UPDATED on 8/21/2019 with the 2018 IFC Commentary…
I am guessing that someone has published a technical paper or article stating that an “electrical shunt” is no longer a code requirement for ammonia refrigeration engine rooms. This has caused several of my clients and subscribers to ask me if this is true as this goes against some of my previous article where I reference the International Fire Code (IFC). This article is NOT an attempt to debate the other paper/article, but I will try to explain why I always design in an “automatic electrical shunt” when I am asked to participate in a design/code review or Process Hazard(s) Analysis (PHA) for a refrigeration process using anhydrous ammonia as it’s refrigerant.
First, let’s be clear, Ammonia will burn. It is a Category 2 flammable gas (GHS); however, under ASHRAE 34 and the IMC it is managed as a “lower flammable” refrigerant with a higher degree of toxicity (e.g. a B2L category). Ammonia has a very high, in relation to most other flammables, Lower Explosive Limit (LEL) of 16% or 160,000 ppm. As well, ammonia has a “higher” Minimum Ignition Energy (MIE) of 680 mJ. Compare that MIE to propane’s <1.0 mJ and LEL of ~2% we can see a stark difference in these refrigerants flammable properties. In other words, it takes a heck of a lot more ammonia and a heck of a stronger energy source to make ammonia go BOOM! And by the way, ammonia does not really go BOOM – more like “whoosh” as it’s flame front does not propagate fast enough for a true “boom”. But it will burn and it will create a pressure wave when it burns strong enough to kill and blow out walls/roofs. We can not forget the 1984 NH3 explosion fatality in Shreveport, LA at a cold storage facility that claimed the life of a FF and severely injured another FF. We have seen other NH3 explosion that caused massive property damage over the years as well.
So why do I always require/suggest an “electrical shunt” at 40,000 ppm?
Simply put, my baseline RAGAGEP, the IFC requires it.
NOTE: this should make it clear that the location of the facility plays a major role in which state/county/city fire code will be in play. If you are in CA, then, by all means, refer to the Uniformed Mechanical Code (UMC). If you are in another state, say Ohio, then this article will be the one you’d want to refer to as Ohio has adopted the IFC.