Imaging highlights explosive atmosphere
IR gas imaging is an accurate, reliable, and cost-effective solution for detecting large clouds of escaping gas. See mo’ evil
By Edward Naranjo
Infrared (IR) gas cloud imaging has made remarkable inroads in industrial safety applications over the last 20 years. Since the mid-1980s, infrared cameras have extended their capacity to identify and quantify a variety of toxic and combustible gases in real time. Advances in resolution, detection range, speed, and richer graphic interfaces have extended their use in oil and gas installations. IR gas cloud imaging offers several benefits that complement other gas detection methods. First, IR gas imaging provides continuous wide area coverage per device, with typical spans of 1 km in length by 0.5 km in width (approximately 0.6 mi by 0.3 mi). With fields of view of 12 degrees to 60 degrees, IR cameras can supervise entire sectors of a plant with detailed spatial resolution.
Second, IR imaging conveys a rich stream of information: The dynamic representation of gas flux allows users to identify not only the specific zones from which gas plumes originate, but also the direction of dispersal, leading to efficient responses to hazardous events. Finally, imaging is immune to major sources of false alarms. Due to the characteristics of the absorption bands for most hydrocarbon gases, IR imaging is unaffected by the absorption of water, carbon dioxide, and other atmospheric constituents present in a plant atmosphere.
Uses thermal background
Loss of gas containment in the oil-and-gas and chemical process industries can have undesirable safety and environmental consequences.
If leaks exist for any length of time at all, they can accumulate into dangerous gas clouds that can ignite and create explosions or inflict harm as toxic agents. Gas leaks produce a significant financial loss as well. A recent report by British Petroleum estimates a single cavity pressure vent at an unmanned offshore platform released approximately 17,970 ft3 (500 m3) of hydrocarbon gas per day or the equivalent of $19,900 (£10,000) over a four week period.
The same report estimates the oil and gas concern loses about 4,000 tons of natural gas per year due to fugitive emissions. There are two methods of optical imaging, active and passive. Active gas imaging uses different types of laser techniques with an intense infrared source and with a laser selected for a spectral line specific to a target gas. Such imaging requires a good backscatter surface in order to return a strong backscatter signal.
In contrast, passive gas imaging uses thermal background radiation within the infrared region from three to 14 microns. One can image gas either against a cold background or against a warm background. The gas itself is a source of radiation.
In this piece, we will examine passive gas imaging as a means of tracing and detecting gas leaks.
Live imaging of hydrocarbons
Gas detection of a hydrocarbon gas via infrared absorption requires the gas absorb optical energy at the wavelength of interest. The absorption bands of hydrocarbon gases center up almost exclusively in the 3.2-to-3.5 micron wavelength region of the mid-wave infrared (MWIR), and also predominate in the 7-to-14 micron wavelength region of the long-wave infrared (LWIR). In the latter case, the absorption bands of hydrocarbons do not overlap as closely as in the mid-infrared wavelength region, thereby making identification of the gas easier in the long-wave infrared region.
A further requirement for an instrument to detect gas via the principle of optical absorption is that atmospheric constituents such as water vapor or carbon dioxide do not absorb the optical energy at the same wavelength. The atmosphere has two regions (windows) of optical transmission in the infrared. These are the 3-to-5 micron wavelength window in the MWIR and the 8-to-14 micron wavelength window in the LWIR.
In fact, hydrocarbon gas absorption occurs within the MWIR and LWIR optical transmission windows of the atmosphere. This is indeed fortunate and has led to the development and widespread use of infrared gas detectors, particularly open path detectors, and IR gas cloud imaging cameras, which measure gas concentration over long optical paths. Both point and open path infrared gas detection are well-developed technologies that employ the strong absorption of gases in the infrared, typically in the near or mid infrared region, along with the transmission of the atmospheric windows previously described. While this technology is fairly mature, there has been considerable interest in a robust technique that can follow and measure clouds of hydrocarbon gas in an open atmosphere. Gas imaging delivers the sensitivity, reliability, and minimal maintenance required of an industrial solution for live imaging and concentration measurement of hydrocarbon gases.
One component of such gas imaging is a two-dimensional array of detector elements known as a focal plane array (FPA). Traditionally, IR cameras have used cooled, infrared focal plane arrays such as cooled mercury cadmium telluride to provide for high sensitivity in the 8-to-14 micron wavelength region.
For IR gas detection in the 3-to-5 micron mid-infrared region, cooled indium antimonide FPAs have been the preferred choice. The cooled, focal plane arrays are expensive; additionally, the Sterling cooling engine required to maintain the array elements at the low temperature of 80K needs periodic maintenance.
The cooled cameras, though having high sensitivity are, therefore, ill suited for continuous monitoring of gas leaks in an industrial environment. The advent in recent years of reasonably priced, high performance microbolometer IR FPAs that do not require cooling to cryogenic temperatures has opened the door to industrial applications of infrared gas imaging.
The gas imaging system here uses a384 x 272 pixel uncooled microbolometer IR FPA designed for the 8-to-14 micron LWIR region.
Another important element of an imaging system is the optics. The lenses, along with the size of the FPA, determine the range and field of view (FOV). The gas imaging system uses a set of germanium lenses coated for optimum transmission in the LWIR region—the horizontal FOV is 30° with a range of 1,000 meters.
The optics can be factory selected to provide a wider field of view at a shorter range: 60° horizontal FOV at 500 meters. Conversely, a narrower FOV is achievable if a longer range is necessary: 12° FOV at 2,000 meters.
An imaging system comprising of LWIR focal plane array and collection optics will be sensitive to all radiation in the 8-to-14 micron wavelength region.
Whereas this may be sufficient for thermal imaging applications that monitor the surface temperature of hot objects, the detection, identification (gas type), and quantification of a gas cloud require the use of specific infrared filters to perform a multi-spectral analysis of the incoming IR radiation.
Infrared point and open path gas detectors traditionally use a single reference filter and a single active filter. The term “active” refers to the fact that infrared radiation transmitted by this filter reacts to the presence of the gas to via absorption, whereas the term “reference” refers to the fact that infrared radiation transmitted by this filter is not affected by the presence of the gas to be detected.
The two filters traditionally used in infrared point and open path gas detectors do not provide for identification or discrimination of the gas type or species, i.e., the detector will respond to any gas that absorbs at the active wavelength without informing the user which type of gas has crossed the optical beam path.
When using an IR gas cloud imaging camera, there is provision for up to six infrared filters mounted on a filter wheel. The use of multiple filters allows the camera to simultaneously identify, quantify, and display the type and amount of gas within a family.
For example, we can detect, quantify, and display on a viewing monitor gas clouds of methane, ethane, propane, and butane—all are important members of the alkane family and constituents of natural gas.
In addition to supplying a large area of coverage and the simultaneous detection of four combustible gases, the gas imaging system tested proved easy to use. The infrared camera is equipped with a visible CCD color camera to enable setup and area surveillance.
A human machine interface (HMI) allows operators to see target gases as the camera detects them. Levels of gas concentrations overlaid on the image provide a unique picture of gas dispersal that allows personnel not only to address the source of a leak, but also to respond more effectively to an alarm based on the direction of motion of the gas cloud.
The gas measurement provided by the camera in our study is in units of percentage LEL (lower emission limit)-m or ppm (parts per million)-m. The concept of percentage LEL-m or ppm-m—a concentration multiplied by a path length—is widely accepted in industry and is the standard for commercial open path combustible gas detectors. This is because both open path and imaging gas detection calculate the presence of gas over a path length rather than at a single point.
Because the sensitivity to a gas partially reflects the IR absorption of molecular species and the sensitivity of the microbolometer, the minimum detection levels are gas specific.
The optical imaging system is for continuous operation, day and night. It has good immunity to common sources of infrared radiation that are potential false alarms. This is because three differential IR imaging processes detect the gas: spatial, spectral, and temporal.
Another unique feature of the gas imaging system that enhances alarm immunity is zoning. Using zoning, a user can select or deselect up to five zones within the camera’s field of view.
Areas with potential sources of false alarms can be deselected; for instance, a patch of sky with a smoke stack that releases a controlled quantity of volatile organic compounds. As another example, an area of a plant under construction could be deselected.
One can split up areas of interest into separate zones: The gas-imaging camera informs the user in which zone the alarm has occurred, while displaying the gas cloud in the image overlay. By this means, one can avoid false alarm triggers and the camera’s attention focused on parts of the field of view that need watching for safety.
Some gas imaging cameras require no gas calibration in the field. An automatic optical check takes place every few minutes and assesses the optical path integrity (window cleanliness), alignment of the optics, position of the focal plane array, and system functionality. This results in systems that require minimal maintenance.
In many gas-imaging devices, a video recorder function allows the operator to play back an alarm event or view gas detection in deferred time. Stored files enable facility management and safety personnel to view video images and concentration profiles of events as they unfold over time.
Users can retrieve and examine alarm events in detail by using the standard commands of a video recorder. The enhanced event logging reduces the time and complexity of managing safety records and informs management of potential improvements at the target site.
Multiple gases, sources
Based on the capabilities of IR gas cloud imaging described above, one can argue the technology offers several benefits that complement conventional gas detection methods.
Indeed, seeing a gas leak in real time enables users to grasp the hazard in a way few other methods do. However, like other detection techniques, IR gas cloud imaging also has its drawbacks. When used to supervise a large terrain, for example, gas imaging can only detect large leaks. Typical amounts of gas detected by the test camera, for example, are about 0.1 to 3 kg of gas at distances of 1.5 km.
Additionally, passive gas imaging requires a temperature difference between the background and the gas in order to produce an image. As this difference becomes smaller, the detected gas loses contrast against surrounding objects and becomes difficult to visualize.
Finally, the wind and humidity influence the sensing technology. Heavy amounts of moisture in the air attenuate the infrared signal, affecting both magnitude of spectral luminance and contrast.
Wind, in turn, can disperse clouds at a rate at which the cameras cannot resolve the moving gas. This is due to the fact most cameras have update rates of several seconds.
Regardless, IR gas cloud imaging systems have found widespread use in a variety of industrial sites. A typical case is one of an LPG storage and bottling plant in the south of France.
A gas visualization system deployed at this facility to detect potential releases of propane and butane. Because of the plant’s proximity to a densely populated housing development, safety managers sought to install the camera as an additional protective measure in the array of safety devices already deployed.
Such protection was paramount, particularly as rising demand for LPG had prompted an increase in the throughput of gas processed at the facility. LPG arrives daily by train or truck, and they store it in underground tanks before bottling and distribution.
Having installed the unit on top of a short building, managers were able to provide coverage to the top of the underground tank storage area and the truck loading and unloading zone.
A large refinery, also in France, faced safety constraints similar to those of the LPG storage and bottling plant. Gasoline output was at capacity, and production of several secondary products (benzene, butadiene, and butane) required careful monitoring.
Making matters more pressing was the concern of a gas cloud dispersing beyond the plant perimeter toward the surrounding community. To address such concern, the company installed an IR gas cloud imaging camera that covered the areas of highest risk, a benzene and butadiene production zone, a benzene storage tank farm, and a ship terminal.
Discrimination of several gas types was one of the advantages of the camera in this set up. These examples underscore several important points about the utility of IR gas cloud imaging.
First, the cameras are a component of a broad array of gas detection solutions, which when interfaced enhance the prospects of early warning should a gas leak occur. The additional protective layer provided by IR gas cloud imaging is particularly important in urban industrial plants.
Second, the wide field of view and long detection range serve well to supervise large sections of a facility. Often they install cameras on top of buildings and aim them towards the grounds, so the bulk of the target detection area is in the field of view.
Lastly, these IR gas-imaging systems can detect several gases simultaneously. Hazards in these complex environments rarely arise from a single gas. As a result, there is an interest in detection solutions where multiple types of gases and sources are present.
ABOUT THE AUTHOR
Edward Naranjo (email@example.com) is an ISA member. He has a BS and a Ph.D. in chemical engineering and an MBA. He is a manager at California-based General Monitors, a developer and manufacturer of high tech gas monitoring and flame detection instruments.
FPA: Focal plane array is a technology used in building the sensors used in IR cameras.
Infrared (IR) is electromagnetic radiation with wavelengths longer than visible light but shorter than radio waves. It is the part of the near-optical spectrum humans perceive as heat rays emitted by stellar objects such as the Sun.
LPG is liquefied petroleum gas or bottled gas or a mixture of butane and propane often supplied to a building. It is usually transports via tank truck and stores near the building in a tank or cylinder until used.