Welding quality inspection - sealability inspection

Welding quality inspection refers to the testing of welding results with the aim of ensuring the integrity, reliability, safety and serviceability of the welded structure. In addition to the requirements for welding technology and welding processes, welding quality inspection is also an important part of the quality management of welded structures.
This time we will talk about the welding quality inspection method: sealing inspection.
So how do you test the hermeticity of a welded joint?
In general, the following methods can be used for testing.
1.Sinker test
For small vessels or pipelines subject to smaller internal pressure. Before the test, the container or pipe is filled with compressed air at a certain pressure (0.4-0.5MPa) and then submerged in water to test the seal, such as right leakage; there must be bubbles in the water. This is also a common test to check whether the bicycle inner tube is leaking or not.
2.Water test
The hydrostatic pressure generated by the weight of the water is used to test the structure for leaks. It is mainly visual and is suitable for general welded structures that are not under pressure but require sealing.
3.Ammonia leakage test
The use and coal pumping leakage test is the same, its sensitivity is higher than the paraffin leakage test. Test before the weld is easy to observe the side of the paste soaked with a 5% mass fraction of HgNO3, aqueous solution or phenolphthalein reagent of white paper or bandage, and then in the container filled with ammonia or compressed air with a volume fraction of 1% nitrogen.
If there are any leaks, the strips of paper or bandages will be discoloured. Soak through the mass fraction of 5% HgNO3 aqueous solution for the black spot, soaked through the phenolphthalein reagent for the red spot.
4.Kerosene leakage test
It is used for welded structures that are subject to small internal pressure and require a certain degree of hermeticity. Paraffin is highly permeable and is ideal for sealing inspection of welds. Before the test, brush lime water on the side of the weld for easy observation, brush paraffin on the other side of the weld after drying, if there is a penetrating defect, the lime layer will be flooded with paraffin spots or paraffin bands. Observation time is 15-30min.
5.Helium mass spectrometry test
The helium mass spectrometry test is currently the most effective means of testing the tightness of the vessel. The helium mass spectrometer is extremely sensitive and can detect helium at a volume fraction of 10-6. Before the test, the container is filled with helium and then the leak is detected on the outside of the container weld. The disadvantages are the high cost of helium and the long inspection period.
Despite the high penetration power of helium, the penetration of very small seams (which cannot be detected by other means) takes a long time, often up to tens of hours for some thick-walled vessels. Proper heating can speed up the leak detection.
6.Gas-tightness test
The gas tightness test is a conventional means of inspection for boilers, pressure vessels and other important welded structures requiring gas tightness. The medium is clean air and the test pressure is generally equal to the design pressure. The test pressure should be incremented step by step.
After reaching the design pressure, soapy water is applied to the outside of the weld or sealing surface and the test is based on whether the soapy water is bubbling or not. Because of the risk of explosion in the gas tightness test, it should be carried out after the hydraulic test is passed.
The airtightness test is different from the pneumatic test in that
1, its purpose is different, gas tightness test is to test the tightness of the pressure vessel, air pressure test is to test the pressure strength of the pressure vessel. Secondly, the test pressure is different, the gas tightness test pressure is the design pressure of the vessel, the air pressure test pressure is 1.15 times the design pressure.
The pneumatic test is mainly to test the strength and tightness of the equipment, the air tightness test is mainly to test the tightness of the equipment, especially the tiny penetrating defects; the air tightness test focuses more on whether the equipment has a small leak, the pneumatic test focuses on the overall strength of the equipment.
2.Use of media
In addition to air, if the medium is highly toxic and does not allow leakage or easy penetration, ammonia, halogen or helium are used.
3.Safety accessories
In the case of pneumatic tests, there is no need to install safety accessories on the equipment; in general, the gas tightness test can only be carried out after the safety accessories have been installed (tolerance gauge).
4. Sequence
The gas tightness test is carried out after the air or water pressure test has been completed.
5.Test pressure
The pneumatic test pressure is 1.15 times the design pressure, the internal pressure equipment also needs to be multiplied by the temperature trim factor; the test pressure is the design pressure when the pneumatic test medium is air, if other media are used, it should be adjusted according to the medium.
6.Using occasions
Pneumatic test: priority is given to hydraulic test, if due to the structure of the equipment or support reasons can not use hydraulic test, or equipment volume is generally used when the pneumatic test. Gas tightness test: the medium is highly or extremely hazardous medium, or no leakage is allowed.
The pneumatic test is a pressure test, in order to check the pressure strength of the equipment. The gas tightness test is a tightness test, in order to test the sealing performance of the equipment.
Source: Reproduced
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