[labnetwork] Conclusion: Heat trace issues on DCS gas lines

[Vito Logiudice]

Dear Colleagues,

Thank you very much to everyone whom took the time to write in with their insights on this issue. Special thanks to John Shott and Tom Britton for the photos and reference documents provided.

So that others may perhaps benefit from our experience, we've concluded that the cause of the premature failure appears to have been the presence of several "voids" where the heat trace was not in intimate contact with the SS tubing. This occurred even though the trace had been taped every 12 inches per the manufacturer's recommendations. We also noted voids at some elbows where maintaining contact was/is difficult.

To keep the issue from repeating itself in the future, our plan is to reinstall two new heat traces along the length of the tubing, one on the bottom and one on the top. One of these will remain active while the backup trace will be kept off and act as an insurance policy should the primary unit fail in the future. If anyone sees a problem with this particular approach, I would be glad to hear from you.

In the new installation, conductive putty will be used to fill any voids before aluminum tape is applied along the entire length of the line much like John showed in his attached photo. The entire assembly will then be re-insulated per the original design specification. Fortunately, the problem occurred under warranty so our only out-of-pocket cost will be limited to the cost of the backup heat trace (a few hundred dollars).

Regards,
Vito

From: Vito Logiudice <vito.logiudice at uwaterloo.ca<mailto:vito.logiudice at uwaterloo.ca>>
Date: Wednesday, 21 January, 2015 12:23 PM
To: Labnetwork <labnetwork at mtl.mit.edu<mailto:labnetwork at mtl.mit.edu>>
Subject: [labnetwork] Heat trace issues on DCS gas lines

Dear Colleagues,

We are experiencing an issue with the heat trace on our Dichlorosilane gas line. The all-welded 1/4" SS line is encapsulated with a 1/2" SS outer containment  line which is itself heat traced with a single strand of heat trace that runs the entire length of the coax assembly. The 120 foot line is insulated as shown in the attached photo. A portion of the heat-trace appears to have failed prematurely (it was installed less than one year ago) and we are wondering if the method of installation may be the cause.

The heat trace was not installed in a spiral fashion around the outer 1/2" tube. Rather it was installed in a straight fashion along its entire length with "heat trace fastening tape" located every four feet or so. A member of my team has suggested that such a straight rather than spiral installation may have caused hot spots (at the fastening locations) which may have in turn caused the failure.

I would appreciate hearing from the community on this point: Are the heat traces around your low pressure gas lines spiral-wound around the lines or are they installed in a straight fashion and somehow fastened along the entire length?

Other insights/suggestions on the proper heat tracing of gas lines by experts in the field as well as comments on possible causes of premature heat trace failure are very much welcome and appreciated. Thank you.

Regards,
Vito
--
Vito Logiudice  P.Eng.
Director of Operations, Quantum NanoFab
University of Waterloo
Lazaridis QNC 1207
200 University Avenue West
Waterloo, ON           Canada N2L 3G1
Tel.: (519) 888-4567  ext. 38703
Email: vito.logiudice at uwaterloo.ca<mailto:vito.logiudice at uwaterloo.ca>
Website: https://fab.qnc.uwaterloo.ca

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