(111c) Effectiveness of Water Sprays in Mitigating Toxic Releases

Chemical manufacturing operations must consider and prepare for release of toxic chemicals, such as anhydrous ammonia, anhydrous hydrofluoric acid, etc. from pressurized storage tanks. Water spray curtains are often advertised as means to mitigate the consequences of released chemicals. Spray curtain effectiveness claims by certain vendors are misleading - a curtain placed at the periphery of a tank will only scrub a puff of a release. A spray curtain that can effectively contain a consequential release will look very different and cost much more. Studies that demonstrate spray curtains to be effective assume low gas velocities. However, calculations show that pressurized liquid NH₃ or HF when released from an orifice to the atmosphere comes out at a high velocity and momentum in the form of a two-phase jet. The jet must travel quite a distance before the velocity drops enough to be effectively scrubbed by a water curtain. The water curtain therefore needs to be at this large distance and consequently the diameter of the water curtain manifold ring needs to be quite large to be effective. It also needs to be quite high to accommodate the expanding jet angle, the potentially high point of puncture or upward angle of jet. Sprays lose effectiveness after a short distance (5-6 m) due to coalescence of the droplets. To avoid the toxic gas from escaping above the spray we may need multiple rows of spray nozzles stacked above each other or some form of barrier above the spray manifold. Finally, large quantities of water are needed to feed all these nozzles at a moment’s notice. This paper presents engineering calculations to estimate jet velocity, diameter & concentration, calculations to evaluate the efficiency of water sprays and discussion of water spray curtain designs that are more effective.