The static electrical charges produced by a turbine rotor create an effect akin to the one that results from dragging your feet across a carpet in the winter when the relative humidity is low. Touch a light switch and you usually can draw an arc. Static charges on a turbine rotor are produced primarily by moisture sliding off the last-stage blades. Being lazy, the charges find the easiest path to the turbine case and, under the right conditions, arc to the nearest component, usually the thrust bearing (Figure 1).
1. Walk a straight line. This is what a thrust bearing of a high-speed turbine looked like after six months of operation without a grounding device. Courtesy: Jim Bothwell Consulting
Ideally, the nearest component is the grounding device that was installed to convey the static charge from the turbine rotor to the turbine case. In most cases—but not always—it restricts the voltage to one or two voltages, so there's no arc to the turbine case.
Static voltages produced on turbine shafts range from 1 V to 150 V. In one famous case, the shaft voltage on a high-speed (20,000 to 30,000 rpm) turbine reached about 600 volts. The static discharge from the rotor to the case through the bearings was picked up by the proximity probes on vibration-monitoring equipment in the control room. An inspection found that the turbine and compressor had large clearances filled with lubricating oil that has exceptional insulating properties.
The unit was not equipped with a shaft-grounding device. To compensate, engineers installed a ground strap, which eliminated the spikes detected by the vibration monitoring equipment and reduced the amplitude of the static discharge voltage to 0.01 volts. Lost in the annals was how much damage was sustained before the turbine was properly grounded.
Circulating currents
The two primary types of damage to babbitt bearings are "frosting" and "turbine worms." Turbine worms or "worm tracks" are also found on welded teeth of geared couplings and gearboxes. They are most often caused by electromagnetic currents or circulating currents produced by magnetic fields in rotating equipment (Figures 2 and 3). It's current, rather than voltage, that damages a bearing. But because measuring the current through the shaft is impractical, we measure the magnitude of the voltage instead.
2. Decorated shaft. A "frosted" gearbox shaft. Courtesy: Jim Bothwell Consulting
3. Modern art. Frosting on a babbitt bearing section. Courtesy: Jim Bothwell Consulting
Under a 30X microscope, a turbine worm looks like small weld beads. Damage to roller and ball bearings appears as fluting-like ripples (such as those seen on a poorly maintained gravel road) or spalling of the bearing race. Gears and couplings can experience "welding" or "arc" marks on the teeth, or as pits or frosting (Figure 4).
5. Bad infestation. Here are typical turbine worms created by electromagnetic activity across the shaft-bearing interface. Notice the arc weld bead effect on the bearing. Courtesy: Jim Bothwell Consulting
6. Night crawlers. With these worm tracks on the thrust bearing of a high-speed compressor, note the change of direction of the electrical tracks. Dirt or contamination would cause straight lines. Courtesy: Jim Bothwell Consulting
Production of an AC voltage requires three elements: a magnet or a magnetic field, a coil of wire, and relative motion. AC current is produced when the AC voltage is given a complete or circular electrical path. In the case of a turbine or compressor, the rotation of the shaft provides the relative motion, and the turbine blades (with their shrouds and/or lashing wires) provide a loop or coil between each blade.
The missing element, a magnet or a magnetic field, is most often supplied by technicians performing magnetic particle testing of turbine blades or compressor components. The technician wraps a few turns of cable around a particular blade group or component and then energizes the cable with a high-amplitude DC current. Once the DC current establishes a magnetic field, the technician sprays Zyglow or some other product on the component and examines it under a black light to see if there are any indications of cracks or imperfections in the surface of the material. Each crack will produce a glow where a north and south pole were formed on the sides of the crack by the strong magnetic field.
After the DC supply has been turned off, the turbine or compressor component retains a certain amount of magnetic field or gauss that depends on the strength of the magnetizing DC current, the number of cable turns or wraps, and the permeability of the component material. The residual gauss is produced by the alignment of polar molecules in the material. Removing it requires returning the molecules to the random state in which they existed prior to the testing.
Some magnetic particle inspection technicians believe that they can restore the particles to their prior state (a process known as degaussing) by reversing the polarity of the DC current in the wrapped cables for a duration equal to the time of its energization. That may happen, but I'd bet that the technician has a better chance of hitting the lottery.
Circulating currents are produced in the rotor and stationary components of a piece of turbomachinery when it is operating. They can result from the residual magnetic fields in the components of a turbine, compressor, or pump. In general, circulating currents are of low voltage but of high current amplitude. If there are large gaps between the rotating and stationary components or insulators on the bearings, an arc will not form but the AC voltage will exist on the component, waiting for a chance to complete the electrical circuit.
Turbine, compressor, or pump components also can be magnetized by:
- Placing magnetic base tools on components.
- Allowing coils of "stinger" or ground cable from a DC welder to rest on, or be close to, a component while welding is taking place.
- Holding a component in place with a magnetic field so it can be drilled or machined.
All turbine or compressor or pump components should be degaussed to a level of less than 2 gauss following magnetic particle testing or any other activity that places a magnet or magnetic field on or near the components. Bid specifications for the following activities and equipment purchases should include the requirement that all components be degaussed to less than 2 gauss:
- Magnetic particle testing.
- Nondestructive testing.
- Inspection or repair work that includes welding or the use of magnetic base tools.
That may seem a lot of trouble to go to every time a pump is overhauled, but experience teaches otherwise. Here's an example.
A 5-hp pump failed at a refinery and required repair work on the impeller and case. The pump was sent off-site to the shop for the repair. The pump was returned, installed, and operated for about nine months before the bearings failed. The pump was removed from service a second time, new bearings were installed, and it was returned to service. About six months later, the bearings failed again. This time, the bearings were examined using a 30X microscope, which revealed the telltale worm tracks of a magnetic field. The pump then was sent to the shop to be disassembled, degaussed, and returned to the plant. It later was reinstalled and has been in continuous operation for the past five years.
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