Dear experts,

I am doing a stress analysis for existing pipe from HP stripper to pool condenser inter connecting piping. In this existing line they are provided four flanges and here the flange gets leakage and the bolts & nuts gets loosened frequently at the time of shut down and start up condition.

The plant has been installed in 2002. The service of the pipe is ammonia and carbon dioxide supply ammonia feed. The line has a design temperature of 150 deg C and pressure 220kg/cm2.g. The pipe is A312 TP316L Seamless pipe, flanges are A182 F316L, Bolts are A193 B8M & gasket is SS 316L and flange rating is 2500#.

We have checked the flange leakage for above materials, and found ok. Kindly advice me is there any other causes for flange leakage. There is no vibration in these lines during operation time.

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.