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INTRIGRITY MANAGEMENT OF PLANT EQUIPMENT WITH THE USE OF HIGH-TECH NON-DESTRUCTIVE TESTING TECHNIQUES
Time-of-flight-diffraction (TOFD) is an ultrasonic inspection method originally developed by the United Kingdom Atomic Energy Authority (UKAEA) for the non-destructive evaluation of components in the nuclear industry. The method relies on the diffraction of ultrasonic energies from 'corners' and 'ends' of internal structures (primarily defects) in a component being tested.
TOFD is now recognised as one of the more rapid, versatile and reliable method of ultrasonic NDT available to industry today. The TOFD method relies on the diffraction of ultrasonic energies from corners and ends of defects in a component being inspected. The technique was originally developed as a method of accurately sizing and monitoring the through-wall extent of welding defects and in-service flaws, primarily in steel components. Maurice Silk, its inventor, released the first report for publication in 1977, stating the main principles of the technique. These original principles have not needed revision since, which is a clear indication of the integrity of the technique.
Tecnovex has, since 2002, adapted the TOFD technique and developed inspection procedures to locate, identify and size defects in lined carrier rolls used in the pulp and paper industry. The procedures cover defects such as disbonding of the lining and cracks contained within the roll. Tests have shown that TOFD can accurately identify and size defects contained in the steel rolls.
Tecnovex has also successfully developed TOFD inspection procedures for wall thickness profiling of the drying cylinders. These inspection methods have been implemented at four pulp & paper plants in South Africa.
THE TIME OF FLIGHT DIFFRACTION INSPECTION METHOD
The TOFD technique uses a single probe pair in a transmitter-receiver arrangement. Longitudinal probes are usually applied for transmitting and receiving the ultrasound through the material that is being inspected (Figure 1).
When the ultrasound is incident at linear discontinuity such as a crack, diffraction takes place at its extremities in addition to the normal reflected wave. This diffracted energy is emitted over a wide angular range and is assumed to originate at the extremities of the flaw. This is in marked contrast with conventional ultrasonic techniques, which rely on the amount of energy reflected by discontinuities1.
The TOFD method detects a surface (lateral) wave travelling directly between the probes and also a backwall echo from energies that reach the back of the test piece without interference from defects. The diffracted signals are received via the receiver probe and are evaluated with the Ultrasonic System to greyscale images (Figure 2).
The study of this phenomenon has led to the successful use of the TOFD method on metallic and non-metallic components for2:
BS 77063, promotes the use of the TOFD inspection method in any inspection task that requires rapid and reliable flaw detection coupled with an accurate assessment of flaw size.
TIME-OF-FLIGHT-DIFFRACTION INSPECTION OF CARRIER ROLLS
The Paper & Pulp industry has, in recent months, had carrier rolls in the paper and board machines that failed during service. These failures could be attributed to:
The inspections of the rolls are made difficult due to the lining that is applied to the outer surface of the roll and also their position in the machines. It is therefore believed that ineffective inspections methods, e.g. ultrasonic testing from the ends of the rolls, are presently being used producing unreliable results due to limited long-range capabilities of the applied inspection methods.
A paper mill in South Africa supplied us with a section of a fractured roll to experimentally develop an inspection technique specifically for the carrier rolls. Subsequently we adapted the TOFD technique to inspect the rolls with linings to detect and size cracks within the shell material and to locate any disbonding of the lining.
Initially concern was raised about the effectiveness of any inspection and the reliability of acquired data due to the thickness of the lining on the roll, i.e. 6 mm. However, we were able to develop an inspection procedure to locate and accurately size defects in the rolls without the lining having any effect on the acquired ultrasonic data. It was also proven that the inspection method could be used to locate areas of disbonding of the lining and used to effectively size and monitor cracks that initiate from either the outer or the inner surface of the roll shell.
Crack detection at the inner surface of the carrier roll
Figure 3 shows the carrier roll sample supplied to us with a TOFD scanner set-up. Two grooves, one at the change of section and one in the shell of the roll, both 2 mm wide by approximately 4 mm and 2.4 mm deep by mechanical measurement respectively, was ground into the wall of the cylinder to simulate cracks from the inner wall (Figure 4),
Figure 5 shows the TOFD data obtained from the roll sample. Point A indicates the simulated crack at the change in section of the roll wall and Point B indicates the simulated crack in the wall of the roll. The cracks were measured, by applying the measurement facility in the Microplus software, to be 4 mm and 2.38 mm deep.
Crack detection at the outer surface of the steel cylinder
We have also demonstrated the capability of the TOFD inspection technique to detect and size cracks that initiate at the outer surface of the steel cylinder. Figure 6 indicates the scan positions for crack detection & verification.
TOFD images of scan positions 1 & 2 are presented in Figure 7 and TOFD data obtained from positions 3, 4, 5 & 6 are presented in Figure 8.
The figures clearly indicate that the TOFD inspection technique is able to detect and size cracks in carrier rolls. The inspection results can be stored in a database for defect size monitoring and management.
TOFD INSPECTION OF DRYING CYLINDERS
Wall thickness measurements taken from a drying cylinder using conventional NDT equipment is believed, and has been proven, to present inaccurate results. This is due to the fact that the cylinders are manufactured from cast iron, therefore having a very coarse grain structure. The grain structure also varies in coarseness from the manhole end to the drive end of the cylinder presenting variations in the sound properties of the material. The coarse grain structure, in most instances, scatters the ultrasound in such a manner that no reliable thickness values can be obtained.
However, we have developed a TOFD inspection technique to obtain a continuous wall thickness profile of the cylinder wall. This technique has been successfully applied at three paper mills in South Africa.
The inspection set-up consists of a purpose-manufactured cart with magnetic wheels that contains the two TOFD probes, the encoder and the ultrasonic couplant feed. The cart is attached to the cylinder and drawn along the length of the cylinder during the collection of the ultrasonic data at encoded intervals of 1 mm (Figure 9 and Figure 10 ).
Figure 12 presents an image of TOFD inspection results of a 6 m long drying cylinder. Note the change in quality of the results at the right-hand side of the image (the drive end). At the centre of the first image a loss in wall thickness is noted, which is located at the rotating siphon tube position.
As can be seen from Figure 10 and Figure 11, this method does not require the NDT technician to walk on top of the cylinder to collect thickness measurements at specified distance increments.
The TOFD inspection method has been successfully applied in the pulp & paper industry for:
An ultrasonic inspection procedure was developed to locate and accurately size and monitor crack like defects in carrier rolls without the lining affecting the quality of the ultrasonic data . The inspection method can also be used to locate areas of disbonding of the lining.
The TOFD inspection method can be used to effectively profile the wall thickness of drying and clupack cylinders. The TOFD inspection data can be stored to accurately compare and monitor the wall thickness in after each inspection.
1. "TOFD Principles", Nondestructive Testing Encyclopedia, NDTnet, www.ndt.net/article/az/ut/info/TOFDp.htm.
2. "Best practice for risk based inspection as part of plant integrity management", Contract research report 363/2001, Health & Safety Executive, 2001, ISBN 0717620905.
3. British Standard, BS 7706: 1993 - Guide to Calibration and setting-up of the Ultrasonic Time of Flight diffraction (TOFD) technique for defect detection, location and sizing of flaws.