May 2016

How Flanges Influence Gasket Material Selection

If flange and enclosure door surfaces were perfectly smooth and perfectly aligned, gaskets wouldn’t be needed. In the real world though, uneven gaps are always present and must be sealed to prevent leaks or contamination. Sealing options range from inexpensive red rubber and buna N materials to advanced silicone rubber gaskets, and include materials as diverse as graphite, PTFE and paper.

When replacing gaskets it’s common to use the same material that’s just been removed. If joints never change, that approach is often adequate. But by considering the nature and design of the sealing surfaces or flanges, it may be possible to select a longer-lasting material.

Impact of flange material

Some flanges can’t take high clamping forces, especially as they age. Plastics tend to become brittle and some metals lose ductility as they age, particularly if put through repeated temperature cycles. This means a soft, easily compressed gasket material is needed.

Impact of flange geometry

Bolt patterns or the position of clamps and latches can distort the mating surfaces, leading to uneven gaps. For example, an enclosure door with a single central latch can leave large gaps at the corners when closed. Also, a flange that’s been assembled and dissembled repeatedly for many years will start to distort, creating uneven gaps.

Flange alignment can change over time. After years of service it’s possible that piping will have moved, with the result that flange faces are no longer parallel. Again, the result is an uneven gap. Another problem is surface imperfections resulting from careless gasket removal.

These problems demand thicker gasket material that provides more compression. But thicker material needs higher loads to compress down in the joint, and those loads can lead to more distortion in the flanges.

Things change

Flanges and mating surfaces change over time and products that performed well, perhaps red rubber or buna N gaskets, may no longer be up to the job. When replacing gaskets, consider the condition of the sealing surfaces or flanges. A different material may last longer in the joint.


ASTM Testing for Creep Relaxation

Open an electrical enclosure and you may see that the neoprene gasket material has taken on the imprint of the door or cover. In technical terms, it’s taken a compression set. In many gasket applications compression set can lead to sealing problems, due to a phenomenon known as creep relaxation.

Introduction to creep

Apply a load to an elastic material and it compresses. This happens because unlike in a metal, the atoms are linked in a way that lets them move. In gasket materials this is good because it lets the gasket deform to take up the irregularities between the two surfaces being sealed. However, there is a downside to this compressibility.

Rubber and rubber-like materials, as used in neoprene gaskets for example, have the ability to spring back. Release the load and the material returns to its original shape, more or less. Some materials do this better than others. The issue is that the material takes on a permanent deformation, or worse still, continues to deform. This behavior is called “creep” or more accurately, “viscoelastic creep.” It’s related to both the strength of the material and the time and temperature to which it’s subjected.

When creep is a problem

In a bolted joint the compressed gasket creates the torque in the securing bolts. But as the gasket material creeps and the gasket thins, the bolts are able to relax. That reduces the torque and the joint begins to loosen.

Polyurethane, silicone and nitrile gaskets tend to have lower creep than some other materials, as quantified by testing to ASTM 38.

ASTM testing

The principle is to measure the thickness of a sample of gasket material, subject it to load, temperature and time, then release the load. The recovered thickness is measured and the difference used to calculate a percentage reduction.

Taking creep relaxation numbers into account when choosing gasket material.

As with testing to ASTM 36, the absolute test values are less important than the ability to make comparisons between gasket materials. It’s a parameter of particular importance when lasting bolt tightness is essential.