MATERIALS
Balanced safety and relief valve with bellows and closed-type bonnet
Balanced safety and relief valve with bellows and with the name of its components
The most common material for bellows construction is AISI 316 L stainless steel . However , it is important to note that the NACE MR 0175 standard requires bellows to be manufactured from Inconel ® X-625 for applications involving hydrogen sulphide ( H 2
S ). Recent developments in valve technology have introduced balanced diaphragms made from elastomers as an alternative to traditional metal bellows . These elastomeric diaphragms can offer advantages in certain applications , providing flexibility in design , greater flow capacity and potentially reducing costs .
Construction materials for studs and nuts
The construction materials for studs and nuts that clamp the bonnet to the body are typically specified in accordance with industry standards . Studs are commonly made from ASTM A 193 Gr . B7 material , while nuts are usually ASTM A 194 Gr . 2H . The studs are constructed from AISI 4140 steel , and the nuts from AISI 4130 steel . This difference in materials results in different hardness values between the components , preventing locking between them . When the body and bonnet are manufactured from corrosion-resistant materials such as CF8 or CF8M , the studs and nuts do not necessarily need to be resistant to fluid corrosion . This is because these components are external to the valve and , in theory , do not come into contact with the process fluid . However , if the external environment is corrosive , alternative materials may be specified for the studs and nuts . In such cases , ASTM A 193 Gr . B8M for studs and ASTM A 194 Gr . 8M for nuts are commonly used , providing guaranteed corrosion resistance for these external components .
Conclusions
Material selection for pressure relief valve components is critical to ensure safe and reliable operation . All components must meet ASME Section II code requirements and ASME Section XIII - 3.3.1 , demonstrate chemical compatibility with the process fluid and comply with pressure and temperature limits . When choosing materials , it is essential to prioritise corrosion resistance . Corrosion in valves can manifest in various forms , including pitting , component failure and the accumulation of corrosive residues . These issues can interfere with the movement of internal parts and potentially lead to valve malfunction . The selection of appropriate materials helps prevent :
• Deterioration of valve components
• Blockage of moving parts
• Compromised valve performance By carefully considering the specific application requirements and potential corrosion mechanisms , engineers can select materials that offer optimal performance and longevity . This approach ensures the pressure relief valve maintains its integrity and functionality throughout its service life , contributing to overall system safety and reliability . Therefore , the user is responsible for providing the process fluid data to the valve manufacturer for the correct selection of its construction materials .
About the author Artur Cardozo Mathias is an Industrial Mechanical Technician and Chemical Technician , Member of ISA and consultant . He has been active since 1985 in the maintenance , inspection , specification and sizing of valves . With extensive experience in the field , Artur provides training and technical courses to share his knowledge and expertise with others in the industry . He is also the author of the book “ Válvulas : Industriais , Segurança e Controle ” ( Valves : Industrial , Safety and Control ), which serves as a valuable resource for professionals working with valves in various applications .
30 Valve World September 2024 www . valve-world . net