[labnetwork] H2 Generators

[John Shott]

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 J.

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