Instrotech News & Articles
Inpecta detects leaks in coal fired power stations around the world
TUBE LEAK DETECTION IN LARGE BOILERS
The conventional methods of detecting tube leaks such as monitoring make-up water, mass balancing or depending on the human ear to recognise a sound change, are not sufficiently sensitive or reliable on large boilers. By using these methods the leak is often large enough to have already caused serious consequential damage - sometimes to an entire boiler face - before anyone is aware of it.
Research on Acoustic Tube Leak Detection for power station boilers started during the 1970's and has since become the most advanced method of tube leak detection world-wide.
HOW DOES THE INSPECTA FFT SYSTEM WORK?
Detection was initially achieved utilising filters to separate the normal background noises from the sound of a tube leak. With the advent of modern digital sound analysis techniques - namely Fast Fourier Transforms (FFT) - frequency spectrums are generated for each audio sensor. The analysis of these spectrums for a combination of frequencies determines the value of the alarm in percentage when a sensor channel is above a pre-set alarm value for a pre-set time when the leak alarm relay is activated.
The computer scans up to 32 sensor points situated around the boiler. Each point is analysed for the sound made by escaping steam. A detected leak is graphically displayed on screen in the form of an audio spectrum and an alarm Bar Graph.
Mimic displays show where the leak has occurred and Trends can indicate how fast the leak is growing. Alarm outputs can attract the operator’s attention and be coupled into the station logging equipment.
With computer advances in the field of voice recognition - applied to industrial tube leak detection to `listen' to the sound within the combustion chamber’ - a highly sensitive method has been developed which can detect leaks of less than 2mm in diameter in 600 MW boilers. These systems are field-proven, the world over and, have provided early warning leak detection on many occasions at such an early stage that no consequential damage has occurred.
This advance warning allows for planned shut-downs and generally reduces downtime to a matter of days instead of weeks.
Experienced plant operators also make use of the system to listen for other abnormal operating sounds such as faulty soot blower valves, exterior flange or pipe leaks and, in one instance, an explosion on start-up which caused a split down the wall. The system adds SOUND to the operator’s repertoire. One operator said that not having a tube leak detection system was like driving a car without a rear view mirror.
THE PARTS THAT MAKE IT WORK
Audio Pipe work
The sound pressure waves from the combustion chamber are conducted through the steel stub pipe, the PVC pipe and the Y-piece to the Sensor. The combination of steel Stub Pipe and PVC extender has a temperature gradient from about 800 Deg C at the boiler wall to about 60 Deg C at the PVC flange and about 40 Deg C at the Sensor.
Audio Sensors/Head Amplifiers
The Audio Sensor is a miniature industrial pressure transmitter encapsulated in a sealed acid and dust-proof capsule. The characteristic is selected to be most sensitive at the tube leak frequencies. Due to the harsh environmental conditions including high ambient temperatures this sensor has a limited life. The sensitivity drops off after a period of typically five years at which stage the sensor capsule can be changed by the station C&I department.
The Head Amplifier is mounted in the cast aluminium Sensor housing and it's function is to amplify the Sensor signal to a high level suitable for transmission to the Control Room. The C&I department can adjust the gain to allow for different background sounds or capsule sensitivities.
The Black circles represent microphone sensors on the left side of a boiler.
The schematics are all viewed from the left side. Microphone placements are duplicated on the right or opposite side.
Multiplexer / Computer
The Multiplexer / Computer is contained in a 19" rack housing situated in the Control Room. All the field Sensor cables terminate on the back of this housing. The Multiplexer / Computer contains power supplies to energise the field sensors and electronic switches to select the sensors in turn to the computer for analysis. An additional selector switches any Sensor required by the operator on to the Loudspeaker.
The built-in single card computer is a standard industrial computer with additional components to enable it to compute the FFT and I/O cards to digitise the audio signals. All signals between computer and field are isolated to improve reliability. The program for running the system is contained in flash memory.
The in-built computer generates audio Spectrums for every Sensor input. The Spectrum is a graph of sound loudness measured in Decibels (dB) verses Frequency (Hz). This sound pattern, also known as a Signature, changes in a specific way when a steam leak occurs.
The in-built computer scans every sensor to generate a spectrum and, according to a formula programmed in from experience, generates an Alarm Bar from each spectrum. The amplitude of the Bar increases with increasing steam leak sounds. In Bar graph display mode, the entire sensor Alarms Bars are shown on one screen to give an overall view of the boiler condition. The computer can also create a Mimic display to show the Operator where on the boiler the leak is taking place.Trends can be selected for any sensor input to show the alarm condition over the last 32 hours.
Visual Display Unit (VDU)
A standard Colour VGA screen is used to display the various sensor Spectrums, Alarm Bar graphs, Mimic and Trends and a printer is used to record any unusual events or to print out audio spectrums of all the Sensors at initial committioning for comparison later.
The 4-way switch on the operator panel can select the display screens. These screens are: SPECTRUM, BAR, MIMIC and TREND.
Full system calibration is required at initial commissioning and at half yearly intervals thereafter.
The INSPECTA FFT system is used in many coal fired power stations around the world including, South Africa, India, Australia and Germany. It has been tested to meet world standards such as CE and is manufactured under the control of an ISO9001 system.