Differential Expansion Measurement for Didcot A Power Station

01 February 2008

One of the challenges facing instrumentation engineers in the power generation sector is the accurate measurement of shaft growth relative to casing on large steam turbine rotor sections. Commonly referred to as differential expansion, this measurement technique is essential at various stages of the turbine.

Differential expansion technique at Didcot A
Differential expansion technique at Didcot A

Obtaining the required linear range over the full operating conditions of the turbine is difficult and common proximity probe techniques utilising a gap measurement against a shaft collar are sensitive to non-axial turbine movement and limited by probe target size.

Didcot A Power Station, since the mid 1980’s, has been successfully utilising a differential expansion technique incorporating a time based measurement method based on a series of plates fitted around the shaft at the appropriate rotor section. However in recent years, maintenance of the original signal processing equipment had become more difficult and condition monitoring specialists Sensonics Ltd were commissioned to update the system.

The MO8612 Mark-Space option, from their Sentry Turbine Supervisory Equipment Series, was fitted on the HP, IP, and LP3 rotor sections of the steam turbine. Software configurable to the shaft plate pattern, the module processes the pulses generated by a standard proximity probe fitted against the shaft, calculating the differential expansion from range settings determined during final commissioning.

Sensonics provided a training course for the station engineers on a simulation rig as part of the system hand over.

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