Besides metallic lens ring gaskets for piping applications the high-pressure processes for LDPE production also require gaskets for bolted girth flange connections. A typical application is the 380 bar (5500 psi) head to tubesheet connection of a recycle gas cooler for the LDPE production process. The most common gasket types for this application are either solid ring or spring energized seals. During startup of an LDPE tube-reactor severe ethylene leaks occurred even though the flanged connection had successfully passed the hydrotest at the fabrication shop. A detailed investigation supported by the exchanger fabricator, gasket manufacturer and LDPE process licensor found that connections having line contact stress need to be treated differently from commonly known surface pressure based flat gasket connections. In this paper the author explains the basic considerations for leak tight flange design using spring energized gaskets supported by FEA based root cause leak investigation results. Conclusions are drawn about the appropriate limits for deflection of flanges as compared to results using bolted flat gasket types like Kammprofile gaskets.