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Shaft displacement probes (non-contacting eddy current sensors) measure the displacement of the rotor or shaft typically adjacent to or inside a hydrodynamic journal bearing. Displacement probes are significantly different from the bearing or casing vibration measurements common on general purpose equipment. The nature of hydrodynamic bearings (both fixed and tilting pads) is such that the shaft can move substantially inside the bearing without transmitting a significant vibration to the bearing housing or casing. This is many times the result of very low rotor to casing mass ratios on turbomachinery
The basic configuration of a displacement probe system consists of a matched set
Oscillator/demodulator (commonly called a proximeter).·
The proximiter excites the probe with a RF signal at approximately 2 MHz. This produces a magnetic field at the tip of the probe. The proximiter demodulates the signal from the probe and provides a modulated DC voltage which is directly proportional to the distance between the probe tip and the target (commonly called the “gap”).
The AC portion of the signal is directly proportional to the vibration of the rotor. Therefore the proximity probe can provide both rotor position as well as vibration(displacement). The rotor position can be used to determine the average journal position in a radial bearing or the axial position in a thrust bearing. This is a very important diagnostic tool since a change in shaft position in a radial bearing may indicate a damaged bearing.
This article contains excerpts from the paper, "Vibration Analysis for Turbomachinery", by Ed Wilcox, consulting machinery engineer of Chevron Energy Technology Company presented at the 2016 Turbomachinery Symposium.
The primary advantages of the proximity probe over other measurement types is that it provides the actual shaft displacement, not the vibration transmitted to some other part of the machinery. Additionally, displacement probes are very rugged and reliable if properly installed. Invariably, when a high vibration is encountered in machinery, one of the first questions by upper management is “is the vibration real?” (i.e. valid). With proximity probes, the vast majority of time the answer is “yes”. While proximity probes are very rugged and reliable, just like any other condition monitoring device, they do have some potential short-comings.
Some of these disadvantages of are:
Target surface condition – imperfections in the target area of the rotor where the displacement probe reads are reported to the measuring device as displacement. The probe targets have to be kept free of scratches, corrosion, and other imperfections that will potentially cause false signals. An example of a proximity probe output where the target has a scratch is shown in Figure 5. Note that the vibration monitor cannot distinguish this defect from actual vibration. Similarly, the FFT will just show this defect as 1X vibration. The time waveform and orbit are the best diagnostic tools to identify such abnormalities. For this reason the probe target area must be protected during rotor repair and storage. Every senior machinery engineer has experienced the frustration of a damaged probe target area from someone’s pocket knife or just simple corrosion. The solution to runout is vector compensation of the displacement data. This is accomplished by collecting a slow roll vector for each probe and subtracting the slow roll vector from the measured displacement vector. The slow roll vector is the actual displacement vector at a very low rotational speed where the shaft is just turning in the bearings (no actual vibration). It is should be noted, that since this is a vector subtraction, the magnitude of the resultant or compensated displacement vector is not always lower than the measured displacement, it just depends on the phase difference between the slow roll and displacement vector (an example of this is provided in the next section). A more rigorous compensation of the vibration data is accomplished with actual waveform compensation, where a time waveform is recorded at low speed instead of just a vector. .
Magnetism – If the rotor has any residual magnetism, this may affect the probe readings and potentially cause runout similar to physical imperfections in the target surface described above. This is commonly referred to as “electrical runout” in the industry.
Target material – the default shaft material for proximity probes is normally 4140 steel. Since the eddy currents are sensitive to the permeability and resistivity of the shaft material, any material other than 4000 series steels must be planned for in the design of the condition monitoring system. Some options include a steel coating in the probe target area or specific probes that are calibrated for the shaft material.
Improper proximiter/cable/probe combinations - as mentioned above, the proximity probe system include three components(probe, extension cable, and proximiter). The cable on the probe itself is 1 m in length.
The proximiter is tuned for a particular probe and extension cable length. Most proximiter probes are supplied in 1m, 5m, and 9m total length options (i.e. extension cable lengths of 0, 4, and 8 m respectively). If the extension cable is mis-matched with the incorrect proximiter it can cause erroneous results. The proximiter was designed for the 1m probe (i.e. no extension cable). The author has also seen a centrifugal compressor factory acceptance test (FAT) where an incorrect extension cable/proximiter combination created a 2X peak that caused the OEM to suspect misalignment before determining the actual problem.
Side wall interference - Because the eddy current is emitted from the tip of the probe, the probe tip must have a clearance that is equal to at least 3x the probe tip diameter to avoid side wall interference.