Bellows are correlated with actual test results to demonstrate predictability of design parameters like rupture pressure, meridional yielding, squirm and cycle life for a consistent series of bellows of same basic design. Minimum five meridional yield rupture tests on bellows of varying sizes are recommended by EJMA
Purpose of testing
To assure a purchaser (user) that the product has been properly designed and manufactured; which requires some method of examination and testing of the product. The user may specify the kind of test required in the acceptance criterion
Various testing being done as a part of bellow manufacturing were
• Pneumatic pressure tests
• Hydrostatic pressure test
• Visible Dye Penetrant Test
• Fluorescent Dye Penetrant Test
• Radiographic Tests
• Helium Leak Tests
This method is based on the principle that extremely high frequency light waves, usually x rays will penetrate solid materials and, when projected on to photosensitive film, will reveal voids, areas of discontinuity, and lack of homogeneity. This examination is widely used to evaluate the soundness of welds. Unless required by the purchaser, radiographic examination of the longitudinal seam of a bellow need not be specified.
Fluorescent Dye Penetrant Test:
This method consists of cleaning a surface, coating it with a dye which contains a flourcentmateria, wiping the dye off and coating the surface with a developer which after sufficient time will draw the dye from the cracks, pin holes, and make them apparent to the observer. Liquid penetrant examination is limited in the scope to detecting the surface defects
Helium Leak Tests:
This kind of Mass spectrometer examination is an extremely sensitive means of determining the presence of a leak. The gas used is helium. This test is only recommended for explosive service requirements
Pneumatic pressure (air) tests:
Low pressure air tests utilizes a probe of suitable design which selectively indicates the presence of halogen gases.This examination is more sensitive than a hydrostatic test or air jet leak examination but since it is done at low pressure, it can only determine the presence of a leak and can not validate the structural integrity of the item being examined.
Hydrostatic pressure (hydro) test:
The hydrostatic pressure testing is necessary to check the pressure withstanding capability of bellow and detection of any leakage in the bellow This test involves filling of the expansion joint with a liquid, usually water. After filling, it can be pressurized up to the test pressure. The test pressure is usually 1.5 times the design pressure at ambient temperature. Expansion joints placed in high temperature service may require the pressure test to be performed at an adjusted pressure. It is imperative that the test pressure does not produce any membrane stress in excess of yield strength or cause permanent deformation or instability of the bellows at test temperature. The observer has to take care about pressure drop in the bellow, leakages if any in the bellow. Bellow should come to its original shape after removal of pressure.