Non Destructive Testing
Direct and remote non-destructive testing:
- accessories and lifting systems
- pressurized systems (PED)
- periodic check of pressure gauges
- periodic check of safety valves
In the industrial sector, every product of critical importance (beams for construction, support screws, aeronautical components, automotive components, pressure vessels) must be checked to verify its integrity and compliance with current regulations. In fact, it is known that a small superficial crack, harmless under normal conditions, if subject to fatigue stresses, may widen until the component breaks. Non-destructive control methods are used for the analysis of each individual piece, replacing the more uncertain “sample destructive test” in many fields.
The experience of the inspection staff is essential for the reliability of the result.
The principle is based on the use of light as a means of detecting defects.
A. Direct visual examinations: at a distance of the surface not greater than about 23.5 inch, with an angle of not less than 30 °. Lenses and mirrors can be used. The lighting is secured with suitable lamps between 150 and 600 lux.
B. Remote visual examinations: if it is not possible to directly access the surface to be examined. used: mirrors, telescopes, endoscopes, optical fibers, cameras, etc. instruments must have a resolution at least equivalent to that of the human eye.
Stages of the operation:
A. Application and penetration of penetrant.
B. Removal of excess by washing with water.
C. Extraction of the penetrating liquid remaining inside the defects by spreading a detector layer on the surface which can be powder or liquid.
D. The penetrating liquid rises by capillarity and leaves in the detector a signal having a larger size than the defect.
The electromagnetic yoke is a very versatile portable instrument for MV controls and is therefore often used in the case of tests conducted directly on site.
The extent of penetration depends on the type of current (alternating, pulsating or direct), the intensity and frequency of the current. However, it is not possible to exceed the value of a few millimeters in depth.
• external (they are the "open" ones on the surface)
• under the skin (up to .005-.007 inch deep)
• in depth (over .007 inch deep)
A. The defect is identified by spraying colored or fluorescent ferromagnetic powders on the surfaces.
B. The particles will concentrate by aligning themselves along the flux lines of the magnetic field, forming a "profile" of the discontinuity (position, size, shape and extent).
C. The particles are made visible by means of a wood lamp.
This method allows to reveal both superficial defects and defects in depth.
The technical standard
The first condition for the correct reading of the technical standards is to place it correctly within our legal system. For this reason it is useful to refer to Article 2, paragraph 1, letter u) of Legislative Decree 81/2008: "" technical standard ": technical specification, approved and published by an international organization, by a European body or by a national standardization body, compliance with which is not mandatory. " The technical standard is a tool that the economic operator has at his disposal to better fulfill his work. Very rarely does it anticipate the best technologies, it frequently welcomes them after market operators have identified better solutions than those envisaged by the technical standard itself. A typical example is precisely the innovation with which the ISO 4309 standard incorporated the magnetic inductive control for ropes intended for lifting equipment (MRT): the test had long been available, for competent people, on rope controls and therefore for users. The technical standard has come to include it among the tests available long after the introduction of its use. There may therefore be a considerable time difference between the use of an advanced technology and its implementation in a technical standard. It is therefore necessary to pay close attention to those superficial readings that assimilate the normative prescriptions to an obligation. The desired result of these readings is to conclude, quite arbitrarily, that the use of tools (such as the MRT) already available for some time has become mandatory, for those who wanted to use them for the safety of ropes and machines. The technical standard must not be confused with the EU harmonized standard, which has a very different legal value. A. The task of the competent person is to examine the condition of the rope, produce a report on it, from which it can be deduced whether the rope can continue to work or not, and finally decide when a subsequent examination is necessary.
B. The technical standard keeps intact the description of the visual checks necessary to verify the state of wear of the rope, underlining the importance of the discretion of the competent person in deciding the frequency and outcome of the checks, and providing for the complementarity of the MRT with respect to the visual checks and physical measurements on the rope (table 1, page 13).
C. The technical standard then, in point 5.6 which is also the only one dedicated to MRT, states that the use of this test is a faculty available to the competent person, and that its usefulness is conditioned by the possibility of having reference data relating to the state of the rope as close as possible at the time of its first use. He reiterates that MRT is complementary to visual checks, performed by a competent person. The complementarity of the MRT with respect to visual checks is repeated in Table 6, which summarizes the criteria for determining the replacement of the rope.
D. The technical standard recalls in several points the priority that the competent person must give to the instructions provided by the manufacturer of the machine with respect to the regulatory prescriptions regarding the use and maintenance of the ropes, as well as for the replacement of the same.