Influence of fire source locations on the actuation of wet-type sprinklers in an office fire

Influence of fire source locations on the actuation of wet-type

sprinklers in an office fire

Ko-Jen Chen 1 Chun-Ta Tzeng 2 Chi-ming Lai 3

Abstract

An experiment is conducted on a full-scale model office space and an actual sprinkler system to explore the influence of fire source locations on sprinkler actuation. The investigated fire source locations include the room center, wall center, three special locations at the room corners, and other locations at different distances from sprinklers. The office space is a brick structure with dimensions of 5 x 6 m and with a net room height of 2.4 m, and contains four installed sprinklers. The fire source is a 100 kW LPG burner. The results show that actuation of the sprinklers is affected by the fire source locations and the heat conduction of the glass temperature sensing ball. Actuation is more rapid if the fire source is closer to corners or walls.

Keywords

fire, office, sprinkler, fire source

1 Ph. D. student, Department of Architecture, National Cheng-Kung University, Taiwan 2 Associate Professor , Department of Architecture, National Cheng-Kung University, Taiwan 3 Assistant Professor,Department of Civil Engineering, National Cheng Kung University, Taiwan

1. Introduction

At a fire scene, the phase change of water from liquid to steam effectively removes heat directly from flames, slows high temperature combustion, and cools the fuel surface directly via the latent heat of evaporation. Large amounts of steam can also reduce the oxygen concentration (particularly effective in enclosed areas) to extinguish fire [1]. Such characteristics make water a preferred extinguishing agent. Inside buildings, automatic sprinklers deliver water drops in the fire protection area to restrain, control, and extinguish fires. When heat or smoke initiates the detection component of the sprinkler system, the system pressurizes water into the activated sprinklers to extinguish the fire. Wet type sprinkler systems are highly recommended since they have a simple structure, low maintenance cost, high reliability, and fast response. Regarding the spatial characteristics affecting sprinkler actuation, the study of fire extinguishment performance of sprinklers in buildings with an atrium is becoming more important as such spaces become more commonplace. Given this, recent literature on the actuation of sprinklers has mainly focused on cases with an atrium [2]. In this study, an experiment is conducted on a full-scale model office space and an actual sprinkler system to explore the influence of fire source locations on sprinkler actuation.

2. Research Method

2.1 10 MW Fire Test Facility

A full-scale fire experiment was done using the 10 MW fire test facility and a combustion gas continuous online analysis system. The device is in the Fire Experiment Center, Architecture & Building Research Institute, Ministry of Interior, located on the Gueiren Campus of National Cheng-Kung University. The combustion gas continuous online analysis system consists of (1) the gas analysis system (including O2, CO, CO2, NOX, and HC

analyzers, as well as a gas sampling/calibration system), (2) an optical density analyzer, (3) a flow rate/temperature monitor, and (4) a data processing system. The 10 MW fire test facility consists of a smoke collection hood, smoke collection bend, mixture tube, measurement section, exhaust bend, and exhaust pipe, as in Fig. 1 and Fig. 2. Large objects or structures can be placed on the floated platform under the hood for testing. Hot gas, smoke, and combustion products are collected with the smoke collection hood, flow vertically through the smoke collection bend, are transferred horizontally into the mixture tube, go through the measurement section, and exit through the exhaust bend and exhaust pipe. The end of the exhaust pipe is finally connected to a waste gas treatment system. A large exhaust fan in the waste gas treatment system offers a maximum 30 m3/s fire gas flow.

Fig. 1：10 MW fire test facility

Fig. 2：Waste gas treatment system

2.2 Investigated Model Office

The investigated model office, as shown in Fig. 3, is located below the smoke collection hood of the 10 MW fire test facility. The plane dimension is 6 m ×5 m (wall core line); the net ceiling height is 3.3 m; the walls are brick-laid in 0.26 m thickness using reinforced bricks. Both northeast and southeast wings have a 2.1 m × 0.9 m single door to be opened or closed, respectively. The ceiling is made of a light rigid frame and gypsum board. To better directly observe restraint of the fire via sprinkler actuation and water droplets, two 2.4 m × 1.2 m fireproof windows are placed at the north wing of the western wall and the east wing of the southern wall. The distance between the window and ground is 0.6 m.

Smoke collection

hood _{The investigated model room is}

located below the hood

Smoke collection bend

Mixture tube

Exhaust bend Exhaust pipe

Waste gas treatment system

(a) Plane view (b) West Elevation

(c) South Elevation (d) 3D View

Fig. 3：Schematic diagram of the investigated model office (not in scale)

2.3 Sprinkler System

Under domestic regulations, the number of sprinklers is based on the effective protection area of each sprinkler. The protection diameter of the sprinklers used here is 2.3 m, meaning the maximum protection area is 10.56 m2; the number of sprinklers is 5 × 6/10.56 = 3. Considering the appearance in actual design, one can have (r=2.3 m, S= 2 r=3.25 m = sprinkler pitch) length: 6/3.25 = 2 sprinklers and width: 5/3.25 = 2 sprinklers. Four sprinklers are installed uniformly in the model room and placed 15 cm below the ceiling, as shown in Fig. 4. The end of each sprinkler pipe is connected to a pressure gauge, and quick joints report independent pressure observations and the stuffing of the water source. The sprinkler’s K-factor is 80 LPM/(bar)1/2, its temperature rating is 68oC, its response time index (RTI) is 131.54 (m s)1/2, and its C-factor is 0.647 (m/s) 1/2.

Fig. 4：Sprinkler layout

2.4 Experimental planning

The fire source used in the experiment is a 100 KW rectangular propane burner with dimensions 18 cm × 18 cm. Various fire source locations, including the room’s corners, walls, and center, are selected for an office fire scenario, as shown in Fig. 5. To understand the effect of the walls on the spread of the fire, the centers of the walls serve as fire source locations, numbered W1-W3. To understand the corner effect, the northwest corner serves as a fire source location, and is numbered C1. The fire source location at the room center is numbered M1. Other locations, numbered X1-X3, Y1-Y2, and Z1-Z2, are selected on symmetrical lines and diagonal lines. Because of symmetry, the experiment focuses on the right side (north wing). Fig. 6 shows the office interior.

Fig. 5：Interior layout of the full-scale office fire experiment

3. Results and Discussion

3.1 Fire source at the center

As shown in the temperature data in Table 1 and curve LPG-M1-W in the top left of Fig. 7,

sprinkler S4 is the first to be actuated at the center fire case, at 142 seconds. It is shown that,

due to the flow pattern, cooler ambient air is introduced from the northern door, flows around sprinklers S2 and S3, and then flows out of the southern door. Sprinklers S1 and S4 show

slightly higher temperatures than S2 and S3 and thus can be actuated more quickly.

Table 1 Experiment results

Number Location of

burner

Activated sprinkler

Sprinkler actuation

time (sec) gas temperature (

o C) LPG-M1-W M1 S4 142 102.7 LPG-W1-W W1 S4 97 102.7 LPG-W2-W W2 S2 91 99.0 LPG-W3-W W3 S3 109 104.3 LPG-C1-W C1 S1 75 106.9 LPG-X1-W X1 S4 141 102.7 LPG-X2-W X2 S1 123 104.8 LPG-X3-W X3 S4 130 99.6 LPG-Y1-W Y1 S1 61 83.4 LPG-Y2-W Y2 S2 96 102.7 LPG-Z1-W Z1 S1 73 99.0 LPG-Z2-W Z2 S2 92 83.7

0 40 80 120

t(s)

T

(

C

)

t(s)

T

(

C

)

Fig. 7：Temperature around the sprinklers with fire sources at the room center and near the walls

3.2 Fire source close to the wall

Temperature curves of the wall fire cases are shown on the top right, bottom left, and bottom right in Fig. 7. Due to the effect of the wall, flames here are higher than those located farther from the wall. When the fire source is located at W1, smoke quickly accumulates in the

upper level, and a strong hot plume is able to actuate the nearest sprinklers, S1 and S4.

Sprinklers S2 and S3 are actuated when the fire source is located at W2 and W3, respectively.

The thermal plume from fire location W2 flows upwards to the ceiling and turns towards the

southern door, passing near and actuating sprinkler S2. Cases LPG-W1-W and LPG-W3-W

illustrate the same situation. Therefore, both the distance and the flow pattern need to be considered in the actuation of a sprinkler for such a condition. The average actuation time of the sprinklers is approximately 100 sec, which is 40 sec quicker than if the fire source is located at the room center M1.

3.3 Fire source at corner

The top left figures in Fig. 8 show the temperature curves. The hot plume coming from the fire in the corner is quite concentrated and rises quickly. After reaching the ceiling, the hot gas spreads and dissipates toward the southern door. At 75 sec, sprinkler S1 is activated.

0 40 80 120

t(s)

T

(

C

)

t(s)

T

(

C

)

Fig. 8：Temperature curves around the sprinklers with fire sources at the corner and other locations

4. Conclusion

This study explores the influence of different fire source locations on the actuation of

sprinklers in an office fire. Twelve fire source locations were tested, including the room center, the wall center, corners, and other locations with different distances to the sprinklers. The findings show that the actuation time of the sprinklers is affected by the fire source locations and heat conduction of the glass bulb of sprinklers. When the fire source is closer to the sprinklers, corners, or walls, the actuation time of the sprinklers is shorter. Because of the wall effect, hot gas is more likely to be induced to flow upwards, and accumulation of upper level smoke is quicker. Average actuation time is 100 sec, around 40 sec faster than if the fire source is located in the room center. For fire sources in corners, sprinklers are quickly activated at 75 sec, showing concentrated hot gas flow.

Acknowledgements

Support from the Architecture and Building Research Institute, Ministry of Interior, Taiwan is gratefully acknowledged.

Reference

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Nam S. Actuation of sprinklers at high ceiling clearance facilities. Fire Safety Journal 2004; 39: 619-642