## IDENTIFYING NATURAL GAS LEAKS TO THE

ATMOSPHERE WITH OPTI CAL IMAGING

Robinson, D.R., ICF Consulting, In.

Luke

- Boone, R.E., ICF Consulting, Inc.

ABSTRACT

Methane is one of the most significant anthropogenic greenhouse gases, whose

global warming potentia l and sources are well documented. The natural gas

industry is a major source of methane emissions. In 2001, unintended (fugitive)

leaks from valves, piping connectors, open

- ended lines, and pump and

compressor seals totaled 3.9 Tg (58% of total U.S. emi ssions from the natural

gas industry). In the U.S. hydrocarbon industry, less than 1 percent (%) of the

thousands of potentially leaking sources contribute over 90% of fugitive

emissions. Finding these few large leakers in a cost

- effective way is a challe nge

for the industry. The current practice in the U.S. and other countries is to visit

each and every potentially leaking component, and “sniff” it with a hand

- held gas

detector. This is time consuming, labor intensive, costly and limited to

components wi thin the reach of the gas detector user.

Optical imaging (OI) is a new technology that allows the user to “see” a live

image of fugitive gas leaks into the atmosphere. Several different types of OI

technologies

- sensitive to fugitive leaks

- are being dev eloped, and one is now

commercially available. These devices allow a user to scan hundreds of

components in minutes, identify those few that are leaking, and video record the

leakers for verification and documentation.

This paper describes different OI l eak detection technologies, how they are more

cost effective than currently used hand

- held gas detectors, what may be possible

in methane emission reductions, and future outlook for OI. The presentation will

show an actual video of gas leaks detected with an optical imager.

1.0 INTRODUCTION

Fugitive leaks can be a major source of methane emissions from natural gas

facilities. Left uncontrolled, fugitive methane emissions can, at the least, add up

to significant economic loss, or can, at worst, pose safety ha zards. In the United

States, companies rely on initiatives such as Directed Inspection and

Maintenance (DI&M) Programs to identify and repair leaks. Current DI&M

techniques and technologies for detecting gas leaks e.g. soap bubble screening,

acoustic det ectors, Toxic Vapor Analyzers (TVAs) and Organic Vapor Analyzers

(OVAs), require an operator to visit and screen every component by placing the

screening instrument in the immediate vicinity of the potential leak. This

approach is time consuming, ineffici ent, and costly, when operators must screen

hundreds or thousands of components at each site.Emerging optical imaging (OI) technologies may enable industry to reduce the

time, labor and costs required to find and repair fugitive leaks. Remote sensing

with OI technologies allows an operator to scan areas containing tens to

hundreds of potential leaks, thus eliminating the need to visit and manually

measure all potential leak sites. Leaks are identified immediately, allowing

quicker repair, and ensurin g efficient use of resources. Two emerging OI

technologies have sufficient sensitivity for methane fugitive leak screening:

• An infrared laser camera based on Backscatter Absorption Gas Imaging

(BAGI) techniques, and

• An infrared camera based on Image Mul ti

- Spectral Sensor (IMSS)

technology.

2.0 BAGI CAMERA

The BAGI camera developed by Sandia

National Laboratory is suitable for detecting

methane. It was developed to detect fugitive

emissions of most organic compounds in the

petroleum and natural gas industr ies. The

current prototype camera weighs approximately

30 lbs and is about the size of a network

television camera. It consists of two systems:

a camera and a power/control unit (Figure 1).

The camera is powered from a backpack borne

power/control (Fig ure 2) that is connected to

either a 28V lithium

- ion battery or a 110

- volt, 60

cps AC power chord through the unit’s 28

- volt DC converter. In the current

prototype, battery lifetime while

operational is between 1 and 11⁄2 hours;

batteries recharge in twelve hours.

Batteries can be changed without shutting

down the camera or greatly interrupting

operations.

The imager can be operated in a shoulder

mounted or a tripod

- mounted position. A

zoom lens allows the operator to adjust

the focal distance to obtain a better view.

The operator can switch between the

infrared view and visible light, and can

record video of the image seen in the

viewfinder in both views. The camera has

simple start

- up procedures requiring 10

– 15 minutes after the power is switched on

. Shoulder

- mounted camera

Backpack power/control

Fig. 1. BAGI Camera

Fiber

l aser

camera

Backpack

with

batteries

Fig. 2. Screening with the BAGI

Camera2.1 BACKSCATTER ABSORPTION GAS IMAGING (BAGI)

Backscatter

Absorption Gas

Imaging (BAGI) produces a live

video image by illuminating the view

area with laser light in the infrared

frequency range. The reflected

(backscattered) laser light is

detect ed with a camera sensitive to

that light. When the chosen laser

wavelength is strongly absorbed by

the gas of interest, a cloud of that

gas does not provide much

backscattered light compared to its

surroundings. Because of its IR light

absorption, the gas is revealed as a

dark image on the camera. Figure 3

shows how BAGI works.

A video camera

- type scanner both

sends out the laser beam and picks

up the backscattered infrared light.

The camera converts this

backscattered infrared lig ht to an electronic signal, which is displayed in real

- time

as a black and white image on the viewfinder (the camera has an auxiliary output

to a video monitor). The same image will be seen whether the scanning is done

in daylight or at night because the scanner is only sensitive to illumination coming

from the infrared light source, not the sun. The visible

- light view is an important

attribute for the operator to identify the leaking component and differentiate

between steam plumes and gas plumes (see di scussion of Atmospheric Window

below).

Figure 4 shows a leaking flange detected by the BAGI camera at a refinery in

Texas, shown in visible and infrared light. In the current state of development, Incident infrared laser light

Incident infrared

laser light absorbed

by chemical plume

Gas Plume

Back scattered

laser light Backscattered

laser light

Incident infrared

laser light

Source: As Adapted from McRae, Tom, GasVue: A Rapid

Leak Location Technology for Large VOC Fugitive

Emissions . (Presentation at the CSI Petroleum Refining

Sector Equipment Le aks Group, Washington, DC, Sept. 9,

1997). U.S. Patent # 4,555,627

Note: Although gas is shown in contact with the background

material, it is not a requirement. The gas plume need only

be between the background and the infrared camera.

Fig. 3. BAGI Pri nciple

hydrocarbon

plume flange Visible light view of leaking flange Infrared view of leaking flange

flange

Fig. 4. Visible and Infrared Views of Leaking Flangethe technology makes visible the size of a vapor clou d of hydrocarbons that are

invisible to the naked eye, but does not quantify the mass emissions rate of the

leak cloud. A detailed technical description of BAGI technology is available in [1].

2.2 BAGI PERFORMANCE PAR AMETERS

Three main parameters infl uence the performance of the BAGI technology:

background, laser wavelength, and atmospheric window.

BACKGROUND. For the technology to image a leak, there must be a reflective,

or backscattering surface close behind the leak. It is not possible to visu alize a

gas plume against the sky or a distant background with BAGI. It is, however,

possible in many circumstances to image a leak against the component itself,

with a distant or sky background appearing dark. The operator knows that the

imager is beyon d detection range when the camera no longer produces a bright

image of the components under inspection: the more distant the component or

background, the darker the image. The operator would also visually check the

equipment under inspection to determine w hether there is an adequate

background surface behind the potentially leaking components.

LASER WAVELENGTH. Gas leak detection sensitivity by OI depends most

strongly on the match between the laser wavelength and the wavelength of

strongest absorption by the gas of interest. T o produce a dark image of a gas

cloud, the hydrocarbon must be capable of absorbing the laser light at the tuned

wavelength.

ATMOSPHERIC WINDOW. An “atmospheric window” is defined as a region of

the spectrum where there is minimal or no light absorption by oxygen, nitrogen,

carbon dioxide, and water vapor that are normally present in air. The major

atmospheric windows in the infrared region are found in the 3 to 4.2 micron and

in the 8 to 13 micron wavelength regions. A laser bea m propagating through the

atmosphere at wavelengths within these atmospheric windows will experience

minimal attenuation.

However, laser light within these IR wavelength windows may still be attenuated

by particulates in the air, including water droplets as in fog and steam.

Consequently, plumes of these particulates will appear as dark clouds in the

BAGI image as do the fugitive gases that absorb the laser light. Since these

particulate plumes are also visible to the naked eye (while the fugitive gases are not), BAGI operators can easily distinguish between the two types of cloud

images.

OTHER PERFORMANCE PA RAMETERS. Wind speed, gas plume motion and

viewing angle affect the fugitive gas cloud, and thereby, the detection sensitivity.

The higher the wind speed, the more quickly the gas is dispersed as it leaves the

leak source, and the less visible it becomes to the optical imager. However,

some motion is beneficial for detecting a leak, as the human eye is particularlysensitive to movement. A stationa ry gas cloud is very difficult to distinguish

against a non

- uniform background.

Viewing angle and optical resolution also influences detection. While the best

(most sensitive) viewing angle is when the gas comes directly towards the BAGI

camera, this i s not important if the camera has sufficient optical resolution to

image the gas as it exits the leaking component. Consequently, the most

favorable leak detection conditions are to view the leak source as close as

possible, under low (but not stagnant) w ind speed conditions, and with sufficient

image resolution to see the escaping gas coming directly out of the leaking

component. Under these conditions, and with a fairly uniform background

surface, the BAGI technology is capable of visualizing smaller le aks.

3.0 The IMSS Camera.

Pacific Advanced Technology (PAT)

developed an infrared camera based on

Image Multi

- spectral Sensor (IMSS)

technology. The camera, originally

developed for leak detection at refineries

and chemical plants, can be adapted to

metha ne detection at natural gas facilities.

The current prototype of the IMSS camera

(Figure 5) weighs 12 lb (including battery)

and is 12 (L) x 6 (W) x 8 (H) inches in size

and is battery operated. The camera,

which holds a patented IMSS lens, contains

the b attery power source and a

microprocessor to acquire and process the

hyperspectral images.

Like the BAGI technology, the IMSS

camera does not quantify leaks. However,

unlike BAGI, it can identify chemical

species within a leak. It can be used in un

mann ed aerial vehicles and controlled from

remote locations. It can also detect

chemical species from great distances,

provided there is sufficient quantity of the

chemical and a significant difference in IR

radiance between the plume and

background. In one test, the camera was

able to screen a flare on an offshore oil

platform 10 miles away and provide a

spectrum showing the composition of the

hot combustion gases. Figure 6 shows the

IMSS camera in use at an oil refinery.

Fig. 6. IMSS Camera in Oil

Refinery Fig. 5. The IMSS CameraFig. 7. IMSS Principle 3.1 THE IMSS PRINCIPLE

Image Multi

- sp ectral Sensing is based on the principal of diffractive optics. It is a

combination of a diffractive imaging spectrometer and an adaptive tunable filter.

The basic concept is shown in Figure 7.

A basic monochrometer has an entrance and exit slit comb ined with a light

dispersing medium such as a prism. In order to obtain high resolution, the

entrance and exit slits must

be very narrow, thus

reducing the optical

throughput of the system.

The light is dispersed

perpendicular to the axis of

the exit sli t and is spectrally

scanned across the slit.

In the IMSS approach, a diffractive element is used to perform both imaging and

dispersion of light. The diffractive optical element disperses the light along the

optical axis and the detector array/lens foca l length is scanned to produce

images of different IR “colors.” The advantage of this approach is that the entire

input aperture collects the light as opposed to the narrow entrance slit and thus

the throughput is greater. The detector element acts as th e exit slit and must be

matched with the blur diameter of the diffractive lens. The disadvantage of this

approach is that micro processing must be done in order to sharpen the image of

equipment and the chemical leak spectrum. A detailed description of t he process

is available in reference 2.

Figure 8 shows an image of a valve leaking methane at 0.1 standard cubic feet

per min (scfm). The test setup is shown on the left. Gas temperature was 63.2

o F, air temperature 63.7 o F. Wind was 5 to 9 mph, gustin g from the left [2].

Fig. 8. IMSS Images of Valve Leaking Methane

4.0 FUGITIVE METHANE REDUCTION OPPORTUNITY

Recent studies in the U.S. have shown that there are significant opportunities to

reduce fugitive emissions at natural gas facilities. A re cent study, co

- sponsoredby U.S. EPA’s Natural Gas STAR Program, showed that the methane reduction

potential using DI&M at 13 U.S. transmission stations averages 70% based on a

payback period of 1 year [3]. The total value of gas saved by implementing the se repairs was estimated at $765K/yr (gas valued $2/Mcf).

Another study at gas processing facilities found the fugitive methane emissions

reduction potential at four plants about 79% based on a payback period of up to

4 years for all components [4]. If gas processing plant emission reduction

opportunities with payout periods between 0.5 to 4 years are considered, the

study estimated percent of total methane savings shown in Table 1.

Table 1. Payback Period For DI&M Program

Payback Period

< 6 mont hs < 1 year < 2 years < 4 years

Methane Emission Reduction 71.9% 78.1% 79.2% 79.5%

The study of gas processing plants achieved an average $20 in gross savings

($13 in net savings after repair costs) per component for leak repairs with less

than 1

- year payback based on $4.50/ Mscf gas price.

OI offers significant savings in labor costs compared with conventional methods

of leak screening. Table 2 compares screening a gas processing facility with

25,000 components using the current practice (TVA or OV A hand

- held gas

detector) and alternatively using optical imaging.

Table 2. Comparison of Screening Gas Plant with TVA/OVA or Optical Imaging

Current TVA/OVA

Leak Detector Optical Imaging

Average Screening Rate 700

components/day 35

components/min

T ime Required to Screen Plant 35 days 12 hr

Total Labor Cost

(DI&M

- $0.50/component)

(1 OI Technician at $25/hr) $12,250 $300

Note: time and cost to tag and repair leaking components is the same regardless

of screening method and are excluded from these estimates. Cost of purchasing

an OI camera or TVA is also excluded.

5.0 FUTURE OF OPTICAL IMAGING

OI is currently used in the U.S. to identify SF 6 at power facilities and has shown

great promise for fugitive emissions control at refineries and chemic al plants.

Several laboratory and field tests have been completed and negotiations arecurrently underway to begin long

- term testing of the BAGI technology at

hydrocarbon facilities. The technology offers a means to overcome cost and

efficiency issues at facilities where hundreds or thousands of components are to

be monitored. An important next

- step in research is a means to quantify the

mass rate of detected leaks. The future of optical imaging appears promising.

As with other video technologies, more usage and time will lead to more efficient

and less expensive methane leak detection and repair.

REFERENCES

[1] McRae, T. G. and Kulp, T. G., “Backscatter Absorption Gas Imaging: a New

Technique for Gas Visualization” Applied Optics , 1993, Vol. 32, 403 7

- 4050

[2] Pacific Advanced Technology, “Sherlock Gas Leak Detection Camera,” Santa

Ynez, CA, 2003

[3] U.S. EPA Natural Gas STAR Program, PRC International , Gas Research

Institute, “Cost Effective Leak Mitigation at Natural Gas Transmission

Compressor St ations,” 1999

[4] U.S. EPA, Gas Technology Institute, Clearstone Engineering, Inc.

“Identification And Evaluation Of Opportunities To Reduce Methane Losses At

Four Gas Processing Plants (USEPA Grant No: 827754

- 01

- 0) ,” Washington,

DC, March 2000