In this paper fundamental equations of sealing analysis for vessels are given and a computer program named SMEC

which considers the change of stud loading

the elastic contact between flange mating surfaces and the transient thermal effects

is developed

accordingly. The SMEC is verified by several tests. On the basis of analysis

a new concept of classifying vessels into three types according to increasing or decreasing of bolt loading with increasing pressure is suggested. Type-A vessel is that in which the bolt loading increases monotonically with increasing pressure

while in type-B

the bolt loading decreases monotonically

and in type-C

the bolt loading changes nonmonotonically. It is important for vessel design to distinguish the types through analysis. The sealing mechanism is also discussed.