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Khaled Hamza · #1 01-08-2011, 08:38 PM

Turbine Supervisory Instruments

The following supervisory Instruments are furnished with the unit and are to be observed during start-up, operati** and shutdown. We will explain the following instruments in detail as follows.

1- Casing Expansi**
As a unit is taken from its cold c**diti** to its hot and loaded state, the thermal changes in the casings will cause it to expand.
The Casing expansi** scale measures the movement of the force pedestal relative to a fixed point (the foundati**). It indicates expansi** and c**tracti** of the casings during starting and stopping period, and for changes in load, steam temperatures.
Should it fail to indicate during these transient c**diti**s, the situati** should be investigated? The relative positi** of the fore pedestal should be essentially the same for similar c**diti**s of load, steam c**diti**s, vacuum, etc.

2- Rotor Positi**
Rotor Positi** instrument measures the relative axial positi** 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 ** both sides of the thrust collar. Wear ** the thrust shoes results in an axial movement of the rotor and is indicated ** these instruments.
This instrument is equipped with an alarm relay which activates if the rotor moves bey**d a predetermined distance. C**tinued movement bey**d a sec**d predetermined distance activates rotor positi** trip relay which trip the turbine via the emergency trip system.

3- Differential Expansi**
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 stati**ary parts are provided to allow for differential expansi** in the turbine, but c**tact between the rotating and stati**ary parts may occur if the allowable differential expansi** limits are exceeded. The purpose of the differential expansi** meter is to chart the relative moti** of the rotating and stati**ary parts. It gives a c**tinuous indicati** of the axial clearance while the turbine is in operati**.
The instrument is equipped with alarm relay which activates if the value reaches the alarm point. As the rotating and stati**ary parts become equally heated after a transient c**diti**, the deferential expansi** 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 ** turning gear, the rotor will be subjected to **** uniform temperature, thereby minimizing bowing.
This bowing of the rotor is recorded c**tinuously 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.

5-Vibrati**

The vibrati** instrument is used to measure and record vibrati** of a turbine rotor at speeds above 600rpm. Below this speed, the rotor bowing is recorded as eccentricity.The vibrati**s are measured ** the rotor near the main bearings. Excessive vibrati**s serve as a warning for abnormal and possible hazardous c**diti**s in the turbine. Each vibrati** instrument is equipped with alarm and trip relays which activates when excessive vibrati**s are measured at any **e of the bearings.