Thursday, March 06, 2008

Who knew ?

When working in a domain outside one's experience one should proceed carefully. There could be unexpected surprises

I have recently finished a project designing an airlock for a high temperature furnace (1600 C, the melting point of steel is ~1500 C). The airlock is a commercially available vacuum gate valve. To protect it from the furnace heat flux there is a set of thermal barrier shutters which rest on a cold plate.

The thermal barrier shutters consist of tantalum and graphite sheets supported by tantalum studs. The multiple layers of tantalum and graphite sheets act as radiant barriers. The studs are supported by thermal isolators. The thermal isolators are fastened into copper shutter plates. Tantalum has a high melting point (3017 C) but is fantastically expensive. One of the goals of my design was to minimize its use.

I looked at a number of materials for the thermal isolators: titanium, molybdenum, and tungsten. But not nickel or ceramics. I didn't want to have to consider dealing with brittleness and fabrication difficulties of ceramics. I didn't consider nickel simply through oversight.

I presented my design to my customer. He was satisfied with the design. Then he added as an aside, "By the way you don't have any nickel touching any tantalum surfaces, do you?"

It turns out that nickel and tantalum form a low temperature (for the purposes of this project) melting point eutectic with a melting point of 1000 C. So wherever tantalum and nickel touch at elevated temperatures there is effectively corrosion.

I had no idea. Worse I did not even imagine the problem to investigate this failure mode.

Postscript: I changed the design from that described here. The thermal flux through the studs was as great as that through the multiple sheets of the radiant thermal barrier. The heat flux through the studs raised the temperature of the copper uncomfortably close to its melting point.


Anonymous Sylvester said...

I liked this post a lot. It reminded me a little of a story my physics teacher used to tell, about building a jumbo jet. The jet was apparently very very large, and was a quite an engineering feat that everyone involved was proud of. One of the more experienced engineers, not involved in the process, took one look at the plane and commented, "It will never get off the ground in the rain." The engineers involved laughed, but when the it came time to test on a watery surface, the plane wouldn't fly. Apparently, its due to the placement of the wheels relative to the engines, the wheels would pickup the water and shoot streams into the engines! Don't know if it's a true story, but it was one of those things that you can imagine why it would never have crossed someone's mind!

Tuesday, May 25, 2010  

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