Hi Donnie!

I would agree, You really need to keep the purge going until you have completed the 'Hot-Pass'!

A thought comes to mind that backing gas may be an essencial variable and if the WPS was qualified with backing gas and it was not removed during intermediate passes, you may be restricted yourself?

Check your code for sure on that.

If you are qualifying your procedure now, in *most* cases, if you have deposited enough filler to remove the risk of sugaring the root on subsequent passes, backing gas is often terminated.

Lawrence makes a good point as to one being able to remove backing gas as long as it meets the WPS, PQR, applicable customer requirements, and applicable codes.

BUT, from an economics standpoint, if a system is set up so that it is sealed (for the most part though probably not perfectly) then it would take longer to be turning the system on and off and constantly refilling the applicable pipe area so you are losing all that gas several times over than to just leave it on and keep going. 

Just seems like there may not be any advantage to worrying about rather to turn it off or leave it on.

There were some great systems for sealing up and purging pipe on display at FabTech in Chicago back in Nov.  I'll see if I can locate some of the info I picked up from a couple of them.

Have a Great Day,  Brent