Once you run the bead and it is sealed up, it is the radiographers problems. The heavy stuff can mess up pictures. If it gets too runny before the bead is in, see if you can put in a bentonite plug on both sides of the weld. Also dry ice can keep the fire down. The last resort is to run the bottom quarter first, putting the fire on top where you can stay out of the way. Yea folks, sometimes you weld on a burning line. Also, you can try a steam purge, but that runs into the problem of heating up the residue, making a bigger fire. It still rings in my head from years ago on crude oil gathering line replacement, the gang would plug off the line withe what ever was handy, claiche, rocks, cow pies, and all the help I would get from the boss would be, "...better run the bottom before she breaks". Oily bell holes, burning pipelines and welding on loaded LPG lines is why I got into inspection. That plus, age, eyes, knees and belly.
Good luck!
BABRT's