Effectiveness of Water Sprays in Mitigating Toxic Releases

Chemical manufacturing operations have to consider and prepare for a loss of containment from storage tanks and associated consequences of releases of toxic chemicals, such as ammonia or hydrofluoric acid. Water spray curtains are often advertised as means to mitigate the consequences of chemicals that are soluble in water. Most studies that support spray curtains assume around 1 m/s discharge velocity. We believe that this assumption is suspect and spray curtains are not even as effective as promoted. In reality, the residence time required for effective mass transfer is not properly considered. Our calculations show that high pressure (150-250 psig) liquid ammonia when released from an orifice to the atmosphere comes out at the speed of sound (about 390 m/s). The jet will have to travel quite a distance (D) before the velocity drops to 1 m/s, for example D ≈ 50’ distance for a 1” hole for 250 psig ammonia. So to be effective, the spray curtain will be a very large diameter ring (150’ for a 50’ diameter tank) of nozzles 0.5’ apart. This water curtain ring needs to be quite high to accommodate the potentially high point of puncture or jet angle. Since each spray nozzle loses effectiveness after 5-6 m due to coalescence of the droplets, we need multiple rows of spray nozzles stacked above each other. Finally, we need large quantities of water to feed all these nozzles (about 14 l/s/nozzle) at a moment’s notice. In this paper we will consider typical water mitigation techniques considered for toxic releases and use engineering calculations to evaluate their actual effectiveness.