Acoustic Emission in CNG cylinder testing
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EVALUATION OF THE DEFECTS IN TYPE-I CNG CYLINDER USING ACOUSTIC EMISSION TECHNIQUE Suparerk Sirivedin (1) , Tonphong Kaewkongka (2) , Jirapong Lim (3) (1) King Mongkut’s University of Technology North Bangkok Bangsue, Bangkok 10800, Thailand Email : [email protected] (2) Department of Physics, Chulalongkorn University Pathumwan, Bangkok 10330, Thailand Email : [email protected] (3) Department of Production Engineering, King Mongkut’s University of Technology North Bangkok Bangsue, Bangkok 10800, Thailand Email : [email protected] ABSTRACT Defects on compressed natural gas (CNG) storage cylinders can often result in damage of costly cylinders. Acoustic emission (AE) testing was performed on type I steel cylinder. These studies suggested that the elastic stress waves or acoustic emissions generated during the microscopic dislocation can propagate across the storage cylinder surface to be detectable by the sensors attached at the ends of the cylinder. This paper presents a preliminary study on the signal transmission and propagation of acoustic emission (AE) signatures across the cylinder with and without a predefined surface crack. During the test, a gradual increase in hydrostatic pressure up to 400 bars was applied to the cylinder. The AE signals were recorded as a function of time and the increased pressure. It is therefore obvious that AE can be used to capture the defects due to crack propagation. Index Terms— CNG Cylinder, Acoustic Emission, Surface Crack Propagation, 1. INTRODUCTION To assure the safety of vehicles using compressed natural gas (CNG) is an important issue in Thailand. There have been several accidents from CNG-cylinder explosions in the past. The periodical inspection requires the first inspection after the cylinder has been using for 3 years. However, the inspection is normally carried out by visual inspection. Consequently, the risk is relied on the inspection by licensed professional engineers since the internal defects cannot be examined. The fully inspection can be accomplished by ultrasonic test but this method requires removal of the cylinder from the vehicles. Therefore, this test method is impractical due to time-consuming and high inspection cost. Recently, acoustic emission (AE) technique has been standardized for gas cylinder inspection as described in ISO/DIS 16148.2 [1]. Using AE technique, the inspection can be quickly performed without disassembling the cylinder from vehicle. However, Craig Webster [2] has found out that the AE test based on ISO/DIS 16148.2 is unable to evaluate defects in Type I steel cylinder. Mark P. Connelly and Han Dinh [3] proposed the AE technique called “Source Location Acoustic Monitoring (SLAM)” to inspect US Postal Service vehicles. They concluded that the inspection cost for acoustic emission SLAM test is about half of the cost for conventional hydrostatic test. They also claimed that the SLAM test has the ability to locate both the external/internal flaws. Stephen J. Hudak [4] has used AE technique to evaluate remaining life-time of the cylinders. He found that at the initial crack of about 25 percent of the wall thickness the cylinder has the remaining life-time for 95 years!, while the deeper crack of 40 percent of the wall thickness the cylinder is last for 5 years under the cyclic load caused by daily refueling pressure ranging from 300 to 3,000 psi. A major benefit of AE inspection is that it can allow the whole volume of the cylinder to be tested non- intrusively in a pressurized operating condition. Generally, the global AE inspection is used to identify areas with the presence of defect problems and other NDT methods are then used to identify more precisely location and the root cause of the AE sources.
Transcript of Acoustic Emission in CNG cylinder testing
EVALUATION OF THE DEFECTS IN TYPE-I CNG CYLINDER USING ACOUSTIC EMISSION TECHNIQUE
Suparerk Sirivedin (1), Tonphong Kaewkongka(2), Jirapong Lim(3)
(1) King Mongkut’s University of Technology North Bangkok Bangsue, Bangkok 10800, Thailand Email : [email protected]
(2) Department of Physics, Chulalongkorn University Pathumwan, Bangkok 10330, Thailand Email : [email protected]
(3) Department of Production Engineering, King Mongkut’s University of Technology North Bangkok
Bangsue, Bangkok 10800, Thailand Email : [email protected]
ABSTRACT Defects on compressed natural gas (CNG) storage cylinders can often result in damage of costly cylinders. Acoustic emission (AE) testing was performed on type I steel cylinder. These studies suggested that the elastic stress waves or acoustic emissions generated during the microscopic dislocation can propagate across the storage cylinder surface to be detectable by the sensors attached at the ends of the cylinder. This paper presents a preliminary study on the signal transmission and propagation of acoustic emission (AE) signatures across the cylinder with and without a predefined surface crack. During the test, a gradual increase in hydrostatic pressure up to 400 bars was applied to the cylinder. The AE signals were recorded as a function of time and the increased pressure. It is therefore obvious that AE can be used to capture the defects due to crack propagation.
Index Terms— CNG Cylinder, Acoustic Emission, Surface Crack Propagation,
1. INTRODUCTION To assure the safety of vehicles using compressed natural gas (CNG) is an important issue in Thailand. There have been several accidents from CNG-cylinder explosions in the past. The periodical inspection requires the first inspection after the cylinder has been using for 3 years. However, the inspection is normally carried out by visual inspection. Consequently, the risk is relied on the inspection by licensed professional engineers since the internal defects cannot be examined. The fully inspection can be accomplished by ultrasonic test but this method requires removal of the cylinder from
the vehicles. Therefore, this test method is impractical due to time-consuming and high inspection cost.
Recently, acoustic emission (AE) technique has been standardized for gas cylinder inspection as described in ISO/DIS 16148.2 [1]. Using AE technique, the inspection can be quickly performed without disassembling the cylinder from vehicle. However, Craig Webster [2] has found out that the AE test based on ISO/DIS 16148.2 is unable to evaluate defects in Type I steel cylinder. Mark P. Connelly and Han Dinh [3] proposed the AE technique called “Source Location Acoustic Monitoring (SLAM)” to inspect US Postal Service vehicles. They concluded that the inspection cost for acoustic emission SLAM test is about half of the cost for conventional hydrostatic test. They also claimed that the SLAM test has the ability to locate both the external/internal flaws.
Stephen J. Hudak [4] has used AE technique to evaluate remaining life-time of the cylinders. He found that at the initial crack of about 25 percent of the wall thickness the cylinder has the remaining life-time for 95 years!, while the deeper crack of 40 percent of the wall thickness the cylinder is last for 5 years under the cyclic load caused by daily refueling pressure ranging from 300 to 3,000 psi.
A major benefit of AE inspection is that it can allow the whole volume of the cylinder to be tested non-intrusively in a pressurized operating condition. Generally, the global AE inspection is used to identify areas with the presence of defect problems and other NDT methods are then used to identify more precisely location and the root cause of the AE sources.
The aim of this research is to study the feasibility of condition monitoring of CNG storage cylinder using AE sensors and its propagation of the stress waves through the type I CNG steel cylinder with and without surface crack.
2. EXPERIMENTAL APPARATUS
Acoustic emission is a natural phenomenon of stress wave generation and propagation spontaneously when a material is subjected under stress. Plastic deformation and growth cracks are the primary sources of acoustic emission in metals. The acoustic signal can be detected by a piezoelectric transducer, which converts the mechanical energy carried by the elastic wave into an electrical signal as shown in Figure 1.
The AE inspection is usually carried out during a controlled loading or pressurization of the material or specimen. A conventional AE parameter, AE hit rate is used to identify the presence of the acoustic emission activities produced during the microscopic failures.
Figure 1 Acoustic emission system
The resonant type of acoustic emission transducers (Holroyd Instruments, UK: Model ASS-1) are mounted on the circumference of the surface of the valve.
It provides the 100 kHz of resonant frequency which
responses well with the material degradation and microscopic crack initiation. The acquired signal is then amplified with 60 dB gain pre-amplifier.
Figure 2 SIMPAL system from Holroyd
The AE signal enveloper converts the amplified signal to ‘rf’ signal which is digitized to personal computer for further data logging and processing. The SIMPAL system equipment (Holroyd Instruments, UK) was used as acoustic emission data logger and processing in this work (see Figure 2). The AE data can then be captured to data logger (PC). In the experiment, four AE sensors were attached to the cylinder without crack (see Figure 3), and with a surface crack (see Figure 4). Characteristic and dimensions of surface crack are shown in Figure 5.
Figure 3 Installation of AE sensors on CNG cylinder
Figure 4 Installation of AE sensors on CNG cylinder with a longitudinal surface crack
The cylinder used in this experiment has a capacity of
60 litres. It has a diameter of 300 mm, the length of 990 mm and the averaged wall thickness of 8 mm. The cylinder was installed in a chamber of a high-pressure hydrostatic testing machine (see Figure 6).
Figure 5 Surface crack characteristic
The threshold was set at 40 dB based on observed background noise. A gradual increase in hydrostatic pressure up to 400 bars was applied to the cylinder.
Figure 6 Installation of CNG cylinder in a test chamber
3. EXPERIMENTAL RESULTS
3.1 Hydrostatic test on CNG cylinder without crack A hydrostatic test with an applied pressure up to 300 bars was performed on a CNG cylinder without a surface crack. The parametric input indicates the pressure level (with the maximum voltage 10 Volt corresponding to 300 bar and minimum 0 Volt corresponding to 0 bar). The first acoustic emission hit rate (AE Hit Rate) was detected at the applied pressure of 90 bars (see Fig. 7).
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Log
Hit
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e Parametric
NGV-Node 0 (11/09/2008 , 1:09:01)NGV-Node 0 (11/09/2008 , 1:09:01)Log Hit Rate vs Recorded Date/TimeLog Hit Rate vs Recorded Date/Time
Recorded Date/Time
Figure 7 AE hit rate v.s. pressure for CNG cylinder without surface crack
3.2 Hydrostatic test on CNG cylinder with a crack
A hydrostatic test with an applied pressure up to 300 bars was performed on a CNG cylinder with a surface crack. A crack length, l = 30 mm and a crack depth, a = 2 mm was introduced in a longitudinal direction of CNG cylinder by hand grinder. The accumulated AE counts of 7 times were detected at the applied pressure between 120-140 bars.
Figure 8 AE counts v.s. pressure for CNG cylinder with a surface crack at hydrostatic test
3.3 Burst test on CNG cylinder with a surface crack
A burst test was performed on a CNG cylinder with a surface crack. A crack length, l = 50 mm and a crack depth, a = 6 mm was introduced in a longitudinal direction on the same CNG cylinder, which enlarged a crack size by hand grinder. At the applied pressure between 100-170 bars, more than 100 AE counts were detected.
AE signal was not detected after 170 bars until the applied pressure was increased to 310 bars. The accumulated AE counts more than 2 million times were detected at the applied pressure between 310-340 bars. The CNG cylinder leaked at the location of surface crack at the applied pressure of 340 bars.
Figure 9 AE counts v.s. pressure for CNG cylinder with a surface crack at burst test
4. CONCLUSIONS Due to “Kaiser effect” in which the material recognizes its service stress, the applied pressure to cause AE hit must be greater than the service pressure. When the surface crack was introduced onto the CNG cylinder, a greater number of acoustic events are likely to occur than CNG cylinder without crack. As the surface crack size is increasing, a large number of acoustic events were detected at the same pressure level. Therefore, acoustic emission technique can be used to evaluate and assess how severe of the defects in Type-I CNG cylinder.
5. ACKNOWLEDGEMENTS The authors would like to thank Energy Technique Development Institute, Department of Energy Business of Thailand for testing facilities. They would also like to thank Iron & Steel Institute of Thailand for financial support throughout this paper.
6. REFERENCES [1] ISO/DIS 16148.2, “Gas cylinders-Refillable seamless steel gas cylinder- Acoustic emission examination for periodic inspection”, International Organization for Standardization, 2006. [2] C. Webster, “Development of Non-Destructive Evaluation (NDE) Techniques for CNG Fuel Tanks”, Report for Transportation Development Centre of Transport Canada, Canada, 2007. [3] M.P. Connolly, H. Dinh, “Fleet Inspection of Compressed Natural Gas Cylinders for Natural Gas Vehicles Using Source Location Acoustic Monitoring” SAE Technical Paper Series No. 961174, SAE International, 1996. [4] S.J. Jr. Hudak, “Assuring the Safety of Natural Gas Vehicles” Technology Today Magazine, Sept. 1991.
