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<div class="moz-cite-prefix">Colleagues,<br>
<br>
I'd like to add another note of caution. I too have worked with H2
for a considerable amount of time. Given most lab's volumes I do
not agree a gas cabinet is "best practice" for 180 cu ft cylinder
of H2. Having said this I'd like to stay of out that fray (note
the LEL's for H2/air are ~17%, the LFL's ~ 5%; it doesn't take a
big room to dilute H2 to below these values. <br>
<br>
Worth adding to this discussion is the routing of H2 lines and the
danger of H2 mixtures in closed chambers. <br>
<br>
H2, when routed through lines near a ceiling can pool H2 in
dangerous amounts. For a mocvd installation, done some years ago
in a room with a t-grid ceiling, an exhaust was added at the
ceiling level to prevent the danger of pooling H2, a lighter gas
than air. <br>
<br>
A mixture of H2 and air or another oxidizer, in a closed chamber
such as a furnace tube or bell-jar, is a significant hazard. The
brisance of H2 reactions can result in significant force. Proper
mixtures of Ar/H2 or N2/H2 eliminate this risk. Ratios on the
order of 20/1 are common. A few decades ago I routinely (daily)
used forming gas (N2 80%/H2 20%) in confined spaces (mircowave
tube processing) with open flames. I never witnessed that ratio
burn. That was best known method at that time.<br>
<br>
Currently, the commonly accepted ratio H2 mixes is 5% in some
other inert gas.<br>
<br>
I agree RFO's are a great line of defense. As per John
Shott/Stanford we use the DISS standard for all of our dangerous
gases and incorporated DISS + RFO's early on. <br>
<br>
Bob<br>
<br>
PS as an added note, the worst lab explosion I've witnessed (after
the fact) was an iced-over LN Dewar<br>
<br>
<br>
<pre class="moz-signature" cols="60">Robert M. Hamilton
Marvel NanoLab
University of CA at Berkeley
Rm 520 Sutardja Dai Hall
Berkeley, CA 94720-1754
<a class="moz-txt-link-abbreviated" href="mailto:bob@eecs.berkeley.edu">bob@eecs.berkeley.edu</a>
(e-mail preferred)
510-809-8600
510-325-7557 (mobile - emergencies)
</pre>
On 7/20/2012 7:34 AM, Weaver, John R wrote:<br>
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<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">John
–<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">My
addled brain said it backward! Yes, the flow rate allowed by
the fixed orifice should be higher than the trip point of
the excess-flow sensor. The key is that you need to allow
the sensor to trip in a downstream failure that causes
excess flow while limiting the maximum allowable flow.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">My
apologies to the group for my confusing remark!<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">John
<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<div>
<p class="MsoNormal"><b><i><span
style="font-size:10.0pt;color:#1F497D">John R. Weaver<o:p></o:p></span></i></b></p>
<p class="MsoNormal"><b><i><span
style="font-size:10.0pt;color:#1F497D">Facility
Manager<o:p></o:p></span></i></b></p>
<p class="MsoNormal"><b><i><span
style="font-size:10.0pt;color:#1F497D">Birck
Nanotechnology Center<o:p></o:p></span></i></b></p>
<p class="MsoNormal"><b><i><span
style="font-size:10.0pt;color:#1F497D">Purdue
University<o:p></o:p></span></i></b></p>
<p class="MsoNormal"><b><i><span
style="font-size:10.0pt;color:#1F497D">(765) 494-5494<o:p></o:p></span></i></b></p>
<p class="MsoNormal"><b><i><span
style="font-size:10.0pt;color:#1F497D"><a class="moz-txt-link-abbreviated" href="mailto:jrweaver@purdue.edu">jrweaver@purdue.edu</a><o:p></o:p></span></i></b></p>
</div>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<div>
<div style="border:none;border-top:solid #B5C4DF
1.0pt;padding:3.0pt 0in 0in 0in">
<p class="MsoNormal" style="margin-left:.5in"><b><span
style="font-size:10.0pt;font-family:"Tahoma","sans-serif";color:windowtext">From:</span></b><span
style="font-size:10.0pt;font-family:"Tahoma","sans-serif";color:windowtext">
John Shott [<a class="moz-txt-link-freetext" href="mailto:shott@stanford.edu">mailto:shott@stanford.edu</a>] <br>
<b>Sent:</b> Friday, July 20, 2012 10:23 AM<br>
<b>To:</b> <a class="moz-txt-link-abbreviated" href="mailto:labnetwork@mtl.mit.edu">labnetwork@mtl.mit.edu</a>; John Weaver<br>
<b>Subject:</b> Re: [labnetwork] H2 Generators<o:p></o:p></span></p>
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</div>
<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
<p class="MsoNormal" style="margin-left:.5in">John et al:<br>
<br>
I am also a big believer in excess-flow switches and RFOs in
virtually all cylinders. We typically select a 0.010" RFO
unless there is a good reason to go larger or smaller. It is
normally installed by the gas supplier in the cylinder valve
before it ever shows up at our site ... although we do try to
confirm that the proper RFO has been installed before we
install it. However, I'm a bit confused by the comment about
setting the excess-flow sensor ABOVE the flow rate allowed by
the orifice .... maybe I'm guilty of the mistake that you are
describing. If the excess flow switch is downstream of the
RFO (which it is in all cases for us), doesn't that mean that
the excess flow switch would never trip if it is set higher
than the RFO-limited flow (at least under steady-flow
conditions)? We typically try to size our excess flow switch
so that it will trip at a flow that is 2-3 times higher than
the maximum expected flow but that is typically several times
LOWER than the RFO-limited flow (at least at full cylinder
pressure). For most of our gases, we would likely have excess
flow switches set to trip at 6-10 SLM. Am I off the mark?<br>
<br>
More directly related to the hydrogen discussion, we've
recently changed our hydrogen sensors from LEL to 0-1000 ppm.
While I believe that the legal requirement is to detect a leak
that reaches 25% LEL, all of our hydrogen sensors are now set
to give us a warning at 200 ppm and an alarm at 400 ppm. Our
thinking is that we are better off catching a leak as early as
possible, since small leaks seem to have a way of becoming
large leaks.<br>
<br>
My apologies to all for not staying on the topic of H2
generators ... but whether coming from cylinders, generators,
or a bulk cryogenic source, safe handling and distribution of
H2 is important to us all.<br>
<br>
Thanks,<br>
<br>
John<br>
<br>
On 7/20/2012 5:51 AM, Weaver, John R wrote: <o:p></o:p></p>
<p class="MsoNormal" style="margin-left:.5in"><span
style="color:#1F497D">I’m also a believer in excess-flow
sensors and fixed orifices. It seems obvious (but I have
seen this mistake), but the excess-flow sensor must be set
above the flow rate allowed by the orifice </span><span
style="font-family:Wingdings;color:#1F497D">J</span><span
style="color:#1F497D">.</span><o:p></o:p></p>
<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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