Tanks and pressure vessels

Tanks are used in innumerable ways in the chemical process iadustry, not only to store every conceivable Hquid, vapor, or soHd, but also ia a number of processiag appHcations. For example, as weU as reactors, tanks have served as the vessels for various unit operations such as settling, mixing, crystallisation (qv), phase separation, and heat exchange. Hereia the main focus is on the use of tanks as Hquid storage vessels. The principles outlined, however, can generally be appHed to tanks ia other appHcations as weU as to other pressure-containing equipment. 

Another type of aboveground tank is the elevated tank. These tanks, elevated by stmctural supports, are almost exclusively relegated to the domain of the municipal water supply companies. Because the municipal water supply is considered a vital pubHc resource, tanks are often elevated for the reason that gravity as a source of pressure is considered more reHable for distribution to the market. Although the same effect can be accompHshed by placing tanks on bills, where this is not possible the tanks are elevated by stmctural steel supports. 

Most tanks store Hquid rather than gases or soHds. Characteristics and properties such as corrosiveness, internal pressures of multicomponent solutions, tendency to scale or sublime, and formation of deposits and sludges are vital for the tank designer and the operator of the tank and are discussed herein. Excluded from the discussion are the unique properties and hazards of aerosols (qv), unstable Hquids, and emulsions (qv). A good source of information for Hquid properties for a wide range of compounds is available (2). 

Density and Specific Gravity. Water has a density, mass per unit volume, of about 62.4 lb/fU (1.000 g/cc) at 0°C, whereas mercury, also a Hquid, has a density of about 842 lb/ft (13.5 g/cc) at the same temperature. AH things being equal, greater densities mean thicker required tank sheU thicknesses. 

Specific gravity (sp gr) is a measure of the relative weight of one Hquid compared to a universaHy familiar Hquid, generaHy water. More speciftcaHy, sp gr is a ratio of the density of a Hquid divided by the density of Hquid water at 16°C (60°F). Specific gravities of selected Hquids are shown in Table 1. 

---