Pressure vessels are containers designed to hold fluids at a pressure substantially lower or higher than the ambient pressure. They are used in a variety of potentially dangerous situations, from industrial boilers to petrol tankers. They facilitate secure storage of pressured liquids and gases, hence are a vital component of many industrial and processing facilities.
However, if not properly designed, constructed and maintained, they can be exceedingly dangerous. In fact, throughout history, poorly designed pressure vessels have been known to cause catastrophic accidents. That led to the invention of the ASME (American Society of Mechanical Engineers) code, also known as the ASME Boiler & Pressure Vessel Code (BPVC).
The code regulates the design, development and construction of pressure vessels, and its rules apply to Australia as well as 60 other nations. In this post, we’re going to be looking at 2 design methods used by engineers to design pressure vessels.
What Is Pressure Vessel Design by Analysis (DBA)?
The ASME code Section viii has three divisions outlining the rules of pressure vessel design. Section viii division 1 focuses on the design-by-rule method while division 2 focuses on a design-by-analysis method. The design by analysis method offers guidelines for designing pressure vessels using numerical analysis (usually finite element analysis).
This requires more detailed calculations than division 1 and seeks to protect pressure vessels against 5 types of failure—collapse from buckling, local failure, plastic collapse, cyclic service: fatigue and cyclic service: ratcheting. Seeing that it focuses on maximum distortion energy, it yields pressure vessels that can tolerate greater strains, though comes with an additional cost.
What Is Pressure Vessel Design by Rule (DBR)?
This is a less comprehensive design method contained in ASME section viii, division 1. It is a conservative method used by engineers to size pressure vessels in accordance with their application requirements and bases its calculation on normal stress energy.
It outlines guidelines for designing pressure vessels with the help of closed-form equations and other stringent regulations. The Design by Rule approach yields pressure vessels that can withstand common failure modes.
A Brief History of Pressure Vessel Design
So where did the concept of pressure vessel design come from? The earliest known theory of pressure vessel design was developed by Renaissance painter, engineer and draftsman, Leonardo da Vinci. He developed a hypothesis in 1495 where he described how he lifted weights underwater using “containers containing compressed air.”
Though his publication was made, his assumption was flawed. After centuries of numerous failed design iterations, and hazardous mishaps to create secure pressure vessel designs, we have now established design methods that work. Today, the ASME code enforces the adoption of standard standards for the design and development of pressure vessels through the BPVC (Boiler and Pressure Vessel Code).
How to Size a Pressure Vessel
To size a pressure vessel, a design engineer should first gather data around parameters that affect pressure vessel performance. These include the vessel’s purpose, its location’s size, shape, and orientation, the type of vessel head to be used, the construction materials to be used, the interior fittings, and the operating circumstances, such as the temperature and pressure of the surrounding environment, as well as process materials and services (deposits and corrosion).
Following preliminary data collection, standard procedures outlined in BPVC Section VIII, which is further divided into subsections, appendices and guides should be used. They help determine material requirements, fabrication requirements and general design requirements. This effectively sizes the pressure vessel.
How to Design a Pressure Vessel: By Analysis
To design a pressure vessel by design, the following steps are employed:
- Data collection for a complete specification of the service conditions.
- Establishment of a preliminary layout as per relevant design codes and information on material properties.
- Conducting an elastic stress analysis to analyse the possible modes of failure. Because the most common modes of failure are stress-dependent, determining the stresses and setting acceptable limits are critical steps in this design process.
- Computational technologies further permit engineers to develop cost-effective pressure vessels.
How to Design a Pressure Vessel: By Rule
Designing pressure vessels by rule involves the following steps:
- Assessment of pressure vessel function, size and other necessary parameters.
- Design preparation using standard vessel geometries using simple formulae and charts. This involves calculating the minimum required thickness of the vessel using design pressure, allowable stress, and a design formula compatible with the geometry of the part.
- Codifying geometries using standards and codes based on the rules proposed in the European Standard prEN13445-3 and ASME BPVC.
- The Code design also outlines basic rules that specify the design method, and design load. The certification requirements, acceptable materials and allowable stress for fabrication inspection for the construction of vessels are also obtained from the code.
Final Thoughts
Because of the ability to consider higher allowable stresses and obtain more real, economic, and reliable results, there has been a significant shift toward using the design-by-analysis method. This provides a more rigorous analysis, especially for vessels subjected to complex loadings such as cyclic, thermal, or localised loads.
Register Your Pressure Vessel Design
Sherwood Design & Engineering provides comprehensive pressure vessel design services. We can produce detailed pressure vessel drawings, ready for manufacture, using our 3D modelling and design drafting technologies.
We also supply the calculation set as well as drawings as part of the package and can facilitate independent third-party verification for all our designs for registration with the relevant authority. Let our qualified engineers help you design pressure vessels in accordance with Australian Standard AS 1210 and the ASME code.
Give us a call at (02) 9437 3566 or leave an enquiry if you would like to learn more about our pressure vessel engineering design and support services.