In the high pressure ethylene polymerization process, pipes transmitting reaction product are often bolt-flange connected with metal gaskets which demands high tightness. However ethylene and polyethylene was found leaking from a flange connection in the pipe out of a polymerization kettle after several pours of rain, which leads to an emergency shutdown. A finite element model was worked out to simulate the ratcheting of the metal gasket during the on and off rains by keeping the temperature of the pipe, flange, and gasket as 270°C, but changing the temperature of bolts from higher temperature 270°C to a lower temperature cyclically. Elastic and perfect plastic model was conservatively employed in the ratcheting analysis. It is found that gasket plastic strains accumulate with the bolt temperature drop cycles. The higher the bolt temperature drop, the more plastic strain accumulation. Consequently, the contact pressure between the flange and the gasket reduces with the bolt temperature cycles. The higher the temperature drop, the less the flange connection tightness. It reveals that bolt temperature drop below 155°C does not destroy the flange connection tightness with a gasket parameter 3.0. When temperature drops are above 155°C, temperature cycles to destroy the flange connection tightness decrease with temperature drops, namely the higher the temperature drop, the less temperature cycles needed to destroy the flange connection tightness. A margin line has been worked out to correlate the bolt temperature drops and cycles to the gasket leakage.