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Welding quality inspection -- tightness inspection

Date: 31/05/2023

Welding quality testing refers to the testing of welding results, with the purpose of ensuring the integrity, reliability, safety and serviceability of welding structure. In addition to the requirements of welding technology and process, welding quality inspection is also an important part of welding structure quality management.

This time we will talk about the welding quality test method: tightness test.



So how to test the tightness of welded joints?


In general, the following methods can be used for detection:


1. Sinking test

Used for small vessels or pipes subjected to low internal pressure. Before inspection, the container or pipe should be 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 the bicycle tube to check for air leaks2. Water holding test

The static pressure generated by water dead weight is used to check whether the structure has leakage phenomenon. Based on visual inspection, it is suitable for general welding structure which is not compressed but requires sealing.


3. Ammonia leakage test

It is used in the same way as coal drainage leakage test, and its sensitivity is higher than kerosene leakage test. Before the test, paste a white strip or bandage soaked with 5% HgNO3, aqueous solution or phenolphthalein reagent on the easy observation side of the weld, and then fill the container with ammonia or add compressed air with 1% nitrogen.

 If there is leakage, it will stain the white paper strip or bandage. The solution of 5%HgNO3 was black spot, and the solution of phenolphthalein was erythema.

 4. Kerosene leakage test

It is used for welding structure with small internal pressure and certain sealing requirement. Kerosene has strong permeability and is very suitable for sealing inspection of welds. Before inspection, brush lime water on the side of the weld for observation, and brush kerosene on the other side of the weld after drying. If there are penetration defects, 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 and can detect helium with volume fraction of 10-6. The container was filled with helium before the test, and the leak was detected outside the weld of the container. The disadvantage is the high price of helium and the long inspection period.


Although helium gas is highly permeable, it takes a long time to penetrate very small gaps that cannot be detected by other means, often tens of hours in some thick-walled vessels. Proper heating can speed up leak detection.


6, air tightness test

Air tightness test is a conventional test means for boiler, pressure vessel 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 is bubbling. Because of the risk of explosion in the air tightness test, it should be carried out after the water pressure test is qualified.

Air tightness test is different from pressure test:


1, its purpose is different, air tightness test is to test the tightness of the pressure vessel, 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 of the design pressure.

Air pressure test is mainly to test the strength and tightness of the equipment, 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 leakage, and the air pressure test focuses on the overall strength of the equipment.

 2, the use of media

 Air is generally used in the actual operation of the air pressure test. In addition to the air tightness test, ammonia, halogen or helium is used if the medium is highly toxic and leakage or easy penetration is not allowed

 3. Safety accessories

Air pressure test, do not need to install safety accessories on the equipment; Air tightness test is generally carried out after the installation of safety accessories (tolerance gauge).

 4. Order

The air tightness test should be carried out after the pressure or hydraulic test is completed.

 5. Test the pressure

The pressure test pressure is 1.15 times the design pressure, and the internal pressure equipment needs to be multiplied by the temperature dressing coefficient; Air tightness test medium for air test pressure for design pressure, such as the use of other media, should also be adjusted according to the medium situation.

 6. Use occasion

Pressure test: hydraulic test is preferred. If the hydraulic test cannot be used due to the equipment structure or support, or the equipment volume is larger, the pressure test is generally used. Air tightness test: medium for high or extremely harmful medium, or do not allow leakage.

 Air pressure test belongs to pressure test, in order to check the pressure strength of equipment. Air tightness test belongs to the compact test, in order to test the sealing performance of the equipment.


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