Handbook-Testing & Inspecting

12 Piping systems and similar structures are frequently subjected to a pressure test. If it is used for a piping system, this would mean capping the open ends and then subjecting the pipe to a hydraulic pressure of the required amount. The part is examined while under the test for any pinhole leaks or evidences of porosity. Usually pressure tests are at least one and one-half times the expected working pressure of the part. Parts under pressure test are frequently further subjected to a stethoscope test. In this method a stethoscope, such as a doctor uses, is employed to listen to the sound of hammer blows in the neighborhood of the weld. Ex- perience in applying this test permits the inspector to locate such defects as lack of fusion or cracks, because of the effect that they have upon the conduction of the sound through the part. Parts that are not under pressure tests may also be inspected with some success by listening to the sound produced by hammer blows. The radiographic or X-ray method provides a means by which it is possible to look through a metal part or weld. X- ray pictures of welds will reveal blowholes, cracks, lack of fusion, or inclusions, provided these are not located in certain restricted positions with respect to the direction of the X- rays. The magnaflux test can be used only on magnetic substances. When an electric current is passed through a conductor surrounding a magnetic substance, magnetic forces are set up in the core material. Any defects at or near the surface, such as hairline cracks, porosity, or blowholes, will result in a disturbance of the lines of magnetic flux. In the practical application of the magnaflux method, small iron filings are dusted onto the part, or they may be suspended in a liquid, usually kerosene, and then the part submerged in the liquid. Discontinuities in the flow of magnetic flux will cause the iron particles to collect at that point on the surface at which a defect occurs and thus call attention to the defect. Frequently parts that are subjected to magnaflux testing are painted a light color so that the iron filings can be more readily spotted. Codes, Standards, and Specifications The industrial experience accumulated over the years that welding has been used, as to the type of test that must be applied for certain types of service, the frequency with which sampling must be done, and the methods of testing that are satisfactory to qualify operators for various classes of work, have been formulated into a series of codes and standards. In general, these codes are published by technical organizations and are available from the interested societies.