Stud Bolt Thread Engagement: A Fitness for Service Approach

In the pressure equipment industry, codes of construction stipulate that fastener assemblies shall exhibit thread engagement through the full depth of the nut. In many older facilities and in locations with poor quality control, assemblies may be found on in-service equipment which do not have full thread engagement and do not meet the requirements of the code of construction. This is often found during the routine visual inspection of equipment and is flagged as a construction code deficiency requiring attention. Current post construction code guidance on fastener assemblies is ambiguous on how to handle fasteners with less-than-full thread engagement exceeding one thread of shortfall. The perceived risk of such a lack of thread engagement may lead maintenance organizations to perform costly and high-risk remediation activities, such as in-service “clamping” or “single stud replacement,” also known as “hot-bolting.” However, in many situations, fastener assembly strength can be proven to be sufficient for a given joint through analytical evaluation. Using a combination of innovative and existing thread strength and flanged joint considerations, the minimum required length of engagement, and, thus, the maximum allowable lack of engagement, can be safely determined for most fasteners on flange connections. This paper reviews some of the most important fastener strength considerations to propose a post-construction evaluation technique to determine the fitness for service of less-than-fully engaged fastener assemblies.