What is differential expansion in a steam turbine?

Differential expansion in steam turbines refers to the difference in thermal expansion between the rotor and the casing of the turbine. This phenomenon is crucial to monitor because excessive differential expansion can lead to mechanical issues, such as rubbing between the rotor and the casing, which can cause damage to the turbine.


  • Differential expansion is the disparity between rotor and casing expansion.
  • Positive (+ve) if rotor expands more than the casing, negative (-ve) if the casing expands more.

Startup Dynamics:

  • During startup, the differential expansion is typically positive (+ve).
  • Rotor mass, being less, expands faster than the casing.
  • After reaching full load, this positive gap diminishes.

Measurement Point:

  • Differential expansion pickup is strategically placed at the farthest point from the anchor.
  • This setup records the maximum difference between rotor and casing expansion.


  • Differential expansion is denoted as “+” when the rotor’s heat expansion surpasses the stator’s.
  • Conversely, it is “-” when the casing’s expansion exceeds the rotor’s.
  • Alarm signals are triggered if the unit differential expansion surpasses prescribed limits.

What is used to measure the difference in expansion between the turbine casing and the turbine rotor?

One common method of measuring differential expansion in steam turbines is through the use of eddy current probes. Eddy current probes are non-contact sensors that work on the principle of electromagnetic induction.

Here is a detailed explanation of how eddy current probes are employed in measuring differential expansion:

Principle of Eddy Currents:

  • When a magnetic field is applied near a conductor (such as metal), it induces circulating currents called eddy currents within the material.
  • The eddy currents generate their own magnetic field, which interacts with the original magnetic field, causing changes in the impedance of the coil.

Probe Placement on Stationary Components:

  • Eddy current probes are typically mounted on stationary components near the rotor(turbine casing).
  • The casing or other fixed structures provide a stable platform for probe placement.

Proximity to Rotor:

  • The probes are strategically positioned to be in close proximity to the rotating rotor without direct contact.
  • This ensures that the electromagnetic fields emitted by the probes interact with the adjacent metal surfaces of the rotor.

Mounting Brackets or Housings:

  • Mounting brackets or housings are used to secure the probes in place on the stationary components.
  • These brackets are designed to withstand the environmental conditions within the turbine and maintain a consistent distance from the rotor.

Avoidance of Mechanical Interference:

  • Care is taken to ensure that the probes do not physically interfere with the rotation of the turbine rotor.
  • Mechanical interference could lead to damage to both the probes and the turbine components.

Adjustable Mounting:

  • Some systems may have adjustable mounting arrangements to fine-tune the position of the probes relative to the rotor, allowing for calibration and optimization of the measurement setup.

Measurement Setup:

  • Each eddy current probe consists of a coil through which an alternating current is passed.
  • The presence of a conductive material, such as the turbine rotor or casing, influences the impedance of the coil.

Differential Expansion Monitoring:

  • As the turbine operates and temperatures rise, the rotor and casing experience thermal expansion. The expansion rates of the rotor and casing are not always identical.
  • The eddy current probes continuously monitor the distance between the probes and the adjacent metal surfaces. Any change in this distance is indicative of the expansion or contraction of the turbine components.

Signal Processing:

  • The impedance changes detected by the eddy current probes are converted into electrical signals.
  • Signal processing equipment analyzes these signals to determine the differential expansion between the rotor and casing.

Alarm System:

  • A control system is set up to compare the measured differential expansion with predefined limits.
  • If the differential expansion exceeds the acceptable range, an alarm signal is triggered, alerting operators or control systems to take corrective action.

Adjustments and Maintenance:

  • Based on the real-time measurements, adjustments can be made to the turbine operation or maintenance activities initiated to prevent any potential issues related to excessive differential expansion.