A Case Study: How the Maximum Spacing of Heat Detectors Should Be Determined

Transcription

1 5C-1 6th Asia-Oeania Symposium on Fire Siene and Tehnology 17-2, Marh, 24, Daegu, Korea A Case Study: How the Maximum Spaing of Heat Detetors Should Be Determined Soonil Nam FM Global Researh, 1151 Boston-Providene Turnpike, Norwood, Massahusetts, USA ABSTRACT The urrent method of assigning the maximum heat detetor spaing based on fire tests omparing the detetor responses to that of a sprinkler does not produe lear performane riteria for the tested detetors. The wide variations of the maximum spaing assigned for the same type of detetors among testing laboratories, whih is another strong indiation of the lak of priniple behind the onept developed for the testing method, simply add more onfusion. Instead, it is proposed in this paper that the maximum detetor spaing should be determined based on a speifi mission that is expeted to be aomplished by using detetors. An example introdued in this work shows: (1) how that an be aomplished, and (2) how the spaing determined here makes a lot more engineering sense than that determined by the urrent method. 1. BACKGROUND Currently, eah heat detetor is haraterized by its temperature rating and its maximum spaing. Measuring the temperature rating of a detetor is quite straight forward. In general, oven tests are used for the task. The temperature of air irulating inside an oven, to whih a detetor sample is exposed, is slowly inreased until the sample responds. Sine there is very little room for any error and the test itself is quite simple, same results are obtained regardless whih testing laboratory onduts the measurement. The maximum spaing assigned to detetors, however, seldom shows agreements among leading listing organizations. Coneptually, the maximum spaing is the maximum distane from a referene fire that a detetor an take while it is expeted to respond either equal to or faster than the response expeted by a referene detetion devie that is installed at a fixed distane from the fire. Currently, an ordinary response sprinkler with a temperature rating of 71 o C is used in the UL test[1], while a sprinkler with a temperature rating lose to the temperature rating of the detetor under onsideration is used in the FM Approvals test[2]. The referene sprinkler is to be installed at the enter of a 3. m by 3. m spaing. Understandably, the maximum spaing 1 Copyright International Assoiation for Fire Safety Siene

2 assigned to a detetor through these tests an vary widely depending on: (1) test room size, (2) eiling height, (3) test fire size, (4) test environments suh as ambient temperature and humidity, and (5) temperature rating of the referene sprinkler, among other things. Not surprisingly, there are large variations in the maximum spaing assigned by UL and FM Approvals for idential detetors. The distanes assigned by UL tend to be a larger value than that assigned by FM Approvals, mainly beause UL onduts the tests in a smaller room than that of FM Approvals while using a larger test fire than that of FM Approvals. It is not unommon that a detetor with a 15-m spaing by UL is given only a 9-m spaing by FM Approvals. In onsequene, a detetor spaing that is perfetly aeptable to one jurisdition may not meet the standards of other jurisditions, depending on what AHJ adopts whih standards. The urrent situation an generate onfusion to end users and an reate a potentially unpleasant issue in ertain fire inident ases. Even if somehow the tests were standardized to eliminate the onfusion surrounding the maximum spaing, still the fundamental question remains. What does the maximum spaing really mean? Does this guarantee that the detetor will respond preisely when it is needed? No one seems to be able to provide affirmative answers to those questions beause the urrent maximum spaing does not have lear objetives behind it. A more relevant question should be, What spaing do I need in order for the detetor to respond before the fire size beomes larger than X kw? A detetor spaing should be mission speifi, and should be determined on a ase by ase basis. A reent study[3] showed that the response time index (RTI) of heat detetors an be used as a means of assessing heat detetors thermal response sensitivity to fires. The study also showed how the RTIs an be measured and how the measured RTI values an be utilized to alulate detetor response times, provided that either the heat release rate of a fire with respet to time is known or it ould be estimated. Thus, the RTI values assigned to detetors would be very handy to answer the questions raised above. By disussing a problem that was enountered reently, this paper intends to show how the maximum spaing of a detetor should be determined. 2. DESCRIPTION OF THE PROBLEM AND THE SOLUTION PROCESS The details of the problem that was asked to be resolved were as follows: A warehouse, whih stores 5.8-m-highheavy-weight-roll-paper staks under a m high eiling, is proteted by a dry-pipe system equipped with a pre-ation valve that is to be tripped by a heat detetor. It was requested to determine the largest allowable spaing without ompromising the full advantage of the pre-ation system. The main advantage of a detetor-tripped pre-ation dry-pipe system over an ordinary dry-pipe system is that the pre-ation system is to be tripped by a detetor well in advane of a sprinkler atuation, so that the system an be ready to disharge water as soon as sprinklers open. A sketh of the feed-main piping system is given in Figure 1 and a sketh of the dry-pipe branh pipe system is given in Fig. 2. As shown in Figure 2, the 2

3 system onsisted of two sets of separate branh lines. The nominal diameter of the branh pipes is 32 mm (1.25 in.) and that of the two ross mains is 76 mm (3 inh). Six sprinklers are attahed at eah branh pipe, 3.53 m apart eah other. The distane between the branh pipes is 2.3 m. Thus, the sprinkler spaing is 3.53 m by 2.3 m. The total volume oupied by ompressed air, whih is the entire volume shown in Figure 2 inluding the riser, is approximately 1.3 m 3. The supervisory air pressure of the system is 129 kpa (4 psig). The design stati water pressure just below the riser is 894 kpa (115 psig). Thus, under the most ideal situation, approximately 87 % of the dry-air volume an be oupied by water before a sprinkler atuates one the pre-ation valve tripped. 8 in. pipe, 122 m 8 in. pipe, 31 m Fig 1. Sketh of the feed-main pipe setion of the dry-pipe sprinkler system in. pipe, 1 in. pipe, 229 m Riser (6 in. pipe, 12 m up) Pre-ation valve 1 in. pipe, 134 m Pump Sprinkler Fig 2. Sketh of the branh lines of the drypipe system. A pre-ation system annot be fully equivalent to a orresponding wet system. The system annot be filled with 1 % water before a sprinkler atuates beause the air inside the system will be ompressed and will math the water pressure after all. Besides, if the atuated sprinklers happened to be loated where the ompressed air poket is, then the air must be disharged first before any water omes out through the open sprinklers. If those onerns an be set aside, the system with its 87 % volume filled with water before sprinkler atuation is the most one an expet from the given system. In order to take this advantage, the valve must trip well before a sprinkler atuates so that the system an be filled with water up to its maximum allowable volume when the sprinkler atuates. Thus, the time differene between the response of a heat detetor that trips the pre-ation valve and the atuation of the first sprinkler must be larger than the time required for the water to fill 87 % of the system, provided that the time required to open the valve ompletely is negligible. It is, therefore, neessary to find out the response times of the detetor and the sprinkler, and the time required for water to fill the system. The following steps will show how those values were obtained. Riser 3 in. pipe, 22.3 m 2. 1 Water Filling Time The time required for water to fill the 87 % of the dry-pipe volume was omputed by using a omputer program developed at FM Global Researh to alulate the water delay times in dry-pipe systems[4]. (The referene 4 and the omputer program in it are proprietary; however, a ommerially available ode [5] an be used for the 3

4 alulation.) In order to alulate the time required to fill the 87 % of the dry-pipe system with water, details of the piping system, dry-pipe air pressure, and the relation between water flow rates and the water pump pressures must be provided to the program. The pump performane urve shows that 1.85 pump pressures were linear to Q & w, where Q & w is the water flow rate in gpm and pressure is in psig. Table 1 shows a few referene points between the water flow rate and the pump pressure. The omputation showed that the time required would be 17 seonds. Thus, the detetor must respond at least 17 s earlier than the first sprinkler atuates. TABLE 1. PUMP PRESSURES VS. WATER FLOW RATES Water Flow Rate Pump Pressure m 3 /s 97 kpa (126 psig) 5.68 x1-2 m 3 /s (9 gpm) 8.2 x1-2 m 3 /s (13 gpm) 1.1 x1-1 m 3 /s (174 gpm) 2.2 Sprinkler Atuation Time 929 kpa (12 psig) 888 kpa (114 psig) 757 kpa (95 psig) The temperature rating of the sprinklers installed on the eiling of the warehouse is 141 o C. The response time index (RTI) of the sprinklers is 234 (m.s) 1/2. The sprinkler response time with negligible ondution effets an be omputed by solving the following equation[6]. 1/ 2 dte u = ( Tg Te ) (1), dt RTI where T e is the sprinkler heat sensing element temperature, t is time, u is the eiling flow veloity surrounding the sprinkler, and T g is the surrounding hot air temperature. In order to solve the above equation, the data of u and T g with respet to time are needed. One of the most onservative fire senarios regarding a pre-ation system is that a fire is loated diretly under a sprinkler, whih happens to be at the enter of the detetor spaing---thereby the most remote loation from the surrounding heat detetors. Thus, the sprinkler an atuate at the earliest possible time, while the heat detetor would respond at the latest possible time, whih would in turn trip the pre-ation valve at the latest possible moment. Following the above senario, the sprinkler is assumed to be loated at the fire plume enter axis where the elevation is the same as the eiling learane height. The following plume orrelations[7] an be used to estimate the enterline temperature and veloity of the plume at the eiling, if the onvetive portion of heat release rate (HRR), q&, is known. ΔT = /3 ( T ) 2/3 5/ 3 ( ) ( ) gc ρ 2 p 2 q& z z 1/3 ( g) 1/3 1/ 3 ( ) (2), V = 3.4 q& z ( z z) C T (3), pρ where Δ T is the exess temperature at the plume enterline (K), T is the ambient temperature (K), g is the gravitational aeleration (m/s 2 ), C p is the onstant pressure speifi heat of air (kj/kg K), ρ is the density of ambient air (kg/m 3 ), q& is the 4

5 onvetive heat release rate (kw), z is the elevation from the soure of the plume, z is the virtual origin of the plume, and Vz is the axial plume enterline veloity (m/s). In order to solve Eqs. (2) and (3), the heat release rate (HRR) of burning 5.8-m-high, heavy-paper- roll staks, whih are stored in the warehouse, has to be found. Beause any diret HRR data from the same fuel were not available, the HRR had to be indiretly estimated by the following way. Old data related to fire tests under a 9.1-m high eiling that used 5.8-m-high, newsprint-roll-paper staks as a fuel were loated. It was found that the plume axis temperature at the eiling height in the dada ould be expressed as ΔT = 57( t 24) (4), where t is time in seond. Then the q& an be obtained by onverting Eq. (2) to the following equation, 1/ 2 3/ 2 5/ 2 3/ g = C ( z z) T T pρ Δ q& (5). Inserting Eq. (4) into Eq (5) yields 3 / 2 q& = 7τ (6), where q& is in kw, τ t 24, and z / z is assumed negligible. Now q& is known, Eq. (2) and (3) will provide u and T g at the eiling height [h=11.6 m; z=5.8 m] for Eq. (1), and then a proper numerial integration until T e beomes the rating temperature will yield the time of the sprinkler atuation. A omputational program using a fourth order Runge-Kutta sheme was developed, and it showed that the sprinkler would atuate when t=5 s with an assumption that the ambient temperature was 2 o C. 2.3 Detetor Response Times A sample of the same type of the detetors that are urrently installed at the warehouse, whih is shown in Fig. 3, was subjeted to the plunge test as desribed in Ref. 3. The test results indiated that the RTI of the detetor was 14 (m.s) 1/2. The temperature rating of the detetor is 57 o C. Sine the RTI value and the temperature rating of the detetor are known, the response time of the detetor an be alulated with a high preision using Eq. (1), as shown in Ref. 3, one the temperature and the veloity of a fire plume at detetor loation are known. Fig 3. The heat detetor urrently installed at the warehouse that is the example of the analysis in is work. The omputations were arried out for two ases of the maximum detetor spaings, 15.2 m by 15.2 m and 7.6 m by 7.6 m. Following the fire senario, the detetor was assumed to be loated at the enter of either the 15.2 m by 15.2 m spaing or the 7.6 m by 7.6 m spaing; thus, the detetor, whih was mounted on the eiling, was loated at either 1.7 m or 5.4 m radial distane away from the fire plume axis. In order to estimate the fire plume veloities and the temperatures at the detetor loations, the orrelations that show the veloities and the temperatures as a funtion of a radial 5

6 distane r from a plume enterline need to be known, preferably in funtional forms. Although there are many eiling flow orrelations, new eiling flow orrelations were devised in this work mainly beause: (1) the degree of sattering of the data assoiated with the orrelations ould be seen at first hand, and (2) many raw data of eiling flows were readily available. Anyone who does not want to go through developing his/her own orrelations, Ref. 8 provides a list of eiling jet flow orrelations and he/she an pik one that seems to be suitable for his/her work. Fig. 4 shows data of Δ T (r), the exess eiling flow temperature at loation r, normalized by Δ T, whih is the exess temperature of an unobstruted fire plume axis at the elevation orresponding to a eiling height h. The radial distane, r, was normalized by b, whih is a plume halfwidth of an unobstruted fire plume at the elevation orresponding to the eiling height h. The plume half width an be alulated by[7], 1/ 2 T b =.12 ( z z ) T where T is ΔT + T. (7), Temperature (ΔΤ/ΔΤ ) Pikard et al. (Ref. 9) Thomas (Ref. 1) Heskestad and Hamada (Ref. 11) Nam Radial Distane from Plume Axis (r/b) Fig 4. Ceiling flow data of temperature vs. radial distane and a orrelation urve. The data in the figure inlude the work of Pikard et al.[9], Thomas[1], Heskestad and Hamada[11], and Nam. Pikard et al. measured eiling flows generated by alohol pan fires the diameters of whih ranged from.15 m to.9 m (HRRs varied between 4.85 and 128 kw) under the eiling heights varying between 1.2 m and 2.4 m. Thomas used.42 m diameter alohol pan fires (HRR=16 kw) under a 1.37-m high eiling. Heskestad and Hamada used propane burners the diameters of whih varied from.15 m to.61 m (HRRs varied between 11.6 and 382 kw) under the eilings varying from.56 m to 2.5 m high. Nam used heptane spray fires ranging from 4 kw to 613 kw under a 4.6-m high eiling. As the fire plumes from spray fires inherits momentum assoiated with sprays from nozzles, the temperatures and veloities from Nam s data were expeted to be slightly higher than the orresponding temperatures and veloities from the other data sets, whih were obtained from purely buoyant fire plumes. The data from Pikard et al. and Thomas were onverted to fit the normalizations shown in the figure. 6

7 A orrelation was developed based on the data and is drawn in the figure. The funtional form is 2 ( x x ) y = y + a ( ) exp 2 w π / 2 w (8), where y log( ΔT / ΔT ), y =-.781, a= , w= , x log( r / b), and x = Veloity (V r / V z ) Kung et al. (Ref. 12) Pikard et al. (Ref. 9) Nam Radial Distane from Plume Axis (r/b) Figure 5 shows a olletion of data showing V r vs. (r/b). V V r is the radial diretional z flow veloity at r and V z is the enterline axial veloity of an unobstruted fire plume at the elevation orresponding to a eiling height h. The data of Kung et al.[12] in the figure are a olletion of nine fire tests burning two- to four-tier rak storage of FM Global Class 2 Commodity, whih is a double tri-wall arton with metal liner on wood pallet, under a 9.1-m high eiling for 3 minutes. The eiling learanes during the tests varied from 1.3 m to 5.9 m and the estimated maximum HRR of eah test varied from 8.4 MW to 14 MW. A orrelation urve is given in the figure the funtional form of whih is the same as Eq. (8), however, with different values of the parameters. Here y log( V r / V z ), y =-.5514, a= , w=.79131, and x = Fig 5. Ceiling flow data of radial veloity vs. radial distane and a orrelation urve. Now all the data neessary to arry out the omputations are olleted. The temperature and veloity variations with time at r=5.4 m or r=1.7 m an be obtained by applying Eqs. (2) through (8). Then Eq. (1) an be numerially integrated until T e reahes the detetor rating temperature, 57 o C. The following Table 2 shows the response times of the sprinkler, the detetor loated at r=5.4 m, whih orresponds to the 7.6 m by 7.6 m spaing, and the detetor loated at r=1.7 m, whih orresponds to the 15.2 m by 15.2 m spaing, under various ambient temperatures. TABLE 2: RESPONSE TIMES OF THE SPRINKLER AND THE DETECTORS Amb. Temp. ( o C) Sprinkler Diretly above Fire Response Time (s) Detetor (7.6 m Spaing) Detetor (15.2 m Spaing)

8 Table 2 shows that the time differene between the response from the sprinkler and the response from the detetor installed with the 7.6 m by 7.6 m spaing is about 19 seonds. It is larger than the time required for water to fill up the system, whih is 17 seonds. Thus, the detetors installed with the 7.6 m by 7.6 m spaing is likely to provide a timely response to open the preation valve and in turn it would provide the maximum advantage of the pre-ation drypipe system. The time differene between the response of the sprinkler and the response of the detetor installed with the 15.2 m by 15.2 m spaing, however, is about 13 seonds, whih is shorter than the time required for water fill up the system. In onsequene, the detetor spaing of 15.2 m by 15.2 m is not likely to allow the system to take its full advantage. Our alulation, therefore, indiates that the warehouse in this example should use the 7.6 m by 7.6 m as its maximum detetor spaing rather than the 15.2 m by 15.2 m spaing. Whenever an engineer should make this kind of deision, all the unertainties assoiated with the engineering orrelations, the experimental data, and the final omputational results, should be arefully weighted. Considering other fators relevant to risk analysis assoiated with fire senarios would be helpful too. 3. CONCLUDING REMARKS The urrent maximum heat detetor spaing assigned by listing organizations does not seem to provide lear performane riteria. The maximum distanes are determined through fire tests, in whih detetor samples are spread out with various distanes from the soure of a fire and the response times are measured. Among the detetor samples that respond prior to the response of a referene sprinkler loated at 2.2 m radial distane away from the fire soure, the distane orresponding to the most remote detetor sample beomes the basis for the maximum spaing. Understandably, it would be unlikely that someone an tell with speifi engineering terms what the signifiane of this spaing is. In addition, the test results an vary widely depending on test onditions. In onsequene, the maximum spaing even for the same type of detetors seldom agrees among the leading testing organizations, whih just add more onfusion to end users. The analysis introdued here shows how the maximum detetor spaing was assigned--- mission speifi and site speifi. The example used here is a real ase problem that was requested to be solved. The proesses used here are quite straight forward and easy to understand, beause the deision was made based on a speifi engineering problem with a lear objetive in the user s mind. The maximum detetor spaing in other sites also an be and even, perhaps, should be determined following similar proesses illustrated in this work--- ahieving a speifi goal by using the detetors. REFERENCES 1. UL 539, UL Standard for Safety for Single and Multiple Station Heat Detetors, 5 th Ed., 2, Underwriters Laboratories In. (UL), 333 Pfingsten Road, Northbrook, Illinois. 8

9 2. Thermostats for Automati Fire Detetion, Class Number 321, FM Approvals, 1151 Boston-Providene Turnpike, Norwood, Massahusetts, Nam, S., Donovan, L. P., and Kim, G., Establishing Heat Detetors Thermal Sensitivity Index through Benh-Sale Tests, Fire Safety Journal, in print. 4. Nam, S. and Kung, H. C., Theoretial Predition of Water Delay Time of Dry-Pipe Sprinkler Systems in the Event of Fire, FMRC Tehnial Report, J. I. TR8.RA, Fatory Mutual Researh Corporation, Norwood, Massahusetts, Golinveaux, J., Calulated Water Delivery Time for Large Dry Pipe Systems, NFPA World Safety Conferene & Exposition, Dallas Convention Center, Dallas, Texas, May 18-21, Heskestad, G. and Bill, R. G., Quantifiation of Thermal Responsiveness of Automati Sprinklers Inluding Condution Effets, Fire Safety Journal, Vol. 14, pp , Heskestad, G., Engineering Relations for Fire Plumes, Fire Safety Journal, vol. 7, 1984, pp Alpert, R., Ceiling Jet Flow, Chapter 2, The SFPE Handbook of Fire Protetion Engineering, Third Ed., Soiety of Fire Protetion Engineers, Bethesda, Maryland, Pikard, R. W., Hird, D. and Nash, P., Note NO. 247, Fire Researh Station, Boreham Wood, Herts, Thomas, P. H., Note No. 141, Note NO. 247, Fire Researh Station, Boreham Wood, Herts, Heskestad, G. and Hamada, T., Ceiling Flows of Strong Fire Plumes, Fire Safety Journal, vol. 21, p. 69, Kung, H. C., You, H. Z., and Spaulding, R. D., Ceiling Flows of Growing Rak Storage Fires, 21 st Symposium (International) on Combustion, Combustion Institute, Pittsburgh, Pennsylvania, p. 121,

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