The following supervisory instruments are furnished with the unit and are to be observed during start-up, operation and shutdown. We will explain the following instruments in detail as follows.
1- Casing Expansion
As a unit is taken from its cold condition to its hot and loaded state, the thermal changes in the casings will cause it to expand.
The Casing expansion scale measures the movement of the force pedestal relative to a fixed point (the foundation). It indicates expansion and contraction of the casings during starting and stopping period, and for changes in load, steam temperatures.
Should it fail to indicate during these transient conditions, the situation should be investigated? The relative position of the fore pedestal should be essentially the same for similar conditions of load, steam conditions, vacuum, etc.
2- Rotor Position
Rotor Position instrument measures the relative axial position of the turbine rotor thrust collar with respect to the first bearing support. The thrust collar exerts a pressure against the thrust shoes, which are located on both sides of the thrust collar. Wear on the thrust shoes results in an axial movement of the rotor and is indicated on these instruments.
This instrument is equipped with an alarm relay which activates if the rotor moves beyond a predetermined distance. Continued movement beyond a second predetermined distance activates rotor position trip relay which trip the turbine via the emergency trip system.
3- Differential Expansion
When steam is admitted to a turbine, both the rotating parts and the casings will expand. Because of its smaller mass, the rotor will heat faster and therefore expand faster than the casings.
Axial clearances between the rotating and the stationary parts are provided to allow for differential expansion in the turbine, but contact between the rotating and stationary parts may occur if the allowable differential expansion limits are exceeded. The purpose of the differential expansion meter is to chart the relative motion of the rotating and stationary parts. It gives a continuous indication of the axial clearance while the turbine is in operation.
The instrument is equipped with alarm relay which activates if the value reaches the alarm point. As the rotating and stationary parts become equally heated after a transient condition, the deferential expansion will decrease.
4- Rotor Eccentricity
When a turbine has been shut down, the rotor will tend to bow due to uneven cooling if the upper half of the casing enclosing the rotor is at a higher temperature than the lower half. By rotating the rotor slowly on turning gear, the rotor will be subjected to more uniform temperature, thereby minimizing bowing.
This bowing of the rotor is recorded continuously as eccentricity from turning gear speed to approximately 600 rpm. The eccentricity instrument is equipped with an alarm signal which activates when the eccentricity reaches the alarm point.
The vibration instrument is used to measure and record vibration of a turbine rotor at speeds above 600rpm. Below this speed, the rotor bowing is recorded as eccentricity.
The vibrations are measured on the rotor near the main bearings. Excessive vibrations serve as a warning for abnormal and possible hazardous conditions in the turbine. Each vibration instrument is equipped with alarm and trip relays which activates when excessive vibrations are measured at any one of the bearings.