Magnetic Particle Examination Electric Current

The electric current for magnetizing the workpiece, with either an electromagnetic yoke or electric prod contacts, must be flowing to produce strong magnetic lines of force when the magnetic particles are applied to the surface. Some residual magnetism is usually retained by the workpiece after the current is turned off, but it is too weak to properly attract the magnetic particles to a flaw.

When electric prod contacts are used, the current should be turned on after the prods are brought in contact with the workpiece, and turned off before the prods are removed from the workpiece to avoid arcing. Arcing can cause small heat affected zones to develop in the workpiece, similar to welding, which could make the vessel susceptible to failure by stress-corrosion cracking in some process environments (see Section 736). Arcing is not a concern when an electromagnetic yoke is used, because the yoke functions as an electromagnet and does not make electrical contact with the workpiece.

Either AC or DC electric current can be used for magnetizing the workpiece with both electromagnetic yokes and electric prod contacts, but the magnetic lines of force created by each type of current have different characteristics. The magnetic lines of force created by a DC current penetrate further below the surface of the workpiece than those created by an AC current. Therefore, DC currents should be used when subsurface flaws must be detected. However, AC currents provide somewhat greater sensitivity for detecting flaws that are open to the surface, and are preferred when only surface flaws must be detected.

The electric current must create magnetic lines of force in the workpiece of sufficient strength for the magnetic particles applied to the surface to be attracted to the magnetic leakage occurring at flaws. When electrical prod contacts are used, SE706 recommends a magnetizing current of from 90 to 110 amperes/inch of prod spacing for workpiece thicknesses less than ¾ inch, and 110 to 125 amperes/inch of prod spacing for workpiece thicknesses of ¾ inch and thicker. It is undesirable for the prod spacing to exceed 8 inches. The criteria for the electric current when an electromagnetic yoke is used are different, because the yoke functions as an electromagnet. The magnetizing strength of a yoke is determined by its lifting power. An electromagnetic yoke must be able to lift a 10-pound steel plate when an AC current is used, or a 40-pound steel plate when a DC current is used.

Mobile electrical power sources are available that provide both AC and DC current. These power sources can be transported to the pressure vessel for in-service inspection.

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