If gasket material isn’t chosen to suit the fluid being sealed, problems are almost inevitable. One reason is that some fluids will make the gasket grow thicker. This is an effect called swell. It increases bolt loads and can lead to material extruding out of the joint. Almost every gasket material has a fluid that will make it swell to some degree.
Bad Combinations
To give one example, an Acrylonitrile Butadiene Rubber (NBR) gasket swells significantly when exposed to acetone or methyl ethyl ketone yet shows almost no growth in the presence of vegetable or mineral oils. Hydrocarbons and petroleum products are a particular problem because they will cause swelling in several widely used gasket materials. EPDM, Styrene Butadiene Rubber (SBR) and Neoprene gaskets are prime examples and should not be used to seal these fluids.
Information on susceptibility to oil swelling is given in the ASTM D2000 classifications for elastomeric materials. This was addressed in “Buna-N (Nitrile) Gaskets and Oil” and “ASTM and Gaskets.” Rubber gasket material sheet properties are essential to know.
Some Exceptions Apply
There are times when a gasket installer might use swell to his advantage. This would be when it’s difficult to get the required level of compression. To give two examples:
- Thinner flanges not meeting ASME/ANSI standards may distort as bolts are torqued, resulting in a variable gap.
- Bolts may lack the thickness or strength to take the necessary loads.
Faced with these problems, choosing a gasket material prone to swelling can be the solution. When exposed to fluid in the pipe the inner region of the gasket will swell, increasing the loading achieved.
Buy the Right Gasket Material!
Harnessing the swell effect doesn’t just mean deliberating selecting the wrong material. This would swell unpredictably, possibly with catastrophic results. However, some gasket material manufacturers produce so-called “controlled swell” material. Often employing Styrene Butadiene Rubber (SBR) binders, these provide predictable growth. (“Controlled swell” material is available for fluids other than hydrocarbons, even water!)
Ask About the Material
If you have a hard-to-seal joint “controlled swell” gasket material might be worth considering. Discuss what’s available with the material specialists at Hennig Gasket & Seals.
