4.1 Classification of instrumentation
4.1.1 Supervisory instrumentation
These are instruments required on a continuous basis to determine the present conditions and trends of the main rotating and stationary components, (a) to ensure safe operation within the defined limits and (b), to give advance warning of deterioration or change in the main turbine-generator behaviour, which may require either maintenance attention or a temporary restriction in the operating role.
The parameters measured are:
- Rotor axial position In conjunction with cylinder expansion measurements, this enables the relative axial movement to be measured so that adequate clearance margins are maintained under all conditions of operation. Typically this measurement is provided on each cylinder of the machine.
- Cylinder expansions In addition to its use in determining clearance margins between the rotor and cylinders, differential measurement systems are provided to monitor the expansion and movement of cylinders relative to each other and their support structures. Measurements are provided on each cylinder of the machine
- Bearing pedestal vibrations These are measured at each bearing to continuously monitor the dynamic behaviour of the machine
- Shaft eccentricity The peak-to-peak radial excursion of the rotor relative to the non-rotating parts is measured on each rotor to monitor qualitative changes in the behaviour of the rotor which may indicate abnormal or unsafe conditions
- Shaft speed A single measurement of shaft speed is required, independent of the turbine governor, for operator reference, particularly during turbine run-up
- Steam valve positions An analogue measurement of the position of each steam valve is useful to the operator for general reference, to check the capability for carrying more load or for assistance in fault finding
- Metal temperature measurements These provide a knowledge of the thermal state of the turbine during both transient and steady state operation. Measuring points are located in the walls of the HP and interceptor steam valve chests, and in the HP and IP cylinders
- Thrust bearing wear This measurement ensures that thrust pads wear is maintained within safe design limits
All of these parameters are displayed to the operator either on continuous, direct-reading instruments or on a VDU format associated with a computer-based data processing system. If a VDU display is used, it is selectable for continuous access and scanned at intervals of less than 10 s to ensure that the operator is kept informed of the current value. Certain parameters, such as steam valve positions and speed, can change substantially in a fraction of a second. It is essential that the operator has available a true value of the present reading, therefore direct reading is always employed.
Because of their close association with the running machine, generally using transducers mounted in a severe environment, highly specialised and thoroughly proven equipment is essential. This aspect is described in Volume F. The processing equipment is cubicle-mounted and includes, where necessary, alarm outputs so that the operator's attention can rapidly be drawn to parameter excursions requiring corrective action. Where computer processing is involved, the whole scheme is subject to careful review to ensure that if the computer fails, the operator still has sufficient information to continue running at a steady load or to permit safe shutdown.
All of the turbine supervisory parameters are used to provide post-incident records. Some are also used in the control equipment for turbine run-up.