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Stray currents manifest in rotating machinery as a result of specific abnormalities in machines related to flux imbalance, residual magnetism, electrostatic build-up and induced voltages. Discharges of these voltages in components can be very harmful and sometimes catastrophic. Typical damage include pit marks, spark tracks, frosting and electric erosion in components such as bearings, seals and gearteeth.
This article contains excerpts from the paper, "Stray currents and their damaging effects on rotating machinery" by Dian J. Hanekom, a rotating equipment specialist with more than 30 years of experience, at the 2018 Turbomachinery & Pump Symposium.
Stray voltages are a consequence of electrical potential between two objects that ideally should not have any voltage difference between them. These voltages can discharge as Stray Currents. Stray currents manifest themselves in rotating machinery as a result of specific abnormalities related to flux imbalance; residual magnetism in parts; electrostatic build-up in machines; and induced voltages in electrical motors.
The field levels due to residual magnetism in machinery occurs not from design, but from manufacturing techniques (magnetic beds), inspection techniques (MPI inspections) or external influences like welding or lightning strikes. Gauss levels are seen to reach levels from 2 to 100. These levels can increase in machines during operation where the magnetic material provides a closed path and the air gap is reduced.
Frosting forms a uniform, continuous, satin-like surface, similar to sandblasting or shot-peened surface. It looks very much like a piece of frosted or etched glass, hence the term “frosting” or “sparketching”. This kind of damage can be found on radial and thrust pads.
Spark tracks could be very fine, 50-125μm deep. They may look as if they have been drawn with a sharp needle. Sometimes they could be as much as 3mm wide. Tracks vary in length, typically 3mm to several centimetres long.Tracks may also zigzag like lightning strikes. Shining melted surface can be found at the bottom of the track.
Electro-discharge-machining (EDM), also called “current etching”, or “sparketching”, or “sparkcoining” refers to noticeable hard metal removal by the spark erosion process. An example is shaft frosting.
An example of EDM is the erosion of floating carbon ring seals, both between carbon and shaft and carbon and carrierring. Stray currents may form a closed loop between the seal, shaft and bearing housing, which can result in welding marks.
Burned spots, welding, and arcing of contacting parts occur at contact areas, for example at the back of pivot points of thrust or tilting pad shoes resulting in electric erosion and oil varnishing. Also, look for connecting parts on the shaft, example inner- and outer ring of timing gears, coupling gears and thrust collar sleeves.
It is important to note that these current loops cannot be grounded since they are short-circuited within themselves. This makes careful de-magnetization of stator and rotor components imperative.
Electromagnetic Activity Testing is conducted by recording the magnetic flux circulating in the machine using an electromagnetic microphone held at the split-line of casings and bearings. When the flux circulates within the housing the tiny gap in the split offers reluctance to the magnetic field and distorts the field. This forces the magnetic field to exit the metal path and rejoins after the split. An audible humming sound is recorded
Shaft Voltage measurements work on the principle that a handheld earth brush is put on the shaft with a wooden stick while the machine is running. The voltage is then recorded using an oscilloscope.
Gauss levels of components can be measured and evaluated in accordance with established industry best practices.This method is quantitative,but the operator needs to disassemble the complete machine (driver and driven) and measure the gauss levels of each component individually.
Remedies: Demagnetization is imperative for new parts and machines with existing magnetism problems. This can be achieved by wrapping a coil around the component and using fullwave DC demagnetization techniques to reverse the magnetic fields.
Earth Brush Installation is essential for machines that have already developed a problem.The location of earth brushes is dependent on the layout of the machine, bearing type, coupling type, train length and accessibility