A pressure vessel is a container engineered to hold gases or liquids at a pressure significantly different from the surrounding atmospheric pressure. These specialized containers are ubiquitous, forming the backbone of numerous industrial sectors, including chemical processing, power generation, and oil and gas refining. Their robust design is necessary to safely manage internal stresses that would compromise standard storage tanks. Because of their diverse applications and strict safety requirements, classification depends on several intersecting engineering and regulatory factors.
Classification by Geometric Design
Pressure vessels are categorized by their geometric shape, which directly influences how internal pressure stresses are distributed across the material walls. Cylindrical vessels are the most common configuration found in industry, offering a pragmatic balance between manufacturing ease and space efficiency. Spherical vessels are often utilized for storing high-pressure fluids because the sphere is the ideal shape for distributing pressure uniformly across its surface area.
This uniform stress distribution allows for a thinner wall thickness compared to a cylinder holding the same volume and pressure. The ends of cylindrical vessels, known as heads, are differentiated based on their curvature, as this is where stress concentrations are highest. Hemispherical heads offer the strongest geometry and are reserved for the highest internal pressures a vessel must contain.
Ellipsoidal heads, which are flatter than hemispherical ones, represent an engineering compromise, providing a good balance of strength while requiring lower manufacturing costs. Torispherical heads, which combine a spherical dome with a toroidal knuckle, are often used for medium-pressure applications. Rarer designs, such as conical or rectangular vessels, are sometimes necessary for specific process requirements, but engineers generally avoid these due to the difficulty in managing concentrated stresses at corners and transitions.
Classification by Operational Function
Pressure vessels are frequently classified according to the specific role they fulfill within an industrial process. Storage vessels are designed simply to hold materials, such as compressed air receivers, and their primary requirement is containment under static pressure conditions. In contrast, reactor vessels are dynamic environments where chemical transformations occur, often involving controlled mixing, heating, or cooling. The design of a reactor must account for complex internal fittings, potential corrosion, and managing thermal dynamics.
Heat exchangers are vessels engineered specifically for the efficient transfer of thermal energy between two separate fluid streams without allowing them to mix. The common shell-and-tube heat exchanger design involves one fluid flowing through internal tubes while the second fluid passes around them within the main shell. Boilers, or steam generators, convert water into high-pressure steam through the application of heat, classifying them as “fired” pressure vessels.
Boiler designs require specialized materials capable of withstanding high temperatures in addition to the pressure. Separator vessels are utilized to physically divide a mixed stream into its constituent components, such as separating oil, water, and gas in the petroleum industry. These vessels rely on differences in density or filtering mechanisms, requiring internal weirs, demister pads, or specialized inlet nozzles to facilitate the phase separation under pressure.
Regulatory and Mobility Distinctions
Classification is also driven by regulatory frameworks and mobility requirements, which dictate design codes and mandatory safety features. A primary regulatory division exists between “fired” vessels, which involve the combustion of fuel like boilers, and “unfired” vessels, such as chemical storage tanks. This distinction is foundational because fired vessels face stricter regulations concerning thermal stress, material creep, and combustion safety.
Vessels are also classified based on their intended movement, separating stationary vessels, which are permanently installed in a facility, from transportable vessels. Stationary vessels are governed by construction codes such as the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code Section VIII. Conversely, transportable vessels are subject to different regulations, often overseen by transportation authorities like the Department of Transportation (DOT).
These mobile containers must be designed to withstand dynamic forces during transit, not just static internal pressure, requiring different testing and inspection intervals. The regulatory context further segments the definition of a vessel “type” based on parameters like maximum allowable working pressure, internal volume, and the hazard level of the contained fluid. This legal segmentation ensures that a small, portable cylinder is designed and inspected under a different set of rules than a massive, stationary reactor vessel.