Section 1: Three-section expansion joints are not simple with one more section of corrugation-structural differences and core parameters
Many people think that the three-section metal expansion joint is to add another section in the middle of the two corrugations, just like stacking Arhats. That would be wrong. True three-section structure usually refers to three independent corrugated segments connected together in series through intermediate or end tubes to form a composite compensation unit that can absorb axial, lateral and angular displacements simultaneously. The biggest difference between this design and a single-or double-section corrugated joint is that it can withstand greater multi-directional displacement while maintaining lower axial stiffness.
To give a specific example,Compound hinge transverse expansion joint(We have this in the product list of our site) is a typical three-section structure-bellows at both ends and a set of hinged tie rods in the middle, allowing the pipe to make lateral displacement in the plane. WhileCompound straight pipe bypass pressure balanced expansion jointIt belongs to a three-section pressure balance structure, which is specially used to deal with the blind plate force of high-pressure large-diameter pipelines. You have to keep an eye on three core parameters: single-wave stiffness, effective area, and the rated displacement of each section. Once these three numbers are miscalculated, there will be pits behind them.
Section 2: Which occasions must it be? Practical cases in power stations, cement and chemical industries
Two days ago, I met a project manager of a cement plant. He told me that the temperature at the corner of the flue gas pipeline can reach 650℃, and the ordinary single-section expansion joint will crack in two months. What about that? ChangeThree-section metal expansion jointYeah. In the cement industry, the thermal expansion of the hot air duct between the rotary kiln and the preheater is often tens of millimeters, and the direction of the duct often has a horizontal corner. At this time, single or double sections can't bear it at all, so three-section structure must be used to absorb the composite displacement. We have a set for a cement factoryCement industry metal corrugated expansion joint, three sections, with guide tube, used for three years without accident.
The power station scene is more typical. Steam turbine exhaust pipe and boiler smoke duct have high temperature, low pressure, large diameter and vibrate at every turn.Corrugated expansion joint for power station industryThe three-section type is the mainstream. For example, we have a vacuum pipe for air-cooled islandsDouble hinge expansion joint for air-cooled island vacuum pipelineIn fact, it is a double hinge with three bellows, which can absorb both axial displacement and lateral displacement, and will not generate thrust. What about the chemical industry? Sulfuric acid pipeline and steam pipeline network, the pressure can sometimes reach 2.5MPa, at this timeExternal pressure single axial expansion jointIt may not be enough, it has to be usedStraight pipe pressure balanced expansion jointOrCurved tube pressure balance expansion joint, they are all three-section structures specially designed to balance the blind plate forces generated by pressure. To put it bluntly, large displacement, high pressure and multi-directional movement-one of these three conditions is missing, so you don't have to worry about three sections. Single sections or double sections save money.
Section 3: Three pits that are most prone to rollover during model selection: matching of pressure, temperature and displacement
Selection, to put it bluntly, is a game of three numbers: design pressure, design temperature and maximum displacement. I've seen too many people get caught up in the temperature-they think that as long as they choose the right material for the bellows, they're done. Don't forget that temperature directly affects the fatigue life and stiffness of bellows. For example, if you use 304 stainless steel as a corrugated pipe, when the working temperature exceeds 400℃, the creep strength of the material will drop by a cliff. At this time, either switch to Inconel 625 or reduce the allowable displacement.
Pressure pits are more concealed. In order to pursue large displacement, some three-section structures make the wall thickness of the bellows very thin, and as a result, the pressure will become unstable and burst. The instability pressure must be checked when selecting the model. This parameter is in ourUniversal corrugated expansion jointIt is in the product manual. What about the amount of displacement? Don't just look at axial displacement. The biggest advantage of the three-section expansion joint is that it can absorb both lateral and angular displacements at the same time, but the displacements in each direction are mutually restricted. The more you pull laterally, the less you can move axially. This relationship has to be usedExpansion Joint Weight TableAnd stiffness calculation formula to check, don't pat your head.
Section 4: Installation and routine maintenance Never ignore the role of tie rod and guide tube
Many people think that the pull rod is just a transportation protection piece. Just put it on and be done. Wrong.Function of expansion joint tie rod(We make it clear in our FAQ) is to adjust to the working length after installation, pre-stretch or pre-compress the bellows to the designed position, and then lock the nut. If you forget to adjust it, the displacement of the three expansion joints will be fully pressed on one section, and the other two sections will be idle, so the life will be greatly reduced.
The deflector is more critical.Specific Function of Expansion Joint Guide TubeIs to guide the streamline of the medium, prevent the high-speed airflow from impacting the bellows, and at the same time, it can reduce the flow resistance. Especially in dusty gases, such as flue gas pipes of cement plants, if there is no guide tube, the dust will directly wash the trough of the bellows, and it will be worn out in a few months. When installing, it should also be noted that the direction of the arrow of the guide tube must be consistent with the flow direction of the medium, otherwise it will be a bad favor. Besides, ourDouble-sealed single-axis circular baffle doorAndFlue gas baffle doorWhen used together, the three-section expansion joint needs to leave enough operating space, and don't let the baffle door hit the pull rod when opening.
Section 5: How long is it normal? Fatigue Life and Prevention of Common Failures
The design life of three-section metal expansion joints is generally between 1000 and 5000 full displacement cycles, depending on the pressure and temperature. Those on the steam pipes in power plants usually last 15 years. But the reality is that many problems go wrong after three or five years. Why? Not because the product is not working, but because the installation or use conditions deviate from the design values.
Bellows cracking and instability. Cracking is mostly caused by excessive fatigue stress, such as too large deflection angle during installation, or extra vibration caused by loose pipeline support. Instability is because the pressure exceeds the design value, or the guide tube falls off and blocks the passage of a section of bellows. The preventive method is very simple: First, check whether the tie rod nut is locked after installation (refer to FAQHow to adjust the tie rod nut of expansion joint）； Second, regularly check the wear of the guide tube; Third, check the displacement once a year. If the pipeline insulation layer becomes thicker, the thermal expansion will become larger. Is the original displacement reserve enough? If it's not enough, you have to change it.
Finally, is the three-section metal expansion joint worth it? It depends on where you use it. Large displacement, high pressure and multi-directional motion-these three conditions are put together, and it is the most economical solution. If it's just normal straight pipe axial compensation, don't waste money,External pressure single axial expansion jointOrdirectly buried expansion jointMore affordable. Choose the right one, one top ten years; If you choose the wrong one, you will have to change it in half a year.