Turbines and compressors operate under demanding conditions, with high rotational speeds and heavy mechanical loads placing constant stress on their components. In this operational environment, vibration becomes an important indicator of machine health. While some level of vibration is normal in rotating equipment, excessive or changing vibration levels often signal that something within the system is no longer operating as intended.
For engineers responsible for reliability and maintenance, understanding the root causes of abnormal vibration is essential for preventing damage and maintaining performance. In this article, we explain five common sources of excessive vibration in industrial turbines and compressors, and explain how continuous vibration monitoring — using sensors and systems compliant with API 670 — supports early detection and intervention.
One of the primary sources of excessive vibration in turbines and compressors is rotor imbalance. This occurs when the mass of the rotating assembly is unevenly distributed around its axis of rotation. The result is a centrifugal force that rotates with the shaft, generating vibration at 1x running speed — one of the clearest and most recognisable signatures in a vibration spectrum.
Imbalance may develop due to blade damage, material buildup, component wear, or manufacturing tolerances. Over time, these forces place additional stress on bearings, shafts, and supporting structures. Eddy current proximity probes — such as the Sensonics XPR04 — provide non-contact relative shaft vibration measurement that is particularly effective for identifying and quantifying rotor imbalance in fluid film bearing machines.
Regular monitoring allows engineers to detect developing imbalance early and correct it through balancing procedures before bearing loads become damaging. Vibration trend data from the Sensonics SentryCMS platform can be used to track the progression of imbalance over time and determine the optimum point for planned intervention.
Misalignment occurs when the shafts of connected machines (such as a turbine and its driven compressor) are not perfectly aligned. This can happen during installation, after maintenance work, or as a result of structural movement and thermal expansion. Misalignment typically produces vibration at 2x running speed, often alongside the 1x component, and may also generate axial vibration — a pattern that distinguishes it from imbalance in spectral analysis.
When misalignment occurs, the rotating components experience uneven forces that generate vibration throughout the system. These forces can lead to accelerated wear of bearings, couplings, and seals if left uncorrected.
Reliable diagnosis depends on measuring both radial and axial vibration simultaneously. The Sensonics Sentry G3 system supports multi-channel acquisition across radial and axial measurement planes, enabling engineers to identify misalignment signatures with confidence. Once detected, misalignment can usually be corrected through the proper shaft alignment procedures, restoring smooth operation and reducing long-term mechanical stress.
These are related but distinct fault modes that are often grouped together, and benefit from being understood separately.
Bearing wear — as bearing surfaces deteriorate, defects on the raceway, rolling elements, or cage generate repetitive impact signals at frequencies directly related to the bearing geometry (BPFO, BPFI, BSF, FTF). These high-frequency signals are best detected using accelerometers such as the Sensonics PZS series, and analysed using envelope analysis or spectral trending in the SentryCMS platform.
Mechanical looseness — loose mounting bolts, worn housings, or degraded supports produce a different vibration signature: typically a series of harmonics at multiples of running speed (2x, 3x, 4x...), often with a noisy, truncated waveform in the time domain. This is structurally different from bearing defect frequencies and should be diagnosed separately.
Continuous monitoring helps your engineers track these changes and assess equipment condition over time. The Sentry G3's API 670 compliant monitoring architecture ensures that both alarm and danger thresholds are continuously evaluated, so that neither fault mode is missed during normal operation.
Fluid film (sleeve) bearings are widely used in large turbines and compressors because of their load capacity and long service life. However, under certain operating conditions they can give rise to rotor instability — a self-excited vibration phenomenon that does not occur in rolling element bearing machines.
Oil whirl occurs when the oil film in the bearing causes the shaft to precess around the bearing centre at approximately 40–48% of running speed (sub-synchronous vibration). If left unchecked, oil whirl can transition into oil whip — a more severe condition where the rotor locks onto a system natural frequency and vibration amplitudes increase rapidly, potentially causing catastrophic bearing or seal damage.
Oil whirl and whip are diagnosed by their characteristic sub-synchronous frequency components, visible in FFT and waterfall displays. The Sensonics SentryCMS waterfall and Campbell diagram functions are specifically suited to identifying these speed-dependent instability phenomena, particularly during run-up and coast-down. Proximity probes are the preferred vibration sensor for this measurement, as they capture the full shaft orbital motion within the bearing clearance.
Every rotating machine has one or more critical speeds — rotational speeds at which the forcing frequency coincides with a natural frequency of the rotor or supporting structure. When a machine operates at or near a critical speed, even small exciting forces can produce disproportionately large vibration amplitudes.
Critical speeds are an inherent characteristic of the machine design and are not faults in themselves — but they become problematic when operating speed ranges shift (due to modifications or changed operating conditions), when damping is reduced (due to bearing wear or lubrication changes), or when a new source of excitation emerges at the resonant frequency.
Bode and polar plots — standard display modes in the Sensonics SentryCMS platform — are the primary tools for identifying critical speeds during run-up and coast-down. Waterfall diagrams allow engineers to observe how vibration amplitude and phase change with speed, making resonance conditions clearly visible. Once identified, critical speeds can be managed through damping changes, structural modifications, or operational speed restrictions.
Whether you are investigating an existing vibration monitoring problem or specifying a monitoring system for a new installation, Sensonics has the sensors, monitoring hardware, and analysis software to support you. Our Sentry G3 API 670 compliant machine protection system and SentryCMS condition monitoring platform are designed for exactly the kind of demanding rotating machinery applications described in this article.
Contact a Sensonics specialist to discuss your application and find out which measurement and monitoring solution is right for your equipment.