Welding quality inspection refers to the inspection of welding results, the purpose is to ensure the integrity, reliability, safety and usability of the welded structure. In addition to the requirements for welding technology and welding process, 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: tightness inspection.
So how to test the tightness of welded joints?
In general, the following methods can be used for detection:
1. Submersion test
For small containers or pipes with low internal pressure. Before the test, fill the container or pipeline with compressed air with a certain pressure (0.4-0.5MPa), and then submerge it in water to test the tightness, such as right leakage; there must be air bubbles in the water. This is also a common section for checking for leaks in bicycle inner tubes.
2. Water test
Use the static pressure generated by the dead weight of water to check the structure for leakage. Based on visual inspection, it is suitable for general welded structures that are not under pressure but require tightness.
3. Ammonia leakage test
The purpose is the same as that of the coal pumping leakage test, and its sensitivity is higher than that of the kerosene leakage test. Before the test, paste a white paper strip or bandage soaked with 5% HgNO3, aqueous solution or phenolphthalein reagent on the side of the weld that is easy to observe, and then fill the container with ammonia gas or add 1% nitrogen compression. Air.
If there is a leak, it will stain the white strip or bandage. The dark spots were soaked in a mass fraction of 5% HgNO3 aqueous solution, and the erythema was soaked in phenolphthalein reagent.
4. Kerosene leakage test
It is used for welded structures that are subject to small internal pressure and require a certain degree of sealing. Kerosene has strong permeability and is very suitable for sealing inspection of welds. Before inspection, brush lime water on one side of the weld for easy observation, and brush kerosene on the other side of the weld after drying. If there is a penetration defect, kerosene spots or kerosene bands will appear on the lime layer. The observation time is 15-30min.
5. Helium mass spectrometry test
Helium mass spectrometry test is the most effective means of tightness inspection at present. The helium mass spectrometer is extremely sensitive and can detect helium with 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 welding seam of the container. The disadvantage is that helium is expensive and the inspection cycle is long.
Although helium has extremely strong penetrating power, it still takes a long time for the penetration of very small gaps (such gaps cannot be detected by other means), and the leak detection of some thick-walled containers often lasts for tens of hours. Appropriate heating can speed up leak detection.
6. Air tightness test
Air tightness test is a routine inspection method for boilers, pressure vessels and other important welded structures requiring air tightness. The medium is clean air, and the test pressure is generally equal to the design pressure. The pressure should be increased step by step during the test.
After reaching the design pressure, apply soapy water to the outside of the welding seam or sealing surface and check whether the soapy water is bubbling. Due to the danger of explosion in the air tightness test, it should be carried out after the hydrostatic test is qualified.
The air tightness test is different from the air pressure test:
1. Its purpose is different. The air tightness test is to test the tightness of the pressure vessel, and the air pressure test is to test the compressive strength of the pressure vessel. Secondly, the test pressure is different. The air tightness test pressure is the design pressure of the container, and the air pressure test pressure is 1.15 times the design pressure.
The air pressure 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 small penetration defects; the air tightness test focuses more on whether the equipment has small leaks, and the air pressure test focuses on on the overall strength of the device.
2. Use medium
Air is generally used in the actual operation of the air pressure test. In addition to air for the air tightness test, if the medium is relatively toxic, no leakage or easy penetration is allowed, and ammonia, halogen or helium is used.
3. Safety accessories
During the air pressure test, there is no need to install safety accessories on the equipment; generally, the air tightness test can be carried out after the safety accessories are installed (capacity gauge).
4. Sequence
The air tightness test needs to be carried out after the air pressure or water pressure test is completed.
5. Test pressure
The air pressure test pressure is 1.15 times the design pressure, and the internal pressure equipment needs to be multiplied by the temperature trim coefficient; when the air tightness test medium is air, the test pressure is the design pressure. If other medium is used, it should be adjusted according to the medium conditions.
6. Use occasions
Air pressure test: Hydraulic test is preferred. If hydraulic test cannot be used due to equipment structure or support, or when the equipment volume is large, air pressure test is generally used. Air tightness test: The medium is a highly or extremely hazardous medium, or no leakage is allowed.
The air pressure test belongs to the pressure test, in order to check the pressure bearing strength of the equipment. The air tightness test belongs to the compactness test, in order to check the sealing performance of the equipment.
