Acoustic emission crack detection

Acoustic emission is defined as the transient elastic waves that are generated by the rapid release of energy. It is the sound resulting from a crack developing in solid material. As this extremely small, low-level sound propagates through the material, it can be picked up by highly sensitive piezoelectric or strain gauge sensors. The advantage of acoustic emission is

that very early crack growth can be detected well before a highly stressed component may fail.

Acoustic emission technology is intended to provide analysts with early indication of the onset of degraded strength in metal components such as pressure vessels. By trending the acoustic emission event occurrence, analysts can track and project the progression of grain structure breakdown. With this information, the plant can remove the metal component from service before total loss of function occurs.

Acoustic emission technology has been successively used on the following equipment:

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Reactor vessels and related piping

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Control rod housings

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Main steam lines

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Transformers

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Fossil high energy piping

Micro-granular material, such as steel, put under tension or compression beyond its yield point, breaks and tears along grain boundaries (inter-granular cracking). This can happen in pressure vessels, structural supports and high-energy piping. Acoustic emission monitors for micro cracks in heavy metal components and specifically, the growth of cracks. The breakdown in plant component’s metal crystalline structures can lead to equipment failure in extreme cases.

Acoustic emission sensors have been very effective when applied to reactor vessels and related piping for the early determination of developing cracks. In these applications, the sensors are monitored continuously during all levels of plant operations. Other applications include monitoring the integrity of control rod housings and main steam lines.

Acoustic emission monitors use many acoustic pickup transducers to detect the energy bursts caused by the inter-granular cracking and other sources. The equipment sends these signals to a computer console for signal conditioning, measuring, and comparison of arrival time.

Arrival time comparison locates the signal source.

Different acoustic emission equipment versions present the data in several ways. Most show the acoustic events per second, cumulative total events over time, change in count rate, or some combination of these three ways. Analysts can then trend this data.

Table 7 shows a correlation of acoustic emission with other technologies.

Table 7. Correlation of Acoustic Emission with Other Technologies

Technology Correlation Method

Indication When Used

Ultrasonic

imaging Time sequence Cracking in heavy metal

weld joints In conjunction with code requirements for periodic (10 year) inspection or after a rise in acoustic emission events in a specific region indicates cracks may be developing.

Dynamic

radiography Time sequence Cracking in heavy metal

weld joints In conjunction with code requirements for periodic (10 year) inspection or after a rise in acoustic emission events in a specific region indicates cracks may be developing.

Stress/strain

measurement Time coincident High levels on strain gauges or distortion indicated by other methods.

When monitoring for pressure vessel or heavy section weld deterioration during hydro testing or other high stress event.

Acoustic emission sensing techniques can be particularly useful for monitoring of in-service power transformers. A major concern of transformer failure is focused on the partial discharge associated with the degradation of insulation inside the transformer. This insulation breakdown causes electrical arcing that deteriorates the oil insulation factor and, if continued, produces highly explosive gases.

Each partial discharge propagates to the tank wall. These stress waves are similar in character to stress waves propagated in solids during crack formation, and generate acoustic emission signals that contain an appreciable amount of energy in the 150 kHz frequency range. These signals can readily be differentiated from other signals emanating from the transformer.

By taking into account the intensity of the acoustic emission signal, the approximate location of the emitting source, and estimated the level of activity involved, it is often possible to estimate the severity of the problem and make a reasonable assessment of its cause.

The detection of acoustic emissions from partial discharge events in transformers is established and instrumentation is available for this detection. However, acoustic emissions from transformers have been detected in the absence of partial discharge. It has been shown that these signals are produced as a result of the inception of bubbles.

The operating transformers can generate acoustic emissions for a variety of reasons. These can be categorized into heating sources, electrical sources, and background noises.

Partial discharge inside a transformer produces acoustic emissions. There are other sources that can produce acoustic emissions prior to the actual occurrence of partial discharge in the transformer. Some of these are:

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Localized heating in oil or paper can sometimes acoustic emissions

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Paper tracking or carbonization produces acoustic emission signals

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Energy released during hydrogen gas evolution by partial discharge either from heating or implosion

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From cavitation, nitrogen is released from solution and generates acoustic emissions.

These mechanisms produce acoustic emission that is directly related to a breakdown in the transformer. There are other mechanisms that have been observed that will generate acoustic emission activity that is not directly related to a problem in the unit. Some of these are:

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Environmental sources due to the impact of rain, snow, ice, or dust against the transformer

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When there are areas or pockets of turbulent oil flow within the unit, acoustic emission can be generated

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Acoustic emissions are observed during pump flow with colder oil. It is believed that these emissions are caused by static discharge and not associated with gassing.

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Oil pyrolysis (chemical change as a result of heat) and core winding irregularities produce acoustic emissions.

Data filtering techniques allow separation of the relevant and non-relevant acoustic emission data. There are several filtering schemes that are used for different situations. Also, by making use of the newer digital signal processing techniques, the accuracy and speed of analysis has improved. Testing transformers in situ can produce a large amount of data if the transformer is acoustically active. The speed and accuracy of data analysis is critical.

There are several different sources of acoustic emission that can be detected within a transformer. They can be classified as burst emissions and continuous emissions.

In document IAEA-TECDOC Implementation Strategies and Tools for Condition Based Maintenance at Nuclear Power Plants (Page 54-57)