24765-01-01  Case Expansion Transducer Systems

An important position measurement in Turbine Supervisory Instrumentation (TSI) is case expansion. Case expansion
(sometimes referred to as shell expansion) is the thermal growth of the machine case as it expands during machine
startup and on-line operations. The case expansion transducer system is typically mounted on the foundation at
the opposite end from where the turbine casing is attached to the foundation and provides information about the growth
of the machine case relative to the foundation.
Case expansion is a parameter that you should use a dual transducer arrangement to measure. This arrangement
provides information about the position of the sliding feet on the machine case. A condition that obstructs or jams one
foot could distort the case and damage the machine. The dual case expansion transducer configuration in conjunction
with the 3300 or 3500 monitors provides an alarm for this condition. The high temperature dual case expansion
transducer configuration is compatible only with the 3500/45 Position Monitor.

Case expansion measurements also enable you to determine whether machine conditions are exceeding
expected temperature growth differentials. This is primarily a startup parameter that determines whether the machine
casing and rotor grow thermally at nearly the same rate. Different rates of thermal growth can cause the rotating and
stationary parts of the machine to rub.
The case expansion transducer assembly consists of a linear variable differential transformer (LVDT), which is housed in a
weatherproof, protective enclosure.

The case expansion transducer system uses the LVDT to measure the machine case thermal
growth.
A rod on the LVDT connects to the machine. As the machine case grows, the rod moves inside
the LVDT and changes the signal in the LVDT.
The transducer electronically conditions the signal and outputs it to a monitor for display
and alarms.
The High Temperature Case Expansion Transducer is designed to meet TYPE 4 requirements.