Ammonia Gas Detector Settings 2

[NorthernLight]

Does anyone have any reference (local/national law or engineering standard/regulation) that provides guidance (or requirement) for setting ammonia gas detectors at ppm levels for alarm and trip (plant shutdown).
Current understanding is somewhere in range of 25-50 ppm for alarm (based on short or long term exposure limits - depending on juristiction) and 300-500 ppm for trip (based on IDLH - again depends on country).
Involved in anhydrous ammonia storage/vapourisation plant design/build and contractor wants to go straight to 300ppm for trip without any pre-alarm on basis that at 1-5 ppm operators etc can smell the leak.
Facility is in UK but so far can't find anything prescriptive from HSE.
All thoughts appreciated - thanks

 

[danw2]

The Canadian Centre for Occupational Health and Safety has numbers like you state and all its employees get gov't paychecks:

http://www.ccohs.ca/oshanswers/chemicals/chem_profiles/ammonia/working_ammonia.html

TIME-WEIGHTED AVERAGE (TLV-TWA): 25 ppm

SHORT-TERM EXPOSURE LIMIT (TLV-STEL) : 35 ppm

TLV Basis - CRITICAL EFFECT(S): Irritation

 

[DRWeig]

About a zillion MSDS sheets and international safety card here:

http://siri.org/msds/gn.cgi?query=anhydrous+ammonia&start=0

For reference,

http://siri.org

is a great safety site.

Your contractor is not thinking well. IDLH means IMMEDIATE DANGER to LIFE and HEALTH. Would you like to be in the room, injured and unable to move due to some unrelated accident, as NH3 approaches a lethal level? Or would you rather have automated action take place to limit the concentration to the TWA level until help arrives?

If there are people in the room, you're pretty well constrained to the TWA figure, since your detector cannot predict concentrations in the future.

I don't think you will find standards that refer to sensor settings. Those require engineering judgement in each case.

Best to you,

Goober Dave

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