(129c) A New Methodology to Evaluate System-Level Performance of Explosion Suppression Systems

An accidental explosion that occurs inside an enclosure will typically generate an overpressure of about 6 to 8 bar, which far exceeds the strength of almost all enclosures in industrial locations (e.g., spray dryers, dust collectors, and rooms with typical wall strengths). Needless to say, an accidental explosion can result in serious property damage, long periods of business interruption, injury to personnel, and loss of life. Although explosion venting has been established as a very effective means of reducing damaging explosion overpressures (e.g., see Tamanini 2001), it is not always possible or desired for various reasons (e.g., not enough surface area of the enclosure is available for venting, space is limited, or there are safety concerns for adjacent personnel). As a result, explosion suppression systems as alternate means of explosion protection have become fairly popular.

An explosion suppression system is essentially designed to detect and quench an incipient explosion inside a protected volume. Once an explosion is detected by a pressure transducer or a visual flame sensor (ideally at the initial stages of its development), a control unit is then prompted to actuate suppressant storage containers that rapidly inject suppressant (such as sodium bicarbonate or monoammonium phosphate) into the enclosure. Successful operation of a system occurs when the injected suppressant quenches the expanding flame and reduces the overall pressure rise to a level below the enclosure strength.

Each explosion suppression system is unique and designed by a manufacturer for a specific application. The type and location of the equipment (i.e., size, number, and placement of suppressant bottles) needed to protect a given volume is determined by using a design methodology that a manufacturer has developed, which is generally an empirical correlation based on many large-scale experiments (e.g., Moore 1996; Chatrathi & Going 2000). The models and computers codes that have been developed by manufacturers are proprietary information, and it is not straightforward to independently evaluate the system-level performance of a given explosion suppression system, particularly at scales larger than those used in tests.

Therefore, a multi-year parametric experimental investigation was undertaken in order to understand explosion suppression phenomena and to develop a methodology that can be used to evaluate the effectiveness of any given system. Open-air discharge tests and full-scale suppression tests were conducted at FM Global’s Research Campus. The open-air discharge tests were conducted to measure the growth of a suppressant cloud as the high-pressure contents inside a suppressant bottle were released into the open atmosphere. The full-scale suppression experiments were conducted in 2.5 and 25 m³ explosion vessels using cornstarch-air or propane-air mixtures in order to understand how the suppression phenomenon is affected by different factors such as the amount of suppressant used, the location of suppressant injection relative to ignition, system-activation pressure, amount of suppression, distribution of suppressant or suppressant bottles, and scale.

A simple physics-based model was also developed to synthesize the experimental data and to help elucidate the dynamics of the suppression phenomenon. Furthermore, this basic model can be used in conjunction with a series of performance tests in order to determine maximum throw distance, protected volume, and surface area limits for a desired reduced pressure and a given suppressant bottle (with a given nozzle configuration and system-activation pressure). As a result, any system that is evaluated using this new methodology can be installed with confidence at larger scales - provided that the maximum performance limits are satisfied.

References:

Chatrathi, K and Going, J (2000) “Dust Deflagration Extinction.” Proc. Saf. Prog. 19:146-153.

Moore, PE (1996) “Suppressant for the Control of Industrial Explosions.” J. Loss Prev. Process 9:119-123.

Tamanini, F (2001) “Scaling Parameters for Vented Gas and Dust Explosions.” J. Loss Prev. Process 14:455-461.