First, what problem does the metal expansion joint solve? — — The core principle of thermal displacement and stress compensation
The pipe looks dead, but in fact, it keeps moving-when the temperature changes, it expands and contracts with heat. A steam pipeline tens of meters long, the temperature rises from normal temperature to 300℃, and it can stretch tens of centimeters abruptly. If there is no cushioning, the flange bolts will collapse directly, and the stress concentration at the pipe elbow will crack, it will be a matter of time. Commonly used metal expansion joints do this job: use the flexible deformation of bellows to eat up the thermal displacement and relieve the stress.
To put it bluntly, the expansion joint is a "breathing" joint. The wall of the bellows is very thin, and the axial expansion and contraction, lateral deflection and even angular rotation can be realized by the press-formed corrugated structure. The larger the amount of compensation, the more wavenumbers are required. But bellows alone is not enough-high pressure, high temperature, and corrosive media will shorten life, so there are various structural variants. The following four types are the most commonly encountered at the engineering site.
2. Four most common metal expansion joints: general-purpose type, pressure balance type, high-temperature axial type and double hinge type
Universal corrugated expansion joint: The structure is the simplest, a bellows is welded with flanges or connections at both ends. It is suitable for low pressure, normal temperature and small displacement occasions, such as air conditioning water pipes and low-pressure steam pipes. But it has a fatal injury-the internal pressure of the bellows will create a blind plate force, which has to be held by the pipe support, otherwise the pipe frame will not be able to stand it. Therefore, the general-purpose model can only be used in systems with strong enough fixed brackets.
Straight pipe pressure balanced expansion joint: Specialized in solving the problem of blind plate force. It has two sets of bellows inside, with a balance ring in the middle, and the thrust generated by the pressure cancels each other internally and does not pass to the bracket. And guess what? This thing is especially suitable for large-diameter, high-pressure pipes, such as the main steam pipe of power plants, and the bracket doesn't have to be made like a bunker. However, the structure is complicated, and the cost is much higher than that of the general-purpose type.
High temperature axial expansion joint: The name tells you what it is used for-high temperature environments. Ordinary corrugated pipes are made of austenitic stainless steel, and the strength collapses when the temperature exceeds 500℃. The high-temperature axial type will use heat-resistant alloys (such as Incoloy 800H), and the bellows will be coated with heat insulation or air injection. Hot air pipelines in the cement industry and flue gas pipelines in catalytic cracking units are the most used. Remember one thing: the high-temperature expansion joint has extremely high requirements for the guide tube, otherwise the high-temperature medium will directly flush the bellows and perforate it in a few days.
Compound hinge transverse expansion joint: This thing comes in handy when there is a turn in the direction of the pipe, or when it needs to absorb lateral displacement. It is composed of two bellows plus intermediate pipe and hinge structure, which can only occur angular displacement and lateral displacement, but cannot expand and contract axially. It is commonly used in L-shaped pipe section or Z-shaped pipe section, and forms a compensation system with fixed bracket. Not to be confused with the double straight tube bypass pressure balanced expansion joint-the latter has an additional pressure balancing function.
3. Four parameters that must be fixed in the selection: pressure, temperature, displacement and media corrosiveness
Many projects have problems, not because the quality of the expansion joint is poor, but because the selection parameters are not stuck. Pressure Don't just look at the design pressure-to calculate the maximum working pressure, consider the instantaneous impact of a water or air hammer. Temperature is more complicated: working temperature, design temperature, transient peak temperature, all three numbers can be obtained. The displacement includes three components: axial, transverse and angular. Only how many works have been pitted with an axial displacement are marked on the drawing. The corrosiveness of the medium is most easily ignored-the medium containing chloride ion, hydrogen sulfide and hydrofluoric acid, ordinary 304 stainless steel will stress corrosion crack in a few days, and 316L, duplex steel and even Hastelloy can be obtained.
Two days ago, I encountered a cement factory project. The temperature design of the hot air duct was 450℃, but the actual operation occasionally rushed to 550℃. The ordinary high-temperature axial type was selected. As a result, the guide tube burned and deformed, and the bellows was directly licked through by the flame. Therefore, when selecting a model, you must ask: "What is the peak temperature? How long does it last?"
4. Don't rush to remove the screw before installation-guide tube, tie rod and pre-displaced doorway
When the expansion joint leaves the factory, the two ends of the bellows are usually fixed by transport screws (also called tie rods) to prevent the deformation of the bellows during transportation and hoisting. Many people quickly screw off the screws as soon as they arrive at the scene-don't. When installing, you should first check whether the screw is loose, and then butt-weld the expansion joint to the pipeline (tighten the bolts for flange connection). When the pipeline system is fully assembled and the fixing bracket is welded, then adjust the tie rod nut or remove it directly according to the design requirements.
Tie rods do more than just transport protection. Some pressure balance expansion joints or large tie rod expansion joints, the tie rod also assumes the function of constraining the displacement direction of the bellows. Adjusting the tie rod nut can control the pre-displacement-for example, when the pipe is installed in the cold state, the expansion joint is pre-compressed or pre-stretched to half of the design value first, and the pipe returns to the free state when it is heated and expanded. This can improve the fatigue life of the bellows. How to tune it? Refer to the question and answer of "How to adjust the tie rod nut of the expansion joint". Remember one thing: the nut should not be tightened too tightly, and space should be left for the thermal displacement of the bellows.
The deflector is also a detail that is easy to overlook. The guide tube is installed on the inner wall of the bellows, and its function is to guide the medium and avoid the scouring and vibration of high-speed airflow. When selecting the model, the material, thickness and opening of the guide tube (for liquid or gas discharge) must match the working conditions. The steam pipe guide tube should be resistant to high temperature impact, and the corrosive medium guide tube should be lined with PTFE-at this time, the experience of lining PTFE metal hoses and PTFE compensators can be used.
V. Actual case: Difference in selection of steam pipeline and cement hot air pipeline in power station
Both the power station industry and the cement industry seem to be high-temperature pipelines, but the selection ideas are completely different. Take the main steam pipeline of the power station as an example. The pressure is often above 10MPa, the temperature is about 540℃, the pipeline diameter is large (above DN500), and the system controls the thrust of the support extremely strictly. Under this working condition, the straight pipe pressure balance expansion joint is the first choice for the commonly used metal expansion joint scheme, which can not only absorb axial thermal displacement, but also internally digest the blind plate force, and the design pressure of the bracket is much smaller. In addition to the main steam pipeline, the cold section and hot section of reheated steam will also use the compound hinge transverse expansion joint to cooperate with natural compensation. The product information of the power station industry is clearly listed-the corrugated expansion joint used in the power station industry has optimized the wall thickness of the corrugated pipe and the reinforcing ring structure specifically for this high-pressure and high-thrust scenario.
What about cement hot air lines? The pressure is low (usually 0.1~0.5MPa), but the temperature is high (450~650℃), the medium contains dust and alkaline gas, and the pipeline specification is often rectangular cross section. At this time, high-temperature axial expansion joint is more suitable, but attention should be paid to the choice of rectangular cross-section or circular cross-section matching, and the guide tube should be thickened to prevent wear. The metal corrugated expansion joint in the cement industry is usually designed with wear-resistant lining, and the guide tube is made of heat-resistant steel or cemented carbide. If the pipeline needs to be compensated in the three-dimensional direction, non-metallic expansion joints (fabric fiber expansion joints) are often used on cement lines to replace metal parts, because non-metals are resistant to high temperatures and compensate heavily, and can also isolate heat-provided that the medium is not strongly corrosive.
There is no universal formula for model selection, but there is an iron rule: first find out how the pipeline moves, how toxic the medium is, and how powerful the bracket can be. After thoroughly understanding the four parameters of pressure, temperature, displacement and corrosivity, and then comparing the structure of the product selection table-general type, pressure balance type, high-temperature axial type or double hinge type, the answer will come out by itself. If you are still unsure, go directly to the product database of this site. The applicable working conditions and limit parameters of each expansion joint are clearly written, and there is always one that can match the number.