Figures 1 and 2

Residual magnetism is found in magnetic materials in various strengths and orientations. It is often neglected as innocuous or unimportant as possible deleterious effects go unnoticed or are accepted as the nature of the beast. However, experience has shown that damaging magnetism can been found and removed in many varying circumstances.

 

Demagnetizing Case Histories

• High residual magnetism caused problems for a manufacturer of automobile parts when steel chips were sticking to and interfering with part alignment.
• During the welding of large pipes, there have been many cases of a welder's inability to lay a weld bead due to arc deflection by residual magnetism.
• Innumerable cases exist where babbitt and steel have eroded away from bearings due to residual magnetically generated currents similar to that shown in Figure 1.
• Thrust bearings and thrust collars of natural gas compressors were virtually eaten away by shaft currents.
• On large screw compressors, timing gears, shaft, and helical compressor elements were severely etched and eaten away by electrical currents.
• Magnetic fields rendered a building unrentable because of residual magnetism in its steel frame that made computer equipment inoperable.
• A cargo jet was grounded because its compass pointed north when the plane was actually directed south. After the jet's cargo of 1500 12 x 0.2 m (40 x 0.09 ft) alloy pipes was demagnetized, the plane took off with accurate compass performance.

Automatic demagnetizing downcycling solved all the above problems, where less thorough methods did not, therefore emphasizing the importance of proper demagnetizing.

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Magnetic particle testing is a main cause for residual magnetism

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Magnetic particle testing (MT) is a main cause for such residual magnetism, but other possible causes include lightning, welding, electromagnets, large direct currents, and permanent magnets. All of these require downcycle demagnetizing for satisfactory removal of residual magnetism.

Residual effects of through current and head shot magnetizing produce residual magnetism in closed rings and cylinders. These are illusive with respect to their presence and effects, as there is no access to the magnetic circuit. Thus, no accurate means of measurement exists to ensure they are removed. The logic in allowing them to exist is that if magnetism remains circular in the member, there is no need to be concerned. On the contrary, tests have shown that circular magnetism may not stay confined. This is especially true when there is an irregularity in the cylindrical magnetic path or when the cylinder inside or outside diameter has magnetic shunting paths adjacent to it. This path may cross a gap where there is relative motion, giving rise to shaft voltage generation in rotating machinery.

Demagnetizing to the lowest levels of magnetism is achieved when machinery is disassembled into individual parts. Once equipment is properly demagnetized and rules which prevent remagnetizing are observed, further demagnetizing should not be required.

In cases where economic or other reasons delay proper demagnetizing, it is possible to shut the unit down long enough to demagnetize the assembled or partially assembled machinery as a stop gap measure. This provides a limited amount of protection until such time that full unit disassembly and demagnetizing can be performed. Piping, baseplates, structures, etc. can also be demagnetized. When the unit is shut down and not disassembled, automatic downcycling is effective locally and in adjacent areas without concern that magnetism might be increased or driven further into the unit. This is not possible with AC or manual DC demagnetizing.

 

Why is Downcycle Demagnetizing so Effective?
AC versus DC in magnetizing and demagnetizing is well understood and documented in standards and literature. AC that can be gradually increased and decreased in magnitude serves well when applied to magnetic materials up to 6.4 mm (0.25 in.) thick. For thicker sections, there is a skin effect that occurs when electric current flows in the surface of the magnetic material preventing penetration of magnetism no deeper than 6.4 mm (0.25 in.) to 12.7 (0.5 in.). To achieve penetration into thicker parts, application of DC magnetism in steps of ever decreasing magnitude that is held sufficient time will provide field penetration at each step. Preprogrammed and structured downcycling removes human judgment and dependency on memory of the operator.

A question sometimes posed is why invest in downcycling when manual demagnetizing can be performed? The answer is that downcycling offers reliability in the result because of the structured control of the demagnetizing step magnitudes and dwell times. With programmed downcycling and proper coil placement, the hysteresis curve of the material is traversed in a cyclical manner and satisfactory residual field reduction can be achieved.

Sequentially reduced readings on a reliable gaussmeter of opposite polarities indicates effective degaussing. Persistence
of one polarity, especially at later auto downcycling steps, indicates ineffective coil placement or insufficient demagnetizing power. If, on the other hand, large magnetism swings continue in lower downcycling steps and the remaining residual polarity favors the polarity of the last step, the coil power is too great, requiring a reduction in turns, amperes, or both. With observations made during downcycling, particularly during the final steps, the operator can determine what changes should be made in the demagnetizing procedure to obtain optimum results.

Magnetic massaging, or repeated automatic downcycling, has been found to be an effective method to gradually reduce residual magnetism to acceptably low values. Persistence in application, paired with the consistency of DC downcycling, has been found to be key in reducing residual magnetism to two gauss or below.

 

Integrating Downcycle Demagnetizing into MT
In the past 15 years we have noticed that the patterned removal of residual magnetism often reveals how it was originally induced. Sometimes it can be traced to coil placement or prod current injection from previous MT. To help avoid costly follow on demagnetization and possibly even costlier forced outages, some recommendations are provided as follows:

• Magnetizing for MT should be conducted as called for in ASTM E 1444 paragraph 6.2.2-5, 6.2.7, 6.4, employing a magnetizing coil. Immediately following and with the magnetizing coil still in place, conduct downcycle demagnetizing per ASTM E 1444 paragraph 6.7.1. Precise guidance is obtained on polarities and appropriate demagnetizing strengths by using an automatic demagnetizing and meter for reading residual magnetism during demagnetization.
• Avoid direct prod magnetizing or head shots (ASTM E 1444, paragraph 6.2.6 and ASTM E 1444, paragraph 6.3.4 and
  5, 6.7.1.3) as residual magnetism remains undetected and is embedded, which is very difficult to remove. Effects in the assembled machine can be very damaging.

 

Cost Savings by Consolidating
During a major turnaround in a petrochemical plant, all rotating machinery rotors and casings were scheduled for thorough MT using a large 4000 A magnetizing machine. Upon completion of MT, parts would be transported to a separate area where Magnetic Products and Services was employed to follow up demagnetize what magnetism remained, following the MT demagnetizing. In fact, fields found were very strong, requiring automatic demagnetizing downcycling. We suggest combining magnetizing for MT with follow-on demagnetizing utilizing automatic demagnetizing for both. Simple tests showed that magnetizing with this approach was equally as effective, and that demagnetizing by downcycling using the identical coil placement was quick and thorough. The result was an overall reduction in manpower and equipment needs, saving more than one half the combined cost for MT and demagnetizing, not to mention the elimination of transporting costs and delays.

 

Summary
Residual magnetism in rotating machinery parts continues to be responsible for costly machinery failures. The practice of using magnetic particle testing without subsequent downcycle demagnetizing is estimated to be responsible for roughly 40 percent of rotating machinery failures that are residual magnetism based. For example, an outage at a petrochemical company can cost approximately $1 million per day. Corporations that have experienced expensive outages due to shaft currents routinely insist on demagnetizing at each shutdown, and vendor contracts increasingly require that repaired and replaced parts have two gauss or less in key components.

The inclusion of downcycle demagnetization into MT affords the service provider with many benefits, including the following:

• protection of both customers and service providers against outages that could result in liability claims
• elimination of rework due to problematic residual magnetism
• an edge over the competition that does not incorporate downcycle demagnetizing practices
• the option of expanded service offerings, such as in field demagnetizing due to the power and downcycle capability now available in a portable unit.

Manufacturers and users not only benefit from the downcycling feature of the automatic demagnetizing during MT, but they also benefit in welding blowout prevention, degaussing of pipes, machine tools, rolling mills, extruders, building steel and rebar, and testing of electric machinery for core lamination shorting.

*  Magnetic Products And Services, Inc. (MPS), 2135 Highway 35, Holmdel, NJ 07733; (732) 264-6651; fax (732) 264-6876; e-mail mps@raven.cybercomm.net.

Copyright © 2000 by the American Society for Nondestructive Testing, Inc. All rights reserved.

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