The Effect of Large Orifice Sprinklers on High-Challenge Fires

Transcription

1 SUPPLEMENT 2 The Effect of Large Orifice Sprinklers on High-Challenge Fires Bill Thomas, P.E. Bill Tomes Editor s Note: Supplement 2 provides background information related to the development and application of large orifice sprinklers to high-challenge fires. The information is intended as an overview of the performance and effect large orifice sprinklers have in comparison to traditional orifice sizes. This supplement is not intended to provide any specific design guidance. For specific application and design requirements, see NFPA 13 and the sprinkler manufacturer s listing materials. INTRODUCTION ft 2 ) wet adjacent sprinklers, even at lower pressures. Over With the completion of the FM test center in the late 1960s, the next years, the NFPA storage standards tried to combat the sprinkler industry had the opportunity to conduct more the skipping by limiting the spacing to no less than 80 ft 2. realistic tests. The test center was large enough to investigate With the advent of the large drop sprinkler in the 1970s and many fire test variables not previously understood in the ELO sprinkler (0.64 in. orifice) in the 1980s, a better smaller-scale testing. For warehouse-type storage, high densities method of achieving high densities was born. This generaprimarily (those greater than 0.35) were found to be necessary, tion of sprinklers largely eliminated the skipping problem. because of the storage of plastics. As the density Larger orifice sprinklers were then combined with other in- requirements increased up to 0.6 for certain high-challenge novations, such as more sensitive sprinklers (quickresponse) commodities, the use of 1 2 in. orifice and then in. orifice and deflector designs that helped optimize the use sprinklers required either higher pressures or closer spacing of existing water supplies. In most cases, the larger the sprinkler of the sprinklers. Both situations caused cold soldering or orifice was, the better the protection. Exhibit S2.1 shows skipping to occur during critical stages of the test fires. (For the pressure requirements for various-sized orifice sprinklers information on skipping, see A.12.6 of NFPA 13.) at various densities (all at 100 ft 2 sprinkler spacing). The high pressures created smaller droplets that were Each of the sprinkler K-factors identified in Exhibit more easily carried to adjacent sprinklers. Close spacing (50 S2.1 creates different combinations of droplet sizes ranging Bill Thomas, P.E., is executive vice president with TVA Fire and Life Safety Corporation in Chicago, IL. Bill Tomes is chairman of TVA Fire and Life Safety Corporation in Chicago, IL. 947

2 948 Supplement 2 The Effect of Large Orifice Sprinklers on High-Challenge Fires Pressure (psi) Operating pressures Density (100 ft 2 ) EXHIBIT S2.1 Pressure Requirements for Various-Sized Orifice Sprinklers at Various Densities. K5.6 (¹ ₂ in.) K8.0 (¹⁷ ₃₂ in.) K11.0 (⁵ ₈ in.) K14.0 (0.7 in.) K17.0 (0.77 in.) K22.0 (0.87 in.) K25.0 (1.00 in.) from very small to large. The larger K-factors result in an increase in the subsequent downward water force created by the velocity and size of the droplet that more effectively penetrates through fire updrafts. This result is true even if the sprinklers are operating at the same pressures. Each of the manufacturers of a specific-size sprinkler has a different deflector, and each produces different combinations of small and large drop sprinklers throughout the pressure range of 7 psi (the minimum required by NFPA 13) to the upper limit of about 175 psi (normally only produced with fire pump installations). Large drops with acceleration downward create bigger forces to penetrate fire updrafts than small droplets but do little cooling above the storage commodity. Small drops do not normally penetrate the storage fire plume and can do some harm by being blown onto adjacent sprinklers, delaying or preventing their operation. Conversely, small drops are good for cooling the ceiling area, depositing drops on exposed structural steel and helping to keep it cool. TESTING PROCESS Listing/Approval Agencies At higher pressures (greater than 50 psi), more small droplets are created and less of the water reaches the top of storage and the burning commodity. The manufacturers, therefore, are presented with a dilemma how much acceleration and downward force to impart versus how many small droplets to produce. Two of the major approval and listing agencies (Factory Mutual and Underwriters Laboratories) have tried to answer that question by requiring each sprinkler to undergo rigorous testing, including full-scale tests of the standard plastic commodity at various storage heights and clearances. The clearance effect will vary depending on the pressure on the sprinklers. Research has consistently shown that for the older, smaller orifice sprinklers, high pressure (greater than about 50 psi) is good if the clearance is small (less than 5 ft), as more water is able to reach the top of the storage and the burning material below. However, high pressure is not as effective if the clearance is greater than about 5 ft, and becomes increasingly ineffective as the clearance increases. Testing has also shown that the large clearance concern has been lessened with the use of larger orifice sprinklers (0.64 and larger). Listing/approval agencies testing at various clearances has clearly demonstrated this. Ignition location in relation to ceiling sprinklers has also been shown by the listing agencies as necessary to show the sprinklers capability to control fires effectively when ignition occurs under one sprinkler, between two sprinklers, and between four sprinklers. As new commodities are stored and new storage arrangements are created, more large size testing will undoubtedly be needed to verify 2007 Automatic Sprinkler Systems Handbook

3 Supplement 2 The Effect of Large Orifice Sprinklers on High-Challenge Fires 949 the proper combinations of large drops, small drops, downward TABLE S2.2 Constant Density Test Comparisons force, and distribution. Sprinkler in. (K-8) 5 8 in. (K-11) Commodity Plastic Plastic Test Result Comparisons Arrangement Palletized Palletized Table S2.1 illustrates the effect changing K-factors has on Ceiling height 25 ft 25 ft a constant density fire test and the results of the fire test, Storage height 15 ft 15 ft including design pressure, steel peak temperature, sprin- Ignition location 4 4 klers operated, and pallet loads of commodity consumed. Pressure Steel peak temp Not measured 203 F The larger orifice sprinkler with lower pressures was able Density to penetrate the updraft and affect the fire faster than the Sprinklers operated smaller orifice sprinkler in this open rack arrangement. Pallet loads consumed 20 2 The larger orifice sprinkler in this palletized arrangement, as shown in Table S2.2, again reached the burning surfaces quicker via better penetration of the fire plume and produced a better result (both tests illustrated in Table the number of sprinklers operated and the pallet loads of S2.2 were successful). commodity consumed with the larger orifice sprinkler. The tests shown in Table S2.3 show a significant difference between the K-8 sprinkler and the other two. Obviously it was not possible in this comparison to show a REAL WORLD FIRE TESTING better result than the K-11 sprinkler when using the larger orifice K-17. Other tests have shown a better result with Warehouse Example the K-17 compared directly to the K-11. Following is an actual approximation of a water supply The test results shown in Table S2.4 come from UL curve with a pump that had a no control situation in a Report EX683-98NK7829, Fire Tests on Wood Pallets, real world hostile fire. With the pump running, the initial which was sponsored by CHEP International and Central pressure was about 175 psi. The first few K-8 ( in. Sprinkler. orifice) sprinklers in the warehouse that operated appar- The difference between the two tests shown in Table ently misted (almost all the drops were small), and the S2.4 was the spread of fire. It was more easily diminished drops did not penetrate the fire plume and were carried by the K-17, which operated less than half the number of upward and outward, thereby cooling adjacent sprinklers sprinklers operated by the K-11 sprinkler. and preventing them from operating. The result was fire Although it is feasible to find other direct comparisons travel through much of the warehouse, destroying the prodshowing the better results with a larger orifice sprinkler at uct and causing a very large loss. Fire testing was initiated the same density, Table S2.5 shows the last direct constant to determine if using a K-17 sprinkler would have helped density comparison for this study. the situation. Two of the tests showed these results as This comparison in Table S2.5 shows the low-pressure indicated in Table S2.6, including the peak steel temperarequirement and much better results as demonstrated by ture, number of sprinklers operated, and whether the fire spread was controlled. TABLE S2.1 Constant Density Test Comparisons Test Findings Three benefits of the larger orifice sprinkler were obvious Sprinkler 1 2 in. (K-5.6) 5 8 in. (K-11) during the tests shown in Table S2.6, as follows: Commodity Class II Class II Arrangement Rack Rack 1. The volume of water was greater. The K-8 sprinkler at Ceiling height 30 ft 30 ft 165 psi provides 103 gpm, while the K-17 sprinkler at Storage height 20 ft 20 ft 130 psi provides 216 gpm. Ignition location With such high initial discharge rates, the friction loss Pressure 43 psi 11 psi Steel peak temp 829 F 331 F on the supply pipes and sprinkler piping would be much Density greater with the K-17 sprinkler. This resulted in lower Sprinklers operated 18 9 first-operating sprinkler pressures with a greater rate of Pallet loads consumed 41 5 pressure reduction as more sprinklers operated, thus creating a self-regulated pressure reduction without a Automatic Sprinkler Systems Handbook 2007

4 950 Supplement 2 The Effect of Large Orifice Sprinklers on High-Challenge Fires TABLE S2.3 Constant Density Test Comparisons Sprinkler in. (K-8) 5 8 in. (K-11) 0.77 in. (K-17) Commodity Plastic Plastic Plastic Arrangement Rack Rack Rack Ceiling height 25 ft 25 ft 25 ft Storage height 15 ft 15 ft 15 ft Ignition location Pressure 55 psi 29 psi 12.5 psi Steel peak temp Not measured 163 F 135 F Sprinklers operate Pallet loads consumed TABLE S2.4 Constant Density Test Comparisons for Idle Pallet Storage Sprinkler 5 8 in. (K-11) 0.77 in. (K-17) Sprinkler temperature rating 165 F 155 F Commodity Idle 4-way wood pallets Idle 4-way wood pallets Arrangement 2 stacks 3 stacks 2 stacks 3 stacks Storage height 12 ft 12 ft Density Steel peak temperature 201 F 129 F Sprinklers operated 13 6 Approx. percentage of pallet damage 90% 23% TABLE S2.5 Constant Density Test Comparisons Sprinkler 1 2 in. (K-5.6) 0.77 in. (K-17) Commodity Plastic Plastic Arrangement Rack Rack Ceiling height 30 ft 30 ft Storage height 9 ft 9 ft Density Pressure 67 psi 7 psi Sprinklers operated 29 5 Pallet loads consumed 14 2 negative impact from high impact from high initial pressures. 3. The use of the lower temperature rated sprinklers with the larger orifice sprinklers showed effective control of the spread of the fire and no need to use hightemperature sprinklers to reduce the number of sprinklers on the periphery of the fire. Exhibit S2.2 highlights the differences of the K-8 and K-17 initial operating pressures. It can be seen from Exhibit S2.2 that as more sprinklers operate, the K-17 sprinkler would move down the curve further with the same number of sprinkler operations, thus reducing the pressure and restricting the misting effect. TABLE S2.6 Comparative Test Results INITIAL DENSITY ADVANTAGES Sprinkler in. (K-8) 0.77 in. (K-17) In the real world, with sloping water supplies and pipe Sprinkler temperature rating 286 F 155 F friction loss, higher initial pressures are inevitable. Since Commodity Class III Class III city water supplies vary considerably from being fairly flat Ceiling height 31 ft 31 ft to extremely steep, it is impossible to come up with a Storage height 22 ft 22 ft Pressure 165 psi 130 psi typical water supply. However, for storage occupancies, Steel peak temperature 421 F 113 F most NFPA requirements are for densities and pressures Sprinklers operated 2 2 well above those required for most buildings. If the city Fire spread controlled? No Yes water supply is flat, it usually requires a fire pump, boosting initial pressures. If the city water supply is steep, the higher 2007 Automatic Sprinkler Systems Handbook

5 Supplement 2 The Effect of Large Orifice Sprinklers on High-Challenge Fires 951 Pressure Available pressure with one K-8 sprinkler activated Available pressure with one K-17 sprinkler activated Flow (gpm) Water supply curve 1000 EXHIBIT S2.2 Differences Between K-8 and K-17 Initial Operating Pressures. USE OF THE QR LINK 1. A cheaper sprinkler piping system by eliminating some sprinkler lines (this usually offsets any possible increase in the cost of the sprinkler itself). 2. A wider distribution, so there is wetting ahead of the fire much quicker than with standard-spaced sprinklers. 3. More initial water delivered to the fire area with the first few sprinklers at pressures that produce the optimum combination of large and small drops. This can be done because of the wide range of sprinkler spacing combina- tions. Over the last few years, the use of the quick-response (QR) link in tests, even with the standard 1 2 in. and in. orifice sprinklers, has been shown to work as well as or better than the standard response sprinklers. However, when the QR link was combined with a certain distribution pattern designed to suppress the fire, the early suppression fastresponse (ESFR) sprinkler was born. It now made sense to put a lot of water on the fire quickly when the fire was small, allowing even less fire, smoke, and water damage to occur than with large orifice sprinklers. The K-11 and K-14 upright ESFR sprinklers have been listed and/or approved by the testing agencies, as has the K-22 pendent ESFR, but have not been included in the following compar- isons. At the time of this writing, two upright ESFRs and four pendent ESFRs are on the market. Table S2.8 shows the partial pressure requirements for each and again shows the advantages of the larger orifice sprinklers. As the pressure requirements are reduced with the use of larger orifice sprinklers, there will be fewer and fewer requirements for fire pumps. Since city water supplies are normally more reliable than having to use a fire pump, a more reliable sprinkler system is normally used if larger orifice sprinklers are installed. SPECIAL DISTRIBUTION CAPABILITIES initial pressures are already there. These high initial pres- sures can be anything over 50 psi up to close to 200 psi. For this example, 81 psi was chosen as a conservative point, the results of which are shown in Table S2.7. Table S2.7 shows the distinct advantage of having larger sprinklers and high initial pressures. There is more water on the fire and a much greater chance of operating fewer sprinklers away from the fire start area. Usually, that translates into a much smaller normal loss expectancy. This smaller normal loss expectancy is good for the owner as well as the insurance companies. Larger orifice sprinklers designed for high-challenge fires with unique distribution methods have been created that produce special protection methods. One is the large drop sprinkler, and the other is the K-25 EC (extended coverage) sprinkler. The large drop sprinkler has certain pressure requirements for certain storage and ceiling heights that are able to produce water penetration capabilities above the norm for upright sprinklers. The K-25 EC can be spaced up to 14 ft by 14 ft and can be used as a density/area sprinkler creating the following three positive features: LIMITATIONS There is not one sprinkler that can be designed that is good for all field situations. Just designing for larger orifice sprinklers is not good enough. Sometimes the cheaper, better protection occurs with smaller orifice sprinklers. Larger orifice sprinklers are not normally suggested in the following situations: TABLE S PSI Initial Operating Pressure (10 ft 10 ft Sprinkler Spacing) K-5.6 K-8 K-11 K-14 K-17 K-22 K-25 Q K P Q K P Q K P Q K P Q K P Q K P Q K P Q Q 8 81 Q Q Q Q Q Q 50 gpm Q 72 gpm Q 99 gpm Q 126 gpm Q 153 gpm Q 198 gpm Q 225 gpm Automatic Sprinkler Systems Handbook 2007

6 952 Supplement 2 The Effect of Large Orifice Sprinklers on High-Challenge Fires TABLE S2.8 Effect on Design Pressure Where Large Orifice Sprinklers Are Utilized Maximum Maximum Nominal Minimum In-Racks Commodity Staging Height Ceiling Height K-Factor Pressure Required? Cartoned No unexpanded plastics No No No No No Yes Yes No K in. orifice K in. orifice K in. orifice 1. When density requirements are less than 0.35 gpm/ft 2, higher with the need for greater densities, these safety as the minimum pressure requirement of 7 psi will cause factors were found not to be conservative at all. overkill for these hazards. Today s testing with the larger orifice sprinklers has 2. For some specialty hazards that cause flash fires or mostly been done by comparing the larger orifice sprinkler hidden fast-spreading fires that normally require in-rack tests directly with the previously conducted 1 2 in. and sprinkler protection, full-scale costly fire testing may in. orifice sprinkler tests meaning constant densities be necessary for acceptance by the fire community. throughout the test. This testing is correct, because it compares apples with apples, but it does not allow for the Also, in the larger orifice sprinkler categories there biggest advantages of the larger orifice sprinklers to be are many choices ( 5 8 in. to 1 in.). This variety of choices displayed. Future testing with large orifice sprinklers may means that careful study should be initiated to determine determine the smallest anticipated initial densities for flat the proper sprinkler for each specific hazard and arrangewater supplies, and may use that density for the first four ment. Not everyone in the fire protection design industry sprinklers operating during the test. If the water supply is is able to determine this. Therefore, some poor designs are known for the specific testing being conducted, of course more likely to occur. that water supply may be followed throughout the test. If the above procedure is followed, the true advantages of the larger orifice sprinklers will more easily be shown FUTURE TESTING PROCESS through the testing process. As the larger orifice sprinklers In the early testing of high-piled storage using 1 2 in. and are used in the field, they will undoubtedly show the same in. sprinklers, almost all the comparison testing was advantages. done with constant densities. This meant that the first sprinkler operated at the same pressure as the last sprinkler. At the time, this was thought to be the conservative approach SUMMARY to testing, as a normal water supply would start at higher Larger orifice sprinklers are normally better for the higher pressures and diminish as more sprinklers operated. This hazard warehouse/storage protection schemes because of meant that in the real world, there would always be more the following: water coming out of the sprinklers initially, and this would be a safety factor built into designs. Also, because public 1. Lower pressures are feasible for the design, allowing water supplies vary from very high initial pressures and fewer small drops to be produced, helping to eliminate step declines to very flat, testing engineers preferred con- skipping and causing better penetration through the stant densities so other variables could more easily be commodity to the fire. analyzed. These reasons for using constant density testing 2. Higher initial densities with a given water supply put were good when testing at lower pressures and a minimum more water on the fire, causing an overkill that often of 70 ft 2 to 80 ft 2 spacing. However, as pressures became produces fewer sprinkler operations outside the fire 2007 Automatic Sprinkler Systems Handbook

7 Supplement 2 The Effect of Large Orifice Sprinklers on High-Challenge Fires 953 area. Also, the higher initial designs cause a quicker For high-hazard applications with density demands higher reduction in pressure for water supplies that normally than 0.35 gpm/ft 2, larger orifice sprinklers usually provided produce very high pressures and misting with smaller a more effective design. orifice sprinklers. This again causes a self-regulating method of producing a better combination of large and small drops. REFERENCE CITED 3. The combination of points (1) and (2) above will nor- UL Report EX683-98NK7829, Fire Tests on Wood Pallets, mally cause a hostile fire in the real world to be smaller Underwriters Laboratories Inc., Northbrook, IL, in size, cause less smoke to be created, and cause less June water damage than smaller orifice sprinklers. It can also, by eliminating the need for a fire pump, create a more reliable system that is less likely to fail. Automatic Sprinkler Systems Handbook 2007

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