Purging closed ends furnace pipe

Hi!
I need some help on how to purge furnace pipe for welding with TIG. Never done it and the situation is this;

Metal:CR 2.5%, MO 1%.
There is no open pipe end to insert a purge tube or any sort of dam. The only opening will be the weld joint.

Questions:
1-How do I purge it?
2-Does a ball of rice paper at each side of the weld joint be enough to contain the gas confined to the welding area?

Thanking you for your prompt answer.

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one idea would be to use purge paper a few inches on each side of the weld joint and then insert a small diameter copper tube until the very last inch of root before removing it. would be best inserted in bottom.

purge paper is water soluble and water soluble tape is also available to make a purge dam.

also that alloy is sometimes tig welded without using a purge. I am sure it will go better with a purge but the need for it depends on code requirements.