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Gasket Thickness Selection

27 Mar 2020 | Posted by Andrew Onions

As a materials manufacturer supplying Rubber Sheeting to the gasket industry, we are often asked for technical advice on the selection of a material for an application.  On occasions this develops further into discussions on Rubber thickness especially where there are challenges with fitting, achieving a sufficient bolt load or the surface finish of the sealing faces.

Gasket installation guides are widely written around semi rigid or metallic gasket materials and recommend the use of thinner gaskets - where possible - however there are circumstances were a thicker rubber gasket is needed, these may include:  

  • Thin flanges that will not be flat when bolted

  • Large diameter flanges on vessels

  • Full faced large diameter flanges built for low internal pressure with limited bolting.

  • Older flanges that may be pitted, warped or damaged in some way.

Thicker rubber gaskets are commonly used in low pressure, large diameter flanges especially when these flanges are not designed with a lot of bolting, and where typically there is no high internal pressure. Consequently, there will be low compression of the gasket, and as thin flanges will often distort when bolted, create little or no compression in the spaces between bolts. In this scenario a thin gasket will not have enough compression available to seal these “less than flat” flanges.

 

When reviewing recommended design installation stresses, these ask for higher loads on the gasket as the thickness increases. But where the design loads are very low, the flange will rarely be thick enough to stay flat and seal a thin gasket. In many cases, the internal pressure on these flanges is also low, so there is not a high risk of blow-out of the thicker rubber gasket.

 

A Typical Application Problem 

A good example of where a thicker rubber gasket material could be better would be a large flange above 1.5mtr diameter with a small number of bolts – say 24, sealing a unit with no internal pressure.

This design will mean that the flange will be quite thin and consequently subject to distortion.

The bolt spacing is so large that there will be very little compressive load in between bolts. In this case thin gaskets would not conform well to bowed flanges and leak paths would be apparent.

There is no disadvantage to using thicker rubber gaskets where there is no internal pressure, because the gasket is not going to blow out.

 

When Thinner is Better 

The situation is very different in flanges that are designed for higher pressures. These flanges have considerably more thickness to them, which means they will typically remain flat when bolted.

In this case the Thinner is Better advice is relevant as the thinner material will have the following benefits. 

  • Higher blow-out resistance due to the smaller surface areas exposed to the internal pressure.

  • Lower leak rates due, again, to the smaller surface areas.

  • Better torque retention in the fasteners, due to the lower creep relaxation characteristics of thinner gaskets.

However, it is not always possible to use thinner gasket materials. Thicker rubber gaskets will conform better to badly damaged or warped flanges. This is because a gasket’s ability to fill in flange irregularities is based on the amount the gasket compresses at a given load. Since compressibility at a particular load is expressed as a percentage of the gasket’s original thickness, a thicker rubber gasket with a larger original thickness will actually compress a larger physical amount.

10% compression of a 1.5mm gasket means a compression of .15mm, where as 10% compression of a 3mm gasket means a compression of 0.3mm.

This extra gasket compression means the thicker gasket will be able to fill-in deep scratches or low spots on the flange surface better than a thinner gasket.

 

It’s not all positives using a thicker gasket

While the thicker rubber gasket will seal more flange irregularities, it can lead to additional problems later. Higher creep relaxation will mean the engineer may need to retorque the bolts to maintain adequate compressive load on the gasket over the life of the joint. This situation is exacerbated by the increased surface area of the gasket exposed to the internal pressure, which creates higher total forces trying to push the gasket out of the joint. Thicker gaskets appear as “taller” surfaces to the internal pressure, which means a larger surface area, therefore pounds per square inch multiplied by greater square inches yields greater pounds of force.

 

Additionally, as all gasket materials are permeable to some degree, media may pass through the body of the gasket. Therefore, thicker gaskets offer a larger path for permeation, and therefore potentially higher leak rates. 

There is an inverse relationship in that if a gasket is too thin to conform to flange irregularities, the media can leak under and over instead of through the gasket, and these pathways tend to generate a higher leak rate than with the thicker gasket.

 

The design criteria for flanges and design calculations for gaskets, including M & Y factors provide scope to eliminate many of the more frequent causes of gasket failure.  Ensuring flanges are well maintained and loading is sufficient for the type of material are of course essential, but ultimately there are factors which cannot be designed in and arguably fall under the term common sense.