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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.