Click here to load reader
Upload
gwenllian-baker
View
103
Download
13
Tags:
Embed Size (px)
DESCRIPTION
Guided Wave Testing. The I.A. University of Ahvaz. December 27, 2012. Mahdi Bavi Hossein Kharazmipour. DEFINITION. Guided waves are ultrasonic waves that propagate along the length of a structure, guided by and confined in the geometric boundaries of the structure. Flange. - PowerPoint PPT Presentation
Citation preview
Guided Wave Testing
December 27, 2012
The I.A. University of Ahvaz
Mahdi Bavi Hossein Kharazmipour
Guided waves are ultrasonic waves that propagate along the length of a structure, guided by and confined in the geometric boundaries of the structure
DEFINITION
Conventional UT measures the wall thickness at a spot , while GWT can identify locations of metal loss along a length of the pipe
Weld Metal loss Metal loss
FlangeConventional Ultrasonic Test
Weld Metal loss Metal loss
Guided Wave
100% Inspection
Localized Inspection
Conventional ultrasonic inspection provides a local thickness measurement
GWUT Inspection provides detection of both internal and external corrosion
The difference in concept between UT and GWT
Guided waves are bulk waves; therefore the entire volume of the pipe is inspected
Frequencies used in guided wave inspection are much lower than conventional ultrasonic testing; therefore the wave lengths are much longer and are scattered instead of reflected from changes in the dimension of the wave guide
The pipe acts as a wave guide, permitting the waves to travel long distances
The waves can be introduced at a single location:◦ When introduced with piezoelectric crystals an array of transducers are used. ◦ Coils of wire are used to create vibrations in the pipe via the magnetostrictive effect
The differences between conventional UT waves and guided waves
Longitudinal
Torsional
Flexural
The waves used in the GWT
Guided waves are introduced into the pipe by one of two systems:
An array of piezoelectric crystals are positioned in modules that typically hold two transducers each. The modules are spaced around the pipe under an air bladder which when pressurized forces the units against the surface. The individual crystals oscillate at the frequency at which they are excited and transmit the wave into the pipe.
Coils of insulated wire are wrapped around the pipe. An alternating current is passed through the coils, and an oscillating magnetic field is produced. Due to the magnetostrictive effect ,this produces a wave in the pipe which can be amplified by using a nickel or cobalt strip bonded to the pipe under the coil.
The power and durability of today’s electronics has made it possible to field the GWUT system in a compact package
Pressurized bladder containing the array of piezoelectric crystals
Monitor
Field electronics
Umbilical cable
connecting electronics to transducers
Weld
WeldArea of corrosion
Example of graphical data display
11
Example of graphical data display
Distance Amplitude
Correction (DAC) CurvesWeld
Minor Anomaly
Welds at two elbows
Can test long distances of pipe from a single access point
Has developed into an effective screening tool useful in locating and ranking areas of corrosion; thereby minimizing the amount of follow-up inspection needed to determine the integrity of piping.
Can be used on in-service pipelines
Both internal and external corrosion can be identified
Current commercial systems are packaged in a small number of durable components. The systems are easily transported and quickly setup in the field with preliminary results available at the time of the test
Some Advantages of Guided Wave Ultrasonic Testing
Complicated evaluation of data by highly trained operators is required because of the complex signals involved
Dimensions of corrosion cannot be directly determined
Significant corrosion can be missed, especially localized damage
The scattered signal cannot be directly equated to a specific area or volume of loss due to a lack of an absolute calibration standard
Many field conditions exist that limit the distances that can be effectively inspected and that cause artifacts which can complicate analysis.
The tests identify change in cross-sectional area, and can miss corrosion that is general in nature, is in the configuration of grooves that pass under the array, or are too small to detect
Some Limitations of Guided Wave Ultrasonic Testing
various coating such as coal tar epoxies, asphalt-tar wraps, concrete, etc, plastic sleeves, particularly those with internal mastics wet insulation, particularly if ice is present rough internal or external surfaces direct buried pipe, particularly in situations where heavy or wet soil is
encountered dense product, internal build up of solids, and situations with variable
product flow system noise created by factors such as turbulent product flow or pumps temperature variations and gradients that can lead to changes in the wave
velocity
The conditions that can limit the distance of a piping segment that can be reliably inspected
15
Test on Buried Pipeline
Pit located on 10” Schedule 80 pipe (0.15 in deep x 4.5 in x 3 in)
This is a photograph of the corroded area which caused the leak in a buried 6” line.
Along the line drawn, the cross-sectional area of the ½” walled pipe is approximately 9.62 square inches, while the area lost to corrosion through the hole is 0.5 square inches.
this is a loss of approximately 5.2% of the cross-section. It would not been seen in a scan since the section was buried. However, if this line was above-ground and exposed the corrosion probably would have been noted as a minor anomaly
Example of corrosion that would not have been noted with Guided Wave on a buried piping segment
Weld profiles are assumed to be uniform along the length of the tested segment, and represent some arbitrary percent change in cross-sectional area, typically 25% CSC. There is no absolute calibration standard.
This can compromise the accuracy of the results and can even lead to miss-calls, as in the case below. The high-low condition extended around approximately one-forth of the circumference, created an asymmetrical response, and was therefore ranked as a moderate anomaly.
1. Guided wave offers valuable new inspection technology if it’s capabilities and limitations are kept in mind.
2. It is a SCREENING tool. Need to follow up with other NDT techniques to quantify / evaluate possible defects.
3. If used without other verification, GWT cannot provide the level of detail needed to ascertain the integrity of piping.
4. Main advantage is the ability to screen long sections of pipe to determine overall, general condition and locate areas that require more detailed examination.
Conclusions