If a valve stem is broken, can it be welded back together in field?

In a sh!tstorm, in the middle of the night, to get through the night, I'll try anything, but I'd rather have a spare.

Just how big and expensive and far away in time is your replacement?

If the welded area goes through the packing, he packing will be destroyed on the first stroke.  If you weld it then chuck it on a lathe and true it up before you reinstall it, you probably have bar/rod on hand that you could use just to machine a replacement. Roller-Burnishing the machined surface both smooths it and puts the surface in compression so it is less likely to crack.   

If the valve is a rotary control valve there is a greater-than-small possibility that the stem is heat-treated (17-4 PH) which will be damaged in the heat-affected zone. You had better get a new, proper metallurgy stem on an airplane to you right NOW.

Since the medium is condensate (as in given steam earlier in the process??), you will have the possibillity of both steam flashing, water hammer and vacuum conditions within the pipeline. All this depending on how the pipeline is equipped for drainage (steamtraps, vacuum relief valves) and how the process is run (in-put -output, amounts and temperature).

All could contribute to extra mechanical forces on the stem, along with temperature variations and corrosion.

Theoretically the disc can under circumstances be forced up.

The stem broken at the packing seems to indicate one, or several in combination, of a series of possibillities:

Prior material weakness / wrong material or construction / faulty machining ?
Broken at point where largest torque forces (?) turning resistance (lower part of stem stuck?)
Stem movement skew (faulty packing, bosses (to wide) etc)
Repetitive forces, weakened material by age / temperature / corrosion at packing?
Valve stem forced by using extra leverage on handwheel?
Valve generally weakened by (repetitive) unfavourable conditions in pipeline.

.... and so on.

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