The Principles of Detecting Refrigerant Leaks
Various sensor types and detection systems can enhance the effectiveness of leak detection in industrial refrigeration systems.
For years, the requirement of installing leak detectors has been incorporated in various standards and building codes such as ASHRAE 15, California Code of Regulations Title 17 and others. The European Union has gone one step further with the new Regulation EU 517/2014, also known as F-Gas Regulation, which came into effect on January 1, 2015. This regulation not only enforces the installation of leak detectors for certain equipment, but it introduces incentives to comply by extending the requirement for periodic leak checks by doubling the frequency for this kind of maintenance. This regulation also moves away from the amount of charge of a refrigerant, and it introduces the GWP (global warming potential) and CO2 equivalent. The smallest charge affected by an annual leak check is 5 tons of CO2 equivalent. This translates to a charge (for example, R-134a with a GWP of 1,430) of less than 3.5 kg, or about 7.7 lbs. The F-Gas regulation also defines an exit strategy to remove HFCs from a number of applications over the next 15 years.
The driving forces for installing a refrigerant leak detection system include:
- Safety – ensuring that a leak of refrigerants will not harm people or assets.
- Reducing operating cost – every leak in a refrigeration system decreases its efficiency and causes increased wear and tear, plus the cost for re-charging the system.
- Green building – reduce ozone-depleting refrigerants and refrigerants with a high GWP.
Most CFC, HFC and HCFC refrigerants listed in the ASHRAE 34 - 2013 standard have an occupational exposure limit (OEL) of 1,000 ppm (parts per million) or less. Even though these concentrations are far from being lethal, the main issue is oxygen displacement. Because these refrigerants are heavier than air, a leak tends to flow to the lowest point and will likely fill pits, trenches or stairwells before the main areas. Depending on the structure of the building/area, a sensor can typically monitor an area with a 15 to 30 foot radius. Any obstructions, airflow, drafts, etc., should be taken into account. Sensors should be placed in areas that are most likely to develop leaks or spills such as valves, gauges, fittings, flanges, joints (brazed or mechanical), seals and filling or draining connections.
For natural refrigerants like ammonia (R-717, NH3) and carbon dioxide (R-744, CO2), toxicity becomes more of an issue. The OEL for NH3 is only 25 ppm and for CO2 is 5,000 ppm. There is another value to keep in mind; the IDLH (immediately dangerous to life and health) is 300 ppm for NH3 and 40,000 ppm for CO2. When entering an area where the IDLH has been reached, increased safety measures through use of personal protective equipment like a self-contained breathing apparatus (SCBA) are required.
Principles of Industrial Leak Detection Systems
|See the related web-exclusive sidebar, "The 3 Essential Parts of an Industrial Refrigerant Leak Detection Program," to learn about the four major sensor principles used in refrigerant leak detection systems.|
There are several different principles of leak detection systems available using different sensing technologies. These can be grouped as either point detectors, where a sensor is mounted at the probable point of a leak, or as aspirated systems, where gas is sampled via tubing from various locations to a central sensor.
Point detectors have the advantage that every instrument continuously monitors the air at the location, resulting in a much shorter response time. These detectors can also be equipped with integrated relays to connect alarm devices such as strobes and horns without having to install wires from a centralized control system. The measuring signal can be transmitted to a building management system for further evaluation and also for initiating safety measures.
Aspirated systems act as a complete centralized control system. Because the sampled air is evaluated within the system, all measured values from the various sampling points are displayed in one unit. With on-board relays, it switches alarm devices, and the measured values can be transmitted to a building management system using various interfaces (e.g. analog 4 to 20 mA, Modbus, BACnet, LON, etc.). For larger systems, an aspirated system offers a cost advantage compared to point detectors. Such systems allow the connection of up to 16 sampling lines, which, at the end of the sampling line (up to 1200 feet), can be split into two points to monitor a larger area (e.g., two points at one chiller). For remote access of such a system, a second display with the complete functionality can be installed outside the monitoring area to get all measurement and status information without having to enter the area.
Operation of Refrigerant Leak Detection Systems
Four major sensor principles are used in refrigerant leak detection systems. Semiconductor sensors respond to a broad range of gases and are usually a lower cost option. More gas-specific types include infrared and electrochemical sensors, which provide higher performance, enhanced accuracy and smaller detection limits. Catalytic bead sensors are typically used for monitoring the lower explosive limit (LEL) for hydrocarbons such as propane (R-290) or butane (R-600).
Semiconductor Sensors. Semiconductor sensors (MOS) are among the most versatile of all broad-range sensors. They can be used to detect a variety of gases and vapors in low parts per million (ppm) range and even combustible ranges. The sensor is made up of a mixture of metallic oxides. They are heated up to a temperature between 300 and 800ºF (149 and 426ºC), depending on the gas or gases to be detected. The temperature of operation as well as the recipe of mixed oxides determines the sensor selectivity to various toxic gases, vapors and refrigerants. Electrical conductivity greatly increases as soon as a diffusion process allows the gas or vapor molecules to come in contact with the sensor surface.
These sensors can be negatively affected by water vapor, high ambient humidity, temperature fluctuations, reducing gases and low oxygen levels, resulting in false readings.
Infrared Sensors. The infrared (IR) sensor principle is based upon the concentration-dependent absorption of infrared radiation in measured gases. This physical measuring principle makes the sensor immune to environmental changes as well as gases that do not absorb at the selected wavelength. The monitored ambient air diffuses into an optical bench. The broadband light emitted by an infrared source passes through the gas in the bench and is reflected by the walls from where it is directed towards a detector element. The reduced sensitivity due to the presence of the target gas is used to determine the gas concentration. Internal electronics and software calculate the concentration and produce a linearized output signal. The size of the optical bench is an essential factor to accurately measure refrigerants. Most refrigerants absorb light at a wavelength of about 9 µm. In order to achieve a high resolution and accuracy, a longer path length is necessary.
Electrochemical Sensors. Electrochemical sensors measure the partial pressure of gases under atmospheric conditions. The monitored ambient air diffuses through a membrane into the liquid electrolyte in the sensor. The electrolyte contains a measuring electrode, a counter-electrode and a reference electrode. An electronic potentiostat circuit ensures a constant electrical voltage between the measuring electrode and reference electrode. Voltage, electrolyte and electrode material are selected to suit the gas being monitored so that it is transformed electrochemically on the measuring electrode and a current flows through the sensor. This current is proportional to the gas concentration. The current flowing through the sensor is amplified electronically, digitized and corrected for several parameters (e.g., the ambient temperature).
Catalytic Bead Sensors. The catalytic bead sensor measures the partial pressure of combustible gases and vapors in ambient air and uses the heat-of-combustion principle. The monitored air diffuses through a sintered metal disc into the sensor. The mixture of combustible gases and vapors and air are catalytically combusted at a heated detector element (pellistor). The oxygen content in the air must be greater than 12 percent by volume. Due to the resulting heat of combustion, the temperature of the detector element rises. This increase in temperature causes a change of resistance in the detector element, which is proportional to the concentration of the mixture of combustible gases and vapors in the monitored air. In addition to the catalytically active detector element, there is a compensator element. Both elements are parts of a Wheatstone bridge. Environmental effects like changes in ambient temperature or humidity are almost entirely compensated.