You will probably by now have found a solution. I will however like to give you some future leads. Even a beginner can find part of a solution by searching the net, even perhaps suggest one or two ways forward. This will greatly help formulating a more specific solution. The second is to be as informative on background of any technical question as possible. In your case fluid type and especially pressure is important.

Answer: In general all soft sealings are recommended changed when flanges and sealings are disconnected. In your case this is probably a cost/safety consideration problem. Alernatives exists, for instance sealing paste applicated on flanges. This is however normally 'permanent' sealings and might be difficult both to demount and rinse and reapply. Serrated flanges might be a possible solution as well as ringtype joint sealings combined with high-temperature sealing rings. I have not checked directly any of the proposed solutions.

Thanks for your answer.
Do you know about the re-usability of metallic c ring?

Your application sounds a lot like the feed lines to our coke drums. On our older systems with unheading equipment, it is necessary to break and make up these flanges about once per day. We use a Grayloc clamp with a re-usable metal sealing element. Our process conditions are about 900 °F and 300 psi. Do an internet search for Grayloc connector or Grayloc seal ring and you should find a lot of information.

Johnny Pellin