1. Principle of folding defect detection for seamless steel pipe
When the steel pipe on the detection line passes through the wall thickness detection area strictly along its axis at high speed, the water coupler rotating around the pipe generates a spray water column with constant pressure to spray on the steel pipe, and the ultrasonic signal perpendicular to the axis of the steel pipe emitted by the ultrasonic probe rotating together is coupled to the pipe through the spray water column. The interface wave and bottom wave formed when the ultrasonic signal passes through the inner and outer surfaces of the pipe are coupled to the ultrasonic probe through the water column. The probe converts the interface wave and bottom wave into electrical signal output, and after processing, analysis, and calculation, a digital curve reflecting the thickness of the steel pipe wall is formed. When the ultrasonic signal encounters folding or other defects in the steel pipe wall, a defect signal representing folding and other defects is formed between the interface wave and the bottom wave, and its thickness pulse width continues to change significantly. The size and range of the folding defect are displayed on the curve.
2. Multi-probe high-energy spray water coupling ultrasonic thickness measurement technology
The above detection technology and device are only suitable for high-speed detection of pipes with smooth surfaces. However, the surface condition of the steel pipes that need to be inspected is usually poor. Many steel pipes are reused, rusted, and have pitting distribution on the surface. Some are also polluted by oil, etc., which causes the ultrasonic signal to be attenuated on the surface of the steel pipe. The interface wave and bottom wave are very weak, and it is difficult to form a stable thickness pulse. Some steel pipes are even equipped with couplings. They run bumpy on the inspection line, and the inspection effect is still very poor.
The “multi-probe high-energy spray water coupled ultrasonic thickness measurement device” mainly adopts the following technologies to solve these problems:
(1) Adopt a “barium titanate wafer” with a pressure resistance of 1500 V p2p and a 10 MHz point-focused ultrasonic probe to improve the energy of ultrasonic signals;
(2) Adopt a high-energy excitation circuit with impact function characteristics to stimulate the probe to generate high-energy ultrasound;
(3) Multi-channel ultrasonic thickness measurement technology and thickness pulse linear broadening circuit to improve the detection speed and accuracy;
(4) “Time division multiplexing, high-speed sampling” multi-channel signal transmission technology to reliably transmit multi-channel thickness signals;
(5) Computer software eliminates the influence of ultrasonic signal instability on the detection results, displays the detection results in numbers and curves, and marks the defect location;
(6) Constant pressure and constant temperature coupling water device to improve the reliability of spray water column coupling to ultrasonic signals;
(7) Precise positioning and driving mechanism to ensure that the steel pipe passes through the probe strictly along its axis in the detection area, Overcome the bumpy operation problem caused by the steel pipe coupling so that each probe maintains a vertical and constant coupling distance with the outer wall of the steel pipe.
3. Experimental results
The sensitivity of the “barium titanate” wafer probe can be increased by 10dB; the wafer is equipped with a concave acoustic focusing lens to form a point focusing probe with a focal length of 15mm and a coupling water column distance of 12mm, so that the focus of each probe falls on the midpoint of the steel pipe wall, effectively reducing the scattering effect of the surface corrosion of the oil production pipe and preventing the steel pipe coupling (thickness 6~8mm) from damaging the probe; the “avalanche triode” is composed of a 6ns high-energy excitation circuit with an impact function characteristic and an amplitude of Vp2p=1000, which excites the ultrasonic probe to generate a high-energy ultrasonic signal, so that the signal-to-noise ratio of the surface wave and the bottom wave reaches more than 20dB; when the probe rotates at 240rmin, the wall thickness pulse on the steel pipe surface is stable, and the uneven data of the circumference can be displayed accurately to 0.05mm; when the steel pipe passes through the detection area at a speed of 15mm in at the same time, Each probe has a spiral scanning, a pitch of 15.625mm, and 4 probes detect 2400 points per second. The distance between points is 0.16mm, and the wall thickness variation accuracy is not less than ± 0.1mm.
4. The channel rotary probe ultrasonic thickness measuring device consists of a water coupler and 4 ultrasonic probes distributed at 90° to each other and strictly perpendicular to the steel pipe axis. The automatic liquid level control high-hanging water tank with a liquid level of 6m and the 40℃ constant temperature coupling water ensure that the 4 probe spray water columns have consistent pressure and reliable coupling at each position, eliminating the influence of bubbles. The water column perpendicular to the axis of the oil production pipe reliably couples the ultrasonic signal between the probe and the pipe body.
The linear thickness pulse stretching circuit composed of constant current source devices stretches each 2Λs thickness pulse by 20 times to 40Λs, improving the thickness detection accuracy and reducing the sampling error; the “time division multiplexing, high-speed sampling” circuit samples 4 width pulses at a sampling frequency of 2.5MHz and merges them into 1 channel, which is transmitted through the slip ring and then synchronously separated into multiple signals by the multiplexer. The signal distortion is ≤1%.
False image interference affects the detection results. The bottom wave of the steel pipe is weak, which makes it easy to form unstable ultra-thick false image pulses; surface wave ringing may also form ultra-thin false image pulses; and folding defects generally form dozens of continuous folding depth pulses. Computer analysis software can distinguish wall thickness, folding, and false image interference, and display stable data and curves.
4. Test results
On the steel pipe flaw detection line, the steel pipes were tested by the “magnetic leakage flaw detection” and then the “spray water coupled rotating probe 4-channel steel pipe wall thickness high-speed detection device”, and the steel pipes with folding defects were eliminated. In the subsequent water pressure test, the steel pipes with folding defects no longer burst.
Post time: Nov-11-2024