Material limits of fibre rubber gaskets

There can be many reasons for leaks at flange joints, but recurring leaks are often the result of not addressing the actual cause. One of the causes may be the effect of the chosen gasket material. In particular, the failure to respect the material limits of a given gasket in relation to operating parameters.

Before the actual installation, several questions should be asked. If all the answers are in the affirmative, it can be assumed that the flange connection will subsequently be safe and reliably tight.

• Has a tightening torque calculation been carried out that takes into account the stresses on all parts of the flange connection in its operating conditions?
• Do I have a suitably designed seal that will safely withstand the operating parameters, i.e. temperature, pressure, medium?
• Does the lubricant shown in the calculation match the one I am going to use to treat the fastener? And does this lubricant have measured friction coefficient values according to EN 16047?
• Are the seating surfaces of the flanges free of visible damage?
• Is the fastener undamaged, cleaned and properly lubricated?
• Will the assembly procedure according to EN 1591-4 be followed? And are the installation personnel trained according to this standard?

Nowadays there is no universal sealing material like the asbestos material "Klingerit" used in the past. Each material has its limits and these are determined by its weakest part.

For fibre rubber materials in general, the weakest part is the rubber, i.e. NBR, CSM etc. In particular, the temperature resistance of these elastomers ranges from -20 °C to a maximum of 150 °C. For this reason, the temperature resistance cannot be increased even by adding e.g. graphite, aramid, etc. Nevertheless, it is often argued that these materials can be used 'short-term' even at 350 °C. But can the term 'short-term' be specified?

The ratio of the different components in fibre rubber materials is as follows.

• approx. 15 % graphite, aramid or synthetic fibres
• approx. 25-45 % are elastomeric binders, i.e. NBR, CSM, etc.
• approx. 40-60 % are fillers

In contrast, asbestos gaskets were made up of approx. 80% asbestos itself, with the remainder being made up of elastomeric binders and fillers.

The high proportion of elastomeric binders in fibre-rubber gaskets shows why their substitution for asbestos gaskets is a common cause of leaks. Especially in applications with temperatures above 150 °C, elastomers suffer from hardening, loss of volume (thickness) and elasticity. For these reasons, leakage subsequently occurs at the flange joint (especially where the temperature changes).

As one example from our experience, we could cite the solution to a leak on a high-end steam heater. This is a pressure vessel with temperatures around 350 °C and a steam pressure of 3.1 MPa. The leak always occurred after the operating temperature was lowered. The cause was the use of a fibre rubber gasket which was totally unsuitable for the parameters. We solved the reliability and tightness of the joint by replacing the original seal with a Dynagraph seal with a corrugated stainless steel core and graphite foil._

Spike Heater

Dyngraph gaskets

The parameters given in the material sheets of the suppliers are rather marketing information.

For this reason, the information in the PT diagrams should be read with reservation and with regard to the specific operating and dimensional parameters of the flange connection. Especially for sealing applications around the extremes indicated in the PT diagrams.

Example PT diagram

Seals made of high quality fibre rubber materials have their place in the flange joint. However, they must be used only for the applications for which they are intended.