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APPLICATION AND DEPLOYMENT OF ADVANCED NDE TECHNIQUES IN HIGH

PRESSURE VESSELS

Jeffrey P. Milligan, Daniel T. Peters, Structural Integrity Associates, Inc., USA

Many advances in Non-Destructive Examination (NDE) have occurred in recent years. Some of

these are becoming common in typical industry applications and are slowly migrating their way into niche

industries, such as high-pressure applications. These advanced NDE techniques include the use of Linear

Phased Array (LPA) ultrasonic examination for volumetric examination and Eddy Current Array (ECA)

technology for surface examination.

Complete periodic assessment of a high pressure vessel’s condition is key to safe, long-term reliable

operation. Structural Integrity provides a comprehensive inspection program that analyzes high pressure

equipment and identifies critical areas where a potential failure mode may exist. Using advanced NDE

techniques, SI is able to overcome some common challenges found in high-pressure equipment, like

access issues of small diameter deep bores, large and thick section components, weld overlays and

examination of thick section welds, complex geometries, and the requirement to detect small crack sizes

due to equipment design and materials used.

ADVANCED NDE APPROACHES

Many traditional NDE methods, such as liquid penetrant testing (PT), magnetic particle testing (MT), eddy

current testing (ET), radiographic testing (RT) and conventional single-element ultrasonic testing (UT)

can be replaced or improved by using an advanced NDE approach.

Eddy Current Array (ECA)

The eddy current array inspection approach provides a surface, or near-surface, inspection of electrically

conductive materials, such as stainless steel, and is therefore well suited for inspection of many high-

pressure equipment materials. ECA provides many advantages over other surface NDE techniques such as

PT and MT. Some of these advantages include increased speed of inspection, digital data storage for a

permanent record, depth sizing, no chemical waste, and ECA can be done remotely using an automated

scanner.

Eddy current inspection is an NDE method that utilizes the principle of electromagnetism,

specifically electromagnetic induction. When an alternating electric current is applied to a conductor,

such as a copper wire (coil or probe), a magnetic field develops in and around the coil. When this coil is

brought close to a conductive material, such as stainless steel, the coil’s changing magnetic field generates

current flow in the conductive material. The induced current flows in closed loops called eddy currents.

Changes in the flow of these eddy currents, like disruption by a flaw, can be detected and quantified on the

eddy current instrument display.

Eddy current array technology provides the ability to electronically drive multiple eddy current

coils, which are placed side by side as an array in the same probe assembly. Each individual eddy current

coil in the probe produces a signal relative to the phase and amplitude of the structure below it. This data

is referenced to an encoded position and time, and can be represented graphically as a C-Scan image. The

ECA probe can be designed to be flat or contoured to fit a specific geometry (Figure 1). Some probes are

sold as flexible arrays that can fit multiple contours. The size, frequency, and amount of coils in the array

probe will be dependent on the inspection requirements like material type, critical flaw size, and part

geometry. The capability of the eddy current array acquisition system will also dictate the amount of coils

available for inspection.

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Figure 1. a) Flat ECA probe; b) Radiused ECA probe

Compared to conventional single-coil eddy current technology, eddy current array technology

drastically reduces inspection time because a large area can be scanned in a single probe pass while still

maintaining high resolution. ECA also reduces the complexity of mechanical and robotic scanning

systems required to inspect a specific surface geometry or surface area. When used with an encoded

scanning system it provides real-time C-Scan image of the inspected region, thereby facilitating data

interpretation, and improving flaw detection, sizing, and probability of detection.[1] Encoded eddy

current array technology also allows for a permanent record of inspection data to be saved and referred to

for future inspections.

Typical for flaw detection, a reference standard calibration block is needed in order to normalize the

individual coils of the eddy current array probe and setup a comparison of known flaw sizes. Figure 2

shows an ECA C-Scan display next to the impedance plane display, or Lissajous, and under that is a strip

chart display. An image of a radiused reference standard can also be seen in Figure 2. The C-Scan

display shows a top down view of the encoded scan area with colors representing signal amplitude in

volts. Further analysis of any position on the C-Scan display can be done using the impedance plane

(Lissajous) or strip chart displays. Signals from individual coils can be analyzed to determine accurate

measurements of voltage amplitude and phase angle in order to categorize and quantitatively size

indications.

Figure 2. ECA C-Scan, Impedance Plane (Lissajous), and strip chart display of a reference standard

with different size EDM notches

Linear Phased Array (LPA) Ultrasonics

The linear phased array inspection approach provides a volumetric inspection of materials and is therefore

well suited for inspection of potential cracking on many high-pressure components. LPA ultrasonic

technology utilizes an array transducer (probe) that contains multiple transducer elements, as opposed to

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the single element of conventional pulse-echo transducers. Each element of an array probe can be utilized

as a transmitter and/or receiver, and when each transducer element is pulsed sequentially with small,

precise timing delays imposed one to the next, ultrasonic beam steering and focusing can be varied and

controlled. A linear array consists of a number of linear elements arranged in a single row, or in a two-

dimensional pattern. Array probes are available in a variety of shapes, sizes, number of elements and

frequencies. All these parameters are important in determining the steering and focusing capabilities of

the probe.[2]

Linear phased array technology provides the ability, by proper phasing, to steer the ultrasonic beam

through a series of different angles covering a sector typically over a range of 60º depending on wave

mode and array parameters. The linear array is used primarily to influence beam direction, electronically

focus the beam, or a combination of the two. A true spatial representation of the linear array data requires

that the data be presented in polar coordinates. The amplitudes of the waveforms, plotted sequentially at

each digitization point along each waveform, are typically presented in colors such that the presentation

provides instant recognition of the position of a reflector as well as its significance in terms of reflection

amplitude. These plots have become known as sectorial scans, or S-scans, because they represent sectors

of the cross-section of the component in the plane of the beam.

One advantage of linear phased array technology includes inspection of small indications over long

metal sound paths. A feasibility study was performed using a 32 element, 5 MHz linear array probe

(commonly referred to as 5L32) which allowed for focusing of the ultrasonic sound beam to over 9.5

inches deep (12.4 inch sound path distance) in a low-alloy carbon steel mock-up block. Figure 3 shows a

beam simulation of this 5L32 transducer at a 40º refracted shear wave angle. As can be seen, the greatest

amount of sound energy is focused near 6.5 inches deep in the part, however the -6dB focal zone stretches

to over 9.5 inches deep. This focusing capability allows for excellent sensitivity and detection of a 0.04

inch wire EDM notch located 8.25 inches deep in mock-up block. The focusing capability of the 5L32

also provides clear detection from a 0.04 inch wire EDM notch that is at a depth of 12.75 inches (18.0 inch

sound path) from the probe inspection position, as seen in Figure 4.

Figure 3. 5L32 probe beam simulation at 40º in carbon steel

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Figure 4: S-scan (35º-55º refracted shear wave beam angles) of 0.04 inch wire EDM notch, 12.75 inches

deep (18.0 inch sound path at 45º) in carbon steel

An advantage of using these advanced NDE techniques is the ability to successfully detect and size

small flaws in areas that traditional NDE techniques would be limited. One example of this would be the

inspection of threads in pressure vessel closures. Figure 5 is an example of a buttress thread setup, similar

to what may be used in a vessel closure. Traditional methods for crack detection at the thread roots would

commonly be a surface exam using either PT or MT, which typically work well, but they offer no depth

sizing information. The flaw depth sizing advantage of LPA can be observed in Figure 5. Artificial

defects are shown to be easily detected and sized between 0.010 inch and 0.020 inch deep in expected

cracking locations.

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Figure 5: LPA Ultrasonic Images of Buttress Thread EDM notches

Computerized Data Acquisition and Scanners

A major advantage of the advanced NDE techniques are the computerized data acquisition and data

storage capabilities. NDE inspections are highly reproducible when inspecting with automated (moved by

encoded motor-controlled drive unit) or semi-automated (encoded movement by hand) scanning systems.

Encoded scanning systems offer speed and versatility when inspecting large areas, as well as precise

movement and adjustment of data collection resolution when dealing with complex geometries. Having a

digital permanent record can provide baseline inspection data, and assist in monitoring discontinuities over

successive inspection intervals. This permanent, digital inspection data can help calculate growth rates of

discontinuities and plan repair or replacement activities. Accurate NDE inspection data is an important

tool for implementing Risk Based Inspection Programs, Fitness for Service Analysis and remaining useful

life programs. Additionally, sophisticated data analysis software can be used to assist in flaw sizing and

interpretation, and in some cases 3-D image presentation of defects.

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An example of an automated encoded scanner for large inspection areas is the custom made SI

Inner Vessel Automated Scanner (SIIVAS), shown in Figure 6. The SIIVAS was designed for inspecting

the inside surfaces of a large high-pressure vessel and utilizes four axes of motion to accurately position an

ultrasonic phased array probe or eddy current array probe on any inside vessel surface. The vertical and

rotational axes are encoded and use a motor control drive unit (MCDU) to control the position, speed, and

zero point. The third and fourth axes are controlled remotely and used to orient the probe for scanning and

maintaining contact on the vessel walls. Both encoded axes have a resolution down to 0.0001 inches;

however, the maximum resolution of the MCDU is 0.001 inches. The scanner is also equipped with a

vision system consisting of two cameras with LED lights, time stamp and recording capabilities.

This type of automated scanning can be very effective for the inspection of the bottom radius area in

deep bore vessels. These deep bore areas can experience high stresses and corrosion, which need to be

inspected and monitored. These types of automated delivery systems are effective in accessing difficult to

reach areas of a vessel and provide the ability for surface and volumetric inspection of previously

inaccessible areas.

Figure 6: Inside Vessel Automated Scanner holding ECA probe

SUMMARY

Advancements in NDE technology have led to more frequent use of phased array ultrasonics and eddy

current array technology in industrial applications, specifically high-pressure equipment inspection. The

use of encoded scanners and computerized data acquisition and analysis programs provide reliable

detection, sizing and permanent recording of critical inspection data. These capabilities can improve

comprehensive inspection programs which will ultimately lead to better life and condition management of

high-pressure equipment critical assets.

REFERENCES

1) Lafontaine, G., and R. Samson. “Eddy Current Array Probes for Faster, Better and Cheaper

Inspections”, NDT.net, Oct 2000, Vol. 5 No.10

2) R/D Tech, 2004, Introduction to Phased Array Ultrasonic Technology Applications, pg. 7-18, R/D

Tech, Quebec, Canada