A shell and tube exchanger is operating with an inlet temperature of almost 900 degree Celsius with a short time from a cold start. The gasket around the tubesheet started leaking after a few of these starts. Red-Bag has executed the analysis of the problem in cooperation with partner Wentzel Dynamics. A FEA model was made for the mechanics of the tubesheet and the flow was implemented to predict the correct thermal effects. The overall FEA investigation was a multi physics quest. The result was that due to the large temperature difference the thick tubesheet started to change shape. Instead of a flat tubesheet the tubesheet became convex shaped. Due to the rotation at the end of the tubesheet gasket facing changed and would deform the gasket. Suggestion have been made to prevent the enormous temperature shock in the heat exchanger. (Petrogas)