Characterization of Twin-Fluid (Water Mist and Inert Gas) Fire Extinguishing Systems by Testing and Modeling

Abstract

In recent years, some twin-fluid water mist systems have been developed to employ a large quantity of inert gas in conjunction with water. As a result, concern has been raised about whether such twin-fluid systems are true water mist systems, considering the fact that the gas used in the traditional twin-fluid water mist system is mainly for water atomization, not intended for fire extinguishment. The basis of the concern is that the requirements for water mist systems and gaseous systems, in terms of safety factor and agent discharge duration, are very different according to the pertinent NFPA standards. Therefore, there is a need to characterize such systems to ensure that appropriate standards are applied. An investigation was conducted to analyze the fire extinguishing characteristics of a twin-fluid system using a large quantity of nitrogen for water atomization, whose fire extinguishing performance had previously been evaluated with four spray fire scenarios in a 260 m3 machinery enclosure as described in FM Approval Standard 5560. Either diesel or heptane was used in the fire tests. Subsequently, a well-stirred reactor model was used to assist in analyzing the test results, and make fire extinguishment assessment for hypothetical protections employing only the water mist or nitrogen component of the twin-fluid system. The analyses showed that either the nitrogen or water mist component alone could extinguish fire for all the investigated fire scenarios. As a result, the twin-fluid system is more appropriately designated as a hybrid fire extinguishing system, instead of a traditional water mist or gaseous system. Based on the analyses, it is proposed that a twin-fluid system may be designated as a gaseous system, a water mist system, or a hybrid system if the dry-basis oxygen concentration in the enclosure at fire extinction is lower than 12.5%, higher than 16%, or between 12.5% and 16%, respectively. Also proposed are the system safety factors and discharge durations for hybrid systems whose fire extinguishing performance is enhanced by coupling water mist with a large quantity of inert gases.