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.
Thank you all for the tips. I had a look in the field. This valve cannot be welded since it broke right at the packing. I don't know if it is broken because of the structure of the stem or the valve got stuck.
MechEMain,
it may always be useful to learn from failures: if you could add some more information about the occurrence in object, probably we may suggest something about possible causes and, in the end, we... all may learn something more ;-)
Is the valve a rising stem or a quarter turn one? Small or big diameter? High or low pressure class? Is the packing made of graphite rings?
How about the materials selection?
And the chemical nature of the medium inside?
Did you contact the valve Manufacturer already?
Sorry if I bother you... even if you can't answer the questions above, thank you anyway!!
This valve is rising stem gate valve. It is used in our 50lb condensate. I cannot find the manufacture of this this valve. So I don't know what the stem material is. But there is no bending on the stem. So I don't think it is broken because of bending.
I have a new question, if we remove the top part of the broken stem, can the pressure of the line push the rest of stem and open the valve? The line pressure is running at 50psig. Valve size is 6".
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.
Globe valves are supposed to be installed with the flow coming in under the disc. Hence the stem will always be in compression. Perhaps this valve was installed backward. The pressure should blow the disc to open position with the stem broken.
Throttling with the valve partially open could have caused the plug to vibrate in resonance and fatigue the stem, or rattle pieces against each other to cause galling or fretting, or even simple frictional wear. Isolation valves do not have the positive guiding that throttling valves have designed into them.
otherwise, something must have seized and the maintenance gorillas used a long cheater to try to break it loose.
