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

Tue Feb 10 22:32:00 EST 2015

Vito:

I have watched with great pleasure the discussion on this topic.  I too agree that much good info has been discussed ... Great feedback from some very knowledgable people indeed.

I need to throw a wrench in the discussion (or prove my ignorance).  I have always resisted heat taping for the following reasons:  1) when the tube enters a VMB or any ventilated enclosure there is a significant temperature drop due to the large purging flow rate within the enclosure ... Tending to cool the line at the worst possible point, 2) the VMB type enclosures tend to have many right angle welds and valves which promote condensation .... Rather than long graceful bends typically used external to the enclosure, 3) SS is a horrible heat conductor ... As is n2 gas ... So if I heat trace a double wall tube how much heat actually gets to the inner tube?  how consistent is that heat?  What is the temperature gradient?, and 4) the actual cold to hot temperature gradient (desired) is difficult to institute along the length of line ... A good feedback loop is required.  Also, heat tape gives me the district impression that it can contribute to an out of control heating failure with a possible fire as a result.

So, not that it solves your problem but here is what I have tried to always implement: 1) short runs (home runs not a distribution), 2) minimum short radius right angles, 3) minimize VMB's ... Mini gas cabinets with multiple outputs in the cabinet, 4) chilled bottles, 5) vacuum delivery, and 6) large radius bends.

Just food for thought ...

Dennis S Grimard, Ph.D.
Associate Director of Operations, MIT.nano

Massachusetts Institute of Technology
60 Vassor Street, Bldg 39-556
Cambridge, MA 02149

C:     (734) 368-7172
EM:  dgrimard at mit.edu

> On Feb 10, 2015, at 1:30 PM, Vito Logiudice <vito.logiudice at uwaterloo.ca> wrote:
> 
> 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>
> Date: Wednesday, 21 January, 2015 12:23 PM
> To: Labnetwork <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
> Website: https://fab.qnc.uwaterloo.ca
> 
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