Because of safety considerations, regulatory agencies (for example, U.S. Department of Transportation) require periodic examinations of vessels used in transportation of industrial gases (see Section 49, Code of Federal Regulations). The AE examination has become accepted as an alternative to the common hydrostatic proof test. In the common hydrostatic test, volumetric expansion of vessels is measured.

An AE examination should not be used for a period of one year after a common hydrostatic test. See Note 2.

Note 2-The Kaiser effect relates to decreased emission that is expected during a second pressurization. Common hydrostatic tests use a relatively high pressure (167 % of normal service pressure). (See Section 49, Code of Federal Regulations.) If an AE examination is performed too soon after such a pressurization, the AE results will be insensitive to a lower examination pressure (that is, the lower pressure that is associated with an AE examination).

Pressurization:

General practice in the gas industry is to use low pressurization rates. This practice promotes safety and reduces equipment investment. The AE examinations should be performed with pressurization rates that allow vessel deformation to be in equilibrium with the applied load. Typical current practice is to use rates that approximate 500 psi/h (3.45 MPa/h).

Gas compressors heat the pressurizing medium. After pressurization, vessel pressure may decay as gas temperature equilibrates with ambient conditions.

Emission from flaws is caused by flaw growth and secondary sources (for example, crack surface contact and contained mill scale). Secondary sources can produce emission throughout vessel pressurization.

When pressure within a vessel is low, and gas is the pressurizing medium, flow velocities are relatively high. Flowing gas (turbulence) and impact by entrained particles can produce measurable emission. Considering this, acquisition of AE data may commence at some pressure greater than starting pressure (for example, ¹/₃ of maximum examination pressure).

Maximum Test Pressure-Serious flaws usually produce more acoustic emission (that is, more events, events with higher peak amplitude) from secondary sources than from flaw growth. When vessels are pressurized, flaws produce emission at pressures less than normal fill pressure. A maximum examination pressure that is 10 % greater than normal fill pressure allows measurement of emission from secondary sources in flaws and from flaw growth.

Pressurization Schedule-Pressurization should proceed at rates that do not produce noise from the pressurizing medium and that allow vessel deformation to be in equilibrium with applied load. Pressure holds are not necessary; however, they may be useful for reasons other than measurement of AE.

Excess background noise may distort AE data or render them useless. Users must be aware of the following common sources of background noise: high gas-fill rate (measurable flow noise); mechanical contact with the vessel by objects; electromagnetic interference (EMI) and radio frequency interference (RFI) from nearby broadcasting facilities and from other sources; leaks at pipe or hose connections; and airborne sand particles, insects, or rain drops. This practice should not be used if background noise cannot be eliminated or controlled.