Give me the answer first: the current national standard is GB/T 12777-2019
The national standard number of metal expansion joint is GB/T 12777-2019 General Technical Specifications for Metal Bellows Expansion Joint. Just remember this sentence, and stop confusing it with GB/T 14525-that is "General Technical Specifications for Corrugated Metal Hoses", which is about metal hoses, not expansion joints. Missing two words, the product is completely different.
A while ago, a customer who was doing HVAC engineering said with the drawings that he wanted to order the "expansion joint of national standard 14525", but the model of metal hose was written on the purchase order. If this is made according to the list and installed on the pipeline, the bellows can't absorb the axial displacement, and it can leak or tear the nozzle, which is no trivial matter. Therefore, understanding the national standard of metal expansion joints is the first threshold for you to communicate with suppliers, which can't be bypassed.
Let's talk about the model correspondence. On the product label you get, it will generally be written in a format similar to "DN100-PN16-T150-200". The first few paragraphs represent the nominal diameter and pressure grade, and the following numbers are the design temperature and displacement. However, what really plays a decisive role is whether the implementation standard number "GB/T 12777" is marked on the label. If there is not even this line of words, then the compliance of this product is in doubt.
There are hard indicators hidden behind the national standard. Don't just check a number and finish it
GB/T 12777-2019, for bellowsDesign pressure, temperature range, fatigue life, stiffness calculationThey are clearly stipulated. It is not a simple certificate of conformity, but a complete set of design and acceptance basis.
Take the general corrugated expansion joint as an example. The national standard requires that the corrugated pipe be subjected to fatigue test under the design pressure, and the number of cycles should not be lower than the design value. How to judge? Look at the manufacturer's calculation book-if the other party can't even tell how to calculate the fatigue life, you dare to quote a low price, and you will have a question mark in your heart.
Another example is the high-temperature axial expansion joint, the kind used in hot air ducts. The national standard has a reduction factor for the allowable stress of materials at high temperatures, which doesn't mean that you just mark it as "temperature resistance 800℃" and you're done. You have to see if it takes into account the effects of high temperature creep and stress relaxation according to GB/T 12777. Some manufacturers take normal temperature parameters to set high temperature working conditions, and the result is that the bellows cracks within a few months.
To put it bluntly, the formula in the national standard is used to reverse product parameters, not for decoration. As a purchaser, you should see at least three numbers clearly: design pressure, temperature and fatigue life. These three numbers don't match, so don't ask for it no matter how cheap it is.
Keep an eye on the displacement and stiffness test during acceptance, and don't be fooled by the test report
For expansion joints shipped according to GB/T 12777, each batch shall have a factory inspection report. But there are parallel imports in this report thing. You should focus on two points:Displacement quantity test and stiffness test.
Let's start with the amount of displacement. The national standard requires that the cycle test should be carried out according to the displacement specified in the design, and the number of cycles should usually not be less than the design fatigue life. Some manufacturers report that "the displacement is up to standard", but do you see clearly whether they are measuring axial displacement or lateral displacement? The difference between the two was a hundred and eighty thousand miles. If you make up the data of lateral displacement for axial expansion joints, you are playing rogue.
Let's talk about stiffness. The stiffness directly affects the force of the pipeline bearing, and the national standard requires a safety factor of twice the stiffness deviation. How to check on site? You can multiply the single wave stiffness by the wave number and compare it with the overall stiffness in the report. If the deviation exceeds 15%, it is abnormal. Without turning over the whole standard, these two items alone can help you pick out a lot of faults during acceptance.
Tsk, the reality is that the acceptance of expansion joints in many projects is to look at the appearance, thinking that "no rust or leakage is good". Think about it, this thing has to endure high temperature, high pressure and repeated displacement when it is installed on the pipeline. If it is removed after something goes wrong, the loss will not be a matter of a pipe.
Key Points of Type Selection in Special Working Conditions: National Standard is the Bottom Line and Industry Standard is the Supplement
GB/T 12777 manages the general technical conditions, but there are many special working conditions in engineering, and other standards have to be superimposed to implement.
In addition to GB/T 12777, the flue gas pipeline and desulfurization system of power plants have to meet the additional requirements of DL/T series standards-such as corrosion-resistant lining and wear-resistant treatment of deflectors. Only the principle requirements are mentioned in the national standard, and the specific parameters depend on the industry specifications. If you have a desulfurization flue gas baffle door project in your hand, it is best to turn out both standards and compare them when selecting.
The cement industry also has its own practices. Metal corrugated expansion joints in cement industry are often used in high-temperature dusty gas pipelines at kiln head and kiln tail, and the wear problem is much more severe than the general working conditions. The requirement of wall thickness in the national standard is the lower limit, but cement plant design institutes usually have their own verification methods. When selecting the model, you have to judge by combining the wear-resistant measures and wall thickness strengthening schemes in the product data. A national standard number can not solve all problems.
Let's talk about structural forms. Directly buried (fully buried) expansion joints are used for underground directly buried pipelines, and external pressure single axial expansion joints are used for high-pressure steam pipelines. These special structures are not necessarily written in every detail in the national standard. What about that? Instead of just giving you a copy of GB/T 12777, you have to ask the manufacturer to provide a formal test report for the structure.
The national standard is tightening, and the 2019 version is much stricter than the old version
Finally, let's talk about trends. This version of GB/T 12777-2019 is better than the 2008 versionFatigue testing and material requirementsRaised a bit. The 2019 edition clearly stipulates the shape deviation and wall thickness thinning rate limit of bellows after forming, which means that it is not so easy for products with inadequate processing technology to muddle through.
Will non-metallic expansion joints be included in the future? Not so far. Non-metallic expansion joints (fabric fiber expansion joints) are now implemented in JB/T 12235-2015, which is two lines with metal expansion joints. The material system and failure mode are different, and it is troublesome to pinch them together. Rectangular non-metallic expansion joints are used a lot in flue gas pipelines. Just look for JB/T 12235 when selecting the model, and don't mix them with GB/T 12777.
Therefore, when buying metal expansion joints, look for GB/T 12777, the national standard of metal expansion joints, check the three data of displacement, stiffness and fatigue life in the factory inspection records against the product data, and then screen it in combination with the supplementary standards of your industry.