GB2167556A - Steam leak detection - Google Patents

Abstract

Steam leaks in a steam raising boiler are detected by microphones 13a-13f connected to the combustion space of the boiler via long narrow listening tubes 10a-10f. The signal from each microphone is amplified and filtered (14e-14f, 15 and 16) so that there is selectively passed a signal of frequency 4-8 KHz. The resultant signal is supplied to a display 17 with bar charts 18 whose range and bias is set so that (a) at a signal level of less than 74 dB corresponding to normal background noise no signal shows on the display, (b) a signal level of 74-94 dB operates to a first portion of the display range to indicate an abnormality, (c) a signal level of 94-114 dB operates a second portion of the display to indicate the presence of a steam leak and (d) a signal beyond 114 dB exceeds the range of the display and indicates that serious secondary damage is occurring. A colour change indicates the change from (b) to (c). <IMAGE>

Description

SPECIFICATION Steam leak detection This invention relates to a method for detecting steam leaks and secondary damage formation in steam boilers, and more particularly in coal fired steam boilers for power stations.

The acoustic detection of steam leaks in such boilers has been described in our patent specification No. EP-A-0108556. But mere detection of a leak or an abnormality preceding a major leak will be insufficient if those charged with responsibility for operating the boiler disregard the warning given or fail to act on it until there is corroborative evidence, by which time major damage will have occurred. A significant difficulty in obtaining corroborative evidence is that the noise level within the boiler is less than the ambient noise level because of the second insulating properties of boiler insulation, so that nothing can be heard from outside the boiler until the steam leak has caused major secondary damage and other instrumentation level such as the boiler water make up level also cannot provide corrobation in time to prevent secondary damage.

Broadly stated the invention provides a method for detecting a steam leak in a steam raising boiler comprising: providing a microphone connected to the combustion space of the boiler via a long narrow listening tube; amplifying and filtering the signal from the microphone so that there is selectively passed a signal of frequency 4-8 KHz; and supplying the resultant signal to a display whose range and bias is set so that (a) at a signal level of less than 74 dB corresponding to normal background noise no signal shows on the display, (b) a signal level of 74-94 dB operates to a first portion of the display range to indicate an abnormality, (c) a signal level of 94-114 dB operates a second portion of the display to indicate the presence of a steam leak and (d) a signal beyond 114 dB exceeds the range of the display and indicates that serious secondary damage is occurring.

The steam leak detector system used according to the invention will typically have several microphones located around the boiler and be sensitive to the acoustic sounds produced by steam escaping from an orifice. This sound is sensed by a microphone sensor and the output of the microphone is firstly filtered then adjusted to a fixed level. This level is set by a potentiometer and is set by using a white noise calibrator (limited to frequencies between 2 and 8 KHz). Each channel's measured sound level is displayed continually by using bar charts. Each bar chart is ranged to give no indication during normal boiler running conditions and full range when a steam leak is causing significant secondary damage. Each bar chart should be identified by its own identification code and should change colour from yellow to red when the alarm level is exceeded.In normal running, a signal level of less than 74dB will not show on the display.

Signals of less than 74dB are normal background noise and must not be presented to the boiler operator who might confuse changes in noise background with an incipient steam leak. A signal level above 74dB is abnormal and must be investigated by the boiler user. Provided no obvious external cause can be identified then a small steam leak is to be suspected. A signal level of 94dB and above indicates a steam leak providing there is no obvious external noise generation. External noise to give a 94dB level inside the boiler would need to be around 120 dBA external to the boiler (at the pain level). Therefore there can be no doubt about the existence of external noises. A signal level of 114dB accompanied by a visible level on a nearby listening sensor indicates that serious secondary damage is occurring.

A cross check of the visual levels of sound may be made by means of a loudspeaker. This acoustic information is of much greater integrity than that obtained by an external listening device because back ground noise outside the boiler is largely eliminated. Also by selecting one listening channel at a time the user is able to make comparisons of noise levels and frequencies inside the boiler without the confusion of varying background noise levels and frequencies outside the boiler.

The display unit may also include an alarm mimic which will indicate the location of all listening parts on a simplified boiler diagram and indicate which, if any, are in the alarm condition.

The whole system should desirably be able to be checked by pressing a single push button from the control room panel. Also the steam leak alarm contact closure is desirably taken via a second push button such that the alarm can be muted. If the alarm is muted then the illumination push button should flash. A switch may be provided such that any one channel can be selected to drive a recorder. This output should be 4 to 20 mA.

A hand held white noise sound generator should be provided. It is essential for correct system calibration that the sound generator used is as close as possible to the sound produced by a steam leak.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of an acoustic steam leak detection system according to the invention; Figure 2 is a circuit diagram of a single listening channel; Figure 3 shows the operation of a bar chart; and Figure 4 is a graph showing noise spectra.

Figure 1 shows a multiplicity of acoustic tubes 10a-10f leading from the combustion space of a coal fired steam raising boiler and connected via isolator valves 11a-11f and elbows 12a-12f to microphone assemblies 13a-13f as described in European Patent Application No. 0108556.

Each microphone 13a-13f must be of such design as to withstand the harsh environment encountered in coal burning boilers. The microphone assembly should be to at least to the following standard.

Sound Pressure Range - 44 to 154 dB.

Temperature Range - 05 to 95" Sealed to IP 67 to the boiler gases Over Pressure - 3 bar Include a sound generator self test facility.

The microphone mounting stub pipe 10a-10f must be directed upwards at an angle such that dust deposite do not build up. This stub pipe must be constructed such that the microphone is shielded from direct radient heat from the boiler and cooled to almost ambient temperature to facilitate easy maintenance. This assembly must have at least 10 dB attenuation to the external noises outside the boiler. Isolator valves 11a-11f must be provided in the air path between the boiler and the microphone 13a-13f in order that the transient pressure changes in the boiler do not cause damage to personnel during microphone removal for calibration.

The microphone leads and connections must be of a waterproof variety. The microphones are connected via head amplifiers 14a-14f to a junction box 15 which feeds a signal processing unit 16.

The output from unit 16 is fed to a display unit 17 that may typically provide up to eighteen channels displayed as bar charts 18, and a speaker output 19. The unit 17 also operates a pen recorder unit 20 and an existing station alarm system.

In Figure 2, which shows a single channel of the system of Figure 1, a signal from microphone sensor 13a is fed to filter 24 and thence via potentiometer 25 to amplifier stages 26, 27 and voltagecurrent converter 28 all forming part of the head amplifier assembly 14a. The converter 28 that drives the outgoing line is terminated by a low impedance to ensure low interference pickup and for minimum crosstalk between channels in the same multipair cable. Maximum crosstalk between channels must be at least 60 dB of the full scale between channels. The microphone 13a is calibrated using a white noise generator set at a level equivalent to 94 dBA at 1KHz so that the bar chart 18 will be at mid scale position at that level.The signal processing unit 16 includes a further amplifier stage and filter stages 31-34 followed by a rectifier 35, a further filter 36 and a logarithmic amplifier 37 that is controlled by bias potentiometer 38 to act effectively as a comparator. The resultant signal is further amplified at 39 and fed via potentiometer 40 to a voltage to current converter and to a bar chart driver 42 controlled by potentiometer 43. The settings of potentiometers 40, 43 may be adjusted to further set the required range and offset of the bar chart 17. Signals from the several channels are further fed from amplifier 30 to audio amplifier 44 and thence via channel selector switch 45 to speaker 19. Thereby a selected one of the output signals of microphones 13a-13f may be heard on speaker 19.

In Figure 3 there is shown the successive stages of operation of one of the bar chart recorders. In Figure 3(a) the signal has activated only the lower part of the scale indicating a possible leak and a "yellow" alert. In Figure 3(b) the scale reading has passed from the yellow into the red zone, indicating a steam leak and in Figure 3(c) the reading has gone off-scale indicating secondary damage. It is considered that a display of the present kind corroborated by the sound on speaker 19 provides the most effective way of indicating to power station staff that action is required without creating false alarm situations that would undermine confidence in the system.

In Figure 4 noise level is plotted against frequency for background noise within the boiler (curve 50) and for steam leak noise 51. It is observed that in the 2-8 KHz range the steam leak noise predominates, so that a useful signal may be derived.

Claims (5)

1. A method for detecting a steam leak in a steam raising boiler comprising: providing a microphone connected to the combustion space of the boiler via a long narrow listening tube; amplifying and filtering the signal from the microphone so that there is selectively passed a signal of frequency 4-8 KHz; and supplying the resultant signal to a display whose range and bias is set so that (a) at a signal level of less than 74 dB corresponding to normal background noise no signal shows on the display, (b) a signal level of 74-94 dB operates to a first portion of the display range to indicate an abnormality, (c) a signal level of 94-114 dB operates a second portion of the display to indicate the presence of a steam leak and (d) a signal beyond 114 dB exceeds the range of the display and indicates that serious secondary damage is occurring.

2. A method according to Claim 1, wherein the microphone that is provided is fitted to a listening tube having an isolator valve between the microphone and the combustion space, said listening tube being angled downwardly towards the combustion space to prevent build-up of dust therein.

3. A method according to Claim 1 or 2, wherein there are provided a multiplicity of microphones at different locations in the boiler each simultaneously operating a respective bar chart.

4. A method according to Claim 3, wherein the amplifying and filtering means further comprises an audio amplifier and loudspeaker, and on detection of an abnormality or leak in a channel, switch means is operated to feed the sound in that channel to the loudspeaker.

5. A method for detecting a steam leak in a steam raising boiler substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.