Rockwell Automation 1440-TPS02-01RB XM-320 Position Module - Manual (Page 10)

Publication GMSI10-UM005C-EN-P - May 2010
2 Introduction
• Cam (Valve) Position - Valve position is a measurement of the main
steam inlet valve stem position. The valve position measurement
provides the operator with the current load on the machine.
The amount of the steam valve opening, speed, governor, and relay
valve position indication assists the operator in controlling the unit
during startup and emergencies. When used in conjunction with data
from other related Turbine Supervisory Instrumentation (TSI)
indicators, load, or steam flow related machinery conditions can be
diagnosed.
• Shell (Case) Expansion - Shell (Case) expansion is a measurement of
the thermal growth of the case from its fixed point outward. Steam
temperature swings widely between startup, shutdown, and various
operating conditions. The high pressure turbine end must move axially
as expansion and contraction occurs. Continuous indication of shell
thermal growth enables the operator to accelerate or change turbine
load without excessive distortion of the machine shell.
• Differential Expansion - Differential expansion (DE) is a
measurement of the differences between the thermal growth of the
rotor compared to the case.
Differential expansion monitoring provides the machine operator with
continuous indication of the critical clearances between the expanding
rotor and blades with respect to the expanding shell or casing. Proper
coordination of machine parameters enables safe machine acceleration
and operation during load changes and emergencies. To prevent axial
rubs it is important that the rotor and case grow at the same rate.
Differential expansion is most important during a turbine “cold”
startup. The rotor is fixed axially by the thrust bearing. The thrust
bearing moves as the case expands – thus the need to monitor the
difference in thermal expansion. Ideally, differential expansion should
indicate zero change in the gap relationship between the two surfaces.
When the shell growth leads or lags the rotor growth, or conversely the
rotor expansion leads or lags the shell growth, an expansion differential
occurs.
The XM-320 accepts input from linear variable differential transformers
(LVDT), non-contact eddy current probes, rotary cam (valve) potentiometers,
or any voltage output position measurement device.