Start with the direction of displacement absorption: axial, transverse, angular, but don't be fooled by the name
When most engineers hear the division of metal expansion joints, what comes to mind is axial type, transverse type and angular type. Yes, according to the direction of displacement absorption, these three categories are basic. But if you think this is all you want to look at, you are underestimating this thing. Two days ago, I met a customer, who took the drawing and said, "I want an axial expansion joint". As a result, when I checked the working conditions, there were three directional displacements on the pipeline, and the light given to the axial direction was equal to white installation.
The axial type is the most common, such as the one in our stationUniversal corrugated expansion jointAndExternal pressure single axial expansion jointSpecially absorb the linear displacement of the thermal expansion and contraction of the tube. What about the horizontal type? And guess what, it actually relies on bellows bending to absorb displacement perpendicular to the axis- -Compound hinge transverse expansion jointThat's what you do. Add a hinge in the middle of the two bellows, and force it to one side. Angular type is more direct, single hinge or double hinge structure, which specifically solves the angle change at the corner of the pipe, such asDouble hinge expansion joint for air-cooled island vacuum pipelineThat kind, it's for turning big vacuum pipes.
What do you do if a ductwork has axial, lateral, and angular displacements at the same time? A single axial type is definitely not good, it has to be used in combination. Alternatively, directly usePressure balanced expansion joint— I'll talk about this later.
Pressure balanced expansion joint: the nemesis of blind plate force, not all expansion joints can pat the chest
What is blind board force? Simply put, the pressure in the pipe acts on the elbow, valve or blind plate, which will generate a thrust. If it is not offset, the pipe and the bracket will be pushed away. Ordinary axial expansion joints do not generate counteracting forces themselves, but may transmit thrust to the fixed bracket. Therefore, there is the classification of pressure balanced expansion joint.
We have them in our stationStraight pipe pressure balanced expansion jointAndCurved tube pressure balance expansion joint, andDouble straight pipe bypass pressure balanced expansion joint。 Their design logic is to let the pressure thrust cancel out internally and not spread out through two or three bellows, plus tie rods or bypass structures. For example, the pressure on the main steam pipe of a power plant is more than ten MPa. Do you want to install an ordinary expansion joint? The retaining bracket deformed within a few days. The pressure-balanced type has to be used. The ingenious arrangement of the bellows group makes the thrust cancel out each other, the pipe expands and contracts quietly, and the bracket is not guilty.
Many people can't tell the difference between the pressure balance type and the double lateral type. The difference is that the pressure balance type must have a tie rod or bypass to absorb the pressure thrust, while the compound transverse type mainly relies on hinges to limit the displacement direction, which offsets not the pressure thrust, but the direction control. If you confuse the selection, the consequence is a loss of real money.
Forms under special working conditions: high temperature, high pressure, vacuum, direct burial-every pit is a technical activity
As soon as the working condition is abnormal, the form division of metal expansion joint has to be adjusted accordingly. Let's start with the heat. For conventional austenitic stainless steel (such as 304 and 316) for corrugated pipes, when the temperature exceeds 400°C, the creep of the material is aggravated, so it is necessary to replace Inconel 625 or high nickel alloy. Structurally, too —High temperature axial expansion jointOften with thermal insulation lining and outer thermal insulation jacket, otherwise the bellows will be baked directly, and the life will be greatly reduced. Used in the cement industryMetal Corrugated Expansion Joints in Cement IndustryThe working temperature is often 800°C, and even the guide tube has to be made of heat-resistant steel.
What about high pressure? Large-diameter thick-walled expansion joint appears. Such asLarge diameter thick wall expansion jointThe wall thickness is more than ten millimeters or even thicker, the number of layers is also increased, and some have reinforcing rings. Note: High pressure does not mean that it can definitely resist fatigue. If the number of cycles is large, it has to be calculated as fatigue life. The calculation formula of bellows stiffness in our product information is to help customers calculate this.
The vacuum is another headache. The common expansion joint is easily unstable under external pressure, so it is necessary to use itSpecial hose for vacuumOrDouble hinge expansion joint for air-cooled island vacuum pipeline。 The vacuum special hose usually strengthens the wall thickness of the bellows or adds a support ring to prevent it from being deflated by atmospheric pressure.
Everyone is familiar with direct buried pipelines, right? Thermal pipelines are buried underground, which is inconvenient for maintenance. At this time, useDirect buried (fully buried) type expansion jointWith an anti-corrosion insulation layer on the outside, the bellows is directly buried in the soil. When choosing this expansion joint, you have to consider soil corrosion, groundwater infiltration, insulation life-all details.
Structural differences: bellows type, sleeve type, metal hose-don't confuse positioning
"What is the difference between metal expansion joint and corrugated expansion joint?" In fact, expansion joint (or compensator) is a collective term, and bellows type is the mainstream, but there are also sleeve type, metal hose and other forms. Different structures have different applicable scenarios.
Bellows-type expansion joints (i.e. our commonUniversal corrugated expansion joint、Corrugated expansion joint for power station industryAnd so on) by elastic deformation of bellows to absorb displacement, good sealing performance, medium compensation amount, suitable for most pipelines. What about the sleeve type?Sleeve type pipe expansion jointThe axial displacement is compensated by the relative sliding of the inner and outer sleeves, and the seal depends on the packing box, which is suitable for scenarios with low pressure and large axial compensation, such as thermal pipelines. However, the disadvantage of the sleeve type is that it is easy to leak and requires regular maintenance of the packing.
What about metal hoses? Many people use it as an expansion joint, but it's not. Metal hoses (e.g.Metal hose、PTFE-lined hose) is the pipe itself bent to absorb displacement, which is mainly used to absorb installation errors, vibration and a small amount of displacement. The compensation amount is small, so it is not recommended for pipelines that need fixed compensation. We often use metal hoses for equipment connection or vibration isolation, but let it be used as an expansion joint? Don't save trouble unless the displacement is very small.
Another category is non-metallic expansion joints, such asRectangular non-metallic expansion joint、Non-metallic expansion joint (fabric fiber expansion joint)They use fabric or rubber as compensation elements, which have average temperature resistance but are also afraid of high pressure. They are mainly used in low-pressure and large displacement occasions such as smoke ducts. The smoke baffle door is also often equipped with non-metallic expansion joints to absorb thermal displacement.
Selection logic: look at working conditions, look at displacement, look at pressure-in fact, it also depends on people
After talking for a long time, how to choose? The purpose of the metal expansion joint form division is to help you make decisions. Three steps:
- The first step is to find out the working conditions.Medium temperature, corrosiveness, whether vacuum, whether direct burial. For example, flue gas desulfurization system, it must beDesulfurization flue gas baffle doorEquipped with anti-corrosion expansion joint; If it's high-temperature dust, you have to use itMetal Corrugated Expansion Joints in Cement Industry。
- The second step is to calculate the displacement.Axial, transverse, angular, what is the amount of each displacement? All three directions? Then consider the duplex or pressure balance type. Don't forget to look at the direction of the pipe. The angular displacement at the elbow is large, and the straight pipe section is mainly axial.
- The third step is to set the pressure.Design pressure, test pressure, pulsation pressure. Select thick wall or multi-layer for high pressure, and use ordinary bellows for low pressure. Don't forget to check the thrust-the pressure balance type is not a free lunch, it is expensive on multiple bellows, but it saves money on large fixed brackets.
Don't just look at the parameter table when selecting the model, and communicate with the manufacturer's engineer is the most reliable. We met quite a few customers and picked one from the tableUniversal corrugated expansion jointAs a result, it was found that the pipeline vibrated abnormally, and the fatigue caused by fluid pulsation was not considered. If I had asked earlier, I wouldn't have.
The division of metal expansion joints is taxonomy on the surface, but engineering experience in essence. Axial, transverse and angular directions are only the introduction, while pressure balance, special working condition and structural variation are the advanced ones. Only when you understand this can you really use expansion joints.