"My bolted flange connection continuously experiences leaks, even after trying several different product types. What can I investigate as a potential root cause?"
Performing diagnostics on your leaking connection can be challenging due to the enormous variations of service conditions that exist at any given facility. In part one of this series (March, pg. 34), we looked at chemical attacks, and in this second part, we will begin to look at mechanical attack modes.
Mechanical damage
Example: The gasket is excessively extruded. Extrusion is typically seen on soft gaskets, though it can be experienced with some semi-metallic gasket designs. Gasket extrusion typically occurs when a gasket is loaded with a seating stress beyond what its supporting matrix can handle, with the result that the gasket creeps or flows radially. Remediation of this situation most commonly involves selecting a replacement gasket with better cold-flow properties or better load-carrying capacity.
For semi-metallic gaskets, extrusion can be reduced by minimizing the clearance between mating flange parts (common for tongue and groove applications), bridging the clearance between mating flange parts (using inner and outer guide rings), and by increasing the gasket ID so that it is greater than the flange bore diameter.
Example: The gasket is extensively crushed. Extreme gasket compressive loads manifest differently in soft and semi-metallic gaskets. In soft gaskets, overloading will often cause the gasket to split in a shear-like pattern as it tries to dissipate the applied load. In semi-metallic gaskets like spiral-wound gaskets, overloading manifests via buckling of the winding element, regardless of the use of industry-recommended inner rings.
Gasket crushing can be reduced by selecting a more robust material able to withstand higher compressive loads; however, the preferred remedy is to analyze the applied torque to the fasteners and determine if a reduced load can be applied.
Example: The gasket is substantially (>25-percent) thinner on the OD than the ID. This condition is the result of flange rotation or bending due to overtightening the fasteners, weak flanges or a combination of both. It is often visible to the naked eye, but the magnitude of the situation is more easily determined by using a micrometer to measure the gasket thickness at various points on the ID and OD.
Flange rotation can be minimized by altering the gasket dimensions such that the gasket-reaction diameter is closer to the bolt-circle diameter. Flange stiffness can also be improved by adding backup rings on the backside of the flanges, where the bolts are inserted. Sometimes the selection of a softer gasket material with lower required seating stresses and/ or reducing the gasket seating area will provide the necessary flange stress relief to minimize rotation.
Understanding the bigger picture
It is important to recognize that the investigation you need to undergo is larger than just looking at the gasket. The gasket can provide many clues as to the root failure cause, and understanding how these failure modes work can significantly improve your future chances of secure and leak-free bolted flange connections. In the final part of this series, we will continue to look at the most common mechanical attack failure modes.
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