Acoustic Emission Testing – Test Pressure

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Acoustic emission tests of a pressure vessel are usually conducted by filling the vessel with water, and slowly increasing the hydrostatic pressure until it exceeds the maximum pressure that the vessel has experienced during recent operation.

Early development of acoustic emission testing showed that significant acoustic emissions are not observed until the recent maximum operating pressure has been exceeded. This phenomenon is known as the “Kaiser Effect,” and it is a very important fundamental of acoustic emission testing. Acoustic emissions are directly related to localized plastic deformation or microcrack propagation at the intensified stress fields associated with flaws in the material. Plastic deformation and microcrack propagation at flaws will not normally recur unless the stress fields at the flaws are increased above the previous maximum levels.

The minimum acceptable hydrostatic pressure for an acoustic emission test is generally 10% above the recent maximum operating pressure. However, it is desirable to have the test pressure reach 20% above the maximum operating pressure, whenever possible, to be certain that all significant flaws have been detected. It is not normally necessary to attain the ASME Code hydrotest pressure, but if hydrotesting of the vessel is required for any other reason, the acoustic emission test should be conducted at the same time at this higher pressure.

An acoustic emission test of a pressure vessel must not be conducted when the ambient temperature is below the minimum design temperature (minimum pressurizing temperature). The material is susceptible to brittle fracture at temperatures below this limit, and the acoustic emissions may not give sufficient warning to stop the test in time to prevent failure of the vessel.

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