September 2018

What is a Constant Seating Stress Gasket?

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.

Uneven Loading

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.

Understanding Stress Relaxation and Torque Loss

Compression is an important part of getting a gasket to work. Closing the joint up tight holds the gasket in place and helps it resist internal pressure. Joints often work loose over time though, and that leads to leaks and even blow-out. Here’s how to reduce the chances of this happening.

Understand the Joint

After putting the gasket between the flanges the bolts are fastened. As torque increases the bolts stretch, creating a load that pulls the flanges together. That compresses the gasket material and pushes it into irregularities on the surfaces.

Over time some of the stretch put into those bolts becomes permanent. Take them out and measure them 24 hours later and you’ll find they’ve lengthened slightly. In addition, the gasket material deforms in a process known as creep, becoming thinner while the outside diameter increases and the bore shrinks.

High temperatures, as might be caused by environmental conditions or the media being sealed against, accentuate these effects. Bolts expand, reducing bolt torque, and creep increases. Temperature cycling can accelerate this loss of torque.

Vibration is another problem. A pulsing pump or water hammer can quickly loosen the joint and lead to leaks.

Bolting Procedure

Counter relaxation by following correct bolt tightening procedure, as detailed in “How to Bolt Flanges”. Conical spring washers can help maintain load as bolts lengthen. In addition, some people suggest torquing-up the bolts, then releasing and retorquing. They argue that this “conditions” the gasket material.

Material Selection

Some gasket materials resist creep better than others. Silicone and nitrile rubber are particularly good, as are compressed non-asbestos materials that incorporate a nitrile binder. Conversely, PTFE is a high-creep material.

Also consider gasket thickness. Creep is proportional to thickness, so using a thinner gasket results in less loss of bolt torque.

Use the right material

A gasket that’s not installed properly will almost certainly leak. Taking steps to counter stress relaxation and torque loss, as detailed here, will help extend joint life. To learn more about the part played by gasket materials, call or email the specialists at Hennig Gasket.