Coke Drum Weld Inspection

Abstract

Conventional Ultrasonic Inspection of Coke Drums may require the use of Automated Pulse Echo or Time of Flight Diffraction Techniques (TOFD). The more recent application of Phased Array ultrasonic technology enables a faster and more accurate location and depth discrimination of the cracks detected in the welds. Pulse Echo ultrasonic inspection requires the use of three transducers from each side of the weld. A zero degree compression transducer and two angle transducers, most likely 60° or 70°. The advantage of this techniques is that it provides positional information as to the location of the crack in the weld and accurate length measurement. The problem is that additional techniques have to be used to determine the depth of any cracks detected. An alternative to Pulse Echo inspection is the Time of Flight Diffraction technique. The TOFD technique uses multimode transducers to insonify the weld region with Lateral, Compression and Shear Wave ultrasound. The technique accurately detects and determines the length and depth of reflectors in the weld region. The technique was initially developed for the Nuclear Industry as a sizing technique. More recently it has become used for detection and sizing of flaws. The TOFD technique does not place the flaw in the cross section of the weld in order to achieve this another technique such as Pulse Echo Ultrasound is required. The TOFD technique is not sensitive to small flaws which are open to either surface. In order to detect small flaws such as “Toe Cracks” a supplementary technique such as ACFM or Eddy Current inspection may be required. The illustration shows the format of the sound generated from a TOFD transducer arrangement. The advantage for welds < 1.50" in thickness is that careful selection of the transducers and appropriate spacing may allow the weld to be inspected in a single pass. The illustration below shows two displays, an unrectified “RF” display which corresponds to which ever cursor is active and a grey scale display adjacent. The Grey Scale Display is a stacked “RF” display where each vertical line correspond to a single location along the line of the weld non-conforming perturbations in the display indicate areas of concern which can be identified by length and depth as shown in the boxes at the lower left of the illustration. The first significant amplitude group on the grey scale display corresponds to the Lateral Wave, the second the Compression and the third the Shear Wave. Flaws detected between the Lateral and the Compression Wave are often repeated between the Compression and the Shear Waves. Phased Array technology has been available for some time, however only recently has the software been able to display the data in a format which provides clear data which can be used to locate and size of the flaws in a variety of weld configurations. Coke Drums have several significant areas of concern, Weld Seams which may be Shell to Shell, Shell to Head or Shell to Skirt format. We will consider the Circumferential Shell weld and the Skirt weld at this time. The photographs show a shell seam which reduces in section for this example the weld was inspected from one side only. The signals were corrected for Beam Path Length and the amplitudes of the signals were equalized for angle. The following data were collected: Two Notches were machined in the plate one either side of the weld on the underside. The plate was then scanned from one, the thicker, side using the Phased Array probe. The reflectors which were the same depth are depicted with a similar amplitude at their correct relative positions, one on the near and the other on the far side of the weld root. With the signals equalized all the reflector were detected from a single scan location and with similar amplitudes. The Skirt to Shell weld was simulated in a solid piece of carbon steel. EDM notch reflectors were machined in the samples at critical locations. The critical angles were calculated which would produce reflections from each of the potential crack areas and the Phased Array inspection was performed to verify the calculations. A single plot is shown as an example, containing the reflector on the Shell side near the crotch on the inside of the weld. The illustration shows the sound path of the Phased Array which detects a reflector close to the crotch on the inside between the Skirt and the Shell. Discriminating this flaw with conventional ultrasonic inspection would be extremely difficult. It is the ability of the Phased Array Sector Scan to use multiple angles on a single pass which enables flaws at multiple locations and angles to be detected by a line scan and imaged at their relative location.