One of the negative technological factors that often have a significant impact on the environment is the leakage of operating media (so-called fugitive emissions), which most often occurs at different separable joints. Therefore, high demands are placed on the joints – they must have sufficient strength to maintain pressure and other loads and also be tight enough to avoid undesired leaks. For a large part of such joints, standard flanges or flanges designed according to standards are used, in the European Union in particular according to EN 1092-1, EN 1591-1 and EN 13445-3. Although the strength of such flanges is sufficient for most applications, the basic design methods do not cover the tightness of flange joints and their emissions completely. A case inspired by industrial practice with a history of insufficient tightness resulting in leaks is investigated using FEA and EN 1591. The flange joint is modelled in software ANSYS 2019 R2 with parts of adjacent shells to take their effect into account. The goal of the analyses is estimation of gasket contact pressures resulting from load history. The pressures are directly related to the seal tightness. Lastly, effect of shell shape modification on the gasket pressures is also investigated and the results are compared with those from the original configuration. Effects of resulting pressures on seal tightness are discussed.