System for Tubes and Pipes

INTRODUCTION: Due to a variety of manufacturing process in the tube and pipe industry, as well as their varied end use, the NDT technique and its procedure employed are different. The technique, selected is based on the expected defects, the sensitivity level desired and the stage of inspection. Today most requirements are met mainly by ultrasonic, eddy current and leakage flux techniques. Other techniques like Radiography and Magnetic Particle Testing are also used. This paper gives general information on the type of NDT techniques employed in the tube and pipe Industry. ULTRASONIC TECHNIOUE: Ultrasonic waves can penetrate through the entire cross section of the tube, hence is employed to detect surface, sub surface and internal discontinuities as well employed for dimension measurements. Commonly used technique for low volume inspection is by direct contact of the probe to the tube surface. For higher throughput speed and for critical inspection, immersion technique is used. The tube under test, either passes under a rotating ultrasonic test head or passes in a helical motion under stationery ultrasonic probes. The type of probe, it's frequency and size, are selected based on orientation of the defects to be detected. Up to minimum five probe orientation are employed to detect axial, transverse, planer discontinuities and dimension measurement. Multi-channel equipment with strip chart recorder, microprocessor, marking device, and sorting device is employed for automatic systems. The number of channels used and the size of reference defects, will decide the speed of inspection. Speeds of 10M/min are normal. Surface finish, heat treatment, manufacturing process and material composition decide the smallest discontinuity detectable. Ultrasonic is mainly applied for full body inspection of Seamless Tubes, weld region or full body inspection of ERW tubes, weld region of Submerged Are Welded Pipes. Usually, parent material and heat affected zone of welded tubes are also inspected. BRIEF FEATURES: Full cross section can be inspected, wall thickness, diameter and ovality can be measured, high sensitivity, defect location and size can be plotted, higher test speeds possible by automation. EDDY CURRENT TECHNIOUE: Electro-magnetic induction based technique is mainly used for detecting surface discontinuities. A material handling equipment traverses the material under the test station. Tube to be inspected passes through a test coil viz. encircling coil, for detection of transverse or spot defects. For longitudinal defect detection the best results are obtained using rotating probe coil principle. For better results with encircling coil, a combination of differential and absolute coil winding arrangement is used. Test speeds in excess of 10M/sec are possible without affecting the test parameters. For rotating heads very high sensitivity to axial defects are possible due to the technology of constructing very small width probes. Test speeds are limited to around 2M/sec. This technique being a non-contact one, there is no wear and tear, hence reliability of test is greater even under very high test speeds. Depth of penetration is limited by the test frequency, material conductivity and material permeability. Higher frequencies are employed for best test sensitivity and lower frequencies are employed for greater depth of penetration. Multichannel equipment is used for rotating beads and a single channel is used for encircling coil equipment. Usually a combination of both gives optimum results. Equipment has facilities to filter out noise and only detect real defects. Recording devices, marking devices and sorting devices are used to complete one installation. Seamless Tubes, ERW tubes are inspected for full surface area. Weld regions of ERW tubes are tested for weld defects. In ERW mills eddy current is used on-line as process monitoring tool. BRIEF FEATURES: High speed of inspection, automatic scanning, sensitive to surface defects, non-contact, widely used for ferrous and non-ferrous material. LEAKAGE FLUX TECHNIOUE: Limitation of ultrasonic and eddy current technique are over come by this technique. Nearly full depth of penetration is achieved in conventionally available tubes. It is more sensitive to surface defects but has adequate sensitivity to detect internal defects in most application. Material with scales i.e. hot finished material without elaborate surface finish can be directly tested. Both A.C. and D.C. flux techniques are employed. A.C. flux is more sensitive to surface defects. The field is employed either longitudinally or transversely to the surface to detect axial and transverse defects. Seamless and ERW welded tubes of ferrous material are inspected. BRIEF FEATURES: No special surface preparation required, high test speeds up to 3M/sec, various sizes of tubes can be inspected. OTHER TECHNIOUES: Radiography, Magnetic Particle are used for inspecting untested ends from other NDT techniques. They are supplementary techniques and not usually employed for the full tube scanning, as other techniques offer better results and cheaper solutions.