Magnetic Particle Examination Systems

The MT systems used for in-service inspection of pressure vessels employ an electric current to magnetize the area of the vessel shell being examined. Either an electromagnetic “yoke” or electric “prod” contacts can be used to magnetize the shell with an electric current, as illustrated in Figure 700-9.

Electromagnetic Yoke and Electric Prod Contacts for Magnetizing Shell of a Pressure Vessel

The electromagnetic yoke functions as a horseshoe magnet, with the magnetic lines of force created by an electric coil in the handle. Magnetic lines of force are developed in the workpiece when the poles of the yoke functioning as a horseshoe magnet are brought in contact. The magnetic lines of force flow through the workpiece from one pole of the yoke to the other. The yoke does not pass an electric current through the workpiece to create magnetic lines of force.

The electric prod contacts do not function as electromagnets. They serve as positive and negative electrodes that introduce an electric current into the workpiece, that flows between the contact points of the prods. Circular magnetic lines of force are created in the workpiece that are concentric around the point of contact of each prod. Prods offer more flexibility for MT of components with complex geometries, such as nozzle welds, and are more frequently used than yokes.

It is very important to note that the direction of the magnetic lines produced by an electromagnetic yoke is different from that produced by electric prod contacts, as can be seen in Figure 700-7. Flaws are most detectable when they are oriented normal to the magnetic lines of force, because this orientation results in the greatest leakage of magnetic lines of force. Therefore, the yoke, or prods must be positioned correctly with respect to the orientation of the flaws to obtain the greatest sensitivity for detection. The greatest sensitivity with a yoke is obtained by positioning its poles normal to the orientation of the flaws, whereas positioning prods parallel to the orientation of the flaws provides the greatest sensitivity. If the flaws appear to have a random orientation, the same location should be examined twice with the positioning of the yoke or prods rotated 90 degrees. One example where two examinations should be made by rotating the yoke or prods is for the detection of both longitudinal and transverse cracks in a weldment.

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