When a piece of metal is magnetized, patterns of magnetic flux fields are established in and around the metal. Smooth, uniform metal provides a smooth, uniform flux field. However, when the metal is not smooth or has a break, the flux pattern is altered. It rises in a bridge pattern to jump the gap created by the defect. The height and characteristic of this bridge can be correlated to the depth and characteristic of the break in the metal. Electromagnetic transducers, designed and manufactured by TUBOSCOPE, are employed to detect flaws by the altered flux pattern. The signal is then amplified and recorded as a permanent record of the location and orientation of the defect.

Transversely oriented magnetic fields: This type of magnetic field allows for the detection of defects that run longitudinally to the length of the pipe.

Longitudinally oriented magnetic fields: This type of magnetic field allows for the detection of 3-dimensional and transverse defects.

MAGNETIC PARTICLE INSPECTION

Most of the defects within the new pipe are established in longitudinal direction. The reason for this can be observed in the manufacturing process.

Seamless pipes are made of a pierced billet by rolling the material to form a pipe. Flaws inside the billet are extended by the rolling process. The most common defects in seamless pipe are seams and / or overlaps, mill slugs and pits. Due to heat treatment cracks may occur, too.

Longitudinal welded pipe are showing different type of defects, i.e. undercuts, lack of fusion, burn marks and inclusions. The most economical method to detect these defects is the MAGNETIC Particle INSPECTION. With this method defects with components in a longitudinal direction and an opening to the surface or situated just under the surface can accurately be detected and identified.

Magnetic lines of force, which are passed through the magnetized pipe, are diverted by non - metallic inclusions or other irregularities within the pipe. When such a defect is located at or near the pipe surface, it will cause part of the magnetic force lines to leave the magnetized pipe. This phenomenon is known as diverted flux or stray flux.

The flux leakage at the defect consists of North and South poles, which attract and hold any fine iron particles or powder at the surface of the tested pipe, thus indicating the presence of a flaw.

However, it should strongly be emphasized that a flaw will only be detected if the major dimensional component of the flaw is oriented transversely or nearly perpendicular (preferably at 900) to the direction of the magnetic field. Thus it is essential to magnetize the pipe to be inspected in a direction diagonally to the general direction of anticipated flaws.

By applying this technique the detectable depth of defects depends not only on pipe size and defect orientation, but also on the method and magnitude of magnetization.

The flux density of AC magnetization remains considerably on the surface of the pipe where as current flow from DC enables detection of Sub-surface defects. Therefore, mainly DC current is used.

With the Magnetic Particle Inspection using a series of powerful lead -acid battery cells or a capacitor discharge unit, a DC electric current of several thousand amperes is conducted momentarily through a central shooting rod or conductor, which is inserted through the tabular in order to induce a strong circumferential field.

With strong residual DC magnetic field transversely to pipe's longitudinal axis delectability of defects below the surface extends to about 5mm. For all type of defects at larger depth the sharpness of magnetic particle indications decreases in direct proportion to depth.

Existing defects with an orientation intersecting the lines of magnetic flux induced into the pipe create a disturbance within the magnetic field.

The external pipe surface is then completely covered with iron powder. The powder is magnetically attracted to the disturbed magnetic field adjacent to a pipe defect and the resulting magnetic powder build -up clearly indicates even invisible defects.

The identified flaw is then probed with a portable grinder and its depth is measured with an accurate gauge. If the flaw's penetration into the tube wall does not exceed the tolerances set by the pertinent specifications, the grind mark is faired into the pipe body. If, however, the defect is found to exceed allowable depth, the length of pipe is rejected.

Except for the larger size of pipe it is only practical to perform MAGNETIC PARTICLE INSPECTION on the external surface of the tube.

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