Optical technologies monitor leak detection, fugitive emissions
Volatile organic compounds and hazardous air pollutants released by industry have been subject to leak detection and repair (LDAR) requirements since the passage of the Clean Air Act Amendment of 1990. Fugitive emissions of these substances contribute to ground-level ozone, a major component of smog and cause of respiratory illness; some are known or suspected carcinogens.
Indeed LDAR itself is a National Air Toxics Enforcement priority for the EPA, “Leaking equipment, such as valves, pumps, and connectors, are the largest source of emissions of volatile organic compounds (VOCs) and volatile hazardous air pollutants (VHAPs) from petroleum refineries and chemical manufacturing facilities…With the large universe of sources subject to LDAR requirements and the high level of non-compliance, EPA will continue to focus on LDAR in the FY 2008-10 Air/Toxics strategy”
With so many sites left to be inspected, LDAR will remain an EPA priority until at least 2013, and an impending mandate to reduce green house gases as well will further broaden the scope of the regulations.
Monitoring for regulatory compliance is a straightforward process, but demands meticulous attention to potentially thousands of individual components, compiling and maintaining databases and generating the requisite reports. The EPA prescribes two major methods for monitoring emissions: the current practice known as Method 21—Determination of Volatile Organic Compound Leaks (M21) and the Alternative Work Practice to Detect Leaks from Equipment (AWP) introduced in December 2008. In addition there are methods not prescribed by the EPA, including fence-line monitoring, mobile optical methods and mass balance analysis.
Method 21 has been used to comply with EPA regulations for over 20 years. It requires that each regulated component be visited and its emissions monitored at monthly, quarterly or annual intervals. Organic or toxic vapor analyzers are used to monitor emissions at each possible point, which in the case of a typical chemical plant or oil refinery can be in the tens of thousands.
Method 21 provides guidelines on the use of the monitoring instruments. When measuring the leakage at a valve stem, for example, the ambient emission level must first be measured. The tip of the analyzer probe is then positioned at the stem and gradually moved around its circumference (Figure 1).
If the meter registers an increase, the motion is stopped until the reading comes to a steady state, after which movement around the stem is resumed. The highest reading is subtracted from the ambient and recorded as the leakage rate, called the screening value. This process is repeated for all valves, pumps, flanges and other connections. Method 21 also addresses calibration and maintenance of the monitoring instruments, the sensitivities of which are capable of detecting emissions down to single-digit parts-per-million.
Alternative Work Practice
A study published by the American Petroleum Institute in 1997 (API Publication 310 Analysis of Refinery Screening Data) found that fewer than 5% of the components in refineries accounted for more than 90% of fugitive emissions. It was further shown that these components were leaking at rates of 10,000 ppm or more, well beyond the leak definition or allowable levels.
To provide a means for quickly detecting and remediating such leaks, the EPA adopted the Alternative Work Practice as an option to M21. The AWP employs optical gas imaging for detecting leaks, most notably forward-looking infrared technology similar to that used in military night vision goggles. Hand-held infrared cameras, the optics and analyzers of which are tuned to the spectra of VOC hydrocarbons, are used to “see” fugitive emissions.
If a component is leaking, the emissions will absorb the IR light, appearing as black smoke on the viewing screen.
This technology does not quantify leaks, so the EPA still requires an annual Method 21 survey to screen a plant’s components for its submission to the toxic report inventory. Nor can it reliably detect leaks less than 5,000 ppm, although the latest technologies purport sensitivities to 2,500 ppm. Not nearly as sensitive as Method 21 analyzers, IR cameras are better suited to timely detection of the small proportion of disproportionately large leaks identified by the API study.
A site planning to use the AWP must declare its intention to do so in writing and retain the documentation. Surveys must be conducted every 30, 45 or 90 days, depending upon the selected sensitivity level of the monitoring instruments, which also must be documented. After repair, a component must be checked regularly for leakage, and all video records of daily instrument checks and survey results must be retained for five years.
It has been demonstrated that conducting 6 to 12 AWP surveys a year is equivalent to using Method 21 one to four times at a leak definition of 500 ppm. Yet few if any plant sites are currently using the AWP, perhaps because it does not obviate the need for an annual Method 21 survey; or it may be due to a reluctance to invite even further regulatory scrutiny.
Although the IR technology associated with the AWP is not being used for regulatory compliance, its use to support leak detection efforts is growing. One Midwest chemical company, for example, has been using IR gas imaging equipment for the past five years to supplement its Method 21 surveys. The technology is particularly useful for monitoring process units prior to startup. Operators use the cameras to find leaks, qualify system integrity and realize smooth startups.
In general, it has been found, AWP will find more leaking equipment than Method 21 within a given timeframe, but that the latter will find more leaks within a given floor area. The table below provides a summary comparison between Method 21 and AWP:
Other optical methods
In addition to IR technology, there are a number of other optical monitoring methods not stipulated in the regulations. Fourier transform infrared, ultraviolet and laser optics are used in fence-line monitors. These open-path monitors utilize differential optical absorption spectroscopy, whereby light of given spectra is emitted from a source and returned to the same point. The difference between what was emitted and what was returned is then analyzed. These methods are capable of identifying types and measuring amounts of air pollutants at distances up to 1,000 meters.
A variation on these methods is dispersive absorption light detection and ranging. First deployed in RV-size vehicles to perform surveys at specific plant sites, this technology also is installed in airplanes to find leaks in gas pipelines. Using a combination of IR and UV laser optics, together with meteorological data and analysis software, a wide range of air pollutants can be detected and quantified. In fact, Scandinavian countries require regular surveys using this method.
Whereas Method 21 is being used primarily for meeting regulatory requirements, AWP is being deployed to proactively look for leaks. As industry continues to invest in IR and other optical technologies, the benefits of AWP and the incremental value it brings will become increasingly apparent. Faster identification of leaks will expedite repairs and reduce emissions for safer, healthier and more environmentally friendly workplaces.
Jim Drago, P.E., has worked in sealing technology for more than 25 years. His work has focused on engineering, applications, product development, business development and management. He has authored numerous articles on sealing to meet fugitive emission regulations and sealing product selection and application, presented papers at technical symposiums and contributed to the formulation of industry standards and guides for API, ASME, EPRI and STLE. Mr. Drago can be reached at 315-597-3070 or email@example.com.