The automatic sprinkler system is the most commonly used fire protection system for industrial and commercial occupancies. Sprinkler systems were first employed in textile mills in New England in the early 20th Century as a means of protecting the equipment and textile goods stored in those early, multi-story buildings.1 Ceilings were low and goods were, for the most part, stored in wooden crates. Designed to project approximately half of the water to the ceiling and half toward the floor, an important function of those early sprinkler systems was to wet and protect the combustible ceiling structure.

This design philosophy was changed when Factory Mutual (FM) introduced the "spray" sprinkler in the 1950s. At this time, it was recognized that applying water directly to the ceiling was not necessary provided that high ceiling temperatures could be avoided, and the spray from each sprinkler could be more efficiently distributed over a larger floor area. The new spray sprinkler was designed to project all the water downward. After several years of successful demonstration of its effectiveness, the spray sprinkler was accepted in 1953 by the National Fire Protection Association (NFPA) as the "standard" sprinkler in the United States. These standard sprinklers, featuring aK-factor (discharge coefficient) of 5.6gpm/psi1/2 (80 l/min-bar1/2) and a nominal orifice diameter of inch (12 mm), were used in the industrial occupancies of that time.

DRAMATIC CHANGES IN MANUFACTURING AND STORAGE PRACTICES

Industrial, manufacturing and storage occupancies have undergone changes in the interim decades. The proliferation of plastics, such as styrofoam, in packaging materials and the increased use of cardboard cartons created new, unprecedented challenges for fire protection sprinkler systems to overcome. These newer, light-weight storage materials allowed for storage racks to be built to greater heights and changed the dynamic of how storage spaces were designed. Taller storage racks create a "chimney effect" when their contents burn, changing the way fires grow and increasing the challenge for adequate sprinkler protection. In addition, plastic materials, now commonly used, generate more heat than previously used manufacturing materials when burned, increasing the hazard.

Overall, fires in a rack storage environment are characterized by fast fire growth, high heat release rate and high plume velocity, and have therefore challenged the standard sprinkler. In some cases, combustibles are stored on solid shelves in rack arrangements, or the storage height and commodity fire challenge are beyond the effectiveness of ceiling-based sprinkler systems. In these instances, in-rack sprinklers are needed to provide sufficient fire protection.

Under these challenging circumstances, a standard sprinkler system is required to supply a relatively large number of sprinklers with sufficient water to control and limit the fire spread within a particular design area by keeping the surrounding combustibles wet enough so that they do not ignite. In the years following the adoption of the spray sprinkler, it became evident that sprinkler system design requirements for each storage condition had to be individually determined.

In 1967, FM built a large sprinkler fire test facility to seek solutions for fire protection challenges of storage environments through large-scale fire tests. Two test programs, rack storage and plastic storage fire test programs, were conducted from 1968 to 1972.

In order to provide the data needed with a reasonable number of fire tests, a concept called "parallelism" was adopted by Factory Mutual, which involved the establishment of a base density (water flux) versus area of demand curve for a standard test commodity and a set of test conditions utilizing a given sprinkler. Additional curves for other stored commodities, storage conditions, and sprinkler variables, such as aisle width, type of storage rack and sprinkler temperature rating, were then constructed by drawing a line parallel to the base curve through a single test point of the new commodity and test variables. All the tests were conducted with the ignition source centered below four sprinklers. By definition, the density/area rule assumes that for a given density, the performance of all listed sprinklers in a given category would be the same, regardless of their manufacturer, orifice size, spacing, or pressure.

Unfortunately, over the years,test results have shown that different sprinkler models and ignition locations can cause significant differences in area demands.2 In addition, the density/area rule, which has been used as the basis of traditional sprinkler system design, is not always appropriate for modern storage protection.

Furthermore, fire tests in the "Plastic Storage Program" at Factory Mutual2,3 revealed that rack storage of a plastic commodity over 15 ft (4.5 m) in height could not be protected with a ceiling-based sprinkler system alone, using standard sprinklers. The standard sprinklers at the ceiling needed to be supplemented with in-rack sprinklers in order to adequately control a fire. In-rack sprinkler systems are susceptible to damage by warehouse operators and create inflexibility in warehouse storage reconfiguration. To warehouse owners looking at cost-effectiveness and future expansion or reconfiguration, it is desirable to be able to use "ceiling only" sprinkler protection.

To respond to this need, new sprinkler technologies came into the marketplace. For protection of 20 ft (6 m) high rack storage of cartoned plastic commodities under a 25 ft (7.6 m) high ceiling, large orifice sprinklers with a K-factor of 8.0(115 l/min-bar1/2) and a nominal orifice of 17/32 inch (13 mm)were developed. As the storage height increases, the fire challenge becomes greater for the ceiling-only sprinkler systems, and more water is required to be discharged from the ceiling sprinklers to protect the stored commodities. With the available pressure from the water source a fixed value, the sprinkler orifice size needs to be increased to provide a higher discharge rate.

MEASUREMENT OF THE EFFECTIVENESS OF STORAGE SPRINKLERS

In response to these ongoing challenges, an additional, more comprehensive series of research programs was conducted by scientists and engineers at Factory Mutual from the 1970s through the 1990s, exploring the principles of sprinkler performance in rack storage fires.2 These research programs included sprinkler sensitivity (Response Time Index) measurement, prediction of sprinkler activation, spray penetration ability through fire (Actual Delivered Density, ADD), and fire suppression requirements of rack storage fires (Required Delivered Density, RDD).

Aided by these scientific principles, the desired effectiveness of sprinkler fire protection could be targeted and the optimal use of water quantity could be determined, resulting in optimized, cost-effective sprinkler protection of a range of commodity storage in warehouses.

KEY CONCEPTS

Response Time Index (RTI): A measurement of the sprinkler's response sensitivity to the gas temperature and velocity in the vicinity of the sprinkler.4

Prediction of Fire Size at Sprinkler Actuation: Correlations of fire plume, ceiling jet flow, and sprinkler response using RTI and fire plume and ceiling jet correlations.5

Required Delivery Density (RDD): The water flux required to be delivered to the top surface of a burning array to achieve fire suppression.6

Actual Delivery Density (ADD): Measurement of the actual water flux delivered by the sprinkler to the top surface of a burning array that penetrates the fire plume. The ADD depends upon water droplet size, spray pattern, discharge rate and fire size.7

DEVELOPMENT OF A "LARGE DROP" SPRINKLER

In response to the need to provide fire protection for 30 ft (9.1 m) high warehouses containing storage factored plastic up to 20 ft high (6 m), beyond what a large orifice (LO) sprinkler could deliver, the "large drop" sprinkler was developed in the mid-1970s. This sprinkler had a nominal orifice diameter of 0.64 inch (26 mm)and a K-factor of 11.0 (157 l/min-bar1/2), as compared with the LO sprinklers that featured an orifice diameter of17/32 inch (13 mm) and a K-factor of 8.0 (115 l/minbar1/2). At a given discharge pressure, this large drop sprinkler delivered a larger quantity of water and larger droplet sizes than the LO sprinkler and demonstrated the superior performance that was expected.7 ADD measurements were used to guide the design of the large drop sprinkler.

The design goal of "large drop" sprinkler systems was to provide a minimum number of sprinklers operating at a minimum pressure for a specific occupancy and commodity class, storage height and storage arrangement. This approach differed from the traditional density/area approach (sprinkler water flux density over sprinkler operation area), allowing the sprinkler design density (sprinkler discharge pressure) to decrease as the sprinkler operation area increases. As the discharge pressure decreases, ADD of the sprinkler spray may diminish to a level at which the effectiveness of the system can no longer be maintained.

THE ADVENT OF EARLY SUPPRESSION FAST RESPONSE SPRINKLERS

In the 1980s, another new technology was developed: Early Suppression Fast Response (ESFR) sprinklers.7 This new class of sprinkler was developed to ensure a higher ADD than RDD while providing hazard protection with cartoned plastic commodity storage as high as 25 ft (7.6 m) under a 30 ft (9.1 m) high ceiling. A fast response link was used in the sprinkler. Therefore, ESFR sprinklers were designed to respond to a fire at its early stage of development and to discharge a large quantity of water over the fire to achieve fire suppression. The first generation of ESFR sprinklers has a nominal orifice diameter of 0.72 inch (18 mm) and a K-factor of 14.0 (200 l/min-bar 1/2). ESFR technology became popular, protecting storage of ordinary combustibles (class I - IV commodities and cartoned unexpanded plastic as defined in NFPA 138). In the 1990s, the K14.0 ESFR sprinkler became a popular technology for storage protection. Shortly after the introduction of K14 ESFR sprinklers, there was a desire to use ESFR technology for protection of greater fire challenges. These challenges resulted from greater ceiling and storage heights, which were beyond the original intended protection objectives of the K14 ESFR sprinkler. Within the next 10 years, ESFR sprinklers with larger K-factors, such as 16.8, 22.4 and 25.2 (240, 320 and 360 l/min-bar1/2) were developed to provide protection for these greater fire challenges. As was expected, the larger the orifice, the larger the drops delivered by the sprinkler. The K25.2 ESFR sprinkler was primary developed for protection of 45 ft high (13.7 m) warehouses with storages of cartoned plastic commodities up to 40 ft (12 m) high.

CLASSIFICATION OF STORAGE SPRINKLERS

Beyond ESFR, storage sprinkler classifications were further expanded to include control mode (density/area) (CMDA) and control mode (specific application) (CMSA).8 CMDA is a system design method based upon the calculation of the density of water discharged in a specified area of coverage (i.e., 0.6 gpm/ft2 (24 mm/min) over 3000 ft2 (280 m2)). This approach is limited to ceiling heights of 25 ft (7.6 m). K-factors of control mode sprinklers include 5.6, 8.0, 11.2, 14, 16.8 and 25.2 gpm/psi1/2 (80, 115, 160, 200,240 and 360 l/min-bar1/2). With increasing K-factor and orifice size, there is an increase in coverage.

CMSA sprinkler systems are designed to provide a minimum number of sprinklers operating at a minimum pressure for a specific occupancy. The large drop sprinkler is the first CMSA sprinkler. After creation of this class, other CMSA sprinklers with larger K factors of 16.8, 19.6 and 25.2 gpm/psi1/2 (240, 280 and 360 l/minbar1/2) were developed.

RECENT STORAGE SPRINKLER INNOVATION FASTER IS NOT ALWAYS BETTER

Today, system designers and contractors typically associate adequate sprinkler suppression performance of rack storage fires only with "fast response" sprinklers. Although a fast-response sprinkler responds to a fire sooner than a standard-response sprinkler (making it easier for water drops to penetrate the fire plume and reach the burning fuel), fast response alone is not necessary and sufficient for a sprinkler system to achieve fire suppression. More importantly, ADD must be greater than RDD. In this situation, superior fire suppression can be expected.

Aided by the scientific principles of studying sprinkler performance in storage fires, a new large K-factor standard-response sprinkler has been developed which can now achieve fire suppression of cartoned plastic commodities under a ceiling up to 40 ft (12 m) high. The sprinkler model is a pendent sprinkler with a nominal one inch (25 mm) diameter orifice and a K-factor of 25.2 (360 1/min-bar1/2). The sprinkler temperature rating is 160F (71C), and the sprinkler has a Response Time Index (RTI) of 235 (ft-s)1/2 (130 m1/2-s1/2). The large water drops generated from this large K-factor sprinkler enhance its penetration ability against the fire plume.

A series of fire tests9 concluded that the standard-response K 25.2 gpm/psi1/2 (360 l/min-bar1/2) sprinkler can be as effective as ESFR sprinklers in providing protection for storage in warehouses with ceiling heights up to 40 ft (12 m), since both ESFR sprinklers and the new sprinkler were evaluated with the same fire scenarios. See Figures 1 and 2. Based on the performance of the new sprinkler, FM Global has treated the sprinkler in the same fashion as ESFR sprinklers, requiring a "12 head" design for the system water demand, identical hose stream demand and water supply duration. The same sprinkler installation rules with regard to physical obstructions and ceiling elements are applied to both the ESFR pendent sprinklers and the new sprinkler. However,the new sprinkler is being classified not as an ESFR sprinkler but as a CMSA sprinkler, because the sprinkler doe snot use a fast-response link.

This type of technology can offer a reduced end-head pressure as compared to traditional ESFR technology and is poised to replace ESFR as a design choice in storage applications. A low-end head pressure system reduces discharge pressure requirements 25-40% for 30-40 ft (9.1-12.2 m) ceilings and 25-70% for ceilings that are up to 30 ft (9.1 m) in height as compared to traditional ESFR products. A benefit of the reduced end-head pressure of this new storage sprinkler is the opportunity to reduce pipe diameters and to even potentially eliminate the need for a pump if the public water supply is strong enough, affording cost savings in material and labor.

THE NEXT DECADE

The broad range and increasing sub-categorization of sprinkler types have made for a confusing palette of fire protection solutions. Complex installation guidelines for each class of sprinklers further complicate the design landscape.

Earlier in 2010, FM Global began an update of its Data Sheets that specify the rules for system design and installation for storage sprinkler systems. The goal is to simplify the variations in sprinkler classes, and base the system design rules on performance of the sprinkler and not on the traditional names of the sprinkler. Hence, greater consistency in system performance can be obtained.

The fire suppression or control performance of sprinklers depends on the combined effects of sprinkler attributes: sprinkler orientation (pen-dent or upright), sprinkler deflector design, volume median diameter of the spray, sprinkler sensitivity (RTI) and temperature rating. FM Global's new Data Sheets base the system design rules on performance of the sprinkler rather than the traditional name associated with the sprinkler. This sprinkler performance is predictable, based upon the parameters of the system.

As storage space design continues to evolve, new technologies continue to be introduced into the marketplace to meet increasing challenges.

H.C. Kung is with Victaulic.

References:

  1. Richardson, K. (Ed.) History of Fire Protection Engineering, National Fire Protection Association, Quincy, MA, 2003.
  2. Yao, C., "Overview of Sprinkler Technology Research," Proceedings of the Fifth International Symposium on Fire Safety Science, International Association for Fire Safety Science, London, 1997.
  3. Yao, C., "Applications of Sprinkler Technology Early Suppression of High-Challenge Fires with Fast-Response Sprinklers," Special Technical Testing Publication 882, American Society for Testing and Materials, West Conshohocken, PA, 1985.
  4. Heskestad, G. & Smith, H., "Plunge Test for Determination of Sprinkler Sensitivity," Technical Report, FMRC J. I. 3A1E2.RR, Factory Mutual Research Corporation, Norwood, MA, 1980.
  5. Heskestad, G., "Engineering Relations for Fire Plumes," Fire Safety Journal 7: p.25-32, 1984
  6. Yu, H., Lee, J. & Kung, H.C., "Suppression of Rack Storage Fire by Water," Proceedings of the Fourth International Symposium on Fire Safety Science, International Association for Fire Safety Science, London, 1994.
  7. Chan, T., Kung, H.C., Yu, H., & Brown, W.R., "Experimental Study of Actual Delivered Density for Rack-Storage Fires," Proceedings of the Fourth International Symposium on Fire Safety Science, International Association for Fire Safety Science, London, 1994.
  8. NFPA 13, Standard for Installation of Sprinkler Systems, National Fire Protection Association, Quincy, MA, 2007.
  9. Kung, H.C., Lee, S., Ide, S. R.and Ballard, R. J., "Full-scale Fire Test Performance Evaluation of Victaulic Model LP-46 Low Pressure Storage Sprinkler," Victaulic Company, Easton, PA, 2008.