The use of ammonia (R717) as a refrigerant has increased substantially over the past several years as a replacement for environmentally unfriendly chlorofluorocarbon refrigerants. While ammonia refrigeration has long been a standard in the food and beverage industry, it also is now found in pharmaceutical production and in electric power generation plants. However, ammonia is a highly toxic gas, and proper safety monitoring procedures and equipment must be in place at all times to avoid serious accidental injury or death.
In the United States, the use of monitoring instrumentation to provide an early warning of releases of “highly hazardous” chemicals such as ammonia is required under 29 CFR 1910.119, “Process Safety Management of Highly Hazardous Chemicals.” It is up to each plant, however, to choose the most suitable type of instrumentation for a particular process. Making the right decision requires evaluating the benefits and limitations of the various sensor types against the needs of the plant.
When evaluating different sensors, be sure to consider both the benefits and drawbacks of each type. For example, substance-specific electrochemical ammonia sensors are extremely selective and do not tend to respond to other interfering contaminants (like the exhaust from a propane-powered forklift) that might be present in the same environment, thereby substantially reducing the chance for a false alarm. Electrochemical ammonia sensors also are extremely accurate, with some sensors able to detect ammonia leaks at concentrations less than 1.0 ppm.
However, these sensors detect gas by means of a chemical reaction that consumes an active ingredient in the electrolyte of the sensor. As a result, the lifespan of the sensor (generally listed in parts-per-million exposure hours [ppmh]) is directly related to its exposure to ammonia. For instance, a 17,520 ppmh ammonia sensor has a lifespan of one year when continuously exposed to a constant concentration of 2 ppm of ammonia. (2 ppm x 365 days x 24 hours = 17,520 ppmh.) However, the same sensor will last only six months if it is continuously exposed to 4 ppm, or three months if exposed to 8 ppm. If the sensor is installed in an area with a background level of 15 or 20 ppm ammonia - a common condition in the immediate vicinity of some ammonia compressors - it might need to be replaced every few weeks. For this reason, electrochemical sensors should not be installed in areas with high background concentrations of ammonia.
CI sensors depend on the adsorption of ammonia by “charge-carrier” molecules in a solid-state substrate. By absorbing ammonia, the charge carriers are “injected” into the sensor element, causing a change in resistance that is proportional to the concentration of ammonia present.
The materials in the sensor element are all carefully selected to maximize the affinity of the sensor to ammonia while minimizing the effects of interfering contaminants. CI sensors can be used over a wide detection range, from 30 ppm concentrations to 30,000 ppm or higher. Because the sensing element is operated at an elevated temperature, the sensor functions as its own “heater” and is suited for operation in flash freezers and other environments subject to extreme cold (-40˚F [-40˚C] or colder) temperatures. Because the detection principle is not based on the oxidation of ammonia, the sensor also can be used to measure ammonia in oxygen-free environments. Most importantly, the sensor is not “consumed” or permanently altered by exposure to ammonia. In the presence of ammonia, the charge carriers are “injected” into the sensor element. As the concentration of ammonia decreases, the charge carriers are just as readily desorbed. CI sensors are stable, with an operational lifespan of five years or even longer. They are not affected by shifts in humidity, and they offer good performance in the extreme low humidity associated with flash freezing operations.
However, like electrochemical sensors, CI sensors also are subject to important limitations. While CI sensors have been designed to minimize the effects of interfering contaminants, they still respond to some volatile organic solvents such as the limonene in citrus oil cleansers and high concentrations of carbon monoxide. CI sensors should never be installed in truck bays or other locations where they will be subjected to high concentrations of internal combustion engine exhaust. Also, CI sensors are optimized for detection at higher concentration ranges and should not be used for applications that require taking action at low concentrations (less than 30 ppm).
Understanding the Environmental FactorsClearly establish the environmental and detection requirements before choosing a detection technology. Following are some of the most important issues to address when making a decision.
Temperature Range in the Areas in Which the Sensors Are Installed. Specifying a single type of sensor for all locations within the facility might not be the best approach. Different sensors (or transmitters) are optimized for use in specific temperature ranges. A sensor that is designed to function properly at 40°F (4°C) might work well in a refrigerated produce storage warehouse, but it might not be able to function at all if installed in a -30°F (-34°C) flash freezer. However, simply switching to a slightly different “freezer” model of the same sensor may be all that is required to ensure good performance. Don’t forget that different areas of the same facility might be subject to different temperature ranges. Discuss the issue with your equipment supplier before making a purchase.
Detection Range Required. Most types of sensors are available in several different detection ranges. Generally, the wider the full range of the sensor, the more limited the ability of the sensor to detect small changes in concentration. For instance, an electrochemical sensor with a full range of 100 ppm can easily detect changes of 1.0 ppm; by contrast, an electrochemical sensor with a full range of 5,000 ppm might not be able to detect changes of less than 20 ppm at a time.
Also, make sure to clarify where the alarm will be set before specifying the type of sensor to use. A transmitter used for health and safety exposure limit compliance typically will have alarms set at 25 ppm. An electrochemical ammonia sensor would be a great choice for this application. Conversely, a transmitter designed to provide an alarm if a sudden large-scale leak occurs (such as in a relief vent) might have alarms that are set between 2,000 and 4,000 ppm. A charge carrier injection sensor with a full range of 10,000 ppm is a much better choice for this application.
Other Background Gases or Interfering Contaminants. Be sure to understand what interfering contaminants might be present. Electrochemical and infrared sensors generally are not affected by the presence of interfering contaminants such as cleaning solvents, process gases or the exhaust of internal combustion engines. Charge-carrier injection sensors have been designed to minimize the effects of interfering gases, but they are still likely to respond if they are installed in areas subject to contamination by internal combustion engine exhaust. Infrared sensors are not affected by interfering contaminants, but they can be expensive.
Cost. An inexpensive system that fails to perform correctly or is responsible for false alarms is a bad investment. An expensive system with capabilities that exceed the requirements of the application is an equally bad choice. The best approach is to understand the requirements of your application and then consider the available technologies with those requirements in mind. Finally, include the purchase price and cost of ownership (including calibration frequency and sensor replacement) in the evaluation. Simply choosing the technology or system with the lowest initial purchase price is rarely the most prudent approach. It might actually turn out that the system with the lowest price is the best choice, but cost should not be the only determinant.
No single type of ammonia sensor is perfect for all applications. The key to success is understanding the monitoring environment, as well as the specific benefits and limitations of the sensors selected.