Hi Phil
You should be able to butt the ends of the pipe tight together. You will need to purge the pipe with argon. Place a feed line in one end and seal it with tape. Seal the other end of the pipe with tape. Poke holes in the tape with your tungsten,3-4 should do. Your purge flow should be around 3-5 CFH. Allow the pipe (depending on it's length)enough time to purge out the air. Fuse around the pipe with out filler. Your amperage will depend on your travel speed. I would reccomend 35-45 amps to start. You'll need to do some tests on your own to see what you are capable of ( travel speed and control of the puddle). Your shielding gas should be argon with a flow of about 15CFH. I would use a 3/32" tungsten with a long taper ground on it. I usually use a gas lens with a number 7 cup , but a regular set up will work fine. this should give you good results with a little practice.
Good Luck
Jim

In addition to JD369's post, it's been my experiece to pay close attention to your puddle, visual indicators for when complete penetration has been achieved when making homogeneous root beads of complete welds in stainless steel. Too much of a good thing is not always good and welding by "feel" alone will also catch you.

There are several visual indicators that work depending on the welder. One is you can just observe the slight sinking of the puddle. Another is as soon as you observe the "dancing star" in your puddle. Thirdly, if you see a "tear-drop" shaped puddle with a visible fusion line as you start to travel forward. In combination, the "dancing star" at the intitial moment of penetration and watching the "tear-drop" puddle during travel always worked for me. Also, a non-weave travel usually will give better results then a weave such as "cup-walking".

You will also be able to control your depth of penetration by not only your amp/travel speed ratio but by the angle of your tungsten as you would when welding an "open" butt joint.

Depending on the type of sanitary service (some are far more critical then others), you may want to be observant of your "pull-out". Fish-eye's, even shallow ones can render a weld unacceptable.

One other possibility if argon purging is a problem would be a backing flux such as Solar B. Good welding, Scott.

All of the comments suggested here are excellent pieces of advice. One other thing that you could look at is your shielding gas. If you are looking for faster travel speeds with smaller heat affected zone and nicer color match, try using a blend with 2% Hydrogen.... Praxair has this as HYDROSTAR H2 for materials under 3/16" or 5% Hydrogen as HYDROSTAR H5. In a recent test, I noticed on a 3/16" stainless plate outside corner joint with no filler metal,an increase in speed of approximately 140%!!! The puddle was easier to control with fine ripples and beautiful appearance.