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.

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