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mixingtank

Understanding the zone area classification method in the NEC

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Highly oxygenated areas are outside the scope of both the zone Classifying Equipment and the division classification systems. These areas have had the ignition and burning characteristics of materials changed by exceeding the normal volume of oxygen of air that is mixed with the flammable gas or vapor. Where highly oxygenated areas are encountered, the user should refer to the specialized documents that deal with these types of areas and comply with the requirements for installation of electrical equipment in those areas.

In zone applications, hazardous locations are classified in accordance with the properties of flammable liquids, gases, or vapors that may be present in the area where electrical equipment is installed. The liquids, gases, or vapors must be likely to form ignitable concentrations and the quantities of the material must be sufficient to pose a hazard when mixed with a sufficient quantity of air. These conditions are similar or often the same as the conditions required for a similar location being designed and installed in the division concept.

Areas where pyrophoric materials are present or handled are also outside the scope of both the zone classification and the division classification systems. Pyrophoric materials can be ignited just by introducing the material to air. Where these chemicals are used, installation of special electrical equipment is usually not necessary. However, care should be taken since there may be other combustible chemicals in the area that may require special electrical equipment.

Each area should be considered individually in determining the classification and care should be taken to not over-classify, as well as to not under-classify. Electrical equipment should be installed and connected in an area that is outside the hazardous (classified) location; however, where this not possible or practical, then special electrical equipment must be installed in the hazardous (classified) location. All of the factors that are normally associated with division area classification would apply to a zone classification, such as temperature, density or molecular weight of the substance, air circulation, quantity, pressure, and so forth.

A proposed operation of a semicontinuous fluidized-bed ion-exchange system was studied. The system splits a liquid current into two currents, one being more concentrated and the other more depleted. This operating technique has been used to split up a mixture of alkaline ions (Na+, K+) using a strongly acidic resin. The equipment operates simultaneously in two multistage columns, one for loading and the other for elution of the resin. The experimetal testing system employs a Hydrometallurgy Equipment containing cobalt and copper as heavy metallic ions, and the resin used was of the chelating iminodiacetic type, Lewatit TP-207. At cyclic steady state, the equipment can split up the wastewater, producing an effluent concentrated in cobalt in the outlet stream of the loading column, and a concentrated stream of copper in the effluent of the elution column. The hydrodynamics and approach to the stationary state of the system were analyzed, and the selective recovery of metals was subsequently tested experimentally. This behavior presents certain similarities with a parametric pumping operation of the system, with the two columns operating at different pH values or temperatures.

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