Turbines and generators are among the most critical and complex assets in industrial operations. Whether used in hydroelectric power generation, steam or gas turbine plants oil and gas processing, or heavy manufacturing, these machines operate under extreme loads, speeds, and temperatures. A Turbine Supervisory System (TSS) plays a vital role in maintaining their safe and efficient operation by continuously monitoring key parameters and detecting early signs of failure.
Accurate measurements also rely heavily on reliable condition monitoring practices and proper sensor selection, and compliance with recognised standards such as API 670. In this article, we look at six critical measurements every turbine supervisory system should track to maintain performance, safety, and long-term reliability.
Shaft vibration is one of the most important indicators of turbine and generator health. Excessive vibration often signals developing mechanical issues such as rotor imbalance, misalignment, bearing wear, or rotor instability. A turbine supervisory system monitors vibration levels continuously to detect abnormal patterns before they escalate into serious faults. When vibration levels exceed predefined thresholds, as defined under API 670, automated alarms or shutdown procedures can be triggered to protect the equipment.
For relative shaft vibration measurement, eddy current proximity probe systems (such as the Sensonics XPR04) are the industry standard, providing non-contact measurement of shaft displacement directly within the bearing clearance. For absolute casing vibration, piezoelectric accelerometers (PZS series) or velocity transducers (PZV/VEL series) are used depending on the frequency range and application.
Monitoring shaft vibration helps you to:
Detect rotor imbalance or misalignment
Identify early-stage bearing faults
Prevent catastrophic mechanical failures
Improve predictive maintenance planning
Axial position monitoring tracks the movement of the turbine shaft along its axis. Excessive axial displacement can indicate problems with thrust bearings, thermal expansion, or rotor movement. If thrust bearings fail or excessive axial movement occurs, severe internal damage may follow. Turbine supervisory systems typically use proximity probes to measure this displacement with high accuracy, with typical alarm thresholds set in the range of ±0.5 mm to ±1.0 mm depending on machine design.
Tracking axial position allows your engineers to:
Detect thrust bearing wear or failure
Monitor rotor stability during operation
Prevent contact between rotating and stationary components
Protect critical internal turbine parts
This measurement is especially important in large industrial turbines and vertical hydrogenerators, where axial forces can change significantly during load variations or runaway conditions.
Rotor speed monitoring ensures the turbine or turbine operates within its safe design limits. Overspeed conditions can occur during sudden load rejection, control system failures, or valve malfunctions and can escalate within seconds.
A TSS monitors rotor speed in real time using dedicated speed sensors. If overspeed conditions are detected, protective systems can initiate rapid shutdown procedures to prevent mechanical damage or catastrophic failure. API 670 defines overspeed protection as a mandatory element of any compliant machine protection system. For hydrogenerators, runaway speed — which can reach 1.8–2.0 times rated speed on load rejection — makes this measurement particularly critical.
Speed monitoring helps:
Maintain safe turbine operating conditions
Trigger protective shutdown mechanisms
Prevent structural damage caused by excessive rotational forces
Ensure compliance with API 670 and applicable safety standards
Because overspeed events can escalate quickly, accurate speed measurement and system responsiveness are critical.
Differential expansion measures the difference in thermal expansion between the turbine rotor and casing as the machine heats up during operation. Because the rotor and casing are made from different materials and have different thermal masses, they expand at different rates — and if the difference becomes too great, internal rubbing between stationary and rotating components can result, causing vibration, damage, and performance loss.
This measurement is most relevant to steam and gas turbines, where large temperature gradients exist between cold start and full operating conditions. It is less applicable to hydrogenerators, where thermal expansion is a much smaller factor.
Monitoring differential expansion allows operators to:
Prevent rotor-to-casing contact
Manage thermal stresses during startup and shutdown
Detect abnormal thermal behaviour
Maintain optimal turbine efficiency
Bearings support the turbine rotor and facilitate smooth, continuous rotation under significant mechanical load. They are, however, highly sensitive to lubrication degradation, shaft misalignment, and overloading — any of which can trigger a rapid rise in operating temperature.
Monitoring bearing temperature is therefore a critical diagnostic tool. Elevated temperatures are often the earliest detectable sign of lubrication failure, increased friction, or progressive mechanical wear — conditions that, left unaddressed, can escalate quickly into catastrophic failure. Turbine supervisory systems track bearing temperatures continuously using sensors positioned at or near critical bearing locations, allowing engineers to establish normal operating baselines and respond promptly to deviations.
Temperature monitoring provides several key benefits:
Early detection of lubrication breakdown before it causes surface damage
Identification of excessive friction from misalignment or contamination
Prevention of bearing seizure, protecting the rotor and surrounding components
Extended equipment lifespan through timely, condition-based maintenance
When combined with vibration monitoring, bearing temperature data forms a powerful diagnostic pairing — vibration reveals the character of a developing fault, while temperature indicates its severity. Together, they give engineers a more complete picture of turbine health and help prioritise maintenance decisions with greater confidence.
Air gap monitoring is a critical measurement for hydrogenerators and large motors that is often overlooked in generic turbine supervisory system specifications — but which can be decisive in preventing serious damage.
The air gap is the clearance between the rotor poles and the stator bore. Variations in this gap — caused by rotor eccentricity, bearing wear, stator deformation, or thermal effects — produce unbalanced magnetic pull (UMP), which in turn generates additional vibration and can accelerate bearing wear. In extreme cases, rotor-to-stator contact can cause catastrophic damage.
Sensonics capacitive air gap sensors (CS series, available in measurement ranges from 7 mm to 75 mm) are installed at multiple points around the stator bore and provide continuous, real-time gap measurement. The Sentry G3 monitoring system processes these signals to deliver minimum, maximum, and average air gap values, with 4–20 mA outputs available for direct integration into your DCS or SCADA system.
Air gap monitoring enables operators to:
Detect rotor eccentricity before it causes bearing damage
Identify stator deformation or movement
Monitor the effects of thermal expansion on magnetic clearances
Provide early warning of conditions that could lead to rotor-stator contact
For any hydrogenerator application, air gap monitoring should be considered a standard element of the supervisory system, not an optional addition.
Looking for a turbine supervisory system that covers all five of these measurements — including air gap monitoring for hydrogenerators? Our Sentry G3 platform is an API 670 compliant machine protection system, designed and manufactured in the UK and deployed in over 60 countries.
Get in touch with a Sensonics specialist to discuss your application and receive a tailored recommendation or quotation.
A turbine supervisory system is only as effective as the measurements it captures and the quality of the data behind them. Discover which parameters should be monitored as standard — including air gap monitoring for hydrogenerators — and how Sensonics's API 670 compliant Sentry G3 system supports earlier fault detection and better operational control. Read the full article on the Sensonics blog.