Distinguishing Between Signal Misjudgment and Real Fault of JNJ3900/55 Bearing Vibration Monitor

During steam turbine operation, vibration monitors are a core tool for ensuring equipment safety. During unit startup, when oil vortex causes vibration to transiently exceed specified limits, triggering a trip protection on the JNJ3900/55 vibration monitor, if no bearing abnormalities are detected, maintenance personnel often face a critical question: how to determine whether this is a true mechanical failure or a false signal? This question not only impacts the risk of equipment downtime but also directly influences the formulation of subsequent maintenance strategies. This article will analyze the physical mechanism of oil vortex, explore the limitations of vibration monitors in signal acquisition and analysis, and provide practical methods for distinguishing true failures from false signals.

I. The Physical Mechanism of Oil Whirl: Challenges for Vibration Monitors

Oil whirl is a common self-excited vibration phenomenon in steam turbine rotor systems, typically occurring when the sliding bearings are underlubricated or when the speed approaches a critical value. Its core mechanism is the asymmetric pressure distribution generated by the bearing oil film under high-speed rotor rotation, which causes the rotor axis to develop an elliptical or even chaotic trajectory, thereby inducing low-frequency vibration. This vibration characteristic is fundamentally different from the high-frequency vibrations of mechanical faults such as bearing wear and rotor imbalance. However, the JNJ3900/55 vibration monitor may misinterpret signals during signal acquisition due to the following reasons:

Signal frequency overlap: The low-frequency vibrations of oil vortex may be close to the characteristic frequencies of certain mechanical faults, making it difficult for the monitor to accurately distinguish them through spectrum analysis.

Transient response amplification: The vibration amplitude of oil vortex can suddenly increase under transient conditions. If the monitor’s alarm threshold doesn’t account for these dynamic characteristics, it can easily trigger a false alarm.

Sensor installation error: If the vibration monitor’s sensor isn’t installed in an area sensitive to oil vortex, it may amplify non-fault vibration signals.

For example, when the JNJ3900/55 vibration monitor suddenly triggers a Level II alarm during a revving cycle to 2800 rpm, the vibration value suddenly increases from the normal value of 50 μm to 150 μm, causing the machine to trip. However, after the shutdown, inspection revealed that the bearing surface was smooth and unworn, and the oil film thickness was consistent with the design value, indicating that the abnormal vibration was caused by oil film turbulence rather than mechanical damage.

II. Limitations of Vibration Monitors: Shortcomings from Hardware to Algorithms

Although the JNJ3900/55 vibration monitor features dual-channel measurement, alarm delay adjustment, and signal filtering, it still suffers from the following limitations in actual use:

Inadequate anti-interference capability: When low-frequency vibration caused by oil film turbulence is superimposed on normal high-frequency vibration, the monitor’s bandpass filter may not be able to effectively separate the two, resulting in erroneous signal amplitude amplification.

Simplified alarm logic: Most monitors rely solely on vibration amplitude thresholds to determine faults, without incorporating multi-dimensional data such as vibration frequency and phase changes. This can easily overlook the dynamic characteristics of oil film turbulence.

Sensor sensitivity deviation: If the sensor sensitivity is not accurately calibrated, normal oil film turbulence may be misinterpreted as abnormal vibration.

Taking the technical parameters of the JNJ3900/55 vibration monitor as an example, its measurement range is 0–500 μm (peak-to-peak), and the alarm delay is adjustable from 0 to 3 seconds. In transient vibrations triggered by oil vortex, if the delay is set too short, the monitor may output an alarm signal before the vibration has subsided, while the bearing is still in a safe state.

III. Practical Strategies for Distinguishing Real Faults from False Signals

To address the issue of vibration monitor misdiagnosis caused by oil vortex, three approaches must be considered: signal analysis, equipment status verification, and process condition optimization.

1. Multi-Dimensional Signal Analysis: Going Beyond a Single Amplitude Threshold

Spectrum Comparison Method: Using the vibration monitor’s spectrum analysis function, observe whether vibration energy is concentrated in the low-frequency range. If the high-frequency component accounts for less than 30%, it is more likely to be caused by oil vortex.

Phase Consistency Verification: Using the dual-channel JNJ3900/55 vibration monitor, synchronously collect axial and radial vibration signals. If the phase difference exhibits periodic shifts during oil vortex periods, this can be used to identify a fluid dynamic disturbance. Trend Comparison Analysis: Compare historical vibration data under the same operating conditions. If the vibration occurs and recovers quickly after shutdown, it is more likely to be oil film vortex.

2. Equipment Condition Verification: Thoroughly inspect the bearings and lubrication system.

Bearing Contact Surface Inspection: After shutdown, disassemble the bearings and use a laser rangefinder to measure the clearance distribution between the journal and the bearing. If the clearance is uniform and no signs of metal wear are found, mechanical failure is ruled out.

Oil Quality and Oil Film Thickness Testing: Samples are taken to analyze the viscosity, moisture content, and particle size of the lubricating oil, and infrared thermal imaging is used to measure the bearing temperature field. If the oil quality is acceptable and the oil film thickness is within the designed range, oil film vortex is highly likely.

Rotor Dynamic Balancing Verification: Use a laser alignment tool to check the rotor axis trajectory during low-speed cranking. If the trajectory is a regular circle with no eccentricity, rotor imbalance is ruled out.

3. Process Condition Optimization: Collaborative Improvements from Design to Operation

Adjusting Alarm Thresholds: Based on the typical vibration amplitude of oil vortex, appropriately increase the II alarm threshold of the vibration monitor and extend the alarm delay to 1-2 seconds to filter out transient interference.

Optimizing the Lubrication System: Install an orifice plate or adjustable flow valve at the bearing oil inlet to improve oil film stability and reduce the occurrence of oil vortex.

Through the synergistic effect of signal analysis, equipment verification, and process optimization, it is possible to effectively distinguish between false signals from the vibration monitor JNJ3900/55 caused by oil vortex and actual mechanical failures.

When looking for high-quality, reliable vibration monitor, YOYIK is undoubtedly a choice worth considering. The company specializes in providing a variety of power equipment including steam turbine accessories, and has won wide acclaim for its high-quality products and services. For more information or inquiries, please contact the customer service below:
E-mail: sales@yoyik.com
Tel: +86-838-2226655
Whatsapp: +86-13618105229

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