Well,I know that hot taps can be safe and they are common.
However, the welder would rather be swimming in a pond full of alligators. The split tee at least "feels" better.
Depending on several things such as
fluid,pressure,time ....weld,weld-o-lets on the vessel walls at 180 degree locations. Thread thru small nylon or teflon rods between the tubes. Tye off and plug with bull plugs.
http://books.google.com/books?id=AOCn1A3B4LoC&pg=PA663&lpg=PA663&dq=flange+leak+rate&source=web&ots=whX8EfWTqz&sig=c-oPNjHfKmI-iIAU10W4UGtYIBI#PPA662,M1
http://www.flowcontrolnetwork.com/issuearticle.asp?ArticleID=191
If you have leaks, you do have problems. It will likely get worse.
What size gas cup are you using? Too much or too little flow.
Do you have any cooling water leaks?
Soap all your gas lines and fittings, may have a pin hole or thread leak.
Is your liner good?
Are you using Anti-spatter? To much is bad.
Any fans or vents in the area?
Leave a bowl of milk...
Over the years I have asked to see all kinds of Certs.
Seldom a problem, but sometimes it was found that what the manufactuer got was not at all what they had ordered.
The paperwork should be part of the deal and if the supplier complains or resists, be very cautious.
Got a fax in one time...
How bad is it? Does it have a seal? Have you contacted the original supplier?
Can you recut to a standard thread? Can you cut off and replace?
If you get in a pinch, sit down at bench and hand work them out, awl, Dremel, bent files. Been there before.
Why not install a vessel that has a MAWP to handle the spikes?
You risk burst or collapsed tubes, seal failures, vessel failure.
Sounds like a poor practice to me.
The calcs. are good to start. But are you bending with a press brake or a leaf? Hot or cold? Free form or die? With the grain or across?
These all make a difference. Go to the Fab Shop and ask. If you need something special, pay for a few samples.
http://www.infratrans.gov.ab.ca/INFTRA_Content/docType306/Production/TechPres28.pdf
http://www.google.com/search?hl=en&q=converting+a+closed+loop+system+to+glycol&btnG=Google+Search
Have a look.
