Possibility of using water mist fire suppression system in Hong Kong
Authors: Chow W.K. , Chan L.Y. 
Published: July 2011

Purpose

The water mist fire suppression system (WMFSS) is an environmentally friendly fire protection system, replacing the total flooding halon system. It is starting to be popular in Hong Kong with five systems approved. However, such systems were tested under some scenarios very different from real fires. Performance under practical scenarios should be evaluated thoroughly. Full‐scale burning tests should be carried out to support that the systems would work in real fire scenarios as expected. This paper aims to briefly discuss whether the water mist system is suitable for use in Hong Kong.

Design/methodology/approach

Five water mist systems were approved in Hong Kong based on the fire test certificate provided by the manufacturers. Four systems were selected in this paper for discussion. Conditions of the fire scenarios in which the approved systems would work are described. The test scenario encountered should match with real fires. Such systems would work satisfactorily in the application area proposed. They are good for enclosed rooms such as machinery spaces and plant rooms. But applying such systems for use in retail areas or libraries should be watched carefully, particularly those with high ventilation rates.

Findings

It is observed that the four approved systems were only evaluated on machinery plant rooms with volume less than 500 m3. They are demonstrated to be effective in extinguishing a wide range of bigger Class B fires with very little residual water in a short time. However, the loss of water vapor under mechanical ventilation conditions might reduce the extinguishing capability of mist. The water mist nozzle should not be installed near to the exhaust and supply positions of the mechanical ventilation system. Note that WMFSS is not effective in handling shielded fires due to the lower mist concentration.

Practical implications

Full‐scale burning tests on pool fires of different shapes under water mist were carried out. It was demonstrated that WMFSS might not work under some fire scenarios. Different scenarios on the amount and arrangement of combustibles, room geometry and sizes would affect the system performance. Further, shop fires were burnt with performance of the water mist system evaluated. Heat release rate curves on the shop fire with and without discharging water mist are measured. Reignition occurred once water supply was shut down, burning up all combustibles.

Originality/value

Effectiveness of WMFSS is determined by other factors, including the spray characteristics, fire size, ceiling height and ventilation rate in the compartment. Their performance in mechanical ventilation rooms should be watched. As a result, the total flooding system might not be so good. Localized water mist systems directed to the hazard might be more reliable to suppress fire. Water nozzle design is a key area as raised by the principal author. Anyway, the possible fire scenarios must be identified in designing workable WMFSS. Full‐scale burning tests should be conducted to demonstrate the system performance.