Exploration of the ultrasonic inspection process for small-diameter pressure pipelines

This paper introduces the pressure equipment pressure pipeline ultrasonic inspection process in the “non-destructive testing of pressure equipment (Part III ultrasonic testing)” (JB/T4730-2005) standard based on the actual ultrasonic flaw detection encountered in some of the problems, the contrast with the ray detection, the advantages and technical difficulties of ultrasonic flaw detection, and put forward the corresponding process solutions to improve the detection of pressure pipeline weld ultrasound has Certain reference significance.

1. Introduction

Pressure equipment, pressure pipeline butt joint nondestructive testing, commonly used radiographic and ultrasonic testing methods. X-ray inspection, double-wall dual projection ellipsoidal imaging, or vertical transillumination. Ellipse imaging for circular defects, strip defects, unfused cracks, and other sensitive, vertical transillumination is particularly effective for the root not welded through. The process generally takes a high voltage short exposure time, which makes defects easy to miss detection. In addition, due to the limitations of the installation conditions, such as the dense tube arrangement and the focal length of the radiographic flaw detection, eccentric distance is difficult to meet the standard requirements and sometimes difficult to shoot. The use of ultrasonic testing, portable detection speed, low cost of detection, and due to the inherent characteristics of ultrasonic testing, sensitive to area-type defects (cracks, unfused), and high detection sensitivity. However, due to the curvature of the pressure pipe wall, the wall thickness is thin, and there are some technical difficulties in the ultrasonic detection of pressure pipe butt joints. In addition, ultrasonic testing for the personnel’s quality and experience requirements is relatively high.

2. Pressure pipeline ultrasonic flaw detection technical difficulties

2.1 Thin wall thickness

Small diameter pipe wall thin, with ordinary oblique probe flaw detection, because the front distance is long, with a wave flaw detection, the main sound beam sweep not small diameter pipe weld root, such as multiple wave flaw detection, because the probe emits a wide sound beam, sound beam diffusion, small diameter pipe wall thin, ultrasonic transverse wave sound range is short, easy to detect in the near field area. Experienced personnel are needed to screen and determine the dynamic waveform of the echo.

2.2 Large curvature

Pressure pipe curvature, surface coupling loss; ultrasonic transverse wave reflected in the inner surface of the small diameter pipe, dispersion is serious, low detection sensitivity; small diameter pipe curvature of the impact, resulting in serious scattering of the sound beam, making the echo wandering range, reflecting the echo chaos.

3. Ultrasonic flaw detection process selection

3.1 Flaw detection instrument

The model should be consistent with JB/T10061 “A-type pulse reflection ultrasonic flaw detector general conditions” provisions.

3.2 Probe

3.2.1 Oblique wedge

The outer surface of the pipe is a cylindrical surface with a small radius of curvature. To better acoustic coupling, the probe must generally be processed into a good fit with the pipe wall surface; the difference between the probe and the pipe radius of curvature is at most 10% of the diameter of the inspected pipe. The use of special probes for surface coupling compensation to improve detection sensitivity and clutter.

3.2.2 Chip size

Probe oblique wedge processing into a curved surface, the probe edge of the sound beam will produce scattering, the larger the size of the wafer, the more serious scattering. Therefore, the size of the piezoelectric wafer should be manageable. The front edge is smaller to facilitate a wave sweep.

3.2.3 Frequency

To provide probe directivity, and improve the detection resolution and detection sensitivity, generally should use a higher detection frequency.

3.2.4 K value

For the pressure pipeline weld, it is necessary to use a larger refraction angle probe to use the first and second wave flaw detection so that the transverse wave beam can sweep the entire weld cross-section. At the same time, using a large refraction angle probe also increases the sound range of the transverse wave in the thin-walled pipe to avoid the detection of defects in the near-field area to locate the large quantitative error disadvantages.
Probe-related parameters are recommended to select the following Table 1.
Table.1 Probe-related parameters table

Pipe wall thickness (mm)	Probe K value	Probe front edge (mm)
4.0-8	2.5-3.0	≤6
>8-15	2.0-2.5	≤8
>15	1.5-2.0	≤12

3.3 Test block

JB/T4730.3 “non-destructive testing of pressure equipment ultrasonic testing” selected pressure pipe butt joint ultrasonic testing and quality grading of GS test blocks.

3.4 Work preparation

Welded seam by the welding inspector after passing the appearance test put can be implemented after nondestructive testing, grinding after the residual height of both sides of the weld so that the roughness of the detection surface and the probe travel distance meet the standard requirements.

3.5 DAC curve production

The instrument used is Nantong Youlian PXUT-350+ digital ultrasonic flaw detector; the probe is 5P5×5K3, the front of the probe is 5mm, the implementation of JB/T4730.3 “non-destructive ultrasonic testing of pressure equipment” standard, adjust the instrument on the special test block of the pipeline, scanning speed for depth 1:1 adjustment, select the assessment sensitivity of Φ2×20-20dB, sweep sensitivity plus 6 The sensitivity of sweep plus 6 dB, coupling compensation 3-5 dB.

3.6 Detection surface and sweep

The detection surface is single-sided and double-sided, the sweeping method takes sawtooth sweeping, and the sweeping overlap area is 10% to ensure all sweeping coverage.

3.7 Typical defect waveform analysis

For small-diameter pipe butt joints, manual argon arc welding bottoming and manual welding cover the welding process; the general bevel form is V-shaped. Various typical defects and pseudo-defects in estimating the judgment method are as follows.

3.7.1 Root not welded through

The root of the small diameter pipe is not welded through perpendicular to the inner surface; In ultrasonic flaw detection, the reflection is similar to the end angle reflection, so the echo is stronger from both sides of the weld detection can be detected, and is located in the center of the weld or near the probe side, along the weld direction echoes have a certain range of travel.

3.7.2 Not fused

Unfused is the weld metal, and the base material is not fused, mostly on the bevel surface of the joint. Due to the angle of the bevel surface, it isn’t easy to detect with a wave, but generally easy to detect with the second wave, its location in the center of the weld near the probe side.

3.7.3 Slag and Porosity

It may appear in any position of the weld, the general signal is weak, and both sides of the flaw detection can be found.

3.7.4 Cracking

Cracks are more complex to determine because the length of the crack, its height, and its location are closely related.
Detectors in detecting the relevant signal should be based on the echo situation for a comprehensive determination.

3.7.5 Control of Clutter

Pipeline special probe due to the refraction angle, if not handled well, it is easy to produce surface waves; detection can be gently knocking the probe with a finger if the disappearance or jump is strong for the miscellaneous waves.

3.7.6 Wrong side

Wave amplitude is low; only from the weld side of the flaw detection can be detected, generally away from the probe side; along the weld direction, echoes have a certain range of wandering.

4. Summary

• (1) The choice of pressure pipeline ultrasonic flaw detection probe, as far as possible, is a small front large K value that can achieve better results
• (2) Pressure pipeline ultrasonic detection of defects should be based on the welding process, welding appearance quality, echo dynamic waveform, and other comprehensive determination, which also has an inseparable relationship with the ultrasonic testing personnel’s quality and experience.

Author: Meng Xiangkun