How to set the zero point for installing the turbine shaft displacement monitoring probe?
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Example: In the case of a centrifugal compressor, the gap between the rotor and the thrust bearing shells can be determined by moving the rotor. However, it is important to ensure that the zero point of the shaft displacement monitoring probe is set at the midpoint of this rotor gap value. Also, I would like to ask: every time the machine stops operating, the zero point definitely changes as well. Does it mean that the zero point needs to be recalibrated each time the machine stops? ； How is the axis displacement alarm value calculated? Is it determined by adding the gap to the wear amount of the bearing shells? I would appreciate some guidance from experts; thank you very much!
Taking the Bentley probe as an example, when there is a gap voltage at the zero point, the displacement is generally -10V; I recall that this voltage can be displayed on the monitoring panel, or by looking at the value in micrometers shown there. The alarm value for displacement should be asked from the compressor manufacturer – it is necessary to know at what level of displacement an alarm should be triggered to protect the machine.
The Bentley displacement monitoring probe does not have a zero point of its own. In practical applications, the midpoint of the linear portion in the gap voltage–gap relationship curve, that is, the gap at which the gap voltage is 10±1 V, is typically designated as the zero point (similar to how Fahrenheit and Celsius temperatures each have a 0 degree marked on their scales). If the gap voltage is within the specified range after installation but the Bentley reading is not zero, the Bentley zero adjustment can be used to set it to zero. Before installing the Bentley displacement monitoring probe, the mechanic should place the shaft in its initial position. Typically, the shaft is rotated in a specified direction to bring the shaft key to the desired position, and then a radial force is applied to the shaft to press it tightly against the thrust bearing shell. The monitoring probe is installed at this position. After the unit stops running, the detection probe is usually not at zero. If it is necessary to disassemble and inspect the probe at this time, the gap voltage and Bentley reading before disassembly should be recorded. When reinstalling it, efforts should be made to keep the gap voltage the same as before disassembly; any slight deviation can be corrected by zeroing the reading so that it matches the value before the machine stopped. After the unit stops following operation, if maintenance is required on the shaft or bearing shells, the probe must be reinstalled in its initial position.
Hello, thank you for your reply. I would also like to ask: since the free position of the shaft each time it stops cannot be exactly at the same point, there is surely a deviation from zero in the displacement reading. So, when the machine starts running again, should the displacement reading be reduced by the value recorded during the shutdown? ? ?
The monitoring data obtained while the equipment is in operation is different from the data collected when it is at rest. The data on installation and shutdown is merely for reference. If the shafts or bearing shells are not inspected after the equipment comes to a stop, the operating conditions before shutdown and after restart should be identical; in such cases, the readings are all dynamic data based on the initial installation conditions. Therefore, if the probe is stopped for individual maintenance, the gap voltage and indication from before it was removed should be maintained during reinstallation, with the aim of restoring the probe to its original installed state. Any changes or modifications would actually be a destruction of the initial state.
This is how I usually carry out the installation and debugging: after the unit has been shut down for maintenance, I ask the maintenance staff to push it at both axial ends, and then I measure that displacement. If it’s 30 units (0.3 mm), then it can be concluded that the displacement from the center point to the axis ends is approximately 15 units. By adjusting the mounting nut, it is possible to set the DCS reading to 0.15 mm when the shaft is at the proximal end (near the measurement point), and to -0.15 mm at the other end (distal end). Then push the shaft in the opposite direction and verify the DCS measurement once again. This method does not require a multimeter to measure voltage or any calculations; it is simple and easy to use, making it suitable for installing displacement probes after the maintenance of units that are operating normally. However, once the unit is in operation, the actual displacement is generally larger than the displacement measured manually, and this is normal. If it is the first time installing the unit, it is best to measure the voltage strictly in accordance with the manufacturer’s requirements during installation. If there are any issues, feel free to discuss them; I can also improve my plan.
The zero point is generally determined by the manufacturer; in some cases, the main thrust bearing is taken as the zero point, while in other cases, the center point of the gap you mentioned is used as the zero point. When zeroing out, have the mechanic set the equipment to zero, and then adjust the shaft displacement probe. Use a multimeter to measure the output voltage; this value may vary depending on the probe – it could be 9V or 10V. This voltage generally corresponds to the midpoint of the probe’s characteristic curve (the linear part). The circuit current at zero point is 12mA, which is displayed as 0 in the control room. The alarm settings in the control room usually range from a negative value to a positive value; in our case, the alarms are set between –0.4 and 0.4, while the interlock settings are between –0.8 and 0.8. These alarm values are determined according to the manufacturer’s specifications, and they vary from one device to another. For our compressors, the normal clearance is 0.3; with the midpoint taken as zero, the shaft displacement during normal operation is around 0.25. Over time, this displacement increases slightly, and it can reach up to 0.6 in some cases. As for whether stopping means resetting again, I think it’s optional; normally, there’s no need to adjust anything unless you disassemble the device for maintenance.
The zero point is generally determined by the manufacturer; in some cases, the main thrust bearing is taken as the zero point, while in other cases, the center point of the gap you mentioned is used as the zero point. When zeroing out, have the mechanic set the equipment to zero, and then adjust the shaft displacement probe. Use a multimeter to measure the output voltage; this value may vary depending on the probe – it could be 9V or 10V. This voltage generally corresponds to the midpoint of the probe’s characteristic curve (the linear part). The circuit current at zero point is 12mA, which is displayed as 0 in the control room. The alarm settings in the control room usually range from a negative value to a positive value; in our case, the alarms are set between –0.4 and 0.4, while the interlock settings are between –0.8 and 0.8. These alarm values are determined according to the manufacturer’s specifications, and they vary from one device to another. For our compressors, the normal clearance is 0.3; with the midpoint taken as zero, the shaft displacement during normal operation is around 0.25. Over time, this displacement increases slightly, and it can reach up to 0.6 in some cases. As for whether stopping means resetting again, I think it’s optional; normally, there’s no need to adjust anything unless you disassemble the device for maintenance.