If you buy or do technology, when you get a sample of metal expansion joint in your hand, your first reaction is mostly to turn over the model, look at the compensation amount and the interface size. But to be honest, what really determines whether this expansion joint can run steadily on your pipeline for ten years is not how beautiful the model is, but which standard it is made according to. The implementation standard of the metal expansion joint is wrong, and there are all pits behind it-the rigidity of the bellows is wrong, the fatigue life is not enough, and the pressure level can't stand up. These problems can't be seen in the workshop at all. When it is installed on the pipeline, it will appear as soon as it is started.
Don't be in a hurry to look at the model: what exactly does the metal expansion joint execution standard tube?
Some people think that the standard is just a few sheets of paper and a stamp. Not really. GB/T 12777 "General Technical Specifications for Expansion Joints of Metal Bellows" governs the whole process of general design, manufacture, inspection and installation; JB/T 6171 "Technical Specifications for Expansion Joints of Metal Corrugated Pipe" is more oriented to the applications in the field of pressure vessels and heat exchangers. These two are often mixed, but with different emphases. If you choose the wrong one, it is like taking high-speed tires to go off-road. If they look round, they will be finished when they run.
- Bellows stiffness-directly determines the stress state of expansion joint
- Fatigue life-calculated by number of cycles, not a pat on the head
- Compressive strength-hydraulic test and airtightness test have specific indicators
These data are the bottom line to judge that an expansion energy saving can't be used.
GB/T 12777 Why Core?
GB/T 12777 is the most widely cited and comprehensive basic standard in the field of expansion joints of metal bellows in China. It covers the mainstream structures of general-purpose corrugated expansion joint, compound hinge type, straight pipe pressure balance type and curved pipe pressure balance type. The general corrugated expansion joint, external pressure single axial expansion joint and curved tube pressure balance expansion joint in our station are designed and manufactured according to this system.
This standard clearly stipulates the geometric dimensions, material selection, forming process, welding joint coefficient, stiffness calculation and fatigue life prediction of corrugated pipes. In other words, if manufacturers dare to say that their products comply with GB/T 12777, it means that the bellows are not casually folded iron pipes, and every corrugation can stand the accounting. The compensation amount data obtained by the customer should also be traceable back to the calculation formula in the standard, instead of randomly printing a number on the sample.
What special standards correspond to different application scenarios?
The national standard is the bottom line, but a bottom line alone is not enough. The working conditions of power stations, cement and desulfurization industries are harsh, especially the superposition of high temperature, dust and corrosive media, which can't be covered by ordinary standards at all. Two days ago, I met a customer from a cement factory, saying that the expansion joint I bought cracked after less than half a year. First, it is a general type made according to the national standard, and the material is not strengthened for the wear-resistant working conditions. In this kind of scene, it is not enough to look at GB/T 12777, but also to look at the industry norms. For example, in addition to the national standard, corrugated expansion joints for power stations have to pay attention to the related requirements of ASME or DL/T; The metal corrugated expansion joint in the cement industry focuses on wear resistance and high temperature resistance, and the implementation standard will have higher requirements for wall thickness, guide tube material and number of bellows layers.
Another example is the expansion joint matching the desulfurization flue gas baffle door. The medium is sulfur-containing wet flue gas, which is extremely corrosive. At this time, the implementation standard must cover the anti-corrosion requirements, and even refer to the selection ideas of corrosion-resistant materials in JB/T 12235 non-metallic expansion joint standard. When choosing a model, take a universal standard to conquer the world, that is, lay a mine for yourself.
How to use criteria to judge whether the product is qualified or not when designing and selecting?
What about that? Get a sample, don't rush to see the compensation amount, first look at three things according to the standard: bellows material, number of layers, and guide tube configuration. There is a clear formula for stiffness calculation in the standard, the fatigue life has the requirement of cycle times, and the pressure value and pressure holding time are stipulated in the pressure test. Take these data to check with product samples one by one, don't be too troublesome. For example, if the customer wants to compensate for an expansion joint of 50mm, the compensation on the sample given by the manufacturer is enough, but the stiffness coefficient is obviously low-this is the number of bellows layers or wave pitch is not calculated well. After installation, the force of the pipe support completely changes, and the weld may have problems after several months of operation.
Non-metallic expansion joints and metal expansion joints are not the same thing. Although they are all called expansion joints, there are huge differences in material system, constraint mode and applicable temperature. For non-metallic expansion joints, refer to JB/T 12235-2015, and for metallic expansion joints, refer to GB/T 12777. Isn't it funny to take a non-metal test report to prove that the metal expansion joint is qualified?
In the installation and acceptance stage, what are the hard requirements hidden in the standard that are easy to overlook?
The design selection has passed the test, and the installation acceptance is more true. How to adjust the nut of the expansion joint tie rod, whether the screw should be disassembled, and where the direction of the arrow points-these are regulated in the standard, but no one at the scene often looks at it carefully. The tighter the nut is not tightened, the better, it plays a role in restraining the displacement of the bellows. Improper adjustment means that the expansion joint becomes rigid, and there is no place for the thermal expansion and contraction of the pipeline. Finally, the bellows is damaged.
Let's talk about the direction of the arrow. The arrow marked on the expansion joint refers to the flow direction of the medium, not the installation direction, and it is not casually drawn. Install backwards, the guide tube loses its function, the medium directly washes the inner wall of the bellows, and the corrosion and wear will be several times faster than normal. The installation requirements of directly buried (fully buried) type expansion joint and sleeve type pipe expansion joint are different: the directly buried type should handle the friction with soil and waterproof, while the sleeve type should ensure the coaxiality of the guide bracket. Check item by item according to the standard during acceptance, so as to avoid problems after several months of operation.
At the end of the chat, it was the same sentence: the standard is not for quality inspectors, but for everyone who uses expansion joints. Only by understanding the implementation standards of metal expansion joints and placing orders can your pipeline system withstand the real big tests of high temperature, high pressure and fatigue.