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An explosion while repairing liquefied petroleum gas (LPG) taxis in a garage located at the ground level of an old residential building constructed in Hong Kong was reported in 2015. Part of the building structures was damaged with the owners staying inside killed. The cause of explosion is still under investigation, but the explosion source can be due to leaking of LPG fuel or flammable clean refrigerants with LPG. A taxi has over 0.5 kg of refrigerant HFC134a (R134a) stored in the air-conditioning unit. A pressure rise exceeding 21 kPa (or 0.21 bar) due to explosion from a small amount of LPG would give damages to the building. As firefighters are always exposing themselves to the risk of explosion when they are carrying out rescue operation in a gas-filled environment, the explosion overpressure has to be more reliably estimated for working out protection schemes during operation. This garage LPG explosion incident was studied numerically using Computational Fluid Dynamics software FLame ACceleration Simulation (FLACS). Three scenarios of different LPG-air mixture volumes and LPG concentrations were investigated. Dispersion of LPG was simulated first with an ignition taken at a position at the garage centre. Overpressure and temperature rise were predicted using a fine grid system with 1657600 computing cells was employed. Results were compared with numerical predictions using coarse grids of 207200 cells. Discussion and conclusions were made with reference to the threshold value of 21 kPa in overpressure.