What are you most afraid of when choosing metal expansion joints? I am afraid that the customer will only say "Quote me the price of DN600" on the phone. After quoting, ask "why is it so expensive". Because the selection parameters are incomplete, manufacturers can only quote according to the most conservative and largest specifications. Can this price not be high?
What about that? If you want to buy a suitable expansion joint, don't rush to report the caliber. Find out the following 5 things.
Don't be in a hurry to report the caliber, first calculate how much displacement the pipeline should absorb
The core function of the expansion joint is to absorb displacement. The amount of displacement was not calculated clearly, and the rest was all for nothing.
The thermal expansion of a pipeline is generally calculated by multiplying the linear expansion coefficient of the pipeline by the temperature difference and then multiplying the length of the pipeline. The linear expansion coefficient of carbon steel is approximately 0.012 mm/m·°C, and the stainless steel is slightly higher. For example: a 30-meter-long carbon steel pipeline, when the medium temperature rises from 20°C to 200°C, the axial thermal expansion is 30×180×0.012=64.8mm.
This is not over, the ambient temperature during installation, equipment vibration, and displacement caused by pipeline foundation settlement have to be counted. If you only know the diameter of the pipe and don't know how to calculate it, ask the manufacturer for a "Quick Check Table of Pipeline Thermal Displacement" and check it. But only if you have to prepare the pipeline direction diagram and clearly mark the positions of the fixed bracket and sliding bracket.
Media, pressure, temperature: three-piece set that determines the material and number of layers of corrugated pipe
Looking at these three parameters together, the material, wall thickness and number of layers of the bellows are basically determined.
At this end of the medium, 304 stainless steel is enough for ordinary steam, hot water and compressed air; For the medium containing chloride ions, 316L is used; In this strong corrosion condition of desulfurization flue gas, corrosion-resistant alloys or non-metallic expansion joints are often selected. The composition of the medium must be written in full, don't just mention a "smoke". The content of SO₂, NO and dust in the smoke is different, and the corrosiveness is several times worse.
If the pressure is high, the wall thickness has to be added; When the temperature is high, the permissible stress of the material decreases, and the compensation amount is also affected. For example, a steam pipeline with a pressure of 1.6MPa and a temperature of 350°C usually uses 2 to 3 layers of bellows; At the same temperature but the pressure dropped to 0.1MPa, a single layer is enough. The more layers, the greater the stiffness and the higher the price, so there is no need to blindly pursue multiple layers.
On the other hand, don't leak the deflector. The flow rate of the gas medium is high, and the bellows are easily eroded without the guide tube. If the medium contains particulate matter, the anti-wear structure at the front end of the guide tube should also be considered.
Axial, lateral or angular displacement? Choose the universal or hinged type
If the displacement direction is not clear, the chosen structure must be wrong.
The pipeline mainly expands and contracts along the direction of the pipeline, that is, axial displacement. Choose general corrugated expansion joint, also called axial expansion joint. If the pipeline takes an L-shaped or Z-shaped bend, and there is displacement in both horizontal and vertical directions, a single axial type can't be handled at this time, so we have to consider the double hinge transverse expansion joint, or the use of two hinge expansion joints together.
Angular displacement is common at equipment connections, such as steam turbine exhaust port and fan inlet and outlet. Hinge type expansion joint or double tie rod type is a good choice under this working condition.
How to judge? To put it bluntly, just one sentence: telescope and contract along the direction of the tube to choose the general-purpose type; If the pipe is bent and the joint is deflected, you have to use one with hinges or duplex structure. Don't expect to eat all the displacements with a general-purpose type. That's called hard carrying, not compensation.
Tie rod, hinge or pressure balance? Constrained structural relationship support stress
The function of expansion joint is not only "telescopic", but also "controllable force".
The bellows will produce a huge thrust under the action of pressure. If this thrust is all thrown to the fixed bracket, how thick must the bracket be? Therefore, there should be a restraint structure inside the expansion joint to share-this is the meaning of the existence of tie rod, hinge and pressure balance.
The universal expansion joint does not come with its own constraints, and the internal pressure thrust must be carried by the main fixed bracket. Where the pipeline is complicated and the bracket is difficult to make, use straight pipe pressure balance expansion joint or curved pipe pressure balance expansion joint, which can offset the internal pressure thrust, and the force acting on the bracket is much smaller. It is often used on pipelines in cement plants and power stations.
Some people think that the expansion joint with a tie rod can replace the fixed bracket. It wasn't. The tie rod constrains the displacement of the expansion joint itself, not the thrust of the pipeline. The bracket has to do what it needs to do, but the force is different.
High temperature flue gas, large diameter, vacuum pipeline: how to feed in special working conditions
Once the working condition is special, the standard products often can't stand it, so they have to be "added".
High-temperature flue gas, when the temperature reaches above 600°C, metal bellows are basically useless-material creep and oxidative corrosion will quickly shorten the life. At this time, the candidate non-metallic expansion joint (fabric fiber expansion joint) is more suitable, with high temperature resistance, good flexibility and certain sound absorption and vibration isolation effect. Non-metallic expansion joints are often used in desulfurization flues, which are used together with flue gas baffle doors.
Large-diameter pipes, such as flues or air ducts above DN3000, metal bellows are expensive to manufacture, heavy in weight, and laborious in construction and hoisting. Rectangular non-metallic expansion joints are more common schemes under such operating conditions. Of course, if the customer specifies metal, it has to use large-diameter thick-walled expansion joints, and the reinforcing rings and side plates have to be thickened.
The vacuum pipe is different again. Under vacuum conditions, the corrugated pipe should withstand external pressure, and its ability to resist instability is the key. Double hinge expansion joints are often used in air-cooled island vacuum pipelines, which have one more constraint in structure and good stability. The special vacuum hose is equipped with metal mesh sleeves outside the bellows, which is also the truth.
It is much more efficient to give the manufacturer a selection parameter table than to call back and forth
When communicating with the manufacturer, give the following information at once:
- Pipe diameter DN, connection mode (flange or welding)
- Type and value of displacement: What are the axial, transverse and angular displacements in mm/degree
- Name and composition of the medium, especially with corrosive components
- Design pressure MPa, Design temperature °C, Operating temperature °C
- Installation position indication (straight pipe section, near elbow, equipment interface)
- Arrangement of fixed bracket and sliding bracket
- Is there any thermal insulation layer and any outer sheath requirements
All this information is given, and the manufacturer can make the plan and quotation in half an hour. The time saved is much more efficient than calling back and forth to urge questions.
There is no "universal model" for metal expansion joints, only "fit or not". Only when the selection parameters are clear and the working condition boundary is clear can the manufacturer give you a reliable scheme. If you are not sure, send the parameter table directly to the manufacturer-let the engineer judge whether the scheme is valid or not.