Low-Cost Standoff Laser Sensing for Smart LDAR

Transcription

1 PSI-SR-1248 Low-Cost Standoff Laser Sensing for Smart LDAR Mickey B. Frish Mickey B. Frish, "Low-Cost Standoff Laser Sensing for Smart LDAR," presented at AWMA 99th Annual Conference (New Orleans, LA), (20-23 June2006). Copyright 2006 Physical Sciences Inc. All rights reserved Downloaded from the Physical Sciences Inc. Library. Abstract available at

2 Low-Cost Standoff Laser Sensing for Smart LDAR Extended Abstract # 06-A-72-AWMA Prepared by Mickey B. Frish, Manager, Industrial Sensors Physical Sciences Inc., 20 New England Business Center, Andover, MA INTRODUCTION The procedures for locating vaporous leaks in chemical and petrochemical plants share much in common with surveying for leaks in natural gas transmission and distribution pipelines. By replacing traditional sensors based on extractive sampling with optical instruments based on infrared spectroscopy, the Smart Leak Detection and Repair (Smart LDAR) initiative is simplifying and accelerating leak inspection in chemical and petrochemical plants. Similarly, the recent introduction of the Remote Methane Leak Detector (RMLD), described in this paper, is simplifying leak surveying of the US natural gas transmission and distribution pipeline system, including over 1 million miles of pipeline, millions of meters, and thousands of compressors and transmission stations. To date, the Smart LDAR focus has been on sensors that pictorially image a leak plume. In contrast, the relatively low-cost handheld RMLD forms no image. Instead, the surveyor manually scans a laser beam, projected like a flashlight, across a surface up to 100 feet distant. When the beam transits a leak plume, an audio tone sounds and a display reports gas concentration. The technique is fast, sensitive, and quantitative. A visible beam or video camera co-aligned with the sensor laser beam provides leak source identification. Computer combination of the detector output with the video image creates a visual display of any leak sources. RMLD technology can be applied immediately for methane leak detection in complex manufacturing plants and, via a simple change in laser wavelength, can sense other specific gases. Advanced development now in progress is combining this proven standoff technique with a rastering mirror and moving platform to semiautonomously scan across surfaces; early tests sensed 2 scfh methane leaks while interrogating 10 m 2 /s of surface. RMLD OVERVIEW Pictured in Figure 1, the RMLD is an eye-safe laser-based standoff natural gas sensor, with a capital cost comparable to a flame ionization detector (the most common tool currently used for natural gas and petrochemical leak detection). 1 The RMLD system includes a 3 lb handheld optical transceiver and a 6 lb control unit that are carried comfortably by a surveyor for an eight-hour work day. An umbilical cable bearing an optical fiber and a few wires connects the two sections. The control unit includes a pair of rechargeable of AA-sized Li-ion batteries that power the system. Figure 1: Natural Gas Leak Surveyor Using an RMLD at a Compressor Station 2

3 The RMLD works much like a lantern a surveyor shines the laser beam onto a passive surface up to 30m (100 ft) distant. Virtually any surface, except a specular reflector like a mirror or standing water, is adequate. By analyzing the signal embedded in the small amount of laser light reflected back to the transceiver, the RMLD deduces and reports quantitatively how much gas is between the surveyor and the surface. It detects methane with 5 ppm-m sensitivity at 10 Hz. The usual mode of RMLD operation is to sound an audible alarm when it detects a high concentration or quickly changing gas cloud indicating a pipeline leak. In addition to its audio output, the Figure 2: Example RMLD User Interface Combining Concentration RMLD communicates gas Data with a Scene and Other Data concentration via an LCD alphanumeric display, and a serial output that may be attached to a separate computer. It is normally equipped with a visible pointer laser that facilitates identifying leak sources, but can also be equipped with a video camera co-aligned with the transceiver. A User Interface that combines the serial output and video display provides a graphical illustration of leak concentration vs position, as shown in Figure 2. The RMLD is based on the established optical measurement technology known as near-infrared Tunable Diode Laser Absorption Spectroscopy (TDLAS). 2,3 The principles are straightforward: Gas molecules absorb energy in narrow bands surrounding specific wavelengths in the electromagnetic spectrum. At wavelengths slightly different than these absorption lines, there is essentially no absorption. By (1) transmitting a beam of light through a gas mixture sample containing a quantity of the target gas, (2) tuning the beam s wavelength to one of the target gas s absorption lines, and (3) accurately measuring the absorption of that beam, the analyzer deduces the concentration of target gas molecules integrated over the beam's path length. The laser source in the RMLD is a distributed feedback (DFB) diode laser, a common type of laser widely used in telecommunications which provides a laser linewidth much narrower than gas absorption lines. Typically, each TDLAS system is built using a laser having a specific design wavelength chosen to optimize the sensitivity to a particular target gas (methane in the case of the RMLD) while minimizing sensitivity to other gases. The wavelength is adjustable (over a range of up to ± 2 nm) by changing the temperature or current that powers the laser. TDLAS exploits this tuning capability to rapidly and repeatedly scan the wavelength across the selected gas absorption line. When the wavelength is tuned to be off of the absorption line, the power transmitted through the gas mixture is higher than when it is on the line. A narrowbandpass phase-sensitive amplifier measures the resulting amplitude modulation, providing very high sensitivity to very weak absorption of the laser power. Absorbances of 10-5 are measured routinely. 4 3

4 FIELD DATA Walking Surveys The RMLD has undergone, and continues to undergo, extensive field testing since the assembly of the first advanced prototype (AP) units in April Between April and December 2003, seven utility companies completed the first field testing cycle, using two APs for leak detection on natural gas distribution systems. Each field test lasted two or three weeks. Surveyors for each company received hands-on RMLD training. Portable FIDs were used to validate RMLD results, and all leaks fully documented the location, readings, weather, wind conditions, backgrounds, etc. To be judged successful, the test criteria required the RMLD to: operate continuously for an eight-hour workday on a single battery charge and be comfortable for the surveyor; have a standoff range of at least 100 ft; within the standoff range detect gas leaks as small as those detectable with current FID units with a 0.1s response time; function in ambient temperatures between 0 and 50 C; be rugged, splash-proof and weather-resistant. During the nine month test period over 200 documented leaks were encountered, representing typical distribution leaks in residential and commercial areas served by both low and high pressure systems. Although the two RMLD AP units exhibited some differences in performance, this initial test program was considered to be a success the better performing RMLD detected more leaks than portable FI, particularly at outside meters and areas with difficult access. Based only on the enhanced ability to detect leaks in difficult-to-access areas, initial estimates show RMLD improving surveyor productivity by more than 25%. 5 Mobile Survey A September 2004 technology demonstration sponsored by the DoE National Energy Technology Laboratory at the Rocky Mountain Oilfield Testing Center (RMOTC) evaluated RMLD performance when used for surveying from an automobile traveling at tens of kilometers per hour. 6 The RMLD found virtual leaks by aiming it out of a car window while driving along a 7.4 mile route simulating a natural gas transmission pipeline. The virtual leaks emanated from methane bottles strategically located along the simulated pipeline. Figure 3 shows the methane measured by the RMLD vs time during one trip along the virtual pipeline route. Figure 4 graphically summarizes the results of nine such trips where each trip had a unique set of leaks. A total of 88 leaks were active during the testing period. Nine of the 88 were at a calibration site. Thirty leaks were located 100 ft or more from the road, beyond the RMLD s specified detection range. 14 of those 30 were at or just beyond the 100 ft range and had small leak rates, 15 scfh or less. The data plotted in Figure 4 indicate that detection of these smaller leaks becomes increasingly challenging as distance increases beyond 70 ft. Eight leaks had flow rates of only 1 scfh, yielding gas concentrations of less than Integrated Concentration (ppm-m) Figure 3: Path-Integrated Methane Concentration Measured by RMLD During Drive Along a Simulated Pipeline Route Time (min) G

5 Indicated ppm-m Leak Rate (scfh) Figure 4. Summary of Leak Detection Success Always Detected Sometimes Missed Not Detected Range to Leak (ft) Figure 5: Scanning RMLD Concept G-8430 Figure 6: Concentration Recorded by Cart-Mounted Scanning RMLD When Moving Past a 2 scfm Leak Time (s) G ppm at 10 ft from the source. Plumes from these leaks are below the 10 ppm-m mobile RMLD detection threshold. Of the remaining 41 leaks, four were not detected. Thus RMLD detected 37 of the 41 leaks within its specified performance range. SCANNING RMLD Figure 5 illustrates a configuration for using the RMLD platform from aboard a moving utility vehicle to scan streets and sidewalks in an urban area. In this scenario, the RMLD laser beam projects from a spinning turret mounted upon a van. As the van travels forward, the laser beam scans an arc to the front and sides of the van so as to survey across streets and to building walls. In an outdoor test, 7 a prototype spinning turret unit, comprising an RMLD transceiver attached to a platform and aimed at a mirror spinning at 120 rpm, was mounted on a rolling cart and pushed past a 2 scfh (57 l/hr) virtual methane leak. The projected beam traversed a 10 ft radius circle across the ground at 125 ft/s, corresponding to areal coverage of 10 m 2 /s. Methane concentration was output to and recorded by a portable computer at 10 points per second. Figure 6 shows an example of the data. Prior to entering the leak area, the unit detects only ambient methane of about 4 ppm-m with an rms noise of about 1 ppm-m. This noise level is comparable to that of the handheld sensor. Upon entering the leak area, approximately 6 seconds after the start of data acquisition, a spike in concentration output occurs each time the spinning mirror directs the laser beam at the gas plume. The spikes increase in amplitude as the cart passes close to the center of the plume, and subsequently decrease as the cart passes the plume. 5

6 CONCLUSION The Remote Methane Leak Detector is a commercially available tool now in use for natural gas leak surveying. Like the active, laser-based technologies considered for Smart LDAR, standoff detection by the RMLD enables detecting leaks from afar, reducing survey time and enabling more efficient use of manpower. Preliminary estimates for walking survey operations show savings in the range of 25% to 40%, resulting in cost savings within the US of millions of dollars annually. The RMLD can also detect natural gas inside a building or confined space with clear access, as well as in difficult to reach areas, such as gas pipelines under bridges, backyard mains and fenced in areas. A key feature of TDLAS is its selectivity to a specific target gas; e.g. the RMLD senses methane only - it will not false alarm on other gases including other hydrocarbons. But with a simple change, the RMLD platform can be converted from a methane sensor to a sensor of, say, a different hydrocarbon or other simple molecules. Emerging mid-ir (3 5 µm) TDLAS systems can sense most hydrocarbon vapors of interest to the Smart LDAR community. RMLD technology offers the potential for a relatively low-cost Smart LDAR sensor. The cost and reduced sensitivity incurred by sensors that actively or passively image a leak plume is absent from the basic RMLD platform, yet a trained RMLD operator can locate leaks rapidly and precisely. The RMLD platform can and has been configured with a low-cost co-aligned visible video camera providing images of the scenes surrounding detected leaks, thus providing much of the information desired by the Smart LDAR initiative. Raster scanning the RMLD to rapidly interrogate or image large areas is also possible; scanning rates of 10m 2 /s have already been demonstrated. ACKNOWLEDGMENTS RMLD development and testing was supported by the US Environmental Protection Agency, the Northeast Gas Association, Heath Consultants Inc., the US Department of Energy s National Energy Technology Laboratory (NETL), Physical Sciences Inc., PSE&G, and Keyspan Energy. REFERENCES 1. Remote Methane Leak Detector (RMLD) Product Literature, Heath Consultants Inc., Houston, TX, Frish, M.B. A&WMA Paper No. 96-TP26B.05, Air and Waste Management Association, Pittsburgh, PA, Frish, M.B.; Klein, F., 5th International Symposium on Gas Analysis by Tunable Diode Lasers, Freiburg, Germany, VDI Berichte 1366, Allen, M.G.; Carleton, K.L.; Davis, S.J.; Kessler, W.J.; Otis, C.E.; Palombo, D.; and Sonnenfroh, D.M. Applied Optics 1995, 34(18). 5. Fabiano, A.G.; Rutherford, J.; Chancey, S.; Frish, M.B. In Proceedings of Natural Gas Technologies 2005; Gas Technology Institute, Des Plaines, IL, Green, B.D.; Frish, M.B.; Laderer, M.C.; Midgley, G. In Proceedings of Natural Gas Technologies 2005; Gas Technology Institute, Des Plaines, IL, Frish, M.B.; Green, B.D.; Scire-Scappuzzo, F.; Cataldi, P.; Burbo, A.H.; Laderer, M.C.; Midgley, G. In Proceedings of Natural Gas Technologies II: Ingenuity & Innovation; Gas Technology Institute, Des Plaines, IL,