piping stress

Your question cannot be answered with so little information.
Please give us a sketch or just provide a written description of what is going on here.

A). "I am working on a long pipe"
How long is long?
What size and schedule in this pipe?
What is the material of the pipe?
Is the pipe one straight run or is it a complex configuration with many turns (elbows)?
How many elbows?
(If you cannot provide a sketch then describe the direction and length of each segment relative to Plant North.)

. "connected through series of flanges."
What do you mean by this?
Is the "Long Pipe" made up of flanged spool pieces?
Why do you have multiple flanges in this line?

C). "The pipe is open at one end."
What direction is the end of the pipe pointed? North, South, East, West, Up or Down?

D). "Under very high gas pressure release (500 bar helium)"
Is this constant or intermittent operation?
If intermittent, what is the timing and duration?
What is the Operating Temperature of this "Long Pipe" when it is in operation?

E). "should I be worried about seals at flange area?"
Do you mean the Gasket between two flanges at each one of your "series of flanges"?