I would like to calculate the bending stress in a flange with pressure on one side, bolting connected to connecting flange and sealed between flanges with a gasket.
What's your question, exactly? What are you asking for help with? So you need to be pointed to an appropriate calculation method? What work have you done on your own for research?
You can easily search the internet. There are plenty of sources.
I suggest you get your hands on Appendix 2 of ASME Sec VIII Div 1. That's the most widely accepted method to calculate flange forces and stress.