Giving explanations about tests of accordion connections
Accordion joint tests are performed for various purposes. To check the design process of the parts and their manufacturing, as well as to ensure the precise performance of the connections, tests are performed on these parts, which are designed according to EJMA standards. The aforementioned tests are generally classified into two groups, non-destructive and destructive. In the continuation of the content, we will mention some of the most important tests.
Review of non-destructive tests
A) Non-destructive tests of accordion joints: Among the most important things that are examined in this test, we can control the parts visually, dimensionally, penetrating liquid, metallographic, hydrostatic, radiographic, pneumatic, ultrasonic, etc. .. Let’s point out. In these cases, by performing the test, the connections will not be damaged.
Test of radiographic accordion joints
- Radiographic accordion joint test: This test usually includes a photo that shows the weld or joint of two metals of the same type and thickness. According to this figure, it is possible to find out how to connect and possible welding defects and the shape of the piece.One of the most important applications of this test is the examination of butt welds in objects of the same material and thickness. Usually, in the construction of accordion joints, much attention is paid to the longitudinal welding seams of the blouses before creating the form. Radiographic tests are not mandatory unless requested by the customer and due to specific sensitivities. In fact, the longitudinal seam of the blouse can be measured with other methods such as the penetrating liquid test. Pay attention that the radiography test must be done before shaping the blouse. Actually, it is not possible to do this test after finding the blues form.
It is worth noting that this method does not play a role in measuring the welding quality in other parts of the accordion joints, except for the longitudinal seams of the pipe. For example, things like welding flanges to pipes, welding blouses to pipes, etc. cannot be measured by radiography and can only be evaluated by penetrating liquid test.In general, the radiographic method is only used in butt welds. If this test is needed, after obtaining the customer’s satisfaction, the location of the seams along with the required path are subjected to radiography. This test must be performed by qualified and expert contractors operating outside the organization (in the form of a third party inspection) and approved by the client. Finally, the original radiograph films are included in the archive files along with the corresponding interpretation.
Liquid penetrant accordion joint tests
- Penetrating liquid type accordion tests: This test is a comprehensive test to check the amount of leakage, the presence of cracks, the possible presence of small holes and surface cavities, etc. The mentioned test is widely used in the welding industry. The penetrating liquid test must be performed by competent and expert personnel and by means of 3 special sprays. Due to the low thickness of the blouses and the characteristics of the penetrating liquid, many problems can be investigated.
Among the most important of these problems, we can point to surface problems such as veins, small holes, surface cracks, the possible return of welds on top of each other, etc. Due to the ease of performing this test, the presence of defects during the operation is almost impossible. This test is used repeatedly to control and measure welds in all joints. Penetrating fluid test, like radiography, should be done before shaping the blouse. The mentioned test can be performed 100% on all products.
Strength test under pressure
- Tests of accordion joints of strength under pressure: In this type of test, the mechanical strength of the joints, the absence of leaks or seams due to the application of hydrostatic and pneumatic pressures, etc. are evaluated. In hydrostatic tests, the interior space of various fittings is filled with water or other liquids. In pneumatic tests, this space is filled with air or other gaseous fluids, and then the necessary pressure is applied to test the connection of the part. Due to the fact that the pneumatic test has an explosive nature, it can be considered more dangerous than the hydrostatic test. Therefore, it is better to use pneumatic tests in special situations.
To ensure the amount of pressure in this type of test of accordion joints, the pressure test is considered as a coefficient of the design pressure. In this way, the stresses created in the connection will not exceed the permissible stresses. The amount of test pressure is determined by the designer according to the standards considered for the design and after the calculation of the amount of stress and is included in the production drawings. According to the ASMEB31.1 standard, the minimum hydrostatic test pressure should be 1.5 times the design pressure. The maximum amount of this pressure should also be lower than the allowable stress. In pneumatic tests, the maximum pressure will be 1.5 times and the minimum pressure will be 1.2 times the design pressure.
In tests of accordion joints of penetrating liquid type, the pressure considered for the test is gradually applied to the joints. This pressure increases over time to reach the final test pressure. After reaching this final pressure, we must keep it constant for at least 10 minutes. During this period of time, the condition of connections are evaluated from various aspects such as leakage, deformation, deformation, etc. Be careful not to apply a pressure higher than the manufacturer’s pressure for the connection test. In order to know the amount of pressure, you can consult the manufacturer. This test can be performed 100% on all connections.
Examining tests of destructive type accordion connections
b) Destructive tests of accordion joints: these tests usually include evaluation of twisting, bursting, fatigue, etc. After performing this type of test, the connections are damaged and destroyed. Therefore, when performing these tests, you should prepare an additional sample of connections.
- Fatigue type accordion joint tests: This test is used to check the blues ability against the number of cycles applied to the joint. Each cycle includes a complete round trip of connections with a certain course. As you know, under the same conditions, the connection life correction factor increases with the increase of the connection diameter. Therefore, it is better to use small size connections with low thickness to perform the fatigue test. When we use different sizes to perform a test, the smallest size will be a good sample for testing. When the pipeline designer considers a certain number of cycles and lifetime for the fittings, it is not necessary for accordion fittings designers to use more cycles in the design.
- Torsion test: As mentioned in the past, the increase in pressure causes plate and column instabilities in joints. Therefore, you should consider suitable fixtures to perform tests of accordion type connections. These fixtures keep the connection in a straight path and seal its two ends. By using these fixtures, the two ends of the connection will not move even when pressure is applied. The accordion part of the connector has fins that should not be tightened with reinforcing rings. Twisting may have different modes. Changing the connection mode from straight to non-parallel, creating a distance between the blades of the blouse, the existence of a diameter difference between the blades, etc. are known as the most important twisting conditions.