A study on fire and explosion hazards in subway system in Hong Kong

Abstract

The number of parallel traders has significantly increased in some railway and subway systems of the Fast East including Hong Kong. In recent years, it has been observed that passengers in Hong Kong not only travel with their handbags on trains as they did in the past 40 years since the commissioning of the city's subway system, but even get aboard with trolleys, suitcases, large luggage and parallel goods. Train cars are often packed with large luggage and parallel goods which include milk powder, electronic products, foods and drinks. The rising trend of parallel traders travelling on train cars with trolleys, suitcases, large luggage and parallel goods poses new fire hazards to the subway system. Although the subway company has imposed a maximum weight limit of 23 kg for the luggage of each passenger, the fire load is still large as more trolleys, suitcases, large luggage and parallel goods are carried on train cars. Any ignition of combustible suitcases, luggage or parallel goods in a train car may result in a ventilation limited fire. A 1/15 scale model of a train car with adjustable openings was constructed for a study on the fire phenomena in a train car under limited ventilation. The fire load of parallel goods was simulated by 10 ml propanol (the effective heat of combustion {439}HC,eff (in kJ/kg) of propanol is 30.45 kJ/kg), which represents 27 kg (about the maximum weight limit) in reality. As the area and volume of the propanol containers can affect the heat release rate and fire duration respectively, two containers of different sizes, namely a 75 mm x 75 mm (5,625 mm2) square pan and a 38 mm (1,135 mm2) round container, were used to contain the same amount of propanol in the tests to analyse the differences in the flame colour, flame shape, temperature, oxygen concentration, heat flux, mass loss rate of fuel and burning duration of the pool fire in an environment with limited ventilation. Seven ventilation scenarios with various degrees of door openings ranging from 0% , 20 %, 40 % (3 different positions), 80 % and 100 % were analysed. The fire phenomena which included steady burning, oscillating fire and ghosting flame in a train car under limited ventilation are also covered in the analysis. Another focus of the study was the fire hazards of using clean refrigerants in subway systems and train cars. What makes the use of clean refrigerants hazardous is that all clean refrigerants contain liquefied petroleum gas with 100 % alkanes having an explosive range between 2.15 % and 9.6 %. As propane, a constituent of clean refrigerants, is flammable and 1.5 times heavier than air, the fire hazards arising from the use of clean refrigerants in subway systems and train cars should not be neglected. To study the phenomena of explosion, a Computational Fluid Dynamics software named Flame Acceleration Simulator was used to simulate three explosion scenarios at different time of diffusion and leakage rates. An enclosure of about 200 m³ used as an electrical and mechanical equipment room in a subway station with the installation of an ventilation and air-conditioning system was taken as an example to illustrate the possible explosion hazards of clean refrigerants. Coarse grid and fine grid simulations were carried out for a better understanding of the explosion phenomena in a compartment. Key aspects of the relevant fire safety standards and codes for subway systems and train cars were studied. Measures to mitigate the fire hazards were also discussed.