Controls and instrumentation - Turbine gas temperature

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Turbine gas temperature
27. The temperature of the exhaust gases is always indicated to ensure that the temperature of the turbine assembly can be checked at any specific operating condition. In addition, an automatic gas temperature control system is usually provided, to ensure that the maximum gas temperature is not exceeded (Part 10).

Fig. 12-7 Turbine thermocouple installation.

28. Turbine gas temperature (T.G.T.) sometimes referred to as exhaust gas temperature (E.G.T.) or jet pipe temperature (J.P.T.), is a critical variable of engine operation and it is essential to provide an indication of this temperature. Ideally, turbine entry temperature (T.E.T.) should be measured; however, because of the high temperatures involved this is not practical, but, as the temperature drop across the turbine varies in a known manner, the temperature at the outlet from the turbine is usually measured by suitably positioned thermocouples. The temperature may alternatively be measured at an intermediate stage of the turbine assembly, as shown in fig. 12-7.
29. The thermocouple probes used to transmit the temperature signal to the indicator consist of two wires of dissimilar metals that are joined together inside a metal guard tube. Transfer holes in the tube allow the exhaust gas to flow across the junction. The materials from which the thermocouples wires are made are usually nickel-chromium and nickel- aluminium alloys.
30. The probes are positioned in the gas stream so as to obtain a good average temperature reading and are normally connected to form a parallel circuit. An  indicator,  which  is  basically  a  millivoltmeter calibrated  to  read  in  degrees  centigrade,  is connected into the circuit (fig. 12-8).

31. The junction of the two wires at the thermocou- ple probe is known as the 'hot' or 'measuring' junction and that at the indicator as the 'cold' or 'reference' junction.  If  the  cold  junction  is  at  a  constant temperature and the hot junction is sensing the exhaust gas temperature, an electromotive force (E.M.F.), proportional to the temperature difference of the two junctions is created in the circuit and this causes the indicator pointer to move.  To prevent variations of cold junction temperature affecting the ndicated temperature, an automatic temperature compensating device is incorporated in the indicator or in the circuit.

Fig. 12-8 A typical double element thermocouple system.

32. The thermocouple probes may be of single, double or triple element construction. Where multiple probes are used they are of differing lengths in order to obtain a temperature reading from different points in the gas stream to provide a better average reading than can be obtained from a single probe (fig. 12-7).
33. The output to the temperature control system can also be used to provide a signal, in the form of short pulses, which, when coupled to an indicator, will digitally record the life of the engine. During engine operation in the higher temperature ranges, the pulse frequency increases progressively causing the cyclic-type indicator to record at a higher rate, thus relating engine or unit life directly to operating temperatures.
34. Thermocouples  may  also  be  positioned  to transmit a signal of air intake temperature into the exhaust  gas  temperature  indicating  and  control systems, thus giving a reading of gas temperature that  is  compensated  for  variations  of  intake temperature. A typical double-element thermocouple system with air intake probes is shown in fig. 12-8.