Find out first: What equipment does GB/T 12777 manage, and why is pipeline design inseparable from it
Old rules, first answer this most direct question: What is the standard of gb/t 12777? The full name is General Technical Conditions for Expansion Joints of Metal Corrugated Pipe, which specifies the design, manufacture, inspection and acceptance requirements of expansion joints of metal corrugated pipe. To put it bluntly, all the corrugated metal compensating elements used in pipes that can absorb thermal expansion and contraction displacement are within its jurisdiction.
But please note that it tubes a "metal bellows expansion joint", which is not non-metallic or rubber. If you use it to cover fabric fiber expansion joints, you will be wrong. The design basis of general-purpose corrugated expansion joints, corrugated expansion joints for power station industry and high-temperature axial expansion joints often mentioned in our industry can't get around this standard.
Then why can't pipeline design do without it? Think about it, the elongation of a steam line from cold to hot may be tens or even hundreds of millimeters. If you don't give it a device to compensate for displacement, the pipeline will transmit force to the equipment interface. This is how the accident of flange cracking and pipe shelf displacement comes from. GB/T 12777 sets rules for this "compensation device", how to calculate the displacement, how to test the fatigue life and how to control the stiffness. With this rule, the design institute can draw drawings and the manufacturer can produce them.
Some of the hardest technical requirements in the standard: How to specify displacement, fatigue life and stiffness
In this standard, the core is nothing more than three things: displacement, fatigue life and stiffness. Let's talk about it one by one.
Displacement, which refers to how much axial, lateral, and angular deformation is absorbed by expansion energy savings. The standard gives the requirement of displacement capacity for bellows with different wave patterns, but its precondition is that the bellows cannot be unstable or excessive plastic deformation. This is not determined by patting the head, it needs to be verified by type test.
Fatigue life is the highlight. When the bellows expansion joint works under repeated displacement, the most afraid is fatigue fracture. GB/T 12777 stipulates that the fatigue life should not be less than a certain number of cycles under design displacement. Do you know what that means? When you select the model, if you only give the displacement, regardless of the fatigue life, the manufacturer can make a "disposable" product for you. It can really move when installed, but it leaks after hundreds of cycles. Therefore, when purchasing, you have to write the number of cycles clearly, such as 1500 times or 10,000 times, which directly determines the number of layers and thickness of the bellows.
The stiffness is directly linked to the force on the pipe. The greater the stiffness, the greater the reaction force when the expansion joint deforms, and the higher the load on the pipe support and equipment interface. There is a calculation formula for stiffness in the standard, and it is also required to measure it at the time of factory inspection. This is why you can't only look at the displacement when selecting the model. Excessive stiffness will transmit the force back to the equipment, and the deformation will not absorb much, but create new stress. These three parameters are tied together, and it is meaningless to take any of them separately. And guess what? Many old masters who have used expansion joints for decades will still stumble on them when selecting models. The problem is often that the fatigue life is neglected.
What is the difference between GB/T 12777 and GB/T 14525 and JB/T 12235? Don't choose the wrong one
To plug in here, many people confuse GB/T 12777 with GB/T 14525. In fact, the latter is "General Technical Conditions for Corrugated Metal Hoses", which pipes metal hoses-that is, flexible pipe sections with mesh sleeves for absorbing displacement or reducing vibration and noise. The expansion joints of metal hoses and bellows look similar, but the use scenarios are different: hoses are mainly used in pump ports, steam turbine inlet and outlet, etc., where vibration isolation or small displacement compensation is needed, while expansion joints are specially used to compensate the axial displacement, lateral displacement and even angular displacement of pipelines.
There is also JB/T 12235, the machinery industry standard for non-metallic expansion joints, which applies to compensators made of fabric fibers, rubber or PTFE. If your project medium is sulfur-containing flue gas, or low temperature but extremely corrosive working conditions, you may want to refer to JB/T 12235 instead of GB/T 12777.
The metallic type is classified as GB/T 12777 pipe, and the non-metallic and rubber have different standards. If you choose a metal bellows expansion joint on the flue gas desulfurization pipeline, it may be corroded for less than two years; On the other hand, it is even more unreliable to install non-metallic expansion joints on the high-temperature and high-pressure steam pipe network.
How to compare this standard in actual selection: landing cases from metal hoses to various expansion joints
Two days ago, a customer who was doing power plant renovation asked with a drawing, saying that a section of his air-cooled island vacuum pipeline needed to compensate for the lateral displacement, and asked me to match it with a model. I opened the drawing and saw that the working condition was vacuum, the temperature was about 200°C, and the displacement requirement was ±50mm in the transverse direction. Under this working condition, it is suitable for you to use a small tie rod transverse expansion joint, because its bellows is constrained by the tie rod, which can only absorb the transverse displacement without generating the main fixed bracket thrust. This selection logic corresponds to the standard, that is, the clause on lateral displacement and angular displacement in GB/T 12777-the bellows cannot bear torque, and the structural design of tie rod and hinge must follow the relevant requirements in the standard.
Another scene is the flue gas pipeline at the head and tail of the kiln in the cement industry. High-temperature axial expansion joint or straight pipe pressure balance expansion joint are often selected under such working conditions as large temperature fluctuation, large dust and limited installation space. The advantage of the pressure balance type is that the internal pressure thrust of the bellows is cancelled by the balance structure, and the blind plate force will not be transmitted to the fixed bracket. This design principle and calculation method are also regulated by GB/T 12777. If you don't look at the standard and choose by experience, and use a large tie rod to forcibly absorb the axial displacement, the stress analysis report of the pipeline may not pass the examination at all.
Back to the metal hose. It is not that with GB/T 12777, any pipe can be used, the flexible connection of the pump inlet and outlet, and the vibration isolation of the instrument pipeline. Where the metal hose should be used, the metal hose should be used honestly. They are complementary relationships, not alternative relationships. The PTFE-lined hose and vacuum special hose in the station take the GB/T 14525 route. The material, mesh structure, pressure level and expansion joint are not the same algorithm.
Pits that are easy to step on during procurement and acceptance: How to read standard versions, material reports and type tests
The last link is also the easiest place to step on pits. Do you think it will be fine if you get the goods and accept them according to the standards? That's naive.
Look at the standard version first. A new version of GB/T 12777 was released in 2024, replacing the previous 2008 version. The old version of the standard has a historical problem-the requirements for fatigue life and safety factor are relatively lax. The new version has made adjustments to the design fatigue life and safety factor requirements. If you only write "in accordance with GB/T 12777" in the contract, without noting the next year number, the supplier will do it according to the 2008 version for you, and you can't find fault. However, the special inspection institute may not recognize it when the project is accepted. Therefore, the full name and year number of the standard must be stated in the contract.
Look at the material report again. Stainless steel plate, the raw material of corrugated pipe, must have a warranty, and the data of material, furnace batch number and mechanical properties must be complete. Here is a detail: many suppliers can't produce the intergranular corrosion test report of stainless steel, but this is precisely one of the main reasons for the failure of bellows under high temperature conditions. You don't want trachoma leakage six months after the tube is installed, do you? It is not excessive to ask the supplier to provide the chemical composition analysis report of raw materials during acceptance.
Finally, there is the type test. GB/T 12777 stipulates that the type test, including hydraulic pressure test, air tightness test, stiffness test and fatigue life test, should be carried out when the new product is finalized. However, note that many factories only do factory inspection and do not do type test, which are not the same thing. When you purchase large diameter thick wall expansion joints or double hinge expansion joints for vacuum pipes for nuclear power plants, it is best to explicitly request a copy of the type test report in the technical agreement. Let me reveal to you the bottom. There are only a few manufacturers in the industry who have really done complete type tests. If you ask to read this report, the supplier's foundation will be figured out at once.
In the final analysis, understanding GB/T 12777 is not to memorize the provisions, but to not be fooled when selecting the model, not missing items when signing the contract, and not being blinded during acceptance. The standard is a tool, and when you use it well, your plumbing system will run steadily for twenty years.