1. Type test is not a "factory inspection", it is a "life and death test" of product design
Many purchases confuse type testing with factory inspection-they are not the same thing at all. Factory inspection is every product is done, and the manufacturing consistency is measured; Type tests are only for new products or after design changes, and the core is to verify the design life, extreme operating conditions and failure modes. To put it bluntly, it is only after the type test that you dare to mass produce; If you fail, change the design and start over, there is no room for negotiation.
This rule isUniversal corrugated expansion joint、A high-temperature axial expansion joint,Straight pipe pressure balanced expansion jointWait for universal use on products. However, the key points of experiments under different structures and working conditions are very different. For example, you let a person doDouble hinge expansion joint for air-cooled island vacuum pipelineManufacturers measure according to the routine of ordinary bellows, and the missed detection rate can be high enough to make you doubt your life.
2. Pressure, displacement and fatigue-none of the three "iron thresholds" can be missed
Usually 1.5 times of the design pressure, hold the pressure for more than 10 minutes, and see if there is any plastic deformation or leakage in the bellows, end pipe and guide tube. Note that the "guide tube" mentioned here is not a decoration-it is directly related to the stability of the flow state. Once it is unstable, the whole expansion joint will be scrapped.
The second item is the displacement compensation ability verification. The axial, transverse and angular displacements must reach the design values, and the wave pitch change and stress concentration point of bellows must be observed at the same time. I met a customer two days ago and brought itCompound hinge transverse expansion jointThe report only measured the axial direction, but not the rotational flexibility of the transverse hinge. As a result, it stuck as soon as it was installed on site.
The third fatigue life test, which is the hardest. Such asCorrugated expansion joint for power station industryThe number of cycles is required to be at least 1000 times or even higher; WhileMetal Corrugated Expansion Joints in Cement IndustryBecause the medium contains dust, abrasion simulation may also be superimposed. The data tells us that many designs look good on paper, but as soon as they are put on the fatigue bench, they crack dozens of times. Do you think it's horrifying or not?
3. Different expansion joint structures have different "pit positions" for type test
Also a metal expansion joint,Compound hinge transverse expansion joint andThe test methods of pressure balanced expansion joints of curved tubes vary greatly. The former mainly tests the pressure-bearing and rotational flexibility of the hinge structure, while the latter also measures the synchronicity of the pressure balance components-if one of these two is wrong, the whole system will have to be reworked.
AgainLarge diameter thick wall expansion jointIf the wall thickness exceeds 12mm, the welding process evaluation must be done separately. During the type test, 100% non-destructive testing of welds is required, and radiation or ultrasound are indispensable. WhileDouble hinge expansion joint for air-cooled island vacuum pipelineIn addition to routine tests, you have to vacuum and maintain pressure to see the leakage rate. I don't understand these differences when I select the model, and I can't understand it when I get the report.
AndDesulfurization flue gas baffle doorFor the matching expansion joint, the medium contains SO₂ and wet chloride ions, and the type test must add stress corrosion cracking test-this requirement is not contained in GB/T 12777. At this time, experienced manufacturers will supplement the test items according to their own enterprise standards, which we call "enhanced type test". Write clearly which standard is based on in the purchase contract, which can save the rear 90% of arguments.
4. How to choose the standard system? National standard, industry standard or enterprise standard?
Type test of metal expansion joints in China mainly refers to GB/T 12777 (General Technical Specifications for Metal Bellows Expansion Joints) and JB/T 12235 (Non-metal Expansion Joints). However, ASME or EN standards are often involved for the expansion joints matched by baffle doors for power stations, cement and desulfurization flue gas.
For example,Desulfurization flue gas baffle doorExpansion joints are used, the medium contains SO₂ and wet chloride ions, and the type test must be added with stress corrosion cracking test. There is no such requirement in GB/T 12777, so what do you do? Either make up the test according to the enterprise standard, or directly quote the relevant provisions in ASME B31.3. Last year, there was a project. Because the contract only said "according to the national standard", the result was on-site corrosion leakage, and the manufacturer and purchasing took half a year.
So, before you sign the contract, find out what you're buyingHigh temperature axial expansion jointStillStraight pipe pressure balanced expansion joint, what is the corresponding working condition, the standard is clearly written. It is not an exaggeration to say that it can save 90% of the fighting at the back.
V. What do you think of the type test report? Three core data determine life and death
Get a type test report, and first find these three sets of numbers: the number of design fatigue life, the number of actual test cycles, and the failure mode. If the actual number of tests reaches more than 1.5 times of the design value, it means that the safety margin is sufficient; If it is just right to press the line, you have to be careful-material fluctuations and manufacturing tolerances may keep the batch product out of reach.
The conclusion of water pressure and air pressure cannot be universal. The air pressure test is risky but closer to the actual working conditions, such as the expansion joint of vacuum pipeline, and the vacuum leakage cannot be detected by water pressure. Another point: Is the measured value of bellows stiffness attached to the report? This data is directly related to the calculation of pipeline support. Don't sign just because the displacement is not in place.
You have this on handHigh temperature axial expansion jointType test report, more than three years ago? The citation value of older reports is to be discounted because standards and materials are iterated. Last year, GB/T 12777 updated the fatigue test clause, and the number of cycles in the old report may not count.
Alas, in the final analysis, type testing is not a formality, it is a "life and death test" of product design. If you fail to pass these five levels, you will be scrapped directly without discussion.