Acoustic Leak Detection System for Railroad Transportation Security
P. C. Womble+a, J. Spadaro b , M. A. Harrison b, A. Barzilov a, D. Harper a, L. Hopper a, E.
Houchinsa , B. Lemoff b, R. Martin b, C. McGrath a, R. Moore a, I. Novikov a, J. Paschal a, and S.
Rogers c
a
Applied Physics Institute, Western Kentucky University, 1906 College Heights Blvd MS 11077,
Bowling Green, KY, 42101-1077
b
West Virginia High Technology Consortium Foundation, 1000 Technology Drive, Suite 1000,
Fairmont, WV 26554
c
Steve’s company
ABSTRACT
Pressurized rail tank cars transport large volumes of volatile liquids and gases throughout the country, much of which is
hazardous and/or flammable. These gases, once released in the atmosphere, can wreak havoc with the environment and
local populations. We developed a system which can non-intrusively and non-invasively detect and locate pinholesized leaks in pressurized rail tank cars using acoustic sensors. The sound waves from a leak are produced by
turbulence from the gas leaking to the atmosphere. For example, a 500 m hole in an air tank pressurized to 689 kPa
produces a broad audio frequency spectrum with a peak near 40 kHz. This signal is detectable at 10 meters with a
sound pressure level of 25 dB. We are able to locate a leak source using triangulation techniques. The prototype of the
system consists of a network of acoustic sensors and is located approximately 10 meters from the center of the rail-line.
The prototype has two types of acoustic sensors, each with different narrow frequency response band: 40 kHz and 80
kHz. The prototype is connected to the Internet using WiFi (802.11g) transceiver and can be remotely operated from
anywhere in the world. The paper discusses the construction, operation and performance of the system.
Keywords: railcar, security, pressurized tanks, acoustic, leak, detection
1.
INTRODUCTION
Annually, approximately 1.7 to 1.8 million carloads of hazardous materials shipped by rail 1, 2/3 of which are
transported by tank car1. In 2005, The US Department of Transportation recorded 737 incidents of unintentional release
of hazardous materials from rail cars, of which 48 incidents were due to train derailment 2. Of the total incidents, 79
were identified as serious incidents*, causing fatalities, serious injuries, major evacuations, or substantial release of
hazardous materials. The greatest concern lies in unintentional release of toxic-by-inhalation (TIH) gases, such as
chlorine and anhydrous ammonia, which predominantly move by rail and recently have been among the top 10 railshipped hazardous materials by volume 3.
Of the solutions proposed to mitigate the risk of TIH release incidents, which include rerouting, shipment notification,
and wayside monitoring, only the latter is supported by the railroad industry as a practical solution. However, current
wayside monitoring systems do not address tank car integrity or even TIH release detection; instead, they focus on
reducing risk of train derailment by monitoring wheel track defects known to cause derailments. Furthermore, the
current wayside monitoring system does not specifically target urban areas or address their concerns not only of
accidental release, but also of terrorism.
+
womble@wku.edu; ph. 270 781 3859; fax 270 781 1104; www.wku.edu/API
PHMSA revised definition (2002) defines a major incident as one that: causes a fatality or major injury due to the release of a
hazardous material, causes the evacuation of 25 or more persons as a result of release of a hazardous material or exposure to fire,
causes a release or exposure to fire that results in the closure of a major transportation artery, results in the alteration of an aircraft
flight plan or operation, involves the release of radioactive materials from Type B packaging, is a release of over 11.9 gallons or 88.2
pounds of a severe marine pollutant, or involves the release of over 119 gallons or 882 pounds of a hazardous material.
*
Worldwide, approximately 181 terrorist attacks on trains and related rail targets occurred between 1998 and 2003,
resulting in approximately 431 deaths and thousands of injuries 4. The March 11, 2004 train bombings in Madrid, Spain
involved 13 IED devices, resulting in 10 near-simultaneous explosions that killed 191 people and injured over 1700. A
similar scenario could be used for an attack on a rail tank car, or an attack could easily occur at a distance and while the
train was in motion. For instance, 50 caliber sniper rifles, weapons readily available in the US and known to be
possessed by al-Qaeda, are known to have the capability to puncture railroad tank cars at distances of 1000 to 2000
yards5.
To reduce the anxieties of urban rail hubs and to mitigate the effects of both unintentional hazardous materials
(HAZMAT) releases and those deliberately caused by terrorism, we have developed a trackside inspection system for
these tank cars. It consists of five narrow frequency band pressure sensors with center frequencies of 40 and 75 kHz
and a broad band microphone for sound normalization. For visual inspection purposes, three cameras are connected to a
digital video recorder and a PC-based frame grabber. In addition, a 5 cm x 5 cm NaI(Tl) radiation detector provides
radiological data on the passing trains. In the following paper, we will discuss our design and its capabilities.
2.
DETECTION OF TANK CAR LEAKS
High pressure gas leaks are known to create broadband acoustic emission in the audio and ultrasound regime.
Turbulence created as the high velocity gas stream enters the surrounding air leads to periodic vortex shedding which in
turn leads to the creation of acoustic waves. The acoustic emission is dependent on the velocity of the gas, the leak size
and shape, as well as the density of the gas and surrounding air. Measurements performed by the our group indicate that
the sound field is directional with a peak acoustic intensity located at an angle that is roughly 45 degrees from the plane
of the pressure vessel.
Airborne acoustic measurements made by our group on a variety of leaks (see Figure 1) show broad acoustic emission
starting at approximately 5 kHz, peaking at about 40 - 60 kHz, and rolling off at about 160 kHz. This roll off is due in
part to the microphone sensitivity and the attenuation of sound in air.
Power (Relative Units)
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10-4
10-5
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Fig 1. The power spectrum of a variety of leak sizes and shapes from 50 psi to 300 psi. The flat curve is
background noises (no leak).
The peaks at 170, 140 and 110 kHz appear at lower pressures and are most likely due to resonances associated with the
aluminum plenum. These peaks are inversely proportional to pressure and tend to broaden as the pressure differential
increases. In general, the acoustic emission varies linearly with pressure differential across the orifice with small
changes in the frequency spectrum as the pressure changes.
We have focused on wayside monitoring since wayside systems already exist for measuring wheel and bearing failures.
In this way, the leak detection system could be integrated with existing systems or at least be modular enough to
compliment these systems. The system could just as easily be mounted on individual tank cars with no effect on the
basic performance of the system.
Excluding the air brakes, there is no significant acoustic energy (as shown in Figure 2) from the train in the ultrasound
regime (>20 kHz) making this frequency regime ideal for detecting tank car leaks.
The measurements utilized a pressurized plenum with an orifice approximately 0.040˝ in diameter. The plenum
consisted of an aluminum block measuring 5.0 cm x 5.0 cm x 2.5 cm with a internal cavity measuring approximately
4.6 cc. The majority of tests were conducted with compressed air with a small number of tests using nitrogen and argon.
Pressures at the regulator ranged from 5 to 140 psig.
The acoustic emission was characterized using a Brüel and Kjær model 4138 1/8” pressure field microphone. The
microphone has flat frequency response with -2 dB roll off at 6.5 Hz and 140 kHz.
The microphone signal was brought into a Brüel and Kjær 2669 preamplifier and then amplified using a Brüel and Kjær
Nexus conditioning amplifier with an output of approximately 100 mV per Pascal. The amplifier output is fed into a
National Instruments data acquisition system with a maximum data rate of 500 ksamples/sec with 12 bit resolution.
Acoustic measurements were made at a railroad switchyard in Connellsville, PA to develop an accurate characterization
of background noise from railcars (shown in Figure 2). Measurements were made with microphones located at a
minimum of 3.5 m from the tracks. The measurement location was along a slight curve which caused an audible
Fig 2. Acoustic spectrum of a train in the presence of a leaking pressurized vessel versus background (no leak).
See text for details.
screeching sound as the train passed the microphones. Measurements were made for a number of different trains with
speeds varying from 1 – 15 mph. During some of the measurements a portable leak source was used to simulate a rail
car leak. The leak source was set parallel down the rail system from a vertically oriented Brüel & Kjær microphone.
The distance from the leak to the microphone was set to equal the distance from the train to the microphone. Figure 3
shows the detection region in greater detail.
The final sensors selected were a pair of piezoelectric transducers from Massa Products Corporation. The sensors
chosen had peak sensitivities at 40 and 75 kHz and were approximately 3 orders of magnitude more sensitive than other
similar piezoelectric sensors. These sensors were approximately $100 each but would be significantly cheaper if bought
0
Power (dB relative units)
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lea
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ai un
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Leak detection
band
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Fig. 3 The acoustic detection region expanded. In both cases a train is present and the leak
is simulated manner described in the text.
in large quantities. Additionally the sensors were significantly smaller than the other vendors measuring less than 1 cm
in diameter and weighing 6 grams. It was determined that the Massa sensors were the best fit for this application.
In order to extend the frequency response of the Massa sensors, 5 mH inductors were added in parallel to each sensor
creating an LC resonant circuit which increased the sensitivity beyond the mechanical response of the system. As a
result the transducers had additional sensitivity at 55 kHz in addition to their mechanical response. This yielded near
complete ultrasound coverage from 35 kHz to 80 kHz.
To mitigate risks from having leaking pressurized tanks near a commercial railway, an electronic surrogate was found
which would emulate the ultrasonic sounds created by a leak. The Ansonics’ Son-Caster II was utilized in later tests.
The Son-Caster II emits tones between 35 kHz and 45 kHz at sound pressure levels of approximately 100 dB.
3.
SYSTEM DESIGN
A 10 m tall tower houses the system and is positioned approximately 10 m from the center of a rail line and siding
located in Bowling Green, KY. The current position is due to concerns with the railroad right-of-way but future
installations will be within a couple of meters of the rails and emplaced on both sides of the train.
Our data indicates that a sound pressure level of below 40 dB can be detected at distances up to 10 m for sound sources
with a frequency of approximately 40 kHz. This frequency band is relatively quiet which allows detection at these low
sound levels.
As described earlier, the system has five transducers, three 40 kHz center frequency and two 75 kHz center frequency
transducers. To facilitate a need for increased sensitivity due to atmospheric attenuation losses, parabolic dishes are
used as shown in Figure 4. These dishes will not be needed once the system is implemented in a closer proximity to the
train. A small microphone is used to normalize the sound levels.
Fig 4. A train passing the rail car leak detection system located on the tower in the center. Four acoustic sensors
(parabolic dishes) can be seen. A video camera is mounted to inspect the passing train. The large metal box at
the bottom of the tower houses the data acquisition system and the WiFi access point. The radiological sensor
(not shown) is typically located just above the lowest acoustic sensor. The tower is located on university
property and does not infringe on the railroad right-of-way.
In order to monitor the trains for special nuclear materials and other radiological materials, a 5 cm x 5 cm NaI(Tl)
gamma ray spectrometer has been added to the system. Spectra are collected every 60 seconds and saved to disk as the
train passes by.
There are three video cameras. As shown in Figure 4, two of these cameras view the approaching trains from different
directions. The third camera is aimed to view the passing train and its images are stored on disk.
The sampling frequency of the ADC is 200 kSamples/s and thus the Nyquist frequency is 100 kHz. Every 20 ms a new
audio spectrum is collected and its power spectrum is computed. Ten of these power spectra are averaged together to
produce an average power spectrum response over a time frame of 200 ms. The averaging helps smooth transient noise
and the resulting spectra are stored within the AVI file with the video data. The video camera produces images at the
rate of 30 frames per second (FPS) but we only store every sixth frame for a rate of 5 FPS (or 1 frame every 200 ms).
Thus every video frame has an audio spectrum associated with it
The system is controlled by a website and server located at the tower and the Internet connection utilizes WiFi
(802.11g) radios transmitted to the Western Kentucky University (WKU) campus. During the inspection process, the
train moves past the tower at normal speeds. An infrared sensor triggers the data collection.
4.
DATA ANALYSIS
Figure 5 shows the user interface for the data analysis. The upper left corner of the interface is a video player which
allows the user to play the video in real time or to step through the video of the train a frame at a time. In the upper
right corner is an audio spectrogram. As each frame is shown in the video, the spectrogram will advance in time. The
corresponding power spectrum for the transducers is shown in the lower left (40 kHz center frequency transducers) and
Fig 5. Data analysis interface for the tank car leak detection system. See text for a complete description.
the lower right (75 kHz center frequency transducers). The power spectrum shown in Figure 5 at the lower left is the
electronic sound source in the presence of the train. The source and the train are at the same distance from the
transducers. The electronic sound source only produces a 35-40 kHz spectrum which is why there is no signal in the 75
kHz transducers in the lower right. Currently, detection is made through visual inspection of the power spectrum.
5.
CONCLUSION
Based on our research, we have shown that using ultrasonic sensors, one can detect the leaks in pressurized rail cars.
The minimum sound level detectable is less than 40 dB at a distance of approximately 10 m. This has allowed us to
build a monitoring system for these tank cars as they pass at normal speeds. Orthogonal sensors such as video cameras,
allow us to visually inspect the passing cars as well. Since the video frames are time correlated to the audio spectra, we
can inspect, one frame at a time, both visually and audibly, the passing cars. A third orthogonal sensor (a radiological
detector) is also correlated to the video frames in order to increase our threat or hazardous material detection capability.
Further work to be done is to 1) automate the audio detection algorithms, 2) correlate the radiation spectrum with the
video image, and 3) create receiver-operator characteristic (ROC) curves to evaluate the system’s effectiveness.
6.
ACKNOWLEGMENTS
This work was supported by the Department of Homeland Security under contract EKU 06-203 and supported in part by
the Applied Research and Technology Program of Western Kentucky University. We also wish to thank the National
Institute of Hometown Security for their support during this project.
REFERENCES
1. Edward R. Hamberger, President/CEO Association of American Railroads, Statement before the US House of
Representatives Committee on Transportation and Infrastructure, Subcommittee on Railroads, June 13, 2006.
2.
Hazardous Materials Information System, US Department of Transportion. Data as of 9/7/2006.
3. United States General Accounting Office, Report to Congressional Requesters, “Rail safety and security: Some
actions already taken to enhance rail security, but risk-based plan needed,” April 2003.
4. J. Riley, “Terrorism and Rail Security,” Testimony presented to the Senate Commerce, Science, and Transportation
Committee, RAND Corporation, March 23, 2004.
5.
Accessed at http://www.vpc.org/graphics/whyregulate50s.pdf in November 2006.