Liquid spray curtain design to contain and mitigate toxic and flammable jets and releases

Liquid spray curtains are considered among the most common devices suitable to mitigate the risk connected to accidental flammable or toxic releases and jets. The effectiveness of a liquid barrier can be evaluated on the basis of several items, i.e.: the release containment effect and its ability to slow down the dispersion into the environment, so to permit the adoption of proper additional protective measures; the enhancement of the dilution of the released gas by means of the air entrained by the liquid sprays; the physical and chemical adsorption of toxic/flammable vapours by the liquid solution. It must be remarked that liquid barrier design is a complex problem to be solved, in that it requires to consider: the expected release/jet properties (flow rate, momentum, time length, chemico-physical characteristics); the meteo-climatic and geo-morphological characteristics of the site, the fluid-dynamics effects and the effectiveness of chemical/physical absorption, as a function of both the liquid flow rate and the droplet size; the optimal reagent concentration in the liquid, suitable to obtain a target removal efficiency. In this experimental and theoretical work, particular attention is dedicated to the adsorption efficiency of the curtain, both chemical and physical, in connection with the different spray characteristics. A mathematical model of a two-phase jet is developed, considering the entrained air rate in connection with the liquid flow rate. The developed model was validated by means of replicated wind tunnel experimental runs, adopting spray nozzles designed ad hoc and able to create a two-blade barrier to contain mechanically the release. The experimental phase evidenced that it is possible to distinguish two regions in the barrier: in the former, dominated by the liquid inertia, the curtain is comparable to a plane jet; in the latter, with prevalence of gravitational effects, the dimensions of the two-phase region do not vary appreciably. As regards the release mitigation, the developed mathematical model correlates both the efficiency and the adsorption rate with the liquid flow rate, the reagent concentration and the droplet size. An analytical solution of the problem is obtained, making reference to instantaneous and non-reversible chemical reactions e.g. chlorine absorption in alkaline solutions. In any case, the developed methodology can be applied to more complex situations, allowing, as well, the attainment of a more generalized approach for the design of a curtain (position, dimensioning, nozzle characteristics, liquid flow rate and characteristics), once given the release parameters, the site layout and the vulnerable target geometrical characteristics.