Blog: The Basics of Pressure Vessel Design and Analysis
Introduction
Pressure vessels have wide applications in power plants, and industries including pharmaceutical & chemical which require the safe storage and transportation of gasses and liquids under high pressure. In order to avoid catastrophic accidents, careful attention to design detail is required. Pressure vessel failures may result in loss of life, health hazards and property damage. Vessels have openings of various diameters to accommodate manholes, handholds, and nozzles, which lead to high stress concentrations that may lead to vessel failure. Typically, pressure vessel failures can be traced to one of the following:
- Material – improper selection for required service; defects such as inclusions; inadequate quality control
- Design – incorrect design conditions; oversimplified or careless design computations; inadequate shop testing
- Fabrication – improper or insufficient fabrication procedures; inadequate inspection
- Service – change of service conditions to more severe ones without adequate provision; inadequate maintenance
Design Process
The design process for pressure vessels is a collaborative effort between designers, engineers and fabricators. The various involved factors include the pressure requirements, type of material, and operating conditions. Operating conditions include:
- Design pressure (internal or external)
- Dead loads
- Wind loads
- Earthquake loads
- Temperature loads
- Piping loads
- Impact or cyclic loads
The design process involves the selecting the appropriate material, calculating the required thickness of the walls, determining the required number of joints and welds and designing a suitable pressure relief system. Once a vessel is designed, an analysis must be performed and documented to prove it is structurally sound according to the specified code. Analysis is performed either by closed-form Design-By-Formula Methods (ASME BP&V Section VIII Division 1) or finite element analysis with Design-By-Analysis Methods (ASME BP&V Section VIII Division 2).
Users of pressure vessels typically order ASME Code vessels because they know that such vessels will be designed, fabricated and inspected to an approved quality control system in compliance to a safe standard.
Materials
Material selection for pressure vessels is critical because it must withstand the specified pressures and temperatures. Materials selection is also influenced by factors as corrosion resistance, strength and cost. Carbon steel, stainless steel and alloy steels are commonly used materials.
Wall Thickness Calculations
Pressure vessel wall thicknesses are typically calculated using the ASME Boiler & Pressure Vessel Code. The wall thickness must be to withstand the maximum design pressure while also factoring in safety margins.
Joints and Welds
Joints and welds influence the both the safety and service life of the vessel. Joints that are designed to minimize stress concentrations are desirable because they are less likely to initiate cracking. Welds must be carefully designed, developed and inspected to ensure structural integrity to the service pressures and temperatures.
References
1. Pressure Vessel Design Handbook, Henry H. Bednar, PE, 1981, Von Nostrand Reinhold Co. New York, NY
2. The ASME Code Simplified – Pressure Vessels, Seventh Edition, Robert Chuse, Bruce E. Carson, Sr., 1993, McGraw-Hill, Inc. New York, NY
Read more about Finite Element Analysis or History of the ASME Code.
O’Donnell Consulting performs pressure vessel analysis & design optimization.
