A fire protection engineer designing sprinklers for a storage/distribution or "big box" retail facility is faced with a bewildering array of new sprinkler types and models from which to choose. The task is not made any easier by the fact that virtually the only guidance available are codes or specification sheets that simply specify the minimum design requirements for the various sprinklers, without providing the performance data that would allow the engineer to match the sprinkler to the particular application at hand. The unfortunate result is that installation cost is very often the driving force behind the choice of sprinklers used to protect storage. In reality, the ultimate cost of the sprinkler system over the life of the building (in terms of effectiveness in the event of fire and flexibility when occupancy changes) may be increased significantly by simply focusing on initial cost of the sprinkler installation.


Historical Perspective1
First, a bit of history is in order to set the stage for more recent developments in sprinkler technology. Control Mode Density Area (CMDA) sprinkler protection is the oldest and still most commonly used sprinkler technology for the protection of storage. (See the sidebar, page 36, for a discussion of control mode sprinkler protection.) This protection is specified in the various curves in NFPA 13. It was developed in the late 1960s in response to a serious problem: large warehouse fires resulting from rapid changes in storage technology. A major full-scale fire test program was undertaken, with the goal of determining how to protect new storage arrangements (such as rack storage) with the sprinkler technology already in place in existing warehouses. As the existing technology called for K5.6 and K8.0 spray sprinklers installed on pipe schedule systems, the expectations were modest. Recognizing that existing pipe schedule systems came in a wide variety of layouts with a wide range of water supplies, the suitable protection options were presented in the form of discharge density/operating area curves. Large fires (with sprinkler operating areas up to 5,000 ft. and more) were considered acceptable, so long as the building occupants could escape safely and the building itself could be saved. Large amounts of fire, smoke, and water damage were considered acceptable.

 

Storage Protection Developments in the 1970s and 1980s

 
Large-Drop Control Mode Specific Application (CMSA) Sprinklers
The first sprinkler developed specifically for the protection of storage was the large-drop sprinkler, which was developed in the 1970s and was a K11.2 sprinkler with a deflector designed to produce a higher proportion of large water drops, enhancing penetration and performance. (See sidebar.) Designs requiring as few as 15 sprinklers were achieved, even with a substantial added safety factor. In actual tests,2 there were a number of cases where fewer than 10 sprinklers controlled fires in challenging storages. As an outcome of the development of this sprinkler, experts recognized that, for high-challenge storage occupancies, sprinkler orifice size and operating pressure were the measure of sprinkler performance, not discharge density and operating area.

 

 

The perceived advantage of large-drop sprinklers was that they permitted protection of rack storage without in-rack sprinklers, which wasn't possible with spray sprinklers. Less obvious, but no less real, advantages include the ability to reduce fire and nonthermal damage, and protect higher-challenge storage occupancies as compared to spray sprinklers.

 

Early Suppression Fast Response Suppression Mode (SM) Sprinklers
The next milestone in the protection of storage came in the late 1980s with the development of the Early Suppression Fast Response (ESFR) sprinkler by Factory Mutual Research. SM technology (see sidebar) was driven by the demand for a means to protect higher-rack storage of higher-challenge commodities without the need for in-rack sprinklers and their associated costs and susceptibility to mechanical damage.

 

Along with the enhanced capabilities of ESFR sprinklers came a new, much more stringent set of application and installation requirements. While the elimination of in-rack sprinklers was easily and widely grasped by users and designers alike, the more stringent requirements were often overlooked, creating problems that still plague the industry today.

 

Recent Sprinkler Technology Developments
Up to the early 1990s, most significant changes in fundamental sprinkler technology applicable to the protection of storage were driven by Factory Mutual Research, which developed the standard spray sprinkler in the 1950s, as well as the large-drop and the ESFR sprinklers. Their efforts were in response to the needs of their insured clients, which were primarily large industrial operations. Then, the sprinkler industry, led by the Central Sprinkler Company (now Tyco Fire Products), began its own development efforts in response to its own customers' needs.

 

Extra-Large Orifice (ELO) CMDA Sprinklers
The ELO sprinkler began life as the Central D64. It was intended to be simply a K11.2 sprinkler that allowed the design of higher densities at lower pressures, purely for economic reasons. But its performance in storage fires was still unverified as compared to the K5.6 and K8.0 sprinklers used in the tests on which the density/area protection guidelines in NFPA 13 were based. In the early 1990s, a fullscale test program3 was undertaken to validate its performance. In this program, benchmark scenarios from the original storage test programs were repeated, with the only change being the substitution of the ELO sprinkler producing the same density at a correspondingly lower pressure.

 

In these tests, the ELO sprinkler not only performed as well as the older types of sprinklers, but in all but one test, it performed significantly better. In a test at minimum pressure of 50 kPa (7 psi) against a relatively highchallenge storage scenario, its performance was comparable to the original test, but not superior.

 

There is, therefore, virtually no practical reason for using K8.0 or smaller sprinklers in designing new storage protection a fact recognized in NFPA 13 by the requirement that K11.2 or larger sprinklers be used for densities of 14 mm/min (0.34 gpm/ft2) or higher.

Larger Orifice CMDA Sprinklers
The Central Sprinkler Company developed the K17-231 sprinkler as a further extension of the ELO concept. In full-scale tests at Factory Mutual Research,4 this sprinkler also demonstrated equivalent (and typically better) performance than K5.6 and K8.0 CMDA sprinklers at densities down to 18 mm/min (0.45 gpm/ft2)[which corresponds to the 50 kPa (7 psi) minimum operating pressure].

 

Larger Orifice CMSA Sprinklers
The Central Sprinkler Company developed the Ultra K17 sprinkler in order to achieve performance comparable to large-drop sprinklers at lower required pressures. In full-scale testing at Factory Mutual Research,5 this sprinkler was able to protect rack storage of a cartoned plastic commodity without the need for in-rack sprinklers.

 

The Ultra K17 sprinkler has not been tested in all of the applications that the large-drop sprinkler has and cannot be used as an across-theboard replacement. Before it is used in a design, the engineer should verify that it is approved/listed for the intended application.

 

Larger Orifice Pendent SM Sprinklers
The original K14 ESFR sprinkler very often creates design difficulties due to the demand for high pressures, so the Central Sprinkler Company developed a K25.2 suppression mode sprinkler at Factory Mutual Research. The sprinkler proved effective in development testing,6 but required higher discharge flows (though still at lower pressures) than the K14 ESFR to meet FM approval requirements. Subsequent testing at Underwriters Laboratories7 produced results equivalent to the K14 ESFR at the same discharge flow, allowing even lower design pressures.

 

More recent testing has shown that the K25 suppression mode sprinkler can protect some challenges greater than the K14 ESFR could handle. For example, it can protect solid-piled/ palletized storage of uncartoned expanded plastics up to 7.6 m (25 ft) high in buildings up to 12 m (40 ft) high. It also can protect rack storage of unexpanded plastics in buildings up to 26 m (45 ft.) high (Fig. 1).8

 

Following the introduction of the K25.2 suppression mode sprinkler, Tyco Fire Products' K16.8 and Reliable Automatic Sprinkler Co.'s K22.4 versions were developed. Again, the driving force behind their development was to reduce the required operating pressure. Testing of the K16.8 suppression mode sprinkler has shown it to be essentially equal in performance to the K14 ESFR.9 To date, there has only been limited full-scale testing of the K22.4 suppression mode sprinkler, and while it may have potential uses on higher-challenge fires like the K25.2 suppression mode sprinkler, none have been identified as yet.

 

Extreme care must be taken when selecting a particular suppression mode sprinkler to assure that it is approved/listed for the intended application. The various suppression-mode sprinkler models are not interchangeable, and each has its own application and installation requirements. At this time, the pendent K14.0 ESFR has the widest range of application, followed by the pendent K16.8 suppression mode sprinkler. The pendent K25.2 suppression mode sprinkler has almost as wide range of applications and can protect some storage configurations in 26 m (45 ft) high buildings that the K14.0 ESFR can't. The pendent K22.4 suppression mode sprinkler has a narrower range of applications.

 

Upright Larger-Orifice Suppression Mode Sprinklers
The limitation on clearance create installation difficulties. In response to this, the Viking Sprinkler Corporation extended their K11.2 upright SM sprinkler to a K14 model via testing at Factory Mutual Research. Although limited as to the commodities and building heights it can protect (due to the reduction in discharge momentum resulting from the upright orientation), it has the advantage of being less sensitive to small obstructions. Typically, locations with respect to the bottom chords of bar joists, individual pipes and conduits, etc., are generally not a concern.

 

More recently, a K16.8 upright suppression mode sprinkler was developed by Tyco Fire Products. This sprinkler offers benefits (and limitations) similar to the K14 upright, with the added advantage of lower pressure requirements.

 

Again, extreme care must be taken when selecting an upright suppression mode sprinkler. In particular, currently available models are limited to use in buildings no more than 11 m (35 ft.) high.

 

K25 Extended-Coverage (EC) CMDA Sprinklers
In an effort to address storage occupancies that could not be practically protected by suppression mode sprinklers, as well as to reduce installation costs, Tyco Fire Products developed a K25.2 CMDA sprinkler that could be installed with a coverage area up to 19 m2 (196 ft2) and spacing up to 4 m (14 ft.) This is a significant increase from the previous limits of 9 m2 (100 ft2) and 3.7 m (12 ft.)

 

The sprinkler underwent a series of full-scale fire tests10 similar to those for the ELO sprinkler and, like the ELO, demonstrated superior performance. One particular benefit is the sprinkler's resistance to "skipping" as a result of its increased spacing. It also was used successfully in a test series at Underwriters Laboratories designed to develop protection for the display/ storage configurations unique to "big box" stores.11

 

James Golinveaux and Joe Hankins are with Tyco.

 

Fire Control vs. Fire Suppression by Sprinklers

Water discharged by sprinklers attacks fire via three distinct mechanisms:

  • Cooling takes place at the roof/ceiling, where relatively small drops are lifted by the fire plume and cool the gas layer at the ceiling. This has the positive effect of preventing collapse of the building structure and sprinkler piping, but also can delay operation of adjacent sprinklers (commonly known as "skipping"). Smaller sprinkler orifices (lower k factors) and higher discharge pressures increase the proportion of small drops, increasing cooling.
  • Prewetting takes place away from the actual fire area, where discharge from sprinklers falls onto unburned combustibles, preventing ignition. Prewetting is primarily a function of the sprinkler discharge density (gpm/ft2 or mm/min) and is not affected by orifice size or discharge pressure.
  • Penetration of the fire plume by water is the only one of the three mechanisms that actually reduces the heat release rate (HRR) of a fire and, if sufficient, can completely extinguish a fire. Penetration is a function of the momentum of water discharge from sprinklers and drop size, as well as the intensity of the fire plume. Larger orifices (higher k factors) increase penetration in control mode sprinklers. In suppression mode sprinklers, larger orifices reduce momentum, requiring higher flows to achieve similar suppression performance.

Sprinklers are divided into two categories based on the mechanism by which they are intended to attack fires.

  • Control mode (CM) sprinklers are designed to rely on cooling and pre-wetting, allowing the fire to continue to burn in the area of ignition while controlling roof/ceiling temperatures and preventing fire spread until firefighters arrive or until the fire burns itself out. Control mode sprinkler protection is characterized by a relatively large area of sprinkler operation (15 50 sprinklers).
  • Suppression mode (SM) sprinklers rely on penetration to stop fire growth quickly and drastically reduce heat release. Far fewer sprinklers operate, typically no more than six.

In practice, the dividing line between control and suppression is not so clearly defined. K5.6 control mode sprinklers regularly achieve effective penetration and suppression when fighting small fires in light-hazard occupancies with low ceilings, such as offices. At the other end of the spectrum, suppression mode sprinklers are unable to fully achieve suppression when protecting some very high-challenge storage occupancies (such as rubber tires and lightweight rolled paper storage) and have been designed to operate in control mode.

The wide array of sprinklers available today can function at virtually any point on the control/suppression continuum, therefore it is incumbent upon the fire protection engineer to understand both the sprinkler and its application in order to design effective sprinkler protection.

 

References

  1. NFPA 13, Standard for the Installation of Sprinkler Systems, National Fire Protection Association, Quincy, MA, 2002.
  2. Chicarello, P., "Large-Drop Automatic Sprinklers,". Technical Report, FMRC J.I. 0G0E7.RR, Factory Mutual Research Corporation, Norwood, MA, February 1982.
  3. Troup, J., "Large-Scale Fire Tests of Stored Commodities Protected by Central Model D.64 Sprinklers," Technical Report FMRC J.I. 0W0R4.RR, Factory Mutual Research Corporation, Norwood, MA, September 1992.
  4. Troup, J., and Chicarello, P., "Full-Scale Fire Test Performance Evaluation of Central's Model K17-231 Sprinklers," Technical Report FMRC J.I. 0D1R8.PR(2), Factory Mutual Research Corporation, Norwood, MA, January 1998.
  5. Troup, J., "Full-Scale Fire Test Performance Evaluation of Central's Model Ultra K-17 Upright Sprinklers," Technical Report FMRC J.I. 0D0R4.PR, Factory Mutual Research Corporation, Norwood, MA, October 1997.
  6. Vincent, B., and Kung, H., "Performance Tests for the Central Sprinkler Company Ultra K25 Fast Response Suppression Sprinkler Under 30 ft. & 45 ft. Ceilings," Technical Report FMRC J.I. 0D2R2.PR/0D1R5.PR, Factory Mutual Research Corporation, Norwood, MA, December 1997.
  7. "Fire Test Data Summary Model ESFR-25 Sprinkler." Underwriters Laboratories Inc., Northbrook, IL, September 1998.
  8. Vincent, B., "Tests for the Central Sprinkler Company ULTRA K25 Fast Response Suppression Sprinkler for Plastic Pallets." Technical Report FMRC J.I. OD3R8.PR, Factory Mutual Research Corporation, Norwood, MA, February 1999.
  9. Ferron, R., "Model 67226 Fusible Type, Pendent Suppression Mode Automatic Sprinkler with a Nominal Orifice Size of 16.8 gal/min/(psi)^1/2." Approval Report FMRC J.I. 3009403, Factory Mutual Research Corporation, Norwood, MA, October 2000.
  10. Troup, J., Vincent, B., and Kung, H., "Performance Evaluation of K-factor 25 Control-Mode (Density/Area) Extended-Coverage Sprinklers for Storage Occupancies," Technical Report FMRC J.I. 3007168/3007169, Factory Mutual Research Corporation, Norwood, MA, May 2001.
  11. "Report of the Retail Fire Research Coalition, A Partnership of Government Agencies, Retailers, Insurance Companies," Underwriters Laboratories Inc., Northbrook, IL.