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Understanding the Mechanics of Pressure Vessels

Found in a wide range of settings, from industrial boilers to scuba tanks, pressure vessels are containers that have been specially designed to hold gases or liquids at high pressures. As these vessels are subject to extreme pressures, they must meet strict safety standards, and they are highly regulated.


What is a Pressure Vessel?

The enclosed containers known as pressure vessels hold gases, vapours, and liquids at much higher or lower pressures than the ambient pressure. Flash drums, heat exchangers, reactors, and separators are some examples of these vessels. They are used widely in a diversity of industries, such as chemical, food processing, oil and gas, and petrochemical industries. Every aspect of these vessels is governed by stringent standards and regulations. The ASME Boiler and Pressure Vessel Code (BPVC) is the most widely used set of universally recognised pressure vessel regulations and standards governing their design and construction, testing, installation, inspection, and certification, as well as that of nuclear power plant components.

Types of Pressure Vessels

The many different types of pressure vessels are classified according to their purpose or their geometry.

Vessel Types According to Purpose

  • Storage vessels: These vessels hold gases, vapours, and liquids temporarily, whether during later processes or for storing finished products such as liquid nitrogen.
  • Heat exchangers: These vessels transfer heat between two or more liquids. They are commonly used in the bioprocessing, energy, food, and pharmaceutical industries.
  • Boilers: Boilers are used mostly to heat liquids. These vessels are often used for phase transformation of liquid to vapour. They use fuel, electrical, or nuclear power as heat sources.
  • Process vessels: A broad classification of pressure vessels, process vessels are containers where industrial processes happen, such as chemical reactions, agitation and mixing, decanting, distillation and mass separation. Examples include distillation columns, industrial mixers, chemical reactors, decanters and gravity settlers.

Vessel Types According to Geometry

  • Spherical pressure vessels: Ideal for holding high-pressure fluids due to their strong structure, spherical pressure vessels are costly and difficult to manufacture. External and internal stress is distributed evenly on the spherical vessel’s surface so there are no weak points.
  • Cylindrical vessels: These vessels are comprised of a cylindrical shell and a set of heads. The shell forms the body of the vessel, while the heads are used as the end caps which cover the vessel’s contents. The heads may have a rounded profile to reduce vessel weakness, or they may be flat. These vessels are more widely used than spherical vessels.

How does a Pressure Vessel work?

The big question now is, “How do pressure vessels work?” These vessels are designed to reach the pressure level required to make an application function, such as containing air in a scuba tank. They can either deliver pressure directly via release gauges and valves or indirectly via heat transfer. The potential pressure levels across the most common vessels range between 15 psi to as much as 150,000 psi, with internal temperatures often higher than 400°C (750°F). Depending on their size, pressure vessels hold between 75 litres (20 gallons) and several thousand litres.

The Mechanics of Pressure Vessels

The mechanical design of pressure vessels includes various internal components which are attached directly to the vessel’s shell. These components include all or some of the following:

  • Baffles
  • Catalyst support grids
  • De-mister pads
  • Distributor trays

The following parameters are used in a pressure vessel’s design calculations and are essential in evaluating the shell and heads’ wall thickness:

  • Design pressure: The vessel specifications are calculated within this value, which is derived from the maximum operating pressure. This is the anticipated pressure surge that occurs during upset conditions such as process abnormalities and emergency shutdown. This value is always higher than the maximum operating pressure.
  • Maximum Allowable Working Pressure (MAWP): MAWP is the highest permissible pressure measured at the top of the equipment. The pressure vessel must operate at this pressure, which is based on its design temperature.
  • Design temperature: The design temperature is always higher than the maximum operating temperature and lower than the minimum temperature. The maximum allowable stress is highly dependent on the temperature, as the vessel becomes brittle at very low temperatures, while strength decreases as temperature increases.
  • Maximum allowable stress: The maximum allowable stress of pressure vessels is obtained by multiplying a safety factor by the value of the maximum stress that the material can withstand.
  • Joint efficiency: This is the ratio of the strength of the welded plate to the strength of the unwelded plate. Usually, the strength is lower at the welded joint.
  • Corrosion allowance: In most cases, corrosion allowance should range between 1.5 and 5mm.

Learn more about our pressure vessel qualities here.

Choosing the best Pressure Vessels

Given the stringent pressure vessel regulations in force as well as the important function of these vessels in numerous industries, your choice of vessel should not be left to chance. Ensure you choose the best pressure vessels for your needs. Contact Steel Plate and Sections and find out how we can help you.

Posted by Killian Ward
21st June 2023

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