Application of on-line airtight leak test technology for steel drum
[United States] Tom Ding Compilation / Yang Wenliang
In the manufacture of steel drums, air tightness testing plays a vital role in ensuring product quality. We use online airtight leak testing technology, through years of testing applications, in the setting of test leak parameters, leak test equipment. The use of other aspects has accumulated valuable experience.
In order to solve the leakage problem, especially the leakage of steel drums, in the steel drum manufacturing process, the former manufacturers used the "water immersion leak detection" to detect the steel drums to find unqualified products. This method can not accurately obtain the leakage of the steel drum to be tested, and the tension of the liquid inside the leakage hole increases the flow resistance. This method is often missed, and there are problems such as drying and automation.
With the improvement of steel drum quality, processing quality and assembly quality, and the application of online airtight leak testing technology since the 1980s, the leakage problem of steel drums has been thoroughly controlled, and the quality of steel drums has been reliably guaranteed. The on-line airtight leak test on the steel drum production line can effectively avoid problems found at the factory, thus reducing unnecessary losses.
In our company's steel drum production line, according to the process layout, we have set up intermediate airtight leak test (bath weld leak test, barrel cover seal leak test, etc.) and final airtight leak test (finished edge test) In the leaking process, part of the leak test process is also equipped with water test equipment, which is mainly used to check and determine the leak point of the unqualified steel drum. Of course, not all leaks can be found by water test. Here, we mainly focus on the application of our company's online airtight leak test technology in the production of barrel and barrel production lines.
First, the principle of air tightness test
Air is compressible and has a low viscosity, which means that air passes through the leak point 100 to 400 times faster than the liquid. Air has essentially no surface tension, a feature that makes it easier to pass small leaks than liquids. However, the leakage that air can pass through does not necessarily leak to the liquid. This is why a maximum allowable air leak is defined when air is used as a medium for leak testing.
The physics study yields the "state equation of ideal gas" for the "ideal gas" state change law, namely:
PV/T=nR
Where P is the gas pressure, V is the volume of the gas, T is the temperature of the gas, and R is the gas universal constant, a constant that is generally applicable to all gases (indicating that the air is close to the ideal gas).
This equation shows that, under constant temperature, after a certain period of time (generally greater than 20 s), its steel drum volume does not change, and if the pressure inside the barrel drops by a certain value ΔP, the steel drum can be judged. There is a leak." This is the basic principle of using gas to test the sealing performance of steel drums. The electronic air leak tester using this principle for leak test has become the fastest, most effective and practical automatic leak test instrument to date.
Of course, steel drum leakage is still leaking. It is a relative concept. Each steel drum has a certain air leakage. Absolute “no leakage†does not exist. When we say that there is no liquid leakage in the steel drum, we can define an air leakage rate accordingly. We test most of the steel drums with air as a medium to ensure no liquid leakage, so we cannot define these workpieces to leak air or other gases.
Second, the main parameters of the air tightness test
To test the volume of the steel drum, the manufacturer of the test leaker needs to combine the requirements of the production line of the user's production line with the actual working conditions of the steel drum, and reasonably determine the following parameters:
Test pressure
The test pressure is generally comparable to the actual pressure of the various media during the operation of the steel drum. We generally test the pressure according to the standard.
2. Test beats
Leak test for steel drums generally consists of four stages, namely aeration, balancing, testing and venting. The setting of each phase first ensures that the object or fixture does not leak, and secondly, set the inflation time to be long enough for testing, then slowly shorten the inflation time to obtain the shortest inflation and balance time when the leakage value is stable and close to zero. .
In theory, the longer the test time, the more helpful it is to obtain accurate test accuracy, but this is not suitable for the actual needs of the production line. The beat of the test leak machine should match the overall beat of the production line while ensuring the test accuracy. In addition, the use of a reduced volume filler in the fixture helps to speed up the test tempo. When the test pressure of the steel drum is high, the test volume of the steel drum is large, the leakage rate of the steel drum is small, and the steel drum and the seal portion of the clamp are deformed by pressure, the test time needs to be extended to ensure the test accuracy of the leakage rate of the steel drum.
3. Leakage rate
The leakage rate standards for the barrel, the lid and the finished barrel should meet the requirements of the actual working conditions of the steel drum, and are closely related to the material and structure of the steel drum (open barrel or closed barrel) and the technical state of the leak test machine. If the leakage rate is too high, it will cause oil leakage, water leakage and air leakage. The low temperature may lead to an increase in the manufacturing cost of the steel drum. Because we know that the quality is costly, the quality will be improved, and the cost will be reduced. increase.
We can obtain the leak rate standard of a steel drum from the suppliers of the test leaker manufacturer and the leak tester, and can also refer to the leak rate of similar products. It is expressed as a parameter commonly used for leak rate testing of various parts and finished products of steel drums.
In addition, in order to ensure the test accuracy, the air tightness test has certain requirements on the material selection and structural design of the test sealing component, the sealing design, the steel barrel positioning tightening device and the sealing fixture. For example, the sealing component should have no elastic creep. , oil and pressure resistance; seal design helps to reduce the stability time of unnecessary seal test; the seal fixture must provide appropriate pressure to seal the steel drum to be tested, the clamping force should be about 3 times the test pressure, while The strength of the clamp support frame is sufficient to support this pressure, and the steel drum should have a reliable guide to ensure accurate sealing position.
Third, the air tightness test should pay attention to the problem
1. Periodic calibration of the leak detector
The leak tester should be calibrated regularly with special instruments, usually once every six months to ensure the test accuracy of the leak tester.
2. Repeated test accuracy of the test leaker
A very important indicator for measuring the test stability of a leak tester is the repeat test accuracy (Cgk). In addition to the accuracy of the leak detector, the test accuracy is closely related to gas lines, seal fixtures and sealing components. Only the repeat accuracy can fully reflect the test stability of the leak tester. Repeatability testing generally requires 50 consecutive measurements on the same steel drum, requiring Cgk ≥ 2, and requires a 10 minute interval between the two tests to fully remove residual air from the steel drum and seal fixture, and the temperature of the steel drum. Lower it and reduce the impact on test accuracy. Otherwise, the leak rate tested will become smaller and smaller.
3. Standard steel drum
A steel drum is used to make a standard steel drum whose actual leakage rate is between the upper limit and the lower limit of the tolerance zone. Its main function is to verify the repeat test accuracy of the test leaker; each test is tested with a standard steel drum and the test leaker is tested. Recording to master the working condition of the leak tester; to verify the leak test effect of the leak tester when performing repair or adjustment of the leak tester.
When the test condition of the leak tester is abnormal compared with the normal time, the standard steel drum is used for testing to understand the current working state of the test leak machine. In order to further understand the state of the leak tester, it is recommended to make a standard load that exceeds the leak standard (must be able to measure the leak rate) and distinguish it by different colors.
4. The quality of compressed air
The compressed air must be clean and dry. If there is water or oil in the leak detector, it will cause the differential pressure sensor to malfunction. In order to completely prevent the incorporation of water and oil, it is required to install an oil mist separator and an air dryer as a pre-filter. Especially in the negative pressure mode, impurities are never allowed to enter the inside of the leak detector.
The stability of the compressed air will also affect the accuracy of the test. The compressed air with a large flow rate, adjusted and higher than the test pressure (at least 105 Pa higher than the test pressure) should be supplied to the pressure regulating valve, which can improve the stability of the test. .
5. Cleanliness of steel drums
For steel drums that need to be tested for leaks, they should be cleaned and dried first to avoid the impact of foreign matter on the test accuracy or the wear of the sealing components. When a gas test is performed on a leak hole in which a liquid leak has occurred, the amount of leakage will become small or disappear, and it is difficult to measure it truthfully. Therefore, in the comparative test of the leak amount, the gas leak test must be performed first.
6. Steel drum and ambient temperature
According to the glass-to-horse law (PV=nRT), the temperature change causes a pressure change under the condition of constant volume, which ultimately affects the test accuracy. In order to ensure the test accuracy of the leak rate, the temperature of the steel drum and the environment should be consistent.
In the barrel and barrel production line, the steel barrel parts are generally heated and cleaned before the test leaks. To ensure that the temperature of the barrels and the environment are consistent, a cooling device is required at the outlet of the washing machine. To ensure the accuracy of the test, a temperature compensation device can also be applied.
7. Test air pipe and accessories requirements
Hard nylon tubes with low pressure expansion and jacketed inserts must be used to reduce the impact on test accuracy. The same piping should be used for steel drums and standard parts to ensure the same amount of variation. In order to prevent movement during short-term tests, the piping should be fixed as much as possible.
Copper pipes and steel pipes are not available for piping, because these materials are easily affected by ambient temperature and cause test errors; and quick-release joints cannot be used because the deformation of the internal O-rings affects the test accuracy.
8. Inspection and maintenance of the leak tester
In order to ensure the accuracy of the leak test, in addition to the above, it is equally important to do the inspection and maintenance work of the usual leak tester to ensure that the test leaker is in a good technical state.
Fourth, the conclusion
In the manufacture of steel drums, the air tightness test plays a vital role in ensuring the quality of the product and is an important indicator for controlling the quality of the engine. Therefore, our company requires “100% testing, 100% qualifiedâ€.
In our actual production, the airtight leak test and related analysis found that the leakage of the unqualified product has defects in material quality, such as loose steel plate, pinhole, crack, crack, stress concentration. And bends, etc. It may also be caused by unreasonable processing techniques, unreasonable structure, and unreasonable assembly. In this regard, we have put forward higher requirements for steel plate suppliers, and at the same time, adopted corresponding measures in the process of processing and assembly, so that the product failure rate is continuously decreasing, thereby improving the efficiency of the enterprise.
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