If flange faces were perfectly smooth no one would need gaskets. Once the faces were bolted together there’d be no leak paths and the joint would seal perfectly. Fortunately for those selling gaskets and gasket materials, perfection is impossible, at a reasonable price anyway. As a result, it’s important to insert some compressible material, or as we like to say, “a gasket”, between the flange faces. This seals surface imperfections and resists internal pressure, ensuring the joint stays leak-free.
When flanges are bolted together the resulting load on the gasket material is uneven. The outer edges of each flange bend inwards towards the pipe centerline, putting more load on the outer edges of the gasket. As a result, the material compresses more at the outside diameter than at the inside.
The load on a gasket is referred to as the gasket sealing stress. Higher load equates to higher gasket stress. Uneven gasket stress is a bad thing, primarily because more creep relaxation is experienced where load is higher. Especially when coupled with high internal pressures, temperature cycling, and vibration, this leads to reduced service life and higher maintenance costs.
Achieving Uniform Sealing Stress
Gaskets are available which even-out the sealing stress. These are sold as “constant sealing stress gaskets”. They work by placing an incompressible metal annulus, (usually steel,) between the flanges. This creates a minimum gap either side of the annulus, which is then filled with compressible gasket material.
The gasket material, typically PTFE, expanded graphite or vermiculite, is layered onto a metal backing thinner than the main annulus. As the flanges are brought together the gasket material compresses, but only until the flanges close up on the annulus. This prevents uneven sealing stress and results in longer joint life.
Sealing Problem Joints
When replacing a failed gasket examine it carefully for signs of uneven compression. If it looks like the outer edges suffered excessive compression, consider replacing it with a constant sealing stress gasket. The material specialists at Hennig can help you understand your options.