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.
(emphasis by me)
2018 IFC, SECTION 605 MECHANICAL REFRIGERATION
605.9.1 Refrigeration system emergency shutoff. A clearly identified switch of the break-glass type or with an approved tamper-resistant cover shall provide off-only control of refrigerant compressors, refrigerant pumps and normally closed automatic refrigerant valves located in the machinery room. Additionally, this equipment shall be automatically shut off when the refrigerant vapor concentration in the machinery room exceeds the vapor detector’s upper detection limit or 25 percent of the LEL, whichever is lower.
We can see that the 2018 IFC has clear intentions that an “electrical shunt” be provided and that it be an AUTOMATIC FUNCTION when the concentration of NH3 rises to 25% of its LEL (40,000 ppm) OR the maximum the NH3 detector can read to accurately. So the requirement for the “emergency shutdown” switch outside the primary door (suggest these be available outside all engine room doors) is the same as the UMC, IIAR, and ASHRAE requirements, but the IFC (note I used the 2018 IFC language above) goes further in requiring the equipment inside the engine room be AUTOMATICALLY SHUT OFF when the refrigerant vapor concentration in the machinery room exceeds the vapor detector’s upper detection limit or 25% of the LEL, whichever is LOWER.
Interestingly enough I was recently working with a refrigeration design company and they had obtained a written letter from the AHJ where the plant was going to be built that stated they wanted, not only the fan and detectors to stay on when the “shunt” was tripped, but they also wanted the lighting and egress signs to remain illuminated. To me, this means that the engine room will indeed be a Class I, Div 2, Group D HAZLOC; at least in terms of the lighting.
SAFTENG Members – See my previous articles where I make the claims that the ventilation fan should always be a rated fan for a Class I, Div 2, Group D location.
The AHJ did respond to our inquiry but only to say they needed to research their position; we still have not heard from them and the design firm chose to follow “industry practice” and not make the lighting meet the HAZLOC requirements, although they did wire the room for the normal lighting to remain powered.
Where did this “shunt” requirement originate from?
For this discussion, we have to recognize that ammonia refrigeration engine rooms have a special exemption that no other flammable gas has. In NFPA 70, ARTICLE 500 — HAZARDOUS (CLASSIFIED) LOCATIONS, CLASSES I, II, & III, DIVISIONS 1 & 2, specifically 505.5 Classifications of Locations these rooms are exempt from being a HAZLOC as long as the area is protected with a ventilation system that will ensure the concentration of ammonia does not exceed 25% (or 40,000 ppm).
505.5 Classifications of Locations.
(A) General. Locations shall be classified depending on the properties of the flammable gases, flammable liquid–produced vapors, combustible liquid–produced vapors, combustible dusts, or fibers/flyings that could be present and the likelihood that a flammable or combustible concentration or quantity is present. Each room, section, or area shall be considered individually in determining its classification. Where pyrophoric materials are the only materials used or handled, these locations are outside the scope of this article.
Informational Note No. 1: See 505.7 for restrictions on area classification.
Informational Note No. 2: Through the exercise of ingenuity in the layout of electrical installations for hazardous (classified) locations, it is frequently possible to locate much of the equipment in reduced level of classification or in an unclassified location and, thus, to reduce the amount of special equipment required.
Refrigerant machinery rooms that contain ammonia refrigeration systems and are equipped with adequate mechanical ventilation that operates continuously or is initiated by a detection system at a concentration not exceeding 150 ppm shall be permitted to be classified as “unclassified” locations.
Informational Note: For further information regarding classification and ventilation of areas involving closed-circuit ammonia refrigeration systems, see ANSI/ASHRAE 15-2013, Safety Standard for Refrigeration Systems, and ANSI/IIAR 2-2014, Standard for Safe Design of Closed-Circuit Ammonia Refrigeration Systems.
When we look at the Ventilation Design requirements from ASHRAE 15-2013 or IIAR 2–2014 we can find one serious flaw… they are NOT based on the LARGEST LIQUID line failing within the engine room, rather the design baseline is a ruptured 1.5-inch high-pressure/high-temperature liquid line. So what happens when the line that fails is larger than a 1.5-inch liquid line? Yep, our ventilation system (when designed to meet the MINIMUM flows per ASHRAE/IIAR, can not keep up; thus the last LAYER OF PROTECTION to prevent ignition is to automatically shut off all ignition sources when the refrigerant vapor concentration in the machinery room exceeds the vapor detector’s upper detection limit or 25 percent of the LEL, whichever is lower.
So we have to ask ourselves… do we have liquid ammonia lines within our engine room(s) that are larger than 1.5″ diameter? How about the Condenser Drain (CD) line? I have seen a lot of 8″ drain lines and some are even 12″ and even bigger. Some of these CD lines are long and should they fail we could drain an entire condenser or two, even with the compressor(s) off. Folks that is a lot of liquid ammonia; thousands of pounds. This scenario would quickly overwhelm the ventilation system and we could pass through the 40,000 ppm range rather quickly and even obtain the LEL (16% or 160,000 ppm) in a matter of minutes. Without a “shunt” we have lost our last line of defense against an ignition event; keeping in mind that the initiating event was the Loss of Primary Containment (LOPC) event.
Bottom line…
KNOW your local code(s)! If you are unfortunate enough to have a facility in a location where there is NO baseline codes such as the IFC, IMC, IBC the engineers should really consider using these ABSOLUTELY bare minimum codes as a baseline for their design. This is doubly important if the process will be over OSHA’s and EPA’s PSM/RMP threshold of 10,000 pounds. Anyone can look up their code obligations at ICC. If the facility is located in a state that has adopted the IFC, check to make sure it is NOT a “revised” version as the state/county/city may have removed this specific requirement; however, even those states that have a “revised code” adopted from the IFC/IMC/IBC, they usually don’t mess with specifics from sections.
UPDATED on 8/21/2019 with the 2018 IFC Commentary…
Some of you have asked about the AHJ in FL that had told the design firm that they wanted the regular lights and egress lighting. This came from the “official code commentary”, sort of like an official interpertation/explanation of the requirement(s). Here is what they say about 605.9.1 Refrigeration system emergency shutoff.
This section of the code is intended to provide a safe environment for emergency response personnel when responding to an incident in a refrigeration room. Shutting down compressors and related refrigeration equipment could be necessary to prevent a hazardous condition from worsening and to allow the room to be safely entered. The emergency “kill” switch must be a tamper-resistant type (similar to manual fire alarm boxes) that requires more than one action to actuate it. To prevent an accidental startup, the switch must be capable of only stopping the controlled machinery, not restarting it. THE SWITCH MUST NOT AFFECT THE OPERATION OF LIFE SAFETY SYSTEMS, SUCH AS DETECTORS AND EXHAUST EQUIPMENT, AND MUST NOT AFFECT ROOM AND EGRESS LIGHTING. In addition to the manual switch, the required refrigerant room detector must also shut down the same equipment when the vapor concentration exceeds the lesser of the detector’s upper detection limit or 25 percent of the refrigerant’s lower explosive limit (LEL).