Magnetic Survey and Degauss
WHAT IS DEGAUSSING?
It is the process of decreasing or eliminating an unwanted magnetic field. Since magnetism is invisible, some say that degaussing (the removal of residual magnetism) is black magic, others refer to it as an art. Regardless of how you describe it, degaussing or demagnetizing needs to be done systematically and thoroughly in order for it to be effective in preventing damaging shaft currents.
WHAT "NEEDS" TO BE DEGAUSSED?
Anything and everything that contains unwanted magnetism. Difficulty welding/arc blow-out is usually associated with magnetism. Removing and/or controlling the magnetic field is the only way for a welder to be able to do his job. Medical facilities with MRI's have exhibited unwanted magnetic fields outside the imaging room that effect patient monitoring systems. Helipads have displayed fields large enough to effect their compasses. These, and many more situations have occurred and will continue to occur. What they all have in common is that they have been successfully dealt with by MPS. Rotating machinery still stands as our most common application.
WHAT SIGNS ARE RECOGNIZABLE IN ROTATING MACHINERY?
The most common signs of shaft current damage, sometimes referred to as electromagnetic discharge, are frosting, pitting , or spark tracks on bearings, collars, journals and even shafts. This type of damage is often passed off as mechanical wear. Replacing the damaged parts can become costly and repairing the parts may introduce more residual magnetism, causing severe damage visible only at the next shutdown. In some machines with shaft current problems, the loss of material can be significant enough to cause radial or axial movement of the shaft. Proximity probes registering high peaks may be caused by the presence of high magnetic fields. These have been incorrectly identified as electrical runout or as vibration harmonics. One machine vendor, after trying typical solutions, such as replacing oil, balancing, adding probes with monitoring, finally described the machine as having "mysterious mechanical damage." MPS has been investigating shaft currents since 1980, and no two machines have shown identical symptoms and signs. If a machine has continued problems of varying nature, shaft currents may be a factor. Shaft currents should only be ruled out after a thorough magnetic survey, and shaft voltage and current analysis have been completed.
WHEN SHOULD YOU CONDUCT A MAGNETIC SURVEY AND DEGAUSS?
On average, most machines need to be surveyed and degaussed every 2 to 3 years, especially at scheduled or forced shutdowns. The need to check for residual magnetism with a reliable instrument like the MPS Gaussometer or MPS Pocket Gaussmeter is vital. Parts subjected to magnetic particle inspection may be highly magnetized and should be degaussed prior to installation. Degaussing is most effective when a machine is disassembled and all parts are accessible. Require that all subcontractors degauss by adding this to all specifications and contracts. Generally, magnetic field levels which are 2 gauss or below will suffice for most rotating machinery parts and applications. Corners, edges and other uniquely shaped parts may display slightly higher residual magnetism values.
WHY USE MPS INSTEAD OF A SERVICE OR REPAIR SHOP?
If degaussing is done incorrectly, more problems can be generated than are solved. Traditional demagnetizing methods using surface AC demagnetizers or yokes will only affect a small area or surface spot, subsurface magnetism may still be left behind. As is often the case, this is not realized until after a machine is put back into operation and magnetic fields start to interact. These magnetic fields may strengthen generating currents causing unforeseen damage. Therefore, if the removal of residual magnetism is skipped, or improperly done, the likeliness of problems will increase. We call ourselves 'experts' because MPS engineers specialize on thorough and lasting, not temporary, solutions.
WHAT WILL THE MPS GAUSSBUSTERS DO ON SITE?
MPS engineers will first conduct a Magnetic Survey. This entails measuring for gauss with the MPS Gaussometer and charting the results. In doing this, MPS engineers are able to identify the strength, locations and polarity patterns of the magnetic fields. The MPS' Auto Degauss cable is then wrapped around the target area as required to generate an adequate field strength. If the part is small or thin, demagnetizing may first be done using the MPS Auto Degauss AC mode to remove the surface magnetism. To reach magnetism found in parts ½ inch or thicker, the MPS Auto Degauss's automatic full wave DC "downcycling" mode is selected. The polarity of the magnetism is checked and aligned to provide the proper starting polarity in the automatic DC "downcycling" mode. When the start button is pressed, the Auto Degauss downcycles through the automated steps, changing polarity and adjusting each power setting to a calculated percentage of the prior setting. This will slowly reduce magnetism that was in the part by predictably and repeatedly reducing the overall magnetic field with each step. Areas with exceptionally high or stubborn levels of residual magnetism may require repositioning of the cables or different demagnetizing field strengths. This is when the MPS GaussBuster's experience is most beneficial and visible. If an incorrect setting is made, an inexperienced operator may inject more ill-positioned magnetism than was removed, resulting in more problems than at the onset. Once all gauss levels are within acceptable ranges, MPS engineers consult with the customer to recommend ways to control and reduce magnetism. Recommendations will be made, as needed, to assist in the future prevention of problems. For Rotating Machinery, solutions may be for the installation of shaft grounding devices and Shaft Condition Monitoring products, such as the MPS VCM-E. All MPS degauss consulting work is ,when ordered by customer, followed up with a thorough report that details all findings and recommendations.
TO LEARN MORE
Please feel free to Contact MPS and ask for an available engineer to answer your questions.