It is a well-known fact that respected and trusted NDT methods such as UT, RT, PT, MT and VT have proven to be effective in finding defects and/or discontinuities in a wide variety of equipment. While effective, these same methods tend to have limitations including, but not limited to, performing a 100% inspection on a structure to make sure there are no defects present, extensive down time to complete tests, loss of revenue through down time, equipment costs, start-up costs etc. Since this is a major concern before or especially during a major shut down, Acoustic Emission Testing (AET) has proven itself as a method of preference in detecting the presence of defects quickly and reliably. AET also provides accurate detail in identifying defects based on their individual level of seriousness and in doing so, permits the manager and/or owner to plan their maintenance schedules in a timely and efficient manner.
WHAT EXACTLY IS ACOUSTIC EMISSION TESTING?
AET can better be described as another form of seismology i.e. micro seismology if you will. Just as the movement of the earth’s crust in the form of an earthquake releases energy as a shock wave detectable by low frequency sensors, so objects/structures themselves under test by AET release energy detectable by sensors
placed at strategic locations. The main difference between seismic and AET sensors are the frequency spectrums used. The seismic industry uses low frequencies between 0 to 10 Hz that would create problems if used in noisy environments. AET uses frequencies between 20 and 400 kHz (in some cases evenhigher), which allows the monitoring of equipment subject to high noise from pumps and liquids flowing in piping etc. Another difference is the distance used between sensors. In the seismic industry the sensors are placed many miles away from the source. Typical AET sensor spacing is in the order of 10 to 15 feet apart.
Thus, AET can be described as the method that uses sensors to detect the high frequency signals resulting from structural defects when the test object is subjected to a load. Such signals may result from the growth of cracks, corrosion, rubbing, leaks, overstress etc. In the case of FRP vessels and/or structures it would be fiber breakage, de-lamination, de-bonding, etc. The AE test as such is normally conducted during a controlled increase in the operating load, which can range from a pressure change to a load being applied or even a cool down or a controlled temperature change where the thermal stresses are monitored. Active defects that could be detrimental to the safe operation of the vessel under test are located and thus structural integrity is kept at a maximum.
As such, AET is used in a vast range of applications such as:
- Metal pressure vessels and piping
- FRP vessels and piping
- Reactors and hot circuit piping
- Heat Exchangers
- De-aerators etc.