In Turbine Supervisory Instrumentation (TSI), case expansion or shell expansion is an important measurement. This is the growth of the machine case with increase of temperature during machine startup and on-line operations.
The LVDT (linear variable differential transformer) is mounted to the foundation at the opposite end from where the turbine casing is attached.
Case expansion should be measured by a pair of LVDTs. This provides information on the position of both the sliding feet on the machine case. This allows for a comparison of readings preventing damage should one foot become obstructed or jammed.
Case expansion measurements also allow determination of whether expected thermal growth differentials are being exceeded on the machine. This is primarily a startup parameter allowing the machine casing and rotor growth to increase at a similar rate.
Thermal growth at different rates can cause internal rubbing between rotating and stationary parts of the machine.
How a case expansion transducer works:
The spring loaded rod in the LVDT presses against the machine, and as the machine case grows the rod moves inside the LVDT.
The change of position of the rod causes a change in the output signal of the LVDT. The signal conditioner has an output for the monitor or for display and alarms.
CMCP's linear variable differential transformers available from Bestech Australia have a range of 0-2 inches. The output is 1-6 VDC, or 2.5 mV/mil. High temperature and extended range models are also available.