This is not the first time I have addressed this matter. Still, somehow, somewhere, a dirty little lie has been circulating among ammonia design and installation contractors that somehow an engine room handling less than 10,000 pounds of anhydrous ammonia (NH3) is incapable of presenting an explosion hazard, thus no need for any engineered ventilation.
This is insane logic and not anywhere near a rational risk analysis! The 10,000 pounds comes from the PSM (and RMP) threshold and has ZERO to do with potential explosion hazards from Ammonia. We could have 3,000 pounds in our system, not be covered by PSM, and yet blow up our engine/machinery room. OSHA does not even have to use their “General Duty Clause” to cite a non-ventilated engine room that is not built to a Class I, Div 2, Group D HAZLOC. And to be clear, the “exemption” that allows an ammonia refrigeration engine room from being a HAZLOC is found in NFPA 70, NOT OSHA; however, OSHA has had a long-standing position on this matter since 1990 (two years BEFORE the PSM standard!). Case in point…