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
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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 01810
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 semiFigure 1: Natural Gas Leak Surveyor
autonomously scan across surfaces; early tests sensed
Using an RMLD at a Compressor
Station
2 scfh methane leaks while interrogating 10 m2/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.
2
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
Data with a Scene and Other Data
RMLD communicates gas
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
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 2003. 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.
Integrated Concentration (ppm-m)
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
Figure 3: Path-Integrated Methane Concentration Measured
88 leaks were active during the testing
by RMLD During Drive Along a Simulated Pipeline Route
period. Nine of the 88 were at a
30000
calibration site. Thirty leaks were located
25000
100 ft or more from the road, beyond the
RMLD’s specified detection range. 14 of
20000
those 30 were at or just beyond the 100 ft
range and had small leak rates, 15 scfh or
15000
less. The data plotted in Figure 4 indicate
10000
that detection of these smaller leaks
becomes increasingly challenging as
5000
distance increases beyond 70 ft. Eight
leaks had flow rates of only 1 scfh,
0
0
20
40
60
80
100
Time (min)
yielding gas concentrations of less than
G-7998
4
3 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.
Figure 4. Summary of Leak Detection Success
Leak Rate (scfh)
10000
1000
100
SCANNING RMLD
10
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.
Always Detected
Sometimes Missed
Not Detected
1
0
50
100
150
200
Range to Leak (ft)
250
G-8430
Figure 5: Scanning RMLD Concept
Indicated ppm-m
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
Figure 6: Concentration Recorded by Cart-Mounted
10 m2/s. Methane concentration was
Scanning RMLD When Moving Past a 2 scfm Leak
output to and recorded by a portable
45
computer at 10 points per second.
40
Figure 6 shows an example of the data.
35
Prior to entering the leak area, the unit
30
25
detects only ambient methane of about
20
4 ppm-m with an rms noise of about
15
1 ppm-m. This noise level is comparable
10
to that of the handheld sensor. Upon
5
entering the leak area, approximately
0
0
2
4
6
8 10 12 14 16 18 20 22 24 26 28 30
6 seconds after the start of data
Time (s)
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.
G-1682
5
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 10m2/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, 2003.
2. Frish, M.B. A&WMA Paper No. 96-TP26B.05, Air and Waste Management Association,
Pittsburgh, PA, 1996.
3. Frish, M.B.; Klein, F., 5th International Symposium on Gas Analysis by Tunable Diode
Lasers, Freiburg, Germany, VDI Berichte 1366, 1998.
4. 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, 2005.
6. Green, B.D.; Frish, M.B.; Laderer, M.C.; Midgley, G. In Proceedings of Natural Gas
Technologies 2005; Gas Technology Institute, Des Plaines, IL, 2005.
7. 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, 2004.
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