Due to safety considerations, the Compressed Gas Association (CGA) and others have produced guidelines which address in-service inspection of NGV fuel containers (see 2.2-2.4). AE examination is listed as an alternative to the minimum three-year visual examination which generally requires that the container be removed from the vehicle to expose the entire container surface. The AE method allows “in-situ” examination of the container.

5.1.1 Slow-fill pressurization must proceed at flow rates that do not produce background noise from flow of the pressurizing medium. Acoustic emission data are recorded throughout a pressurization range (that is, 50 % to 100 % of AE test pressure).

5.1.2 Fast-fill pressurization can be used if hold periods are provided. Acoustic emission data are recorded only during the hold periods.

5.1.3 Background noise above the threshold will contaminate the AE data and render them useless. Users must be aware of the following common causes of background noise: high 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 particles, insects, rain and snow. This test method should not be used if background noise cannot be eliminated or controlled.

1.1 This test method provides guidelines for acoustic emission (AE) examination of filament-wound composite pressure vessels, for example, the type used for fuel tanks in vehicles which use natural gas fuel.

1.2 This test method requires pressurization to a level equal to or greater than what is encountered in normal use. The tanks' pressurization history must be known in order to use this test method. Pressurization medium may be gas or liquid.

1.3 This test method is limited to vessels designed for less than 10 000 psi (689 bar) maximum allowable working pressure and water volume less than 2.5 ft³ (0.07 m³).

1.4 AE measurements are used to detect emission sources. Other nondestructive examination (NDE) methods may be used to evaluate the significance of AE sources. Procedures for other NDE methods are beyond the scope of this test method.

1.5 This test method applies to examination of new and in-service filament-wound composite pressure vessels.

1.6 This test method applies to examinations conducted at ambient temperatures above 70°F (21°C). This test method may be used at ambient temperatures below 70°F if provision has been made to fill to the tank's rated pressure at 70°F.

1.7 The values stated in inch-pound units are to be regarded as the standard. SI units which are in parentheses are for information only.

1.8 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Specific precautionary statements are given in Section 8.