Dispersion Water Sprays

Cooling of water can be carried out on a small scale either by allowing it to stand in an open pond or by the spray pond technique in which it is dispersed in spray form and then collected in a large, open pond. Cooling takes place both by the transference of sensible heat and by evaporative cooling as a result of which sensible heat in the water provides the latent heal of vaporisation. 

Water spray systems have been demonstrated to assist in the dispersion of vapor releases. The sprays assist in the dilution of the vapors with the induced air currents created by the velocity of the projected water particles. They cannot guarantee that a gas will reach an ignition source but do improve that probabilities that dispersion mechanisms will be enhanced. 

Activate fixed fire extinguishing systems or vapor dispersion mechanisms (i.e., water sprays). 

There are a variety of nozzles that can be provided to hoses and monitors. They are capable of projecting a solid, spray or fog stream of water depending on the requirements and at varying flow rates. Straight stream nozzles have a greater reach and penetration, while fog and water sprays will absorb more heat because the water droplets absorb more heat due to greater surface area availability. Fog and water spray nozzles are sometimes used to assist in the dispersion of vapor releases. 

Vapor Dispersion Vapors from certain materials can be dispersed or moved using water spray or air movement. Reducing the concentration of the material may bring the material into its flammable range. 

Prevention of fire spread or explosion is sometimes possible by the use of water spray to dissolve, dilute, disperse, or cool flammable or combustible materials. 

Gas-continuous impinging streams with a liquid as the dispersed phase has wide application, such as in the combustion of liquid fuel droplets, absorption, water-spray cooling of air, etc. [9]. In such systems the dispersity of liquids plays a very important role affecting heat and mass transfer rates, because it influences both the interface area and the mean transfer coefficient. Wu et al. [68] investigated the influence of impinging streams on the dispersity of liquid. 

Tolson, P., Examination of possible electrostatic hazards arising from the use of water spray barriers to disperse flammable vapors, J. Electrostatics, 23, 89-98, 1989. 

Some toxic chemicals have a strong affinity for water or other liquids. Sprays in the path of the cloud will absorb some of the chemical and will induce more turbulence in the cloud which can lead to enhanced dispersion. Water is used for vapors that have an affinity for water, for example halogen acids, and ammonia. 

Dilution is attributed to the large amounts of air that are entrained by the spray. As a result of air entrainment, dispersion behavior is altered for materials that exhibit negative buoyancy upon release. This is found to be effective in controlling flammability hazards that are located close to the release source. If an explosion does occur, some of the energy will be absorbed in the breakup of the water spray droplets, thereby mitigating the explosions impact. 

Martinsen, Muhlenkamp, and Olson 1977 To test varied spray nozzles and water flow rates to determine their effectiveness in dispersing LNG vapors. Water sprays do affect concentration reduction of LNG vapors... 

Moore and Rees 1981 To force dispersion by water and steam. Eight experiments were conducted. Best water-spray configuration is upward, with conical pattern and narrow angle. Best steam-jet configuration is downward. Water is more effective downstream in reducing 15-m downwind concentrations. 

Van Doom 1981 To investigate the use of water sprays in dispersing a hazardous cloud by using air entrained by the spray to dilute the material to safe limits. A semiemperical model was developed of describe transfer of momentum between spray curtain and air. Induced air movement is turbulent and strongly dependent on initial water velocity. Nozzle design and operating parameters influence spray-curtain performance. When two spray-curtains are used, the one with the larger flow predominates. 

Moodie 1981 To study the effectiveness of different nozzle sizes and arrangements in full-scale water-spray barriers to disperse heavy-gas vapor clouds. Amount of air entrained by a water-spray barrier may be dependent on nozzle inclination. Results demonstrated the effectiveness of the basic arrangement tested and the experimental method of assessment. 

Schatz and Koopman (1990) reported on the Hawk series, 87 tests conducted at the DOE Nevada test site. These experiments were large-scale chamber releases of HF, as well as laboratory experiments. The objective of these tests was to study the effect of the water-to-HF ratio, water spray geometry, water application via a fire monitor, acid type (anhydrous HF and alkylation unit acid (AUA)), acid temperature and pressure, water additives, relative humidity, wind speed, and steam as an acid jet dispersant on HF removal efficiency. Figure 4.2 shows removal efficiencies ranging from 25 to 90% for water-to-HF volumetric flow ratios ranging from 6 1 to 40 1. Fire monitors provided removal efficiencies comparable to those of water sprays. Some of the conclusions reached by the authors were ... 

Maximum Predicted Impact of Water Spray on Dispersion Distances at a Water to Ammonia Ratio of 5 (water flow is 19 kg/s from all 20 nozzles)... 

Water sprays from monitor nozzles and hose lines can be used for vapor mitigation. Tests have been conducted in which monitor nozzles and hose lines have been used to create a chimney effect through which the gas is forced upward and dispersed at a high elevation (Beresford, 1981). Application techniques and flow rates are facility-, installation-, and material-specific. Careful planning, analyses, and testing should be conducted prior to deciding on the use of a mobile water spray as a proven means of mitigation. Preventive maintenance of this equipment is key to reliable operation. Hose lines, typically, are hydrostatically tested annually. Flow tests should also be conducted periodically. 

Beresford, T. C., 1981. The Use of Water Spray Monitors and Fan Sprays for Dispersing Gas Leakages. Northwest Branch Papers, 5 6.1-6.3. London Institution of Chemical Engineers. 

Meroney, R. N. and D. E. Neff, 1984. Numerical Modelling of Water Spray Barriers for Dispersing Dense Gases. Journal of Boundary-Layer Meteorology, 31 (March) 233-247. 

Moodie, K., 1985. The Use of Water Spray Barriers to Disperse Spills of Heavy Gases. Plant Operations Progress, 4 (October) 234-241. 

Van Doom, M., 1981. The Control and Dispersion of Hazardous Gas Clouds with Water Sprays. Ph.D. diss., Department of Applied Physics, Delft University of Technology, Netherlands. NTIS DE82-902736. 

In the analysis it was assumed that the water-spray curtain was activated within one minute of the start of the incident and that 86% of the HF vapor from the evaporating pool that reached it was removed. The balance of the HF not removed by the water sprays became the material that formed the reduced hazard zone and the input for the dispersion modeling. For the F/l meteorological conditions the mitigated hazard zone for the centerline concentration of 20 ppm was 750 m. The effectiveness of water spray as postrelease mitigation measure is shown in Table 7.6. 

Figure 7.21. HF input rate to the dispersion model for the mitigated (water-spray curtain) and unmitigated scenario.

Water, steam, and air curtains and water sprays are primarily effective in dispersing and/or diluting vapors with air to reduce the severity of effects of a hazardous vapor release. In some cases, vapors can be partially knocked down or absorbed after release. ... 

This procedure has been used to determine droplet size in sprays. Oseillations in the curve relating x and D can be smoothed out by the use of an incident laser beam having a broad speetral bandwidth [83]. An accumulation of independent scattering intensities from multiple scatterers ean be used to measure the mean droplet size of a group [84]. This procedure has been applied to water sprays and the experimental data confirmed by phase Doppler anemometry [85]. The applicability of the polarization ratio technique is strongly influenced by the complex refractive index of the dispersed media and is limited to media having a relative refractive index below about 1.44 [86]. 

Explosivity Fine dust dispersed in air in sufficient concentrations and in the presence of an ignition source, is a potential dust explosion hazard. Minimum concentration for explosion is 0.25 oz. ft The recommended fire extinguishing media is water spray, dry chemical, alcohol foam, or carbon dioxide. Acetaminophen is capable of generating a static electrical charge. Processes involving dumping of acetaminophen into flammable liquid, inert atmosphere in the vessels, or temperatures of flammable liquid should be maintained below its flashpoint. 

Meroney, R.N., and Neff, D.E. (1985) Numerical modelling of water spray barriers for dispersing dense gases, Boundary-Layer Meteorol. 31, 233-247. 

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