[labnetwork] H2 Generators

[Tom Britton]

Something to remember -

The trip point of the excess flow switch is a function of switch size and inlet pressure, so the location where you place it (before or after the regulator) makes a difference. If you place the switch on the pigtail, size the switch so it won't trip when your bottle decays in pressure. Depending on range selected, a switch that is fine at 60 slpm when the cylinder is at 1000 psig, could trip at as little as 10 slpm if your cylinder pressure decays to 100 psig. Always size for the worst case scenario.

Best Regards,

Tom Britton
Sales Manager
Critical Systems, Inc.
7000 W. Victory Road
Boise, ID 83709
Direct: 208-890-1417
Shop:   877-572-5515

[cid:image003.png at 01CD666D.0CFE1920]

www.criticalsystemsinc.com<http://www.criticalsystemsinc.com/>




From: labnetwork-bounces at mtl.mit.edu [mailto:labnetwork-bounces at mtl.mit.edu] On Behalf Of Weaver, John R
Sent: Friday, July 20, 2012 7:35 AM
To: labnetwork at mtl.mit.edu
Subject: Re: [labnetwork] H2 Generators

John -

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.

My apologies to the group for my confusing remark!

John

John R. Weaver
Facility Manager
Birck Nanotechnology Center
Purdue University
(765) 494-5494
jrweaver at purdue.edu<mailto:jrweaver at purdue.edu>

From: John Shott [mailto:shott at stanford.edu]<mailto:[mailto:shott at stanford.edu]>
Sent: Friday, July 20, 2012 10:23 AM
To: labnetwork at mtl.mit.edu<mailto:labnetwork at mtl.mit.edu>; John Weaver
Subject: Re: [labnetwork] H2 Generators

John et al:

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?

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.

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.

Thanks,

John

On 7/20/2012 5:51 AM, Weaver, John R wrote:
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 :).

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