Let's talk about the conclusion: Don't be fooled by the parameter table
The more parameters of corrugated metal expansion joints are not the better, the key is those few. Nominal diameter, pressure, temperature, compensation amount, stiffness, these five are fixed, and the rest are all additional items revolving around them. You bring a selection table, which is packed with dozens of lines. Look at these five columns first, and you will have a spectrum in your heart.
As soon as many purchases come up, they ask "How much is your expansion joint?" This question can't be answered. The same DN300, with pressure levels of 1.0MPa and 2.5MPa, is not unusual to double the price difference. Not to mention with or without a guide tube, tube or flange, single layer or multi-layer-parameters determine the price, and working conditions determine the parameters.
Pressure vs. Temperature: This pair of parameters is a 'twin'
For the same expansion joint, as soon as the temperature goes up, the allowable pressure goes down. This is determined by the high-temperature creep characteristics of metal materials, not by manufacturers cutting corners. Take the national standard GB/T 12777 and turn it over. There is a temperature correction coefficient table in it. At 200℃, the allowable stress of 304 stainless steel is about 70% of that at normal temperature, and it is directly cut in half at 500℃.
What is the design temperature? What is the corresponding pressure correction factor? Two years ago, there was a case in a chemical plant. A nominal 1.6MPa expansion joint was selected for the steam pipeline, and it was fine to install it in summer. When driving in winter, the bellows leaked. Why? Because the design temperature is selected according to 150℃, there is no problem, but the steam temperature rushes to 250℃ during actual driving, and the pressure does not change. Once the correction coefficient is calculated, the actual allowable pressure is only 1.2MPa, and it runs under overpressure. Who takes the blame? I can only blame you for not putting temperature and pressure together when selecting the model.
How to calculate the amount of compensation? Don't pat yourself on the head
Single wave compensation amount That's the ideal value, from the lab. The actual pipeline has cold tightness, installation errors, and the synthesis of multi-directional displacements-axial tension, compression, lateral misalignment, angular deflection, which do not appear alone, but are often superimposed.
A thermal pipe network, the pipeline length is 80 meters, the medium temperature rises from 20℃ to 200℃, and the temperature increase expansion amount is about 80×0.012×180 ≈ 173mm. This is without counting the lateral displacement caused by the settlement of the pipe support. If you only choose the model according to the axial compensation amount, you will find that it doesn't match when you install it and test the pressure.
What about that? It is recommended to use software to calculate, or directly ask the manufacturer for a calculation book. Especially when there is lateral displacement and angular displacement, don't pat your head to estimate a number. The transverse displacement absorbed by the general corrugated expansion energy saving is limited. For complicated working conditions, the transverse expansion joint of compound hinge or the pressure balanced expansion joint of straight pipe should be selected, which are specifically aimed at the working conditions of multi-directional displacement. You call the manufacturer to report the working conditions, and people will tell you which one to choose with a calculation book, which is much more reliable than thinking about it yourself.
Stiffness: affects the force and also the life
The smaller the stiffness, the smaller the reaction force, and the friendlier it is to the fixed support. There's nothing wrong with this sentence, but it's only half said. If the stiffness is too small, the bellows is prone to instability-that is, lateral bending occurs between the corrugations, which is called column instability and directly scrapped.
There's a balance here. When you ask the manufacturer for axial stiffness and transverse stiffness, by the way: Is it calculated according to actual measurement or theory? How much deviation is allowed in the standard? GB/T 12777 stipulates that the stiffness deviation is allowed to be ±30%, you heard it correctly, ±30%. This means that the theoretical stiffness of the product is 100N/mm, and the measured stiffness may reach 130N/mm. For fixed bearings, the 30% reaction force deviation may be a gap of several hundred kilograms or even several tons. Therefore, when you encounter a pipeline with high precision requirements, you have to ask the manufacturer to provide the measured stiffness value, and don't just look at the theoretical data on the sample.
Material and number of layers: Don't just look at the word "stainless steel"
304, 316L, 254SMO, all stainless steel, corrosion resistance is far worse. 304 pitted in the chloride ion environment soon, 316L can carry it for a while but it is not a panacea, and 254SMO is the correct solution to deal with high chloride ion working conditions. The selection of material depends on the composition, concentration and temperature of the medium-the expansion joint used in flue gas desulfurization pipeline should be made of at least 316L, and 254SMO or higher nickel-based alloy should be used in cases of severe corrosion.
Let's talk about the number of layers. The number of layers is large, the withstand voltage is high but the flexibility is poor; The number of layers is small, the compensation amount is large, but the wall thickness should be increased. The same general-purpose corrugated expansion joint of DN400, with a single layer wall thickness of 1.5mm, can compensate 50mm, and a third layer of 0.5mm can only compensate 30mm-but the withstand voltage level is up a bit. There is no absolute good or bad here, only the matching of working conditions.
There are also deflectors, which are easily overlooked. When there is a flushing medium, no guide tube is equivalent to letting the bellows be beaten directly. The condensed water in the steam pipe and the dust particles in the flue gas pipe flow through the bottom of the corrugated valley at a high speed, which is called erosion wear. The wall of the corrugated pipe is thinned to a certain extent and then perforated. The function of the guide tube is to let the medium flow through the inner tube without directly contacting the bellows body. Don't remove the deflector to save that little money. The later maintenance cost is enough for you to buy several new ones.
Connection Method and Installation Length: The Easiest Pit to Overlook
The sealing requirements of each connection mode of flange and tube are different. Should flange connections consider flange standards-JB/T 81 or HG/T 20592? Is the bolt hole spacing right? Welding connections should take into account site welding conditions and NDT requirements.
The installation length determines whether you can adjust it on site. Some expansion joints come out of the factory with a tie rod, pulled to the installation length to lock, and the pipe is in place before being loosened. If your scene space is narrow, the wrench can't reach in, and the tie rod bolt can't be removed-then the picture is sour and cool to think about.
So don't just look at the parameter sheet. Placing an order after figuring out the on-site working conditions is more important than any parameters-pipeline direction, bracket position, flange standard, insulation layer thickness and operating space. These things that can't be seen on the parameter table are often the key to deciding the success or failure of model selection.
The expansion joint is the flexible link of the pipeline system. If the selection is not accurate, it will be the first problem. When you are unsure, send the working conditions to the technical department of the manufacturer and ask them to produce a calculation book. Professional matters are handed over to professional people. This is true when it comes to the selection of expansion joints.