GB2118715A - Validating rotor segment temperature - Google Patents

Abstract

In a method and apparatus for validating rotor segment temperature, means (not shown) are provided for monitoring the temperature of the or each segment each time the segment passes, in use, a preselected position. Means (14) generates a condition signal depending on the result of the monitoring step, and means (5) validates the monitored temperature only if substantially the same condition signal is generated at the rotation frequency of the rotor. <IMAGE>

Description

SPECIFICATION Validating rotor segment temperature The invention relates to a method and apparatus for validating rotor segment temperature and in particular for validating the rotor peak temperature of a gas turbine.

The operation of a gas turbine involves a compromise between efficiency and turbine blade life. The efficiency of a turbine is greater the higher the temperature of the inlet gas but blade life correspondingly decreases. It is therefore important to know the turbine blade temperature in operation so that an optimum compromise can be made. Also, with cooled blades, a cooling failure can lead to one blade being much hotter than the rest. Thus, accurate monitoring of rotor peak temperature is also necessary.

Traditionally, turbine blade temperature has not been measured directly. Instead gas temperature downstream from the turbine has been monitored using thermocouple arrays. This measurement gives only a rough, indirect assessment of blade temperature and moreover this is only an assessment of mean blade temperature and does not give information on blade to blade temperature variations. This is a serious drawback in turbines with cooled blades since if cooling of a given blade is reduced (for example due to a partial blockage of cooling holes) then that one blade can run excessively hot and eventually break off with disastrous consequences.

More recently, various attempts have been made to monitor rotor temperature directly. A pyrometer is aimed at the blade array as the rotor turns. The response time of the pyrometer can be fast enough to resolve blade to blade temperature variations and in principle, therefore, a pyrometer permits the turbine to be controlled on the basis of the temperature of the hottest point on the rotor (the rotor peak temperature). Equally, excessive blade to blade temperature variation, indicating a blade cooling failure, can be detected.

However, in practice there are difficulties. In particular, hot particles flying through the thermometer sight tube path or other noise sources can give spurious "spiking" on the pyrometer signal. This will give spurious results if a simple peak-picking circuit is used to give the rotor peak temperature.

In one proposal to deal with this problem, the rate at which the pyrometer signal amplifier can slew is severely restricted. This effectively chops off the particle noise. However, this is accompanied by loss of individual blade information and is only really useful for improving mean rotor temperature measurement and it cannot detect over-heated blades.

In another proposal a pyrometer is used which can operate simultaneously in two wavelength bands. It is argued that the two temperature readings will be the same where no particles are in the field of view but different when particles are present. Comparison of the two signals thus permits "gating" of an output derived from one signal to "miss" periods when particles are present. This particular proposal has never been used in practice and there are numerous practical difficulties in providing signals in two wavebands.

A further proposal uses the principle that all the particle noise is positive going-so if one minimum picks the pyrometer signal in synchronisation with the rotor revolution then the result is a true signal. This method is generally satisfactory but it needs a lot of hardware. The pyrometer signal needs to be digitised (at about 1/10 of a blade intervals) and the complete digitised rotor profile stored over many revolutions-to permit a minimum profile to be selected. A large memory is required. Furthermore a minimum picking system has the worrying aspect that any negative going (electrical) noise getting into the system would be "picked" and erroneously low temperatures recorded.

In accordance with one aspect of the present invention, a method of validating rotor segment temperature comprises monitoring the temperature of the or each segment each time the segment passes, in use, a preselected position; generating a condition signal depending on the result of the monitoring step; and validating the monitored temperature only if substantially the same condition signal is generated at the rotation frequency of the rotor.

With this method, the problems of spurious readings due to hot particles are avoided by comparing the monitored temperature during at least two successive turns of the rotor, the monitored temperature only being validated if that same temperature is detected on each occasion. Any spurious peak due to a hot particle or other noise will not be repeated.

The condition signals may be representative of the actual temperature monitored but preferably the monitoring step includes the step of comparing the temperature of the or each segment with a reference temperature, the condition signal depending on whether or not the monitored temperature is greater than the reference temperature. This enables faulty cooling to be detected in a particularly simple manner while also allowing the reference temperature to be set to any desired value. Conveniently, the condition signal is in the form of a logical 0 or a logical 1 representing that the monitored temperature is greater than the reference temperature or vice-versa.

In one convenient arrangement, the method further comprises scanning the reference temperature from a high value to a low value in order to determine the rotor peak temperature.

Such scanning can be carried out manually or automatically and preferably is continuously repeated in order to monitor the rotor peak temperature.

Each segment of the rotor corresponds to a portion of the rotor and is defined by the timing of the monitoring steps. Preferably, a plurality of monitoring steps are carried out for each rotation of the rotor, whereby condition signals are generated for a corresponding plurality of segments. Each segment may correspond to a rotor blade so that discrimination between segments allows discrimination between blades and in the case of faulty cooling, the particular blade can be isolated and identified.

In view of this simple way of defining segments, the resolution of this method can be very easily adjusted as required simply by altering the timing of the monitoring steps.

A further advantage of this method is that it can be operated at any required level of confidence simply be selecting an appropriate number of successive condition signals which are to be the same for validation.

In one arrangement, the method may further comprise the step of triggering an alarm or switching off the rotor if a validation is obtained.

In accordance with a second aspect of the present invention, apparatus for validating rotor segment temperature comprises means for monitoring the temperature of the or each segment each time the segment passes, in use, a preselected position; means for generating a condition signal depending on the result of the monitoring step; and means for comparing validation condition signals generated at the rotation frequency of the rotor and for generating a validation signal only if the compared condition signals are substantially the same.

Preferably, the monitoring means comprises a temperature sensing device, and means for generating a peak signal representative of the peak value of the temperature sensed by the temperature sensing device in a preselected time interval.

The apparatus may utilise simple, conventional, and well proven (single waveband) pyrometers. Furthermore, conventional shaft transducers are suitable for providing timing signals for the monitoring steps.

Preferably, the condition signal generating means includes means for comparing the monitored temperature with a reference temperature, and for generating the condition signal as a logical 1 or a logical 0 according to whether or not the monitored temperature is greater than the reference temperature. The provision of a condition signal which can only take two values is particularly useful to enable simple processing to be carried out.

Conveniently, the apparatus further includes a memory for storing successive condition signals, the validation means including means for comparing successive condition signals corresponding to the same segment and for generating a validation signal if all the compared condition signals are the same.

The memory need not be particularly large and may typically be 128x8 bits.

One further advantage of the apparatus is that it can operate at any engine speed with slow (cheap) hardware by choosing an appropriate number of rotor segments. In other words, a slow system permits less resolution on a fast engine but operates nevertheless.

An alternative method is to feed the condition signal in pulse form to an analogue arrangement.

This arrangement may comprise two filters. One filter (B) is a tracking bandpass filter whose bandpass is centred on the rotor rotation frequency (sensed from a shaft signal). The other filter (A) is a filter with a fixed bandpass which does not correspond to the rotor rotation frequency. The pulsed signal is fed in parallel to each bandpass filter and the power outputs (PA, PB) of each filter are sensed. A discriminator of the general form: PEA PUB D= PA+PB is calculated. When the threshold value is above the rotor peak temperature, the pulsed signal is due wholly to noise. The discriminator D will then produce a value of +1. However, as the threshold value is reduced to a little less than the rotor peak temperature, power will be fed to the pulsed signal at the rotor frequency so that the value of D will become -1.

Clearly, this analogue apparatus can be used to operate an alarm or on/off controller.

Furthermore, by repeatedly "scanning" the threshold value while monitoring the discriminator D, the true rotor peak temperature can be latched and displayed (in the same way as with the digital apparatus) or more complex control arrangements can be implemented.

Some examples of methods and apparatus in accordance with the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a block diagram illustrating one example of apparatus for use in an alarm mode; Figure 2 illustrates schematically the signal bit validator shown in Figure 1; Figure 3 is a timing diagram; Figure 4 is a view similar to Figure 1 but of a second example; and, Figure 5 is a view similar to Figure 1 but of a third example.

In these examples, it is assumed that the apparatus is to be used for monitoring a six blade rotor of a turbine (not shown). In the alarm mode illustrated in Figure 1, the apparatus provides an alarm if any part of the rotor exceeds a preset "alarm" temperature and also identifies where on the rotor the excess temperature occurred.

An analogue voltage signal is fed from a pyrometer (not shown) along a line 1 to a conventional peak-picking device 2. The pyrometer signal is related to, but is non-linear with, target temperature. A transducer (not shown) senses rotation of the rotor shaft and provides a pulse for each revolution of the rotor. This pulse is fed along a line 3 to a frequency multiplier 4 and to a signal bit validator 5. The number of blades on the rotor (in this case 6) is manually set and input (as indicated by a line 6) to the frequency multiplier 4 and to the signal bit validator 5. The frequency multipler 4 processes the inputs of the lines 3 and 6 to provide a signal on a line 7 which comprises 6 pulses per revolution of the rotor.

Thus, the number of pulses on the line 7 per revolution of the rotor is equal to the number of blades in the rotor. By adjusting the output of the frequency multiplier 4, the degree of resolution (that is the size of the segments) can be altered.

For example, the number of pulses may be increased in order to increase resolution and distinguish between different points on the same blade. The signal on the line 7 is fed to the peakpicking device 2.

The timing signal on the line 7 is illustrated in Figure 3 (d) where it is seen to comprise a number of equally spaced discreet pulses. The incoming pyrometer signal on the line 1 is illustrated in Figure 3 (a). This signal is a continuously varying signal of roughly saw-tooth form, each tooth corresponding to the profile of one blade.

In general, the peak value of the signal from the pyrometer does not exceed a preselected reference level (to be described below) shown by a dashed line in Figure 3 (a). However, occasionally, the reference level is exceeded and in this example, the first occurrence is due to the presence of noise as illustrated at 8 in Figure 3 (a). The next succeeding peak in the pyrometer signal also exceeds the reference level, as shown at 9, and this corresponds to the presence of a "hot" blade i.e. a blade whose temperature exceeds the reference temperature. Each time the hot blade is monitored by the pyrometer, the corresponding signal exceeds the reference temperature. As may be seen by comparing figures 3 (a) and 3 (d) the signal from the hot blade is detected between the fourth and fifth timing pulses of the timing signal on the line 7.

Additionally, noise occurs randomly in the signal and this also exceeds the reference level.

The timing signal on the line 7 defines temperature "windows" each corresponding to the time of passage of one blade. The peakpicking device 2 detects the maximum pyrometer signal occurring in each window and is subsequently reset by the timing signal. The output of the peak-picking device is shown in Figure 3 (b). Thus, initially the output from the peak-picking device 2 is substantially level but when the first peak exceeding the reference level is detected the peak-picking device 2 outputs a pulse 10 of correspondingly greater magnitude followed by a pulse 11 of slightly less magnitude corresponding to the hot blade. The duration of each pulse is equal to the time between successive pulses of the timing signal.

The apparatus also comprises an alarm temperature setting device 12 which allows the operator to set the reference temperature which should not be exceeded by the rotor blades. The device 1 2 is connected to a converting device 1 3 which converts the set temperature to a voltage level equivalent to the voltage level output from the pyrometer corresponding to that temperature (i.e. the reference level in Figures 3 (a) and 3 (b)).

The output from the peak-picking device 2 (illustrated in Figure 3 (b)) and the "alarm temperature" voltage are both fed to a comparator 1 4. If the magnitude of the output of the peak-picking device is greater than that of the reference voltage then the comparator outputs a signal on the line 1 5 representing a logical 1, otherwise the output is logical 0. The output signal on the line 1 5 is illustrated in Figure 3 (c). Thus, the incoming pyrometer signal on the line 1 has now been reduced to a signal representing either logical 0 or logical 1 depending on whether or not the rotor blade temperature exceeds a reference temperature.

The signal on the line 1 5 is fed to the signal bit validator 5. The validator 5 comprises a microprocessor and random access memory and is represented schematically in Figure 2. The random access memory 1 6 is best visualised as a two dimensional array each row corresponding to a separate blade and each column corresponding to one full rotation of the rotor. The signal on the line 1 5 is thus stored in the memory 1 6 as a series of bits. Figure 2 illustrates a typical range of stored values but not corresponding to the input signal shown in Figure 3 (a). As may be seen the fourth row corresponding to blade number 4 contains all "1 "s meaning that the alarm or reference temperature has consistently been exceeded.Rows corresponding to other blades also contain "1 "s but these are randomly arranged and correspond to random noise. In this case data from eight successive rotations of the rotor are stored and this required number of validations 8, is preset by the operator.

The signal bit validator 5 also contains AND logic 1 7 which is controlled by a timing signal on the line 3 so that at each full revolution of the rotor, each full row of 8 values are ANDed together, the output being fed to an output register 1 8. As may be seen, the output register 8 contains a single "1" corresponding to the fourth blade.

On each full revolution of the rotor, the output register 1 8 is updated, and each column is moved one position to the right, the right hand column being lost and new data being written in to the left hand column. In practice, the memory width does not physically exceed 8 bits. The validations in excess of 8 may be handled in practice by a recursive technique through a memory 8 bits wide.

The values in the output register 18 are fed to OR logic 1 9 so that if any element of the output register 18 includes a "1" a signal is fed to a buffer 20 and from there to an alarm (not shown).

At the same time, the output register contents are decoded by an address decoder 21 to indicate which blade was too hot. This number is latched by a latch 22 and may be displayed until the alarm system is reset.

The operations of the signal bit validator may be carried out, as described, by micro-processor software or alternatively a hardware system may be utilised.

In the examples illustrated in Figures 4 and 5, the majority of the apparatus is the same and the same reference numerals are used.

In the example shown in Figure 4, the apparatus is set up in a diagnostic mode. The output from the signal bit validator 5 is fed via buffers 23 to a light emitting diode (LED) array 24. A device 24' is connected to the array 24 to control the resolution of the device. In practice, the array will have a fixed number of LEDs. If the rotor has N blades and it is desired to resolve to 1 blade then the device 24' causes N LEDs in the array 24 to activate as appropriate, the remaining LEDs being inactive. If, however, it is only desired to resolve to 1/1 0 of the rotor then the device 24' will still cause N LEDs to activate but they will be divided into 10 groups corresponding to the 10 segments and will thus turn on and off in blocks.

In this example the reference temperature set by the device 1 2 is slowly (relatively to the rotation frequency of the rotor) decreased so that a picture of any developing hot spots or individually overheated blades may be rapidly gained. Thus, as soon as the reference temperature reaches the temperature of the hottest blade, the appropriate LED or block of LEDs in the array 24 will light. Futhermore, it should be noted that because of the presence of the peak-picking device 2 in the input circuitry, a blade is diagnosed as "hot" if any part of the blade is above the reference temperature.

The example illustrated in Figure 5 enables the temperature of the hottest viewed point on the rotor to be continuously displayed. In this example, the device 12 is replaced by an automatic scanning device 25 which generates a continuously scanning reference level, the scanning rate being slow compared with the rotation frequency. As soon as the reference level reaches the level of the hottest blade the output from the OR logic 19 is fed to a latch 26 which latches the reference level at that point. The latched reference level is fed to a lineariser 27 and from there to a display device 28 which displays the temperature of the hottest blade in degrees C or F.

In any of these examples, the number of validations can be set to any appropriate value but clearly the larger the number of validations the longer the response time of the system.

Typically, a one second response would permit of the order of 100 validations.

Claims (15)

Claims

1. A method of validating rotor segment temperature, the method comprising monitoring the temperature of the or each segment each time the segment passes, in use, a preselected position; generating a condition signal depending on the result of the monitoring step; and validating the monitored temperature only if substantially the same condition signal is generated at the rotation frequency of the rotor.

2. A method according to claim 1, wherein a plurality of monitoring steps are carried out for each rotation of the rotor, whereby condition signals are generated for a corresponding plurality of segments.

3. A method according to claim 1 or claim 2, further including storing the condition signal, at least temporarily, in a memory.

4. A method according to any of the preceding claims, wherein the monitoring step includes the step of comparing the temperature of the or each segment with a reference temperature, the condition signal depending on whether or not the monitored temperature is greater than the reference temperature.

5. A method according to claim 4, wherein each condition signal is in the form of a logical 0 our a logical 1.

6. A method according to claim 4 or claim 5, further comprising the step of triggering an alarm or switching off the rotor if a validation is obtained.

7. A method according to any of claims 1 to 6, further comprising the steps of determining and indicating the segment or segments whose temperature has been validated.

8. A method according to claim 4, claim 5 or claim 7, wherein the reference temperature is scanned from a high value to a low value in order to determine the rotor peak temperature.

9. A method according to claim 8, further comprising repeating the scanning step so as continuously to monitor the rotor peak temperature.

10. A method according to claim 1, substantially as described with reference to any of the examples illustrated in the accompanying drawings,

11. Apparatus for carrying out a method according to any of the preceding claims, the apparatus comprising means for monitoring the temperature of the or each segment each time the segment passes, in use, a preselected position; means for generating a condition signal depending on the result of the monitoring step; and validation means for comparing condition signals generated at the rotation frequency of the rotor and for generating a validation signal only if the compared condition signals are substantially the same.

12. Apparatus according to claim 1 wherein the monitoring means comprises a temperature sensing device, and means for generating a peak signal representative of the peak value of the temperature sensed by the temperature sensing device in a preselected time interval.

13. Apparatus according to claim 11 or claim 12, wherein the condition signal generating means includes means for comparing the monitored temperature with a reference temperature, and for generating the condition signal as a logical 1 or a logical 0 according to whether or not the monitored temperature is greater than the reference temperature.

14. Apparatus according to claim 13, further including a memory for storing successive condition signals, the validation means including means for comparing successive condition signals corresponding to the same segment and for generating a validation signal if all the compared condition signals are the same.

15. Apparatus according to any of Claims 11 to 14, further including indicator means for indicating the segment or segments whose temperature has been validated.

1 6. Apparatus according to claim 11, substantially as described with reference to any of the examples illustrated in the accompanying drawings.

1 7. A turbine having a rotor; and apparatus according to any of claims 11 to 16 for validating the temperature of one or more segments of the rotor.