What exactly are expansion joints "expanding"? Understand what it does first
When many customers hear the name "expansion joint" for the first time, their first reaction is: Can this thing become bigger and smaller by itself? Actually, you guessed half right-the expansion joint does "expand", but strictly speaking, it is not getting bigger, but absorbing the thermal displacement of the pipe. Simply put, when pipelines transport high-temperature media, they will expand and contract by heat. Pipelines of tens or even hundreds of meters may elongate by tens of millimeters when the temperature rises. If you don't find a way out for this stretch, pipe joints, valves and even brackets will be broken. At this time, the expansion joint comes in handy, and it "eats" this part of the displacement through its own corrugated structure.
Two days ago, I met a friend from a power plant, who told me that their main steam pipe burst welds three times a year, and finally replaced itCorrugated expansion joint for power station industryIt never went wrong again. You see, the core value of an expansion joint is not to expand itself, but to give a "safe amount of deformation" to the piping system.
Bellows is the real "actor"- -Detailed explanation of three deformation modes in axial, transverse and angular directions
How do metal expansion joints expand? The answer is all in the bellows. The bellows is made of layers of stainless steel sheet pressed, like the bellows of an accordion. It has three ways of "performing":
- Axial expansion: Bellows are stretched or compressed along the length of the pipe, most commonly. For example, when the steam pipe heats up, the bellows is compressed and absorbs the elongation. If you are usingUniversal corrugated expansion jointOrHigh temperature axial expansion jointIt mainly depends on this action.
- Lateral expansion: When the pipe is offset horizontally or vertically, the bellows will bend like a snake. However, it is difficult to complete the lateral displacement at a large angle by one section of corrugated pipe alone, so two sections of corrugated pipe are usually connected in series, and a section of connection pipe is added in the middle to makeCompound hinge transverse expansion jointOrDouble hinge expansion joint for air-cooled island vacuum pipeline。 This construction allows the pipe to sway from side to side, but the bellows themselves do not twist.
- Angular expansion: The bellows rotates at a certain center point and is usually used with a pair of hinges or universal hinges. Such asCompound hinge expansion joint、Curved tube pressure balance expansion jointAll can realize angular compensation. You see the kind of expansion joint with "a short tube in the middle of two bellows and tie rods around it" at the scene, which is mostly angular.
These three deformation modes are not mutually exclusive-many expansion joints bear axial and lateral displacements at the same time, and the selection depends on the direction of the pipeline and the position of the fixed bracket.
Different types of metal expansion joints, what is the difference between expansion methods?
Although they are called expansion joints, their internal structures are different, and the ways of "expansion" are worldly different.
Axial type
The most simple and crude, such asAxial expansion joint、External pressure single axial expansion jointThey have only one section of corrugated pipe and both ends are welded directly to the pipe. Used in large straight pipe sections, where the temperature changes greatly and the pipe has enough space to expand and contract. But the downside is also obvious-once the pipe has a lateral displacement, it is prone to "twist twists".
Complex hinge type
Just mentionedCompound hinge transverse expansion joint, with two bellows connected by a short pipe in the middle and a set of hinges on each side. The function of the hinge is to limit the axial expansion and contraction of the bellows, only allowing it to swing angularly or laterally in the plane. This structure is particularly suitable for L-shaped or Z-shaped pipes, which can absorb thermal displacement without producing huge blind plate forces.
Pressure balance type
There is a trouble in pressure pipes called "blind plate force"-the medium pressure acts on the end face of the bellows and will push the pipe like a piston. To solve this problem, there areStraight pipe pressure balanced expansion joint、Curved tube pressure balance expansion joint、Compound straight pipe bypass pressure balanced expansion joint。 They typically have two sections of bellows inside, one that absorbs displacement and the other that counteracts pressure so that there is no additional thrust on the fixed bracket. And guess what? The pressure of the main pipeline of many chemical enterprises can reach more than 4.0MPa. If there is no pressure balance structure, the bracket can't hold it at all.
In addition, rectangular flue ducts in coal mines and power plants are generally usedMetal rectangular expansion jointOrRectangular non-metallic expansion jointTheir expansion principle is similar to that of a circle, but the structure is an integral frame plus a non-metallic skin, and the folds and elasticity of the skin absorb the displacement. One thing to remember: rectangular expansion joints cannot withstand internal pressure and are only used for low-pressure or atmospheric pressure pipes.
Pits often stepped on at the installation site: how to deal with the direction of the tie rod, guide tube and arrow?
When the equipment arrives at the site, it is easy for the installer to make mistakes if he is not careful. Say a few of the most common questions:
Do you want to remove the tie rod and nut?When the expansion joint leaves the factory, it usually has a transportation pull rod to press and fix the bellows to prevent transportation bumps. Once installed in place, this tie rod should generally be loosened-but don't disassemble it completely. If it isPressure balanced expansion jointOrCompound hinge expansion jointSome tie rods are permanent structural pieces used to transmit blind plate force. Dismantle or not? Look at the nameplate or instructions, which will say "Installation: loosen tie rods after installation" or "remove shipping bars". You remember: All threaded tie rods, any with adjustment nuts, are mostly transport tie rods; If both ends of the tie rod are welded to the end plate, it is a permanent piece.
Direction of guide tubeA guide tube is usually welded inside the expansion joint, which guides the flow of the medium, reduces the resistance, and protects the inner wall of the bellows from being washed by the high-speed medium. How to judge the direction? The opening of the guide tube is directed in the direction in which the medium flows. If you install it backwards, the deflector will be held up and the bellows won't expand or contract properly at all. There was a client who putDesulfurization flue gas baffle doorThe expansion joint at the back was installed backwards, and the guide tube blocked the flue gas flow. After changing the bellows three times, it was not found out... Later, it was discovered that it was a direction problem.
What's the direction of the arrow?Many expansion joint shells have arrows printed on them or the words "flow direction". This arrow represents the flow direction of the medium, not the installation direction, let alone the displacement direction. Once installed backwards, the guide tube and the sealing structure will fail. Especially forFlue gas baffle door、Desulfurization flue gas baffle doorFor such equipment, the flow direction arrow must be consistent with the actual flue gas flow direction.
It is enough to focus on these four points when selecting a model: temperature, pressure, medium and displacement
"How exactly do you choose the expansion joint?" I generally only ask them to provide four parameters:
- Temperature: Maximum operating temperature and minimum temperature. Temperature determines the material of corrugated pipe-austenitic stainless steel (304, 316L) for high temperature pipeline, and ultra-high temperature (above 700 ℃) should be consideredHigh temperature axial expansion jointOrMetal Corrugated Expansion Joints in Cement IndustrySometimes thermal insulation is needed. For cryogenic pipelines below-196℃, austenitic stainless steel or copper alloy should be selected.
- pressure: Design pressure and test pressure. Pressure determines the number of layers and wall thickness of the bellows. Generally, the pressure of steam pipelines in power plants is 1.0~2.5MPa, and some chemical pipelines reach more than 10MPa.Large diameter thick wall expansion jointOrPressure balanced expansion joint。
- Medium: Gas, liquid, corrosive medium? If it is a corrosive medium, such as the wet flue gas behind the desulfurization tower, ordinary stainless steel cannot be used, it must be usedPTFE-lined hoseOrPTFE compensator。 If the medium contains particulate matter, such as cement raw material conveying pipelines, the bellows need to be wear-resistant bushings.
- displacement amount: What are the axial, transverse and angular directions respectively? Calculated by pipeline stress analysis software such as CAESAR II, or estimated by empirical formulas. The magnitude of the displacement directly determines the wave number and structural form of the expansion joint-for large displacement, use multi-wave bellows or connect multiple joints in series.
These four parameters are clear, and the selection will basically not overturn. As forMetal hose、rubber compensator、Non-metallic expansion jointThe principle of these special products is similar, but the applicable boundary conditions are different.