Pressure Vessels serve critical functions across numerous industries
Introduction
The safe design, analysis, fabrication, inspection, and testing of pressure vessels are performed in accordance with the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code. Therefore, great emphasis should be placed on analytical and experimental methods for determining their operating stresses.
The actual installation, registration, and periodic safety inspections are governed by state or municipal boiler and pressure vessel laws and often require registration with the National Board of Boiler and Pressure Vessel Inspectors (NBIC). In-service inspection, repair, and maintenance fall under the NBIC and API 510.
Pressure vessels serve critical functions across numerous industries, including petrochemical processing, power generation, pharmaceuticals, food processing, and manufacturing. These vessels routinely contain substances at elevated pressures and temperatures, creating inherent risks that demand rigorous engineering analysis. A failure can result in catastrophic consequences, including loss of life, environmental damage, property destruction, and significant financial liability. The following table describes a few benefits of performing stress analysis of thin-walled pressure vessels.
| Benefit | Description |
|---|---|
| Safety & Reliability | Handles extreme pressures/temperature; prevents failures |
| Cost Efficiency | Reduces material usage, operating and maintenance costs |
| Long-Term Durability | Optimized for lifecycle; easy maintenance |
Pressure Vessels
Pressure vessels hold gases or liquids at a pressure substantially different from the ambient pressure. They have a variety of applications in industry, including petrochemical and energy. Pressure vessels can be any shape, but typical shapes are spheres and cylinders.
The spherical vessel is preferred for storage of high pressure fluids. For the same wall thickness, a spherical pressure vessel has approximately twice the strength of a cylindrical vessel. The distribution of stresses on the sphere’s surfaces, both internally and externally are uniform. A spherical storage has a smaller surface area per unit volume than any other shape of vessel. However, a spherical vessel is more expensive to manufacture than a cylindrical vessel.
Stress Analysis of Thin Walled Vessels
The stress in the walls of a thin-walled pressure vessel is proportional to the pressure and radius of the vessel. They are considered thin-walled when the ratio of the radius to the wall thickness is greater than 10. Pressure vessel failure occurs when the stress state in the wall exceeds a certain criterion. Stress analysis of thin-walled pressure vessels involves evaluating the normal and shear stresses in the vessel walls.
For example, in a cylindrical pressure vessel, the circumferential stress and axial stress are important parameters.;The circumferential (hoop) stress is given by: σh = Pr / t where P is the internal pressure, r is the radius, and t is the wall thickness.
The axial stress is given by σa = Pr / 2t.
This analysis is valid when the thickness is much less than the radius of the vessel.
For a spherical pressure vessel we cut the sphere into two hemispheres. The free-body diagram gives the equilibrium condition
σ = σ a = σ h = Pr / 2t
In the spherical vessel the double curvature means that all stress directions around the pressure point contribute to resisting the pressure. Understanding the stresses developed in these vessels is crucial for ensuring their structural integrity and preventing failure. The analysis involves evaluating the normal and shear stresses in the vessel walls, taking into account the vessel’s geometry and internal pressure.
ASME Section VIII, Division 1 does not use the textbook membrane formulas directly. Paragraph UG-27 gives the required thickness of a cylindrical shell under internal pressure, based on circumferential stress, as t = PR/(SE − 0.6P), where R is the inside radius, S is the allowable stress at design temperature, and E is the joint efficiency of the longitudinal weld. The −0.6P term adjusts the thin-wall result toward the Lamé thick-wall solution, so the formula stays accurate well beyond the r/t > 10 limit of membrane theory. Paragraph UG-27 permits its use when t does not exceed R/2 or P does not exceed 0.385SE; thicker shells fall under the thick-wall formulas in Mandatory Appendix 1. Corresponding formulas cover longitudinal stress in cylinders, t = PR/(2SE + 0.4P), and spherical shells, t = PR/(2SE − 0.2P), each with its own applicability limit.
Fatigue Analysis of Thin Walled Vessels
Experience shows that fatigue cracks in such vessels are caused by cyclic thermal and pressure stresses. They typically occur at structural discontinuities and weldments. Residual stresses at weldments play a major role in the initiation and propagation of such cracks. Stress corrosion cracking and fatigue at nozzles and other structural discontinuities should be evaluated using materials science and finite element analysis (FEA).
Finite Element Analysis is a powerful computational method used to predict how equipment reacts to real-world forces, vibration, heat, and other physical effects. Read more about Fatigue Analysis of Pressure Vessels.
Understanding Material Properties
The successful design of thin-walled pressure vessels depends fundamentally on selecting materials with appropriate mechanical and chemical properties. Yield strength represents the stress level at which a material begins to deform permanently, establishing the upper limit for safe operating stresses. ASME codes typically require design stresses well below yield strength to provide adequate safety margins against unexpected loads or material variations.
Ductility measures a material’s ability to deform plastically before fracture, providing a critical safety characteristic. Ductile materials exhibit visible deformation before failure, offering warning signs of impending problems. Materials with low ductility can fail suddenly and catastrophically, making them unsuitable for most pressure vessel applications. The ASME code requires minimum elongation values to ensure adequate ductility.
Corrosion resistance impacts vessel longevity and safety. Materials must withstand the specific chemical environment they encounter, including the contained fluid, external atmosphere, and any cleaning or process chemicals. Inadequate corrosion resistance leads to wall thinning, pitting, stress corrosion cracking, and eventual failure. Additional properties requiring evaluation include creep resistance at elevated temperatures, fracture toughness for low-temperature service, weldability, and thermal expansion characteristics. Each application demands careful consideration of these interconnected properties to ensure safe, reliable, long-term operation.
Material Selection Guidelines
Material selection for pressure vessels involves balancing multiple technical and economic factors. The decision process begins with defining operating conditions: internal pressure, temperature range, contained fluid chemistry, and external environment. These parameters immediately eliminate unsuitable materials and narrow the selection to qualified candidates.
For moderate-temperature service (up to about 650°F), carbon steel plate is the usual choice, most often SA-516 Grade 70. Pressure does not set the material grade in this range. It sets the wall thickness, and SA-516 Grade 70 is used for both low- and high-pressure vessels. SA-516 is produced to fine-grain practice, which gives it better notch toughness than the coarse-grained SA-515 Grade 70. SA-515 was intended for intermediate- and higher-temperature service, but SA-516 is now commonly specified across the full range because its toughness makes minimum design metal temperature requirements easier to meet. Above about 650°F, the allowable stress for carbon steel in ASME Section II, Part D begins to fall as the material approaches its creep range, and long-term service above about 800°F introduces a risk of graphitization. Corrosion is managed with a corrosion allowance added to the calculated thickness, a protective coating or lining, or both.
High-temperature service (above 650°F) demands materials with superior creep resistance and oxidation resistance. Chromium-molybdenum alloy steels (such as SA-387 Grade 22) perform well in petrochemical and power generation applications up to approximately 1,100°F. For even higher temperatures, austenitic stainless steels like Type 304H or 316H provide excellent properties, though at significantly higher cost.
Corrosive environments require special consideration. Stainless steels (Type 304L, 316L) resist many chemical attacks but remain vulnerable to chloride-induced stress corrosion cracking. Nickel alloys (Inconel, Hastelloy) offer superior corrosion resistance in extremely aggressive environments but at premium costs. Titanium provides excellent corrosion resistance in oxidizing acids and seawater applications.
Low-temperature applications (below -20°F) require materials with adequate fracture toughness at service temperature. Fine-grained carbon steels, 3.5% nickel steel, austenitic stainless steels, or aluminum alloys serve effectively depending on the specific temperature range. Impact testing verifies adequate toughness at minimum design temperature.
The selection process must also consider fabrication requirements, including weldability, heat treatment needs, forming characteristics, and availability in required sizes. Economic factors balance initial material cost against expected service life, maintenance requirements, and consequences of premature failure. Consulting with materials engineers and reviewing similar successful applications provides valuable guidance for optimal material selection.
Reference
- ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 — Rules for Construction of Pressure Vessels, Paragraph UG-27 (American Society of Mechanical Engineers)
Read More
- ASME Design and Analysis for Pressure Vessels and Piping Systems Covers design and analysis of pressure vessels, heat exchangers, and piping systems to ASME Section VIII, B31.1, and B31.3. Includes FEA-based evaluation of nozzle stress concentrations, thermal cycling, and fitness-for-service assessments for aging equipment.
- Design by Analysis vs. Design by Rule Design by Rule applies fixed formulas from Section VIII, Division 1 — simpler, but conservative by design. Design by Analysis, under Section VIII, Division 2, Part 5, evaluates the vessel against five specific failure modes directly, often allowing reduced weight and material cost on complex geometries.
- Overview of ASME Design By Analysis Rather than calculating a required thickness, Design by Analysis evaluates the exact vessel geometry against expected loads. Applies to conditions Division 1 formulas don’t cover well — complex nozzle reinforcement, non-standard heads, external loads, and high-cycle fatigue service.
- Introduction to ASME Design Approval Process A stalled or rejected application usually traces back to incomplete documentation or an unprepared quality system — not the design itself. This article covers the certification process required for the ASME “U” stamp: quality control documentation, design records, NDE and hydrostatic testing, and the ASME/AIA audit that leads to a Certificate of Authorization.
