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Stainless thermowell galling at high temperatures


We have a pilot plant for a chemical process, and expect to build a commercial-sized one soon. Two of our thermocouples cannot be pulled out of their thermowells. Normally, the probes slide in and out of the t-wells with negligible friction. The probes are both nominally 1/4" in diameter and 8 or 10 inches in length. They are mounted horizontally with a spring pressing them against the end of the t-well. One regularly sees temperatures of 960C with peaks of 1000C on occasion. The other sees 550-560C. They also cool to ambient temperature (as low as 10C). I would estimate that they have cycled from 200C to their operating temperatures 80 times, and from room temperature another 40 times over two years. They are subject to some vibratrion. The sheaths and thermowells are all made of 316 stainless steel, and come from a reputable manufacturer. The manufacturer says that this sort of thing is rare.

My question is how to prevent this in our commercial scale plant. I assume that it is a galling issue (well, it definitely galls my boss!) Is electroless nickel plating viable? The probes are also available in:

Hastelloy B
Hastelloy C
304 SST
304L SST
316L SST
Chrome Molybdenum F22
Nickel 200
Alloy 20
Inconel® 600

If I bought a probe with one of these sheaths and inserted it in a 316 t-well, could I be reasonably assured of avoiding seizure? Any other suggestions? Sadly, we have no materials people on staff - and my specialty is electronics.

Ron Stewart
DynaMotive Energy Systems - Vancouver, BC, Canada

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