When a high temperature bolted flange assembly was found to be leaking during a nitrogen regeneration process at a petrochemical refinery, two modes of joint failure were identified. There was excessive relaxation of the B16 alloy used in the bolt, and oxidisation of the gasket’s graphite filler material.
At this stage, it was recommended that the bolt be upgraded to B80A Nimonic material and that the gasket design be modified. The modification of gasket was simple enough, but the upgrade of the bolt material would be the cost of the system by over £10,000. Because of this, FEA (finite element analysis) was requested to confirm the initial evaluation and explore alternative solutions.
Flexitallic provided the FEA service. Analysis of the B16 assembly indicated massive bolt stress relaxation and subsequent gasket unloading during the regeneration process, which had caused the joint to leak.
Before changing bolt material, a further analysis of the B16 flange assembly was performed with the lagging surrounding the flange removed. This showed that the bolt temperature was reduced to within operating limits, resulting in more acceptable stress relaxation and minimisation of gasket unloading.
Flexitallic. Tel: 01274 851273.