What Are Pressure Vessels?

Pressure vessels are closed containers designed to hold fluids or gases at internal pressures that differ substantially from the ambient pressure. They range from small compressed-gas cylinders to massive industrial reactors with wall thicknesses of several hundred millimeters, and from simple cylindrical shells to complex multi-compartment assemblies with hemispherical heads, nozzles, and internal fittings. Because the consequences of a pressure vessel failure can include explosion, fire, and toxic release, their design, fabrication, inspection, and operation are governed by some of the most detailed engineering codes in existence.

The discipline of pressure vessel engineering draws on structural mechanics, fracture mechanics, materials science, and non-destructive evaluation. The fundamental design problem is to select a wall geometry and material such that the hoop stress and axial stress generated by the internal pressure remain below the allowable stress of the material with an adequate safety margin throughout the vessel's service life, accounting for temperature cycles, corrosion, fatigue, and the possibility of pre-existing defects.

Design Codes and Standards

The ASME Boiler and Pressure Vessel Code (BPVC) is the primary design standard for pressure vessels in the United States and is recognized internationally. Its Section VIII covers unfired pressure vessels and is divided into three Divisions: Division 1 specifies mandatory requirements for vessels operating above 15 psi using design-by-formula rules; Division 2 permits thinner walls through design-by-analysis methods that require more detailed stress calculations; and Division 3 addresses very high-pressure applications above 10,000 psi. The ASME BPVC standards page describes the code as the single largest source of technical data used in the manufacturing, construction, and operation of boilers and pressure vessels, with editions updated every two years to incorporate new materials and construction methods. European practice is governed by the Pressure Equipment Directive (PED) and the harmonized EN 13445 standard, which use risk-based approaches to categorize vessels and specify inspection requirements accordingly.

Materials: Steel and Concrete

Carbon steel and low-alloy steel are the most widely used materials for pressure vessels because they combine high yield strength, good weldability, and predictable fracture behavior. Chromium-molybdenum steels such as 2.25Cr-1Mo are used at elevated temperatures because they resist creep and hydrogen embrittlement. Stainless steels are selected when corrosion resistance is the primary concern. For very high pressures, such as those in isostatic pressing equipment and certain chemical reactors, forged or wire-wound vessels may be used to pre-stress the wall in compression. Concrete pressure vessels are used primarily in nuclear power applications; prestressed concrete reactor vessels (PCRVs) were used in gas-cooled reactor designs and offer excellent radiation shielding and the ability to withstand internal pressure through the compressive prestress applied by high-strength steel tendons embedded in the wall.

Nuclear Pressure Vessels

The reactor pressure vessel (RPV) in a light-water nuclear power plant is one of the most demanding pressure vessel applications in engineering. It contains the reactor core and primary coolant at pressures of approximately 15 MPa (pressurized water reactors) or 7 MPa (boiling water reactors) and temperatures up to 325 °C, while enduring decades of neutron irradiation that gradually embrittles the vessel steel. The RPV must maintain structural integrity even under postulated accident conditions, including rapid cooldown events that impose severe thermal shock on the wall. The ScienceDirect overview of pressure vessel codes discusses how nuclear vessels require code cases and supplementary requirements beyond those in the standard BPVC to address irradiation embrittlement monitoring and in-service inspection. Surveillance programs remove small coupon specimens from the vessel wall periodically and test their toughness to track the accumulated embrittlement and establish the adjusted reference temperature used in fracture assessments. The Paul Mueller Company overview of ASME boiler and pressure vessel codes explains how the ASME certification mark on a vessel signifies third-party inspection to applicable code sections, a requirement that nuclear vessels must also satisfy.

Applications

Pressure vessels have applications in a wide range of industries, including:

• Nuclear power generation, as reactor pressure vessels containing the core and primary coolant
• Petroleum refining and petrochemical production, for reactors, heat exchangers, and separation columns
• Industrial gas storage and transport, for compressed oxygen, hydrogen, nitrogen, and liquefied natural gas
• Food and beverage processing, for autoclaves, pasteurizers, and carbonation vessels
• Water treatment, for pressure filters and reverse osmosis membrane housings