Ammonia Detection – CTI
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Following is a discussion of ammonia detection system design for facilities utilizing ammonia refrigeration systems. Regulations for the ammonia refrigeration industry have dramatically changed in the last 10 years. Beginning with 2021 editions of the Uniform, International, NFPA, and ASHRAE codes and standards, you only need to consider recommendations from IIAR when you evaluate your ammonia detection system for compliance. In addition to IIAR standards, you should always consider any local amendments or supplemental requirements of your AHJ, RAGAGEP, and your insurance company. Many insurance carriers impose their own requirements to mitigate the risk of loss of life and product in a facility. What follows is a system design that meets the requirements all of the above-mentioned influences. This document is updated periodically, and it is recommended that you check our website (www.ctiengineering.com) for the latest revision.
Compressor Room (0-250 ppm sensors)
Code requires audio-visual indication inside the compressor room and outside each entrance to the compressor room at 25 ppm. From the gas detection control panel or PLC, the warning outputs can be set at 25 ppm to activate a horn / strobe unit inside the engine room and outside each entrance. Entrance monitor display units can be located outside each doorway to warn personnel of ammonia concentrations prior to entry. Audio-visual alarms can automatically reset if the ammonia concentration drops below 25 ppm. Code requires emergency ventilation at 150 ppm. The alarm setpoints should be set at 150 ppm and trigger the emergency ventilation fan starter. Emergency ventilation and visual alarms should be latched until manually reset by a switch located in the machinery room. Audible alarms shall continue to operate until they are manually reset by a switch located in the machinery room, or an area remote from the machinery room. The compressor room is the highest risk location in most plants. It has the most potential leak sources, and the most ammonia available for disastrous concentrations. Using a minimum of two 0-250 ppm sensors is necessary for complete coverage and redundancy. Use two 0-250 ppm ammonia gas sensors in Engine Rooms 4,000 square feet or less. Install an additional sensor for each additional 2000 square feet. Locate sensors in the breathing zone ~ 5 feet off the floor. Locate one sensor below the ventilation fan so it samples airflow from throughout the room when the fan is on. Locate other sensor(s) evenly distributed throughout the room.
Compressor Room Shutdown (0-2% sensor)
In the compressor room, code also requires shutdown of compressors, pumps, and normally closed solenoid valves at a very high concentration (20,000 ppm is industry standard) and IIAR allows up the highest output of a fixed sensor not to exceed 4%). The alarm output can be used to shut down at 2% (20,000 ppm). For redundancy at no cost, the warning output can be used to also trigger emergency ventilation at 1% (10,000 ppm). Use one 0-2% sensor located ~ 5 feet off the floor below the emergency ventilation fan so it samples airflow from throughout the room in an emergency condition. Note that a sensor in this range cannot be used to detect lower concentrations covered by the 0-250 ppm sensor.
In refrigerated rooms, code requires alarming to a monitored location. Some insurance companies require shutdown of liquid feed and hot gas solenoids in the event of a leak (but the major codes currently do not). Use 0-100 ppm sensors in these rooms. This range gives the best accuracy at very low concentrations which is appropriate in these unrestricted areas. From the gas detection panel or PLC, the warning output can alarm to a monitored location at 25 ppm. Additionally, the alarm output can be used to shut down the liquid feed and hot gas solenoids at 35 ppm to mitigate the leak. Locate sensors in the breathing zone ~ 5 feet off of the floor. The quantity of sensors should be determined by locating at least one sensor within 30 horizontal feet of each potential leak source (one sensor located
between 2 evaporators could cover them both if they are 60 feet apart). In large, open cold storage warehouse rooms where this results in more than 3 sensors in a room, distances can reasonably be relaxed to 50 horizontal feet from a potential leak source, with a minimum of 3 sensors