What Firefighters Should Know About Automatic Sprinkler Choices
Automatic sprinkler technology has come a long way since the early 1850s. when New England mill owners developed crude, perforated pipe systems to protect their facilities. Despite the technological advances that have been made, however, the sprinkler system’s mission remains the same —to be on duty 24 hours a day, 365 days a year as a facility’s first line of defense against fire.
Today’s most common types of automatic sprinklers include the standard sprinkler, residential sprinkler, large-drop sprinkler, and early-suppression fast-response sprinkler. Newcomers to the sprinkler scene also include the extra-largeorifice sprinkler, limited-water-supply sprinkler, and tine-spray (water-mist) sy stems They differ according to the type of occupancy they are designed to protect.
The standard sprinkler is by far the most widely used. Its effectiveness is based largely on the ability to prewet adjacent materials the fire has not yet reached and to cool adjacent areas of the building.
Fire control with the standard sprinkler occurs as the original fuel burns out. Fire spread is slowed because the fire is much less likely to ignite surrounding areas that have been prewet by the sprinklers.
The diameter of the orifice (opening through which the water flows) of a standard sprinkler is ½ or ‘79 of an inch. The size selected depends on the amount of water (density) needed to protect a particular occupancy—the larger the orifice. the more water delivered at a given pressure.
In addition, all sprinklers’ thermal sensing elements, whether glass bulb or metal link, are set to operate at a predetermined temperature. The temperature ratings of standard sprinklers range from 135°F to 650°F. although the most common ratings are 165°F, 212°F. and 286°F.
Another measurement relating to sprinkler operation is the K-factor, a numerical constant used in tile following formula to relate a sprinkler’s flow and pressure: Q -the square root of K x P. where Q = the discharge in gallons per minute. P = the pressure in pounds per square inch, and K = the constant K values used for standard sprinklers are 5.6 for a:-inch orifice and 8.0 for a o-inch orifice.
Standard sprinklers continue to be the mainstav of industrial and commercial fire protection. In fact, the standard sprinkler was so effective that, after its development. research came to a near standstill until the 1960s. when new fire challenges were limiting the sprinkler’s effectiveness. For example, as warehouses continued to grow in size and storage fires became more challenging, it was determined that water from standard ceiling sprinklers could not penetrate the strong updraft, or fire plume, of a fast-growing warehouse fire. Water droplets were blown upward and sideways; some even evaporated.
Another limitation of the standard sprinkler was its response time in residential settings, where it is necessary to maintain a livable environment during a -fire long enough for occupants to escape or be rescued. These new challenges led to renewed interest in automatic sprinkler , research.
In 1970. Factory Mutual Research Corporation (FMRC) began a sprinkler technology research program to study the’ basic principles of fire protection. This research reexamined the basic concepts of sprinkler protection and led to the development of fast-response residential sprinklers to maintain a survivable environment in residential areas; large-dropy sprinklers to control highly challenging warehouse fires; and early-suppression fast-response sprinklers to suppress severe storage fires.
Until the mid-190s. most sprinkler research concentrated on the protection l industrial and commercial properties. Not until 1976, when the l ,S. Fire Administration (IJSFA) began sponsoring programs, did research into residential sprinklers take off.
From 1976 to 1982, the USFA sponsored several residential sprinkler research programs. The programs concluded that, because of the low ceilings and small compartments in residential buildings. hot gases generated by the fire descended quickly to a level that hindered a person’s ability to see and breathe. Thus, a residential sprinkler that could respond while the fire was in its early stages—to maintain a survivable environment —had to be developed. The element of time as a critical factor in sprinkler response was a new concept. Historically, sprinkler operation for industrial fire protection was measured in terms of water discharge density (the water discharge rate in ratio to the designed coverage area), sprinkler spacing, and fusible element temperature. How quickly a sprinkler activated was tied to the temperature rating of the head.
The traditional method used to decrease sprinkler actuation time was to lower the temperature rating of the sprinkler’s fusible element. However, the fusible links being evaluated for residential sprinklers already had the lowest practical temperature rating.
As a result, FMRC conducted extensive research into sprinkler sensitivity and determined that to maintain a survivable environment, the fusible element of a residential sprinkler had to be five times faster than that of the standard sprinkler.
FMRC then developed a new measure of the sensitivity of a sprinkler’s thermal element sensitivity, termed the response time index (RTI). The more responsive the element is to temperature change, the lower its RTI value. The ability to test sensitivity made it possible to develop sprinkler operating elements with far lower RTIs than were previously available. This work led to the development of the fast-response residential sprinkler, whose operating element has an RTI of about 50 (the RTI of industrial sprinklers ranges from 225 to nearly 700).
Residential sprinklers have varied orifice sizes and, as a result, varied K-factor values.
The most challenging and often most damaging fires faced by business and industry occur in warehouses. Warehousing practices in the post-World War II years changed significantly. As rack storageheights rose, warehouses increased in height and size, and the combustibility characteristics of the stored materials worsened, standard sprinklers at the ceiling no longer were enough.
A fire often grew out of control because water from ceiling sprinklers could not find its way down to the seat of the fire— especially if the fire started in a lower rack. One answer was to install additional sprinklers within the rack structure at one or more levels, allowing the in-rack sprinklers to promptly deliver water much closer to the fire.
This protection scheme, however, lacks flexibility. In-rack piping and sprinklers are inconvenient when racks must be moved or rearranged. Higher-hazard commodities and an increase in storage height may require repiping the in-rack system. Thus, although it provides good protection. a standard ceiling and in-rack system is expensive to install and is potentially more costly if modifications must be made to deal with storage changes.
In addition, despite data that prove otherwise. some warehouse owners and managers worry that in-rack sprinklers are more vulnerable to damage as products are put into or removed from the racks. They fear the damaged sprinklers will leak and damage surrounding storage.
To increase flexibility and maintain effective fire protection, the large-drop sprinkler—the first sprinkler designed specifically to deal with high-challenge fires—was introduced. Its unique design incorporates a larger orifice (0.64 inches in diameter, with a K-factor of I 1.2), and its more effective discharge pattern enables larger drops of water to penetrate the upward blast of a strong fire plume while providing adequate cooling for the surrounding area. A large-drop sprinkler has an RT1 of approximately 300 and requires a minimum head pressure of 25 psi.
The large-drop sprinkler is well-suited to a wide variety of commodities and storage arrangements. It was developed for fires requiring sprinkler discharges of 60 gpm or more per sprinkler. At a given pressure, large-drop sprinklers will discharge approximately 40 percent morewater than/42-inch standard sprinklers.
EARLY-SUPPRESSION FAST-RESPONSE SPRINKLERS
The quick-response and large-drop technologies, in their own right, significantly advanced the effectiveness of fireprotection. In addition, FMRC further refined these concepts to create the early -suppression fast-response (ESFR) sprinkler—a ceiling sprinkler that provides early suppression for highly challenging storage fires, such as those involving plastics.
Using a more heat-sensitive fusible element and a 0.70-inch orifice, an ESFR sprinkler installed at the ceiling allows rapid discharge of a large quantity of water in a very efficient discharge pattern to suppress, not just control, a fire in its early stages. Suppression occurs before severe fire plume velocity develops and the heatrelease rate accelerates.
Backed by a strong water supply (the system is designed based on a 100-gpmper-head minimum, with 50 psi end head pressure), an ESFR sprinkler system eliminates the need for in-rack sprinklers and puts out. not just controls, a fire with fewer heads operating than standard or large-drop sprinklers. The K-factor of an ESFR sprinkler is 14.0.
A relative newcomer to the fire protection scene is the extra-large-orifice (1:1.0) sprinkler. FMRC approved its first automatic sprinkler under the new extra-largeorifice sprinkler classification in early 1993Although this particular sprinkler has an orifice diameter of 0.64 inches and a nominal K-factor of 11.2. it is not a largedrop sprinkler. Its most appropriate application is in facilities where the protection of standard sprinklers is desired but the available system water pressure is low.
By installing the ELO sprinkler in place of standard heads, a facility may be able to forgo purchasing a fire pump to boost pressure and still be adequately protected.
As previously mentioned, the I’SFA has long been interested in the development of effective residential sprinkler systems. Based primarily on the USFA-sponsored research conducted from 1976 to 1982. the National Fire Protection Association adopted the residential sprinkler standard NFPA 13D. Sprinkler Systems in Oneand Two-Family Dwellings and Mobile Homes. While this research and the resulting standard have proven successful, the I I’SFA. in the early 1990s, identified anoth| er residential sprinkler area needing more i investigation: protection for dwellings with water supplies insufficient for NFPA i 1314 systems.
Once again FMRC, under USFA sponsorship, developed a new residential sprinkler. the limited-water-supply (LWS) sprinkler. It provides viable protection for self-standing dwellings such as manufactured (mobile) homes with water supply quantities limited to 100 gallons, using a total freestanding water supply.
LWS heads are spaced eight instead of the typical 12 feet apart, reducing by about 50 percent the heat release rate at first sprinkler actuation. By reducing sprinkler response time, less water is needed to suppress the fire.
With the accelerated phaseout of halon production, developing an effective and environmentally acceptable halon substitute has become a fire protection priority for industry. As a result, FMRC is investigating fine-spray (water-mist) systems to determine whether this emerging technology will offer options not only for replacing halon but also for other more traditional fire protection methods.
The fine-spray delivery system is similar to a sprinkler system, but there are key differences. For example, water is delivered through a nozzle under high pressure (in some cases more than 100 psi) or by air atomization to generate droplets significantly smaller than those generated by sprinklers. The resulting spray drop size, concentration, and momentum are critical for extinguishment.
The water flow from a nozzle potentially extinguishes the fire faster than a sprinkler head because droplets are smaller and vaporize more quickly. This rapid vaporization extracts the heat from the flames and results in cooling beyond the surface of the fire.
There are various types of nozzle hardware. Orifice sizes of high-pressure nozzles range from just smaller than the standard sprinkler size to only thousandths of an inch in diameter. Air-atomization systems, in which both water and air are supplied to create droplets, typically have orifice sizes comparable to the larger orifice sizes of high-pressure systems.
Proper fine-spray system design, including the spacing of these nozzles and the flow rates, depends on the fire hazard, ceiling height, and building size.