

The first test was used to investigate protection for Exposed Expanded Group A plastics stored in racks. The test setup included polystyrene meat trays stored on pallets in doublerow racks up to 35 ft. high under a 40 ft. ceiling. The extended coverage in-rack sprinklers were located only in the longitudinal flue. One sprinkler was located in each bay, with the sprinklers spaced approximately 8 ft. 3 in. apart. A horizontal barrier consisting of 3/8 in. plywood was provided immediately above the in-rack sprinklers. In-rack sprinklers were supplied with an operating pressure of 30 psi.

Protection criteria are not currently provided in NFPA 13 for Exposed Expanded Group A plastics stored in racks. Two criteria discussed above are likely to be included in the next edition of NFPA 13, which would protect Exposed Expanded Group A plastics as follows for a double-row rack:

• Ceiling-only K25.2 ESFR sprinklers hydraulically calculated for 12 sprinklers at 60 psi, with vertical barriers provided in the racks at approximately every 16 ft. on center; or
• K8.0 or K11.2 in-rack sprinklers and horizontal barriers at a maximum of every 12 ft. vertically, with sprinklers provided at a maximum of every 5 ft. on center at the face and a maximum of 10 ft. on center in the longitudinal flue.

The extended coverage in-rack sprinkler configuration eliminates the face sprinklers that are required for the conventional system and significantly reduces the potential for damage to the in-rack sprinkler system. The spray pattern from the extended coverage sprinkler located in the longitudinal flue is adequate to wet not only the face of the commodity at the aisle, but also to spray water into the aisle to protect adjacent racks of commodity.

Because of the improved droplet distribution versus conventional in-rack sprinklers, fewer levels of in-rack sprinklers were included. In the new NFPA 13 in-rack design discussed above, levels of in-rack sprinklers are required at a maximum of every 12 ft. The extended coverage in-rack sprinklers were tested with sprinklers and barriers at 20 ft. for Exposed Expanded Group A plastics. In tests of less hazardous Cartoned Unexpanded Group A plastic commodity, extended coverage in-rack sprinklers were spaced up to 30 ft. vertically.

The first fire test was ignited at the base of the commodity at the aisle, which was the most remote location between the in-rack sprinklers. Test criteria were as follows based on requirements in UL 199 for storage sprinklers:⁹

• No sustained combustion at the outer edges of the target arrays
• No sustained combustion at the far end of the main test array
• No sprinklers should operate at the outer edges of the installed sprinkler system
• 1,000°F (538°C) maximum 1 minute average steel temperature measured above the fire.

The results of the first fire test demonstrated the performance of the extended coverage in-rack sprinklers. The fire was contained within the two initial rack bays that were ignited. Two in-rack sprinklers activated and, in part because the fire was ignited at the face of the aisle, one ceiling sprinkler activated. No ignition of the target array occurred and the maximum one-minute average steel temperature above the fire was limited to 102°F. The success of the first test showed the performance and potential advantages of extended coverage in-rack sprinklers.

Further fire testing has investigated not only additional commodities, but additional storage configurations. For example, protection for 15 1/2 ft. deep multiple-row racks of Exposed Expanded Group A plastics was investigated where in-rack sprinklers were provided in the longitudinal flue and in the rack uprights nominally 18 in. from the face. The sprinklers were able to contain the spread of the fire within the main storage array through the activation of five in-rack sprinklers at an operating pressure of 30 psi. Further fire testing is planned for additional commodities and storage configurations.

Extended coverage in-rack sprinkler systems provide several potential advantages over conventional in-rack sprinkler systems:

• Lower cost by reducing the amount of piping and number of sprinklers needed.
• Less piping and fewer sprinklers to coordinate with racking.
• Less piping and fewer sprinklers to interfere with loading and unloading of commodity.

Compared with currently available ceiling-only sprinkler systems, extended coverage in-rack sprinkler systems have the following potential benefits:

• Protection for high hazard commodities such as combustible liquids, aerosols, and Exposed Expanded Group A plastics.
• Reduced water supply requirements.
• Unlimited storage and building heights.
• Limited impact on the building configuration, including sloped roofs and ceiling-level obstructions.

The fire sprinkler industry must continue to evolve to address new storage fire challenges. While many storage and distribution buildings could be protected with ceiling-only sprinkler protection, an increase in the height of these buildings has led to renewed research into more efficient in-rack sprinkler systems.

Steve Wolin is with the Reliable Automatic Sprinkler Co., Inc.

References:

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