Differential Expansion - Allen-Bradley 1444-TSCX02-02RB User Manual

Chapter 4 Measurement Definition
Table 36 - Configurable Parameters for Rod Drop Measurements (continued)
Parameters Values
Target Angle 0...359 degrees
Angular Range 2...20 degrees in 2 degree
steps
Decay Time 1...60.0 seconds
172
Comments
The target angle for the rod drop measurement (the mid-point of the range).
The angular range of the rod drop measurement. This is the allowed range, around the target angle, in which the
measurement can be taken.
Represents the time for the measured value to move 50% nearer the target/new value. Increases and decreases, and
the magnitude of the change, have no effect on the behavior.
For example, if a step change occurs, the measurement moves 50% of the way to the new value in the initial time
constant period. In the next TC period, it moves another 50%, totaling 75% of the step change, etc.

Differential Expansion

Used in steam turbine monitoring, Differential Expansion is the measure of
the difference between the thermal growth of the rotor and the thermal growth
of the case. During machine startup, it is used by operators to help verify that
the heat up is managed so that the rotating and stationary components of the
machine do not touch.
In a steam turbine, the rotating blades are affixed to the turbine rotor while its
stationary blades are connected to the machine casing. As steam turbines
operate at high temperatures, these components experience significant thermal
growth from their non operating (cold/shutdown) state. Because of differences
in the mass, material, and construction of the rotor and case, the rate of thermal
growth of each as the machine heats during startup is different. A rotor always
expands faster than the case. This difference in thermal growth rates manifests
in a varying distance between the rotating and stationary blades, with the
potential for the distance to reduce until the rotating and stationary
components touch. So when starting these machines it is important to do so in
a manner that makes sure that the differential between rotor and case
expansion never exceeds design tolerances.
To monitor Differential Expansion, transducers are mounted to a fixture
attached to the machine case and measure the distance to a collar or ramp that
is machined onto the turbine rotor. Consequently the difference in expansion
between the two structures is merely the measured distance.
Because the distance necessary to measure is further than the range of small
diameter probes, in most cases two probes are applied. In these cases, for
complementary measurements, the combined ranges of the two probes, minus
some overlap range, is considered. Or, for ramp measurements, the change in
distance to a ramp, which is always increasing for one probe while decreasing
for the second probe, is considered. For either method, the Differential
Expansion measurement provides the functions necessary to measure one
distance by using two probes that are mounted in either of the two ways –
complementary or ramp.
Differential Expansion is measured using either an Axial or a Radial method.
The terms "Axial" and "Radial" refer to the orientation of the sensors relative to
the shaft, either in line axially, or are oriented radially to the shaft.
Rockwell Automation Publication 1444-UM001D-EN-P - June 2018