Welding quality testing refers to the testing of welding results, the purpose is to ensure the integrity, reliability, safety and serviceability of the welding structure. In addition to the requirements of welding technology and welding process, welding quality inspection is also an important part of welding structure quality management.
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. Submerged test
For small vessels or pipes subject to low internal pressure. Before inspection, the container or pipeline is filled with compressed air at a certain pressure (0.4-0.5MPa), and then submerged to check the tightness, such as right leakage; Bubbles must occur in the water. This is also a common part of checking the bicycle inner tube for air leaks.

2. Water test
The static pressure generated by the water's own weight is used to check the structure for leakage. Based on visual inspection, it is suitable for general welding structures that are not under pressure but require sealing.

3. Ammonia leakage test
It is used in the same way as coal extraction leakage test, and its sensitivity is higher than kerosene leakage test. Before the test, paste a white strip or bandage impregnated with 5% HgNO3, aqueous solution or phenolphthalein reagent on the side of the weld for easy observation, and then fill the container with ammonia or add 1% nitrogen compressed air.
If there is a leak, it will appear on the white paper strip or bandage. The solution of 5%HgNO3 aqueous solution was black spot, and the solution of phenolphthalein was red spot.

4. Kerosene leakage test
It is used for welding structures that are subject to small internal pressure and require certain tightness. Kerosene is highly permeable and is very suitable for sealing inspection of welds. Before inspection, brush lime water on the 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, the lime layer will spill coal oil spots or kerosene belts. The observation time was 15-30min.

5. Helium mass spectrometry test
Helium mass spectrometry test is the most effective means of sealing test at present, helium mass spectrometer is very sensitive, can detect the volume fraction of 10-6 helium. Before the test, the container is filled with helium and the leak is detected on the outside of the weld of the container. The disadvantage is the high price of helium and the long inspection cycle.

Despite the extremely penetrating power of helium gas, it takes a long time to penetrate very small cracks (such cracks cannot be detected by other means), and the leak detection of some thick-walled vessels often takes tens of hours. Proper heating can speed up the 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 on the outside of the weld or sealing surface and check whether the soapy water bubbles. Because of the risk of explosion in the air tightness test, it should be carried out after the hydraulic test is qualified.

Air tightness test is different from air pressure test:
1, its purpose is different, the air tightness test is to test the tightness of the pressure vessel, the pressure test is to test the pressure 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, and 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 a small leak, and the air pressure test focuses on the overall strength of the equipment.

2. Use media
Air pressure test is generally used in actual operation, air tightness test in addition to air, if the medium is relatively toxic, do not allow leakage or easy penetration, use ammonia, halogen or helium

3. Safety accessories
When the air pressure test, it is not necessary to install safety accessories on the equipment; In general, the air tightness test can only be carried out after the installation of safety accessories (capacity gauge).

4. Order
The air tightness test should be carried out after the air pressure or water pressure test is completed.

5. Test the 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 dressing coefficient; When the air tightness test medium is air, the test pressure is the design pressure. If other media are used, it should also be adjusted according to the medium condition.

6, the use of occasions
Pressure test: hydraulic test is preferred, if the hydraulic test cannot be used due to the equipment structure or support reasons, or the equipment volume is generally used when the pressure test. Air tightness test: medium is highly or extremely hazardous medium, or leakage is not allowed.
Air pressure test is a pressure test, in order to check the pressure strength of the equipment. The air tightness test is a compact test, in order to check the sealing performance of the equipment.