OVERVIEW

Vessels in chemical processing service are of two kinds: those substantially without internals and those with internals. The main functions of the first kinds, called drums or tanks, are intermediate storage or surge of a process stream for a limited or extended period or to provide a phase separation by settling. Their sizes are established by process calculations or by general rules of thumb based on experience. The second category comprises the shells of equipment such as heat exchangers, reactors, mixers, fractionators, and other equipment whose housing can be designed and constructed largely independently of whatever internals are necessary. Their major dimensions are established by process requirements described in other chapters, but considerations of adequate strength of vessels at operating pressures and temperatures will be treated in this chapter.

The distinction between drums and tanks is that of size and is not sharp. Usually they are cylindrical vessels with flat or curved ends, depending on the pressure, and either horizontal or vertical. In a continuous plant, drums have a holdup of a few minutes. They are located between major equipment to supply feed or accumulate product. Surge drums between equipment provide a measure of stability in that fluctuations are not transmitted along a chain of equipment, including those fluctuations that are characteristic of control instruments of normal sensitivity. For example, reflux drums provide surge between a condenser and its tower and downstream equipment; a drum ahead of a compressor will ensure freedom from liquid entrainment and one ahead of...