Tightness and Stress Analysis of GRP Flange Connections of an Extraction Column

Abstract The trigger for the paper was leaks in column flange connections which could not have been eliminated without the measures described below. Pipe and equipment flange connections based on glass fiber reinforced plastics (GRP flange connections) are usually sealed with rubber-based gaskets. These gaskets require comparatively low gasket stresses of around 3 MPa (450 psi) to seal reliably over the long term. However, if, for example, chemically very aggressive media are processed and conveyed, rubber gaskets are often not sufficiently resistant. The only sealing option then remains PTFE based gaskets, which, require significantly higher gasket stresses than rubber gaskets. Depending on the type of PTFE gaskets, it can be assumed that at least 10 MPa (1,450 psi) are required to create a permanently tight connection. Currently, common analytical calculation methods for GRP flange connections only allow low gasket stresses that roughly correspond to those of rubber gaskets. The use of PTFE gaskets is therefore difficult or even impossible. However, if PTFE gaskets could be used across the board, this would open up the possibility for the chemical industry to use GRP connections on a much larger scale. For the reasons mentioned above, the paper shows which measures and procedures are required to be able to use PTFE gaskets without reducing the safety of the connections, so that permanently tight connections can be made. In the first step, the causes of the above-mentioned leaks in the column flange connections are described in an anonymous form. On this basis, the analytical calculation methods currently used for GRP flange connections are examined and their limits regarding the sealing surface pressure are described. At the same time, the different requirements for rubber and PTFE gaskets are discussed. It is inevitable that analytical calculation methods will continue to be used in the future, as FEM calculations, for example, are too complex and cost-intensive. It is therefore shown that a slightly modified proof of tightness and strength according to EN 1591-1 does indeed allow a significant increase of the bolt forces and thus the gasket stresses. In the present case, this was achieved for several connections of sizes DN700 to DN1200 (28″ to 48″) by at least a factor of 4. The analytical calculation was accompanied by intensive experimental investigations at ambient and operating temperatures. FEM calculations were carried out to further validate the results. The results of the analytical calculation, the FEM calculation and the experimental results are compared. Based on the results, it is shown which measures will be required in the future to enable calculation according to EN 1591-1 using PTFE gaskets, with the aim of expanding the area of GRP flange connections. One example of these measures is that metal loose flanges should be used. There are also optimization options when using thinner bolts, which increase the flexibility of the connection and can thus counteract a loss of bolt force. It also shows what optimization options there are when using the gaskets themselves.