[labnetwork] on RFOs

[Iulian Codreanu]

Good Afternoon.

What is your take on having an RFO installed in the gas pigtail while 
also asking the gas supplier to have one RFO installed in the cylinder 
valve?

I used to think it was a good idea (especially if the supplier forgot to 
install one) until someone mentioned that it may hinder the complete 
removal of the hazardous gas from the space between the two RFOs during 
the "purge down" sequence before the cylinder removal.

Thank you for your input.

Iulian
---
iulian Codreanu, Ph.D.
Director, Penn NanoFab
200 South 33rd Street
Room 376 GRW Bldg
Philadelphia, PA 19104-6314
P: 215-898-9308
F: 215-573-2068
www.seas.upenn.edu/~nanofab


On 7/20/2012 1:29 PM, Robert M. Hamilton wrote:
> Colleagues,
>
> 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.
>
> Worth adding to this discussion is the routing of H2 lines and the
> danger of H2 mixtures in closed chambers.
>
> 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.
>
> 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.
>
> Currently, the commonly accepted ratio H2 mixes is 5% in some other
> inert gas.
>
> 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.
>
> Bob
>
> PS as an added note, the worst lab explosion I've witnessed (after the
> fact) was an iced-over LN Dewar
>
>
> Robert M. Hamilton
> Marvel NanoLab
> University of CA at Berkeley
> Rm 520 Sutardja Dai Hall
> Berkeley, CA 94720-1754
> bob at eecs.berkeley.edu
> (e-mail preferred)
> 510-809-8600
> 510-325-7557 (mobile - emergencies)
>
> On 7/20/2012 7:34 AM, Weaver, John R wrote:
>>
>> 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/*
>>
>> *From:*John Shott [mailto:shott at stanford.edu]
>> *Sent:* Friday, July 20, 2012 10:23 AM
>> *To:* 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 J.
>>
>>
>>
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