Storage Tank Design Guidelines Caustic soda solution users are responsible for building and maintaining a properly designed storage and handling system which meets applicable safety and regulatory requirements. Each caustic soda solution user is responsible for design and implementation of a storage and handling system that is suitable for its individual facility. In this section side navigation links are provided so you can find additional information related to the design and maintenance of storage systems for caustic soda solution.
Storage tanks should be located to minimize all piping, especially exterior pipes in which caustic soda solution can freeze if a malfunction occurs in the heating equipment. Of equal importance is the need to locate storage and piping in a low-traffic zone to minimize potential exposure of workers.
Vertical tanks are preferred over horizontal tanks since they take up less space and are easily supported on concrete slabs. Tank bottoms should be set in an asphalt grout for protection from exterior corrosion. All connections at the top of a tank should be grouped in one small area near the edge of the tank to permit servicing from a single location.
All caustic soda solution storage facilities should have a spill containment system. The containment system should be designed to confine, collect and recover all spills, using sloped floors and a sump or reservoir. Separation of incompatible chemicals into separate containment areas must be considered. Governmental regulations may also require diking/secondary containment to hold a specific volume of caustic soda solution related to the capacity of the storage tank(s).
Over time, precipitation reactants such as calcium carbonate, magnesium hydroxide, etc, will accumulate. The tank discharge nozzle should be located at least 1-foot (30-cm) above the tank bottom to avoid entrainment of these solids into the pump suction. A second tank discharge nozzle should also be provided at the bottom of the tank to facilitate out-of-service cleaning. A 2 to 4 year out-of-service tank cleaning schedule is recommended to prevent excessive build-up of sediments.
The tank inlet nozzle should be above the highest anticipated tank level with a dip pipe that extends to below the lowest tank level. Freefalling or splashing liquid may cause additional carbon dioxide absorption and form insoluble calcium carbonate. The inlet dip tube should have a syphon break and be on the opposite side of the tank from the discharge.
A vent should be provided which is open to the atmosphere at all times. In general, the vent cross-sectional area should be sufficient to prevent overpressure or vacuum (this is generally considered to be at least four times that of the fill line). A dedicated overflow line should also be installed. If the vent line is also used as the overflow line, it should extend to within 3-feet (0.9 meter) above the grade level. Valves should not be installed on the vent/overflow line to ensure it is always open. If possible, the discharge of the overflow line should be visible from the unloading area.
A tank lining should be considered if maintaining low iron concentration in the caustic soda solution. For more details see Tank Linings.
http://www.dow.com/causticsoda/safety/linings.htm
Storage tanks should be heated and insulated if sustained ambient temperatures below 65°F (18°C) are expected. The insulation must be well protected with jacketing to keep it dry.
Heating can be accomplished with steam coils or electrical heating tape. For internal tank heating, a nickel bayonet-type heat exchanger is recommended. The heater should be attached to a nickel flange that can be fitted to a tank nozzle opening close to the bottom of the tank. The horizontal heater should be supported about 8-inches (20-cm) above the bottom of the tank and extend across the storage tank to within 1-foot (30-cm) from the opposite side. As the caustic soda solution around the heater is warmed, a thermal agitation pattern will form in the tank, resulting in uniform heating of all caustic soda solution. A maximum of 15-psig (104-kPa) steam is recommended for heating the caustic soda solution.
A temperature controller should be installed to maintain a caustic soda solution temperature of 85° to 100°F (29° to 38°C). The controller should include a high-temperature alarm and a thermocouple at the same liquid level as the steam coil. This guards against the thermocouple reading a false air temperature causing the coils to overheat the remaining caustic solution when the tank is nearly empty.
If a source of steam is not available, the storage tank can be heated using an electrical horizontal bayonet-type heat exchanger similar to the one described earlier. The temperature of the caustic soda solution can also be maintained or increased slowly using electrical heat-tracing tape attached to the outside of the storage tank. If electric heat-tracing tape is the only source of heat, the storage tank should be well insulated to ensure a minimum heat loss from the caustic soda solution.
In addition to a solution high-level alarm, the tank should have continuous liquid-level measurement and a vent. Sight glasses should never be used on 50% caustic soda solution storage tanks or transfer lines due to the potential for breaking or clouding.
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