Material limits of fiber rubber seals

There can be many reasons for leaks on flanged joints , but a recurring leak is usually due to failure to eliminate its true cause. One of the causes may be the influence of the selected sealing material. In particular, non-compliance with the material limits of the seal in relation to the operating parameters.

Before mounting, we should ask ourselves a few questions. If all our answers are positive, it can be assumed that the flange connection will subsequently be safe and reliably tight.

Has a tightening torque been calculated that takes into account the stress of all parts of the flange joint in its operating states?
Do I have a suitably designed seal that can safely withstand the operating parameters, ie temperature, pressure, medium?
Does the lubricant listed in the calculation match the one I am going to use to treat the fastener? And does this lubricant have measured values of coefficients of friction according to EN 16047?
Are the flange seatings free of visible damage?
Is the fastener undamaged, cleaned and properly lubricated?
Will the installation procedure according to EN 1591-4 be followed? And are the assembly staff 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 fibrous rubber materials, it is generally true that the weakest part is rubber , ie 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 eg graphite, aramid, etc. Nevertheless, we often encounter the argument that these materials can be used "for a short time" at 350 ° C. But is it possible to specify the term "short-term"?

The ratio of the individual components in fibrous rubber materials is as follows.

Approx. 15% consists of graphite, aramid or synthetic fibers
Approx. 25-45% are elastomeric binders, ie NBR, CSM, etc.
Approx. 40-60% are fillers

In contrast, asbestos seals were formed from approx. 80% with asbestos alone and the rest was then supplemented with elastomeric binders and fillers.

The large proportion of elastomeric binders in fiber rubber seals shows why their replacement by asbestos seals is a common cause of leaks. Especially in applications that exceed 150 ° C in temperature, elastomers harden, lose volume (thickness) and be elastic. For these reasons, a leak subsequently occurs on the flange joint (especially where the temperature changes).

From our practice, we could cite the solution of a leak on a top steam heater as one example for all of them. It is a pressure vessel with temperatures around 350 ° C and a steam pressure of 3.1 MPa. Leaks always occurred on this device after the operating temperature had dropped. The reason was the use of a fibrous rubber seal, which was completely unsuitable for the stated 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.

Top heater

Dyngraph seal

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

The information in the PT diagrams must therefore be read with caution and with regard to the specific operating and dimensional parameters of the flange connection. Especially for sealing applications around the extreme values given in the PT diagrams.

Example of PT diagram

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

At the end of a series of articles devoted to the topics of sealing and tightness of flanged joints, we are preparing a small surprise for you. We will bring you more information in future issues of newsletters.