Is the material bolts A193 B8M CL.1 or CL.2?
Bolts A 193 B8M CL.2 (an austenitic stainless material which is carbide solution treated and strain hardened) is classified by B16.5/5.3.3 as Intermediate Strength Bolting, and ASME B16.5 says that such grades "may be used in any flanged joint provided the user verifies their ability to seat the selected gasket and maintain a sealed joint under expected operating conditions."
Well, you verified that ability...
Why not use "High-Strength Bolting" as defined by the same ASME standard?

By the other hand try to have an engineering judgement on what is happening in shut-down and start-up condition and why a strong bending moment(or/and tensile force) would appear in the flange connection.