What is a Constant Seating Stress Gasket?

18 November 2019

Constant seating stress gaskets function as flange-interface fulcrums. That’s a difficult term to interpret, especially for non-engineering types. Picture a gasket with a uniform carrier ring. This incompressible metal annulus absorbs the bulk of sealing stress as a ring of pattern-tightened bolts pulls two flange faces together. Cleverly placed at a stress-neutral location on the gasket face, the annular acts as a ring-shaped pivot zone, one that evens out any and all mounting stress.

Following Flange Face Trajectories

It’s a simple enough movement, isn’t it? Two flanges come together, bolts tighten, and a gasket compresses. A perfect seal is produced. Only, that’s not really what happens in real-world gasketing applications. When those two faces meet, they actually deform slightly, right at the outer edges of the flanges. They bend slightly as they compress a gasket. With more strain pushing the outer surfaces, the inner section of the joint experiences a reduction in seating uniformity. It’s like the seating load is splitting into pressure bands. Out at the furthest edges, the load is highest. Moving inwards, though, the seating load drops off precariously.

Constant Seating Stress Compensation

Granted, this effect is imperceptible when measured on a pipe and gasket joint that uses small diameter flanges. What, however, if the flanges are wide and flat? Flange deformation is the cost here. By fitting constant seating stress gaskets, we counter this seal undermining effect. The steel annular, the centrally positioned ring, which protrudes a few key millimetres outwards, performs as a seating compensation feature. Of course, the ring isn’t meant to function as a pliable seal. This is a rigid section of incompressible metal. To complete the gasket, the annular needs one or more additional rings, which are typically fabricated out of PTFE or some other similarly high-functioning gasketing rubber. At any rate, once the flange sections on a stress-susceptible pipe joint do tighten, this artificial fulcrum is right there, centrally positioned as a stress-mitigating fulcrum, and those flanges won’t deform.

Different issues crop up when flange forces aren’t uniformly distributed. Creep relaxation problems and flange compressibility effects climb dangerously high because of the uneven loading. On-site technicians can see the consequences. They’ll see that the flange edges are pulled tighter towards one another. There’s no way the gasket between those faces can be uniformly compressed. On taking the joint apart, more signs of uneven compression are spotted on the gasket and the flanges. If these sealing defects are confirmed, the fitter really should replace the seal with a constant seating stress gasket, one that uses a steel annular as a load balancing fulcrum.

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