Effect of heat collector plate on thermal sensitivity of sprinkler heads in large terminal Halls

https://doi.org/10.1016/j.jobe.2019.100787Get rights and content

Highlights

  • Heat collector plate HCP is commonly installed above sprinkler head in large halls.

  • Thermal sensitivity of sprinkler head with and without HCP was investigated.

  • HCP gives a shorter activation time for sprinkler heads tested in a fire chamber.

  • HCP does not shorten the activation time in standard plunge test in a wind tunnel.

  • Suggested to place HCP above sprinkler heads in tall halls for enhancing fire safety.

Abstract

Heat collector plates (HCP) are commonly installed above sprinkler heads in large halls with perforated ceilings, such as in high-speed railway terminals. The thermal sensitivity of a pendent type 68 °C liquid-in-bulb sprinkler head operating with and without the HCP was investigated in this paper. Sprinkler heads were first tested in a fire chamber under a common scenario at an early stage of a fire. Air temperatures next to the sprinkler head were measured by thermocouples. It is observed that HCP gives a shorter activation time.

Additionally, the standard plunge test in a wind tunnel for evaluating the thermal response of sprinkler heads was also carried out. The wind tunnel was adjusted to give air velocity of 1 ms−1 or 3 ms−1 and temperature at 90 °C. Sprinkler head was put in the work section either in parallel or perpendicular to the air flow direction. However, experimental data show that HCP is not able to shorten the sprinkler head activation time in the standard plunge test. Results suggest that the plunge test is not appropriate for studying the thermal sensitivity of sprinkler heads with HCP. This is because the HCP will change the physical environment assumed in the plunge test.

Introduction

Perforated suspended ceilings with various shapes, profiles, materials and installation methods are commonly found in large terminal halls in the Asia-Oceania region [1]. As an example, a railway terminal in Taiwan is shown in Fig. 1. Materials such as aluminum, paper, wood, steel and fabric are used for these perforated suspended ceilings. Such a design would give an open visual impression though there are concerns on disturbing the performance of fire safety systems, particularly the smoke extraction systems. In view of this, fire protection systems should be carefully considered. For example, automatic sprinkler system should be installed to suppress fire. Proper thermal activation of sprinkler head is very important in controlling fire [[2], [3], [4], [5], [6], [7], [8]]. Sprinkler heads are commonly installed under the ceiling and would be immersed in the smoke layer during a fire. The thermal sensing element would be heated up and break at the activation temperature. Many full-scale experiments had been reported [9] to evaluate the thermal response of sprinkler heads under a flat ceiling.

However, the presence of a perforated ceiling [1] poses challenges in fire protection since it might affect fire growth and smoke distribution, and hence the performance of the fire safety systems. Very limited studies [10,11] concerning the effect of perforated ceiling on sprinkler activation and smoke filling process have been reported in the literature. It is difficult to quantify these effects in the wide range of design configurations of perforated ceilings conforming to sprinkler design standards (e.g. BS EN 12845[12]). The modern interior designs of perforated ceilings may affect the performance of sprinkler system. The effect of perforated ceiling on the performance of sprinkler system was reported in an experimental study employing a 500-kW fire [1]. The results in that study suggest that perforated ceiling would affect the room fire development and hence the thermal activation of sprinkler heads. The sprinkler heads are commonly put at lower positons closer to the fire so that high water flow rate is not required to deliver water uniformly at the ground level. An HCP is recommended [13,14] to be put above the sprinkler head for heating up the thermal sensing element faster. Furthermore, the effect of perforated ceiling on convective heat transfer can be kept to a minimum. In this way, the sprinkler heads can be heated up to for faster activation before the smoke layer falls to very low positions below it.

Thermal response of sprinkler heads installed in a tall hall with perforated ceiling such as in high-speed railway stations deserves careful investigation. For example, the sprinkler head is far below the ceiling as in the Tainan station of the high-speed railway system of Taiwan (i.e. Taiwan High Speed Rail THSR), and a longer time is required to build up the smoke layer to activate the sprinkler. In order to shorten the sprinkler response time, a heat collector plate (HCP) [[14], [15], [16], [17], [18], [19], [20], [21], [22], [23]] is installed above the sprinkler head as in Fig. 1 to collect heat and smoke from the uprising air for faster activation of sprinkler heads [20].

Sprinkler heads are expected to be immersed in a smoke reservoir or hot air layer formed under the ceiling and become activated [[2], [3], [4], [5], [6], [7], [8]] when the temperature reaches the operating temperature. The maximum air temperature of a ceiling jet above ambient at a distance away from the vertical axis of a fire can be estimated [9] by equations reported in the literature. However, sprinkler heads installed far below the ceiling in many large terminal halls are not immersed in the smoke or hot layer. Putting an HCP above [20,22] the sprinkler head would collect heat to activate the thermal sensing element. Heat emitted from a fire can be collected and immediately transferred [24] to the thermal sensing element of the sprinkler head. HCP usually takes the form of a circular metal baffle, or ‘pie plates’, as in Figs. 1(b) and Fig. 2.

There are some standards associated with HCP in some Asia-Oceania areas [13,14]. HCPs are required when the distance between the deflector of the nozzle and the ceiling or floor slabs is more than 30 cm. The HCP shall be made of metallic materials with the distance between the deflector and HCP [18] not more than 30 cm. Putting in HCP would allow installing sprinkler head at taller position above the ground. The installation position depends on the design fire accepted by the Authority. Bigger the design fire, taller can be the sprinkler head. Putting in HCP can further reduce the height above the ground. However, very few studies on the effect of HCP on the thermal sensitivity of sprinkler heads have been reported. On the other hand, there is no regulation on the necessity of using HCP with sprinkler in many Asia-Oceania cities including Hong Kong.

The present paper reports an experimental investigation of the thermal response of the standard glass bulb sprinkler heads with and without an HCP in a fire chamber. Sprinkler heads were installed at higher levels and exposed to micro-environment on air temperature and air speed induced by a fire below. Therefore, fire category, the height from the ceiling and wind speed, etc. are not the issue. Air temperature and speed near to the ceiling would be used to evaluate the thermal response of sprinkler heads, irrespective of where the fire is and the fire strength. A typical HCP of diameter 10.5 cm as in Fig. 2(a) was selected in this study. Furthermore, the standard plunge test [2,3] was carried out. The objective is to investigate whether standard tests are appropriate for assessing sprinkler head with HCP. In this test, hot air with constant temperature and speed acts on the sprinkler head to transfer heat through convection.

Section snippets

Fire chamber tests

Sprinkler heads with and without an HCP were installed in a fire chamber as in Fig. 3. The experimental setup with thermocouples around the pipe bar is shown in Fig. 3(a). Locations of the fire source and thermocouple tree are shown in Fig. 3(b) and (c).

A total of 20 sprinkler heads were tested with and without an HCP above the fire as in Fig. 3(b), labelled as tests F1 to F10 under a propanol pool fire of 40 cm diameter as in Table 1. Propanol fire was used because of occupational safety and

Standard plunge test

The plunge test was developed using a wind tunnel to study the thermal sensitivity of sprinkler heads by measuring the Response Time Index (RTI) [[2], [3], [4], [5], [6], [7], [8]], a quantitative measure of the thermal sensitivity of a sprinkler head. The theory behind is based on convective heat transfer between hot air and the thermal sensing element of the sprinkler head, without applying boundary conditions of combustion. Note that this test is not a scale-model experiment, but being part

Discussion

HCP is effective in shortening the activation time of sprinkler heads in the fire chamber test. The temperature near the sprinkler with an HCP is higher than that without an HCP as shown in Fig. 4. The activation time with HCP is thus shorter. Scenario in the fire chamber test matches better with the scenario in a tall railway tunnel with perforated ceiling shown in Fig. 1. HCP is useful to reasonably shorten the activation time. It appears useful to put HCP above the sprinkler head in tall

Conclusions

Based on the experimental results in the present study, the following concluding remarks can be drawn on the effectiveness of heat collector plate (HCP) on sprinkler head sensitivity:

  • Results of fire chamber experiments indicate that HCP is effective in collecting heat and then transferring it to the sprinkler head. Faster increase in temperature around the HCP gives shorter activation time of sprinkler head. According to Table 1, the activation time was reduced from 121 s to 65 s for test F1.

Declaration of interest

None.

Funding

The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region for the Theme-Based Research Scheme Project “Safety, Reliability, and Disruption Management of High Speed Rail and Metro Systems” (Project No. T32-101/15-R) with account number 3-RBAC.

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