Protection of the environment and low consumption of resources is more important than ever. Sealed joints produce relatively high emissions, which must be reduced as far as possible for the reasons mentioned above. Since 01.12.2021, for example, the new “Technical Instructions on Air Quality Control (TA Luft)” have come into force and must be implemented. In the course of this, sealing connections made of thermoplastics, which are used in natural gas pipelines, for example, are becoming more important, as proof of tightness must be provided. At present, it is only possible to prove the tightness of thermoplastic sealing joints by experimental means to a very limited extent. Experimental proofs are both cost-intensive and can only be transferred to other sizes of sealing elements to a limited extent. The aim is therefore to develop a cost-effective analytical calculation method for the proof of tightness and strength based on an existing standard for metallic sealing joints (EN1591–1), taking into account the special material properties of thermoplastics. Furthermore, an overriding aim of the work is to show which measures can be taken to reduce emissions as far as possible. In order to adapt the analytical calculation method to the special features of the mechanical and thermal behavior of thermoplastics, extensive experimental investigations and FEM analyses are required for the final modification of the calculation algorithm and the verification of the algorithm. Based on an existing set of rules for metallic sealing joints (EN1591–1) and considering the special elastoplastic and thermal material properties of thermoplastics, the energy-elastic material parameters of steel must be replaced by the elastoplastic material parameters of thermoplastics and the calculation algorithm adapted accordingly. In order to adapt the analytical calculation method to the special features of the mechanical and thermal behavior of thermoplastics, extensive experimental investigations and FEM analyses are required for the final modification of the calculation algorithm and the verification of the algorithm. The first step is therefore extensive research into the state of the art. This includes both the current handling of thermoplastic flanges as well as the underlying standards for the design of steel flanges. In addition, research work must be carried out to model the demanding material behavior in the FEM software. An elementary part of this work will be to investigate the creep behavior in more detail, as this is responsible for the decrease in surface pressure on the gasket. With the help of the simulation results and on the basis of the experimental findings, the analytical calculation algorithm will then be adapted. In the course of the experimental tests, the focus will be placed on investigating the thermoplastic material and its characteristic values. Once the calculation method has been adapted, the tests and simulations can be used again to validate the analytical approach. This allows the possibilities and limits of the developed method to be demonstrated and evaluated. In addition to the simple and cost-efficient design method, optimization potentials for the future should also be better identified and exploited.