12.2.3 Stress monitors
The life of the turbine-generator is based on its operational requirements and the assumption that a particular stress level related to each start is reached, but not exceeded. During runs-up it is beneficial that thermal stress levels within critical components, e.g., HP and IP rotors, are monitored so that the maximum allowable thermal stress levels are not exceeded.
This should ensure that thermal fatigue damage will not occur in the operational life of the turbine. This monitoring requirement brought about the development of stress monitors or life expenditure monitors, as they are sometimes called.
The monitoring is achieved by the use of temperature probes with a central core which simulates the critical section of the rotor. Figure 2.95 shows a typical probe arrangement. One end of the core is in contact with the main steam flow and the other end is insulated. Because it cannot be in contact with the rotor it is situated to measure the temperature of the steam as admitted to the rotor. For correct simulation, the probes are positioned so that during natural cooling, the probe and critical areas of the rotor cool at the same rate. This means that steam/metal temperature mismatch is the same for both probe and rotor. This data is then fed into a computer, where detailed stress analysis is carried out to evaluate the thermal stress within critical areas of the rotor.