Design of Pressure Vessel using COMPRESS – Basics of Metallurgy and Material Selection

A: Material selection for pressure vessels hinges on several critical factors: mechanical strength to withstand operational pressures, corrosion resistance depending on the service environment, temperature tolerance both for high and low extremes, weldability for fabricating the vessel, and cost-effectiveness. Common materials include carbon steels, stainless steels, and nickel alloys. Metallurgical considerations such as grain size, toughness, and fabrication history also influence performance. For further reading on materials selection, refer to ASM International’s guide on pressure vessel materials: https://www.asminternational.org/materials-resources/pressure-vessel-materials

A: Metallurgy significantly affects pressure vessel performance and safety by dictating material properties like tensile strength, toughness, ductility, and resistance to brittle fracture. Proper heat treatment processes refine grain structures to improve toughness, while alloying elements can enhance corrosion resistance. Metallurgical factors also determine susceptibility to embrittlement or fatigue, which are critical in cyclic or extreme temperature operations. Understanding these properties allows engineers to select and fabricate vessels that maintain integrity under service conditions. For deeper insight, the book 'Metallurgy for the Non-Metallurgist' by Arthur C. Reardon offers a solid introduction.

A: Pressure vessels commonly fail due to yielding or plastic deformation, brittle fracture, fatigue crack initiation and propagation, creep at high temperatures, and corrosion leading to thinning or pitting. Design mitigation includes selecting appropriate materials with sufficient toughness, ensuring adequate wall thickness through design calculations, applying safety factors, incorporating corrosion allowances, enabling proper inspection and maintenance, and accounting for fatigue life by limiting cyclic stresses. Software like COMPRESS helps quantify stresses and identify critical areas, enabling engineers to design against these failure modes effectively.

A: Corrosion allowance refers to an additional thickness of material added to a pressure vessel to compensate for expected material loss due to corrosion over the vessel's service life. It ensures that even with corrosion, the vessel maintains sufficient mechanical integrity to operate safely. Determining the corrosion allowance depends on the corrosiveness of the media, operating conditions, and inspection frequency. Neglecting corrosion allowance can lead to premature failure or unsafe operation. ASME Section VIII provides guidelines on incorporating corrosion allowance during design.

A: Post-weld heat treatment (PWHT) is crucial to relieve residual stresses induced by welding, reduce hardness, and improve toughness around the heat-affected zone (HAZ). It prevents brittle fracture, hydrogen-induced cracking, and uneven mechanical properties that can compromise vessel integrity. PWHT parameters depend on the material grade and thickness and must conform to standards such as ASME Section VIII or specific material codes. Proper PWHT ensures long-term durability and safety of the pressure vessel.

A: Operating temperature and pressure directly impact the mechanical properties required from the vessel material. At high temperatures, materials may lose strength and become susceptible to creep deformation. Conversely, very low temperatures risk brittle fracture, especially in carbon steels. High pressure increases stress on vessel walls, requiring materials with higher yield strength. Therefore, materials must be chosen based on their performance envelope matching the service conditions to ensure safety and longevity. Refer to ASME BPVC Part II for material specifications based on temperature and pressure ranges.

A: Toughness is the ability of a material to absorb energy and deform plastically before fracturing, which is critical to prevent brittle failure of pressure vessels. A material with good toughness can resist crack initiation and propagation, especially in low-temperature or impact conditions. It is typically evaluated using impact testing methods, such as the Charpy V-notch test, which measures the energy absorbed during fracture at specified temperatures. Materials for pressure vessels must meet minimum toughness requirements defined by codes like ASME to ensure safe operation.

A: Key standards and resources include the ASME Boiler and Pressure Vessel Code (BPVC) Section VIII for design rules, ASME BPVC Part II for material specifications, and API 510 for in-service inspection. Books like 'Pressure Vessel Design Manual' by Dennis R. Moss and 'Materials for Pressure Vessels' by Samuel Kelly provide comprehensive coverage. Additionally, manufacturer guides, research papers, and software manuals such as for COMPRESS are invaluable. Online platforms like the ASM International website (https://www.asminternational.org/) offer technical articles and materials databases for continued learning.