Designing a pressure vessel is a highly technical process that must balance safety, efficiency, durability, and compliance. Pressure vessels are integral to industrial operations across sectors such as energy, chemicals, food processing, and pharmaceuticals. Whether storing steam, gas, or liquids, these vessels must operate reliably under pressure while complying with relevant design codes and safety regulations.
At Holdens Engineering, pressure vessel design is approached as a multidisciplinary task—where mechanical strength, process conditions, fabrication methods, and site-specific requirements all play a part.
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Design Pressure and Operating Pressure
The most fundamental parameter in pressure vessel design is the internal pressure the vessel must withstand. Design pressure is typically set higher than the vessel’s maximum operating pressure to provide a margin of safety. This ensures the vessel can tolerate transient spikes or minor overpressure events without structural failure.
Where applicable, external pressure must also be considered, especially for vessels subject to vacuum conditions or external loading. The design pressure affects wall thickness, head shape, reinforcement requirements, and choice of stiffeners.
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Design Temperature and Operating Temperature
The temperature range the vessel is expected to encounter also influences material selection, stress calculations, and potential for creep or thermal expansion. At elevated temperatures, materials may lose strength or become more susceptible to deformation. At lower temperatures, materials may become brittle and require special toughness considerations. Thermal gradients and cyclic temperature changes can introduce fatigue loading, which is addressed during the design stage through appropriate stress limits and material allowances.
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Contents and Corrosiveness
The nature of the vessel contents—whether liquid, gas, vapour, or a mixture—affects internal pressure dynamics and material compatibility. Vessels containing corrosive or abrasive media often require specialist materials, linings, or coatings to prevent premature degradation. Hydrocarbons, acids, steam, food-grade products, and cryogenic fluids each present unique challenges. The design must account for chemical compatibility, potential for erosion, and cleanliness requirements. For example, stainless steel or duplex alloys may be selected for aggressive media, while food processing vessels may require polished internal surfaces to prevent bacterial growth.
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Material Selection
Material choice is governed by mechanical properties, corrosion resistance, temperature performance, and compliance with recognised standards such as BS EN 10028 or ASTM specifications. In most cases, the design code being used will also dictate acceptable material types and grades. Common materials include carbon steel, stainless steel (304, 316, 316L), duplex stainless, and high-performance alloys such as Inconel or Hastelloy. The chosen material must be weldable, suitable for the required thickness, and capable of maintaining structural integrity over the vessel’s service life.
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Design Code and Compliance
All pressure vessels must be designed in accordance with a recognised code. In the UK, the most commonly applied standards are:
- PD5500 – UK specification for unfired pressure vessels
- ASME Section VIII Division 1 – widely used international code
- EN 13445 – European standard for unfired pressure vessels
The chosen code influences how wall thickness is calculated, how welds are qualified, and what inspection or testing methods are required. Holdens Engineering routinely designs vessels to all three standards, depending on project scope and client requirements.
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Volume, Orientation and Geometry
The size, shape and orientation of a pressure vessel are dictated by both the process design and the installation environment. Vertical or horizontal orientation can affect nozzle location, support design, and internal pressure distribution. Larger vessels require additional consideration for transport, foundation loading, and installation.
Cylindrical vessels with dished ends are most common due to their ability to distribute stress evenly. However, for some processes, spherical or rectangular vessels may be needed, requiring more complex stress analysis and fabrication techniques.
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Nozzles, Supports and Attachments
Nozzles, flanges, inspection ports, and other attachments are integral to pressure vessel function, but they also introduce local stress concentrations that must be accounted for in the design. The location, size, and reinforcement of nozzles are carefully engineered to prevent overstressing and to ensure safe integration with pipework. In some cases, finite element analysis (FEA) is used to evaluate the impact of large or off-centre connections.
Similarly, supports—whether skirts, saddles, or legs—must be designed to withstand dead load, wind, seismic forces, and any applied piping loads. Improper support design can lead to local buckling or long-term fatigue failure.
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Manufacturing and Inspection Requirements
A vessel must not only be designed to perform safely, but also to be practical to manufacture, inspect and maintain. This includes considering access for internal inspection, space for lifting and welding, and allowances for tolerances during Pressure Vessel Fabrication.
Post-fabrication testing—such as hydrostatic pressure testing, radiography, or ultrasonic weld inspection—must be incorporated into the design. Similarly, internal coatings, linings, or insulation systems may affect weld accessibility or require dedicated mounting points.
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Location and Installation Conditions
The site where the vessel will be installed may affect certain design choices. For example, vessels located outdoors must be designed for wind and snow loads, while vessels in hazardous areas must accommodate ATEX compliance or zoning considerations. Limited access, height restrictions, or lifting constraints may also influence the vessel’s size or sectional build-up. If the vessel is destined for use within a modular plant room or containerised system, the design must account for restricted space, skidding requirements, and clearances for pipe connections.
Pressure vessel design is a complex, interdependent process that demands input from engineering, fabrication, and operational teams. By understanding the key influencing factors—from pressure and temperature to materials, geometry, and regulatory compliance—clients can ensure their vessels are not only code-compliant but also practical, durable, and fit for purpose.
At Holdens Engineering, we provide full in-house design, supported by decades of experience and an understanding of real-world installation and performance challenges. To discuss your pressure vessel requirements or to learn more about our design capabilities, contact our engineering team today.
