[labnetwork] point of detection - Gas sensors

Thu Feb 12 10:49:19 EST 2015

Hi Matthieu,
While I understand the arguments against environmental monitoring noted below by John and Vito, in many cases it is required by the building and fire codes.  It will depend upon the occupancy classification of the spaces (cleanroom, gas room, and any intermediate spaces where you are piping the gases).  And of course it depends upon the building and fire codes applicable to your location.

The International Building Code and the International Fire Code will require sensors in the breathing air of the room if you using toxic or highly toxic gases in an H-5 Occupancy.  H-5 Occupancies are defined as “semiconductor fabrication and related or similar research operations”.

Before omitting the breathing air sensors, please check building code requirements at McGill.

In all other respects, I agree with comments below, and with John Schott’s summary as well.

Great discussion!

Cheers,
Jack

From: labnetwork-bounces at mtl.mit.edu [mailto:labnetwork-bounces at mtl.mit.edu] On Behalf Of Vito Logiudice
Sent: Wednesday, February 11, 2015 7:10 PM
To: 'Matthieu Nannini, Dr.'; Labnetwork
Subject: Re: [labnetwork] point of detection - Gas sensors

‎Hi Matthieu,

I share John's opinion in regards to environmental monitoring. A personal experience many years ago while working in the private sector reinforced my opinion that room sensors may not be sensitive enough under some conditions and thus give people a false sense of security as John points out. Someone managed to tear into a H2 line in a class 10 ballroom style cleanroom and the room sensor in that particular instance did not pick it up.‎

We've stayed away from room sensors in Waterloo and we've placed our sensors in  the same locations John described. You'll have the opportunity to see these for yourself when you come visit us.

Cheers,
Vito

From: Weaver, John R
Sent: Wednesday, February 11, 2015 7:51 PM
To: 'Matthieu Nannini, Dr.'; Labnetwork
Subject: Re: [labnetwork] point of detection - Gas sensors

Matthieu –

I have quite strong feelings in this regard, based on a lot of experience in this area.

I use, and highly recommend, the following scheme:

A sensor in the exhaust of the gas cabinet, a minimum of four feet from the gas cabinet outlet. This maximizes turbulence in the exhaust to ensure that the sensor “sees” the leak. Note that the purge line into the exhaust should be at least two feet beyond (downstream of) the sensor connection. Otherwise you could see alarms every time you purge :).

A sensor in the exhaust of each VMB using the same criteria.

A sensor in the exhaust of each piece of equipment using the hazardous gas. If the gas has the potential of being in multiple chambers of the equipment with different exhausts, each exhaust needs a sensor.

The lines in between the gas cabinet, VMB, and equipment should be doubly contained. I highly recommend a static, inert atmosphere in the interstitial that is set at ½ the delivery pressure of the supplied gas. This pressure is sensed real time. There are three possible excursions: 1. The pressure decreases. This indicates in a leak in the outer containment. Automatic notification of appropriate staff results. 2. The pressure increases. This indicates a leak in the delivery line. Automatic notification of appropriate staff results, with a priority to assess the potential hazards. 3. The pressure immediately drops to atmospheric pressure. This indicates a catastrophic failure (such as a fork truck catching the line and ripping it down :(), resulting in immediate evacuation of the facility.

I have had no success with “environmental monitoring.” With the air changeover rates, it is very rare that a leak would be detected –in the cleanroom, chase, subfab, or the gas rooms. In my experience, these sensors lead to a false sense of security and are therefore more dangerous than not having sensors.

That’s my two cents,
John

From: labnetwork-bounces at mtl.mit.edu<mailto:labnetwork-bounces at mtl.mit.edu> [mailto:labnetwork-bounces at mtl.mit.edu] On Behalf Of Matthieu Nannini, Dr.
Sent: Wednesday, February 11, 2015 12:40 PM
To: Labnetwork
Subject: [labnetwork] point of detection - Gas sensors

Colleagues, first thanks Vito for initiating this discussion. Very important points where made which led me to explore the labnetwork archives about sub-atmospheric setup and TGMS. Fore those interested I will save you the search:
https://www-mtl.mit.edu/pipermail/labnetwork/2012-August/000541.html
https://www-mtl.mit.edu/pipermail/labnetwork/2013-August/001004.html
https://www-mtl.mit.edu/pipermail/labnetwork/2014-July/001346.html
https://www-mtl.mit.edu/pipermail/labnetwork/2013-August/000998.html

Since we are in a gas discussion timing,

If you had to prioritize the following position of the sensors, what would you recommend ?

- exhaust of gas cabinet
- gas cabinet at the tool
- VMB if any
- exhaust of pump after the tool ?
- exhaust after abatement system ?
- free space sensors scattered around most sensitive areas: where human presence is usually high

Thanks

-----------------------------------
Matthieu Nannini
McGill Nanotools Microfab
Manager
t: 514 398 3310
c: 514 758 3311
f: 514 398 8434
http://mnm.physics.mcgill.ca/
------------------------------------

Le 2015-02-11 à 10:53, Vito Logiudice <vito.logiudice at uwaterloo.ca<mailto:vito.logiudice at uwaterloo.ca>> a écrit :

Hi Dennis,

Great insights – thanks very much for sharing.

Our aim is to avoid cold spots and keep the entire system at 19C to 20C, especially since the DCS line traverses a loading dock between the gas bunker and the fab. The two roll-up dock doors are equipped with heated air curtains but we wanted the added insurance of a heated/insulated line.

In our particular case, we've got a single 120 foot line between the gas cabinet and the point of use (no VMB's) and we did our best to stay true to the use of large radius bends all along the run. The DCS panel design was kept as simple as possible (no regulator) and the entire cabinet is located in a heated bunker in which temperature trends are monitored.

Good point about the possible risk of fire. While the heat trace controller is capable of outputting a limited amount of power, we did see some odd "burn" marks at some locations which lead us to conclude that the Armaflex insulation's upper use limit of 105C may have been exceeded at some of the void locations. In light of these findings we've decided to use fiberglass insulation instead of Armaflex for the repair.

Best,
Vito

From: Dennis Grimard <dgrimard at umich.edu<mailto:dgrimard at umich.edu>>
Date: Tuesday, 10 February, 2015 10:32 PM
To: Vito Logiudice <vito.logiudice at uwaterloo.ca<mailto:vito.logiudice at uwaterloo.ca>>
Cc: Labnetwork <labnetwork at mtl.mit.edu<mailto:labnetwork at mtl.mit.edu>>
Subject: Re: [labnetwork] Conclusion: Heat trace issues on DCS gas lines

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<mailto:dgrimard at mit.edu>

On Feb 10, 2015, at 1:30 PM, Vito Logiudice <vito.logiudice at uwaterloo.ca<mailto: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<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<https://fab.qnc.uwaterloo.ca/>

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