1. Structural disassembly of double-section metal expansion joint: how to match two sections of bellows, and what is the difference between single-section and double-hinge transverse expansion joint
Double-section metal expansion joint, to put it bluntly, is to connect two sections of corrugated pipes in series on a pipeline, connected by an intermediate pipe in the middle, and fitted with pipes or flanges at both ends. These two sections of bellows do not do their own work. They cooperate to convert the displacement in one direction into two bellows to bear a part each. For example, when the pipeline is displaced laterally, the two sections of bellows will bend in opposite directions, and the displacement will be shared, so the burden of a single section of bellows will be much smaller.
Then what is the difference between it and a single-section expansion joint? Single-section bellows mainly absorbs axial displacement. If you force it to carry lateral displacement, the bellows will crack fatigue in a few days. The two-section type is different, it is born to do the work of lateral displacement. As for the double hinge transverse expansion joint, that is a higher-order game-it adds a hinge structure between two sections of bellows, which can constrain axial thrust and only allow lateral and angular displacement. The double-section metal expansion joint has no hinge, and relies on a tie rod or an intermediate tube to guide the displacement. The structure is simpler and the price is more affordable. Which one to choose depends on the pipeline constraints and the allowable blind plate force.
Second, under what working conditions must double-section type be used? From absorbing lateral displacement to reducing pipeline blind plate force, it is clearly explained in combination with actual cases of power station and cement industry
Two days ago, I met a customer who was doing a power station project. Their condenser inlet pipeline was DN1400, and the temperature changed greatly, so the pipeline turned two more corners. Originally, I wanted to use the universal corrugated expansion joint to carry it hard, but as a result, the lateral displacement couldn't be absorbed at all, and the pipe support was pulled to squeak. Later, it was replaced with a double-section metal expansion joint, and the two sections of bellows were matched, so the lateral displacement was easily eaten, and the force of the bracket was normal. This is the typical scenario of double section-when the pipeline is restricted in direction, the lateral displacement is large, and there is no natural compensation.
The cement industry is more typical. The flue gas pipes at the head and tail of the kiln are all high-temperature and big air ducts, and the direction of the pipes often has to go around the equipment. When a single expansion joint is installed, the bellows is quickly tortured by lateral displacement to leak air. Because the two sections of bellows share the displacement, the deformation of the single section is small, and the fatigue life of the double-section metal expansion joint doubles up. In addition, the double-section type can effectively reduce the blind plate force. The blind plate force is the thrust generated by the medium pressure acting on the section of the bellows, and the single-section expansion joint must be carried by the main fixed bracket. What about the double section? The blind plate force of the two sections of bellows partly cancels each other on the middle pipe, so the force transmitted to the fixed bracket is much smaller, and the cost of the bracket can be saved a lot.
Then when not to use the double section? The pipeline has only pure axial displacement, and the space is tight, so it is more cost-effective to use a single axial expansion joint. If it is required to absorb both the lateral displacement and not transmit the blind plate force, it is necessary to look at the compound straight pipe bypass pressure balanced expansion joint or the straight pipe pressure balanced expansion joint. In short, the more complicated the selection is, the better, it depends on the matching of working conditions.
Third, how to determine the selection parameters: pressure, temperature, compensation amount, fatigue life, these data don't pat your head, it depends on how to calculate the material and wavenumber of the bellows
When choosing a double-section metal expansion joint, the most taboo thing is to pat your head and report the parameters. You have to think clearly first: What is plumbing media? How stressful? How hot is the temperature? Which direction and how big is the displacement? These parameters directly determine what material the bellows is made of, how many layers and how many waves are made.
For this material, 304 austenitic stainless steel is enough for ordinary steam pipes; If the flue gas contains sulfur and the temperature is high, consider 316L or more corrosion-resistant alloys. When the temperature exceeds 550℃, the creep strength of ordinary stainless steel can't bear it, so it has to be replaced with high-temperature resistant alloy. If the material is selected wrong, the bellows will slowly become "soft" at high temperature, and it will start to deform before the design life.
How to calculate wave numbers? The compensation amount is divided by the single-wave compensation capacity and multiplied by the safety factor. The single-wave compensation ability depends on the geometric parameters and material fatigue curve of the bellows. Note that the fatigue life here is a key constraint-whether you ask for 1000 or 10000, the wave number, wall thickness, and layer number are all different. There is a fatigue life calculation formula in the national standard GB/T 12777, so don't blindly estimate it yourself. The number of layers also affects the pressure bearing capacity. If the pressure is high, use multi-layer bellows, each layer is thinner, with good flexibility and pressure resistance. If you don't understand these, go directly to the manufacturer for a model selection calculation book and let the manufacturer come up with a plan, which is much more reliable than your own guess.
4. Installation and pre-displacement pits: How to adjust the tie rod nut and whether the direction of the guide tube is correctly installed directly affects the use effect of the double-section expansion joint
When the double-section metal expansion joint leaves the factory, the tie rod nut is generally locked or adjusted to the pre-displacement state. The most afraid thing when installing is forgetting to adjust the tie rod nut. The function of the tie rod nut is to fix the bellows in the pre-displacement position during installation, and then loosen the nut to the working position after the pipe is in place and the fixing bracket is welded. If you forget to loosen it, the expansion joint will become a rigid connection, and the pipe displacement will all be pushed onto the bracket. Sooner or later, the noise and vibration will go wrong with the equipment.
And the deflector tube. The direction of the guide tube must be consistent with the flow direction of the medium, and the arrows must be clearly marked. Install backwards, the medium directly washes the inner wall of the bellows, and the bellows quickly wear out. Especially for flue gas pipes with particles, the guide tube is the "body armor" of the bellows, which is equivalent to going to the battlefield shirtless when installed backwards. When installing, it should also be noted that there should be enough gap between the guide tube and the pipeline, so that the guide tube cannot limit the thermal expansion and contraction of the pipeline.
In addition, be sure to check whether the pre-displacement amount is correct before installation. Double-section metal expansion joints usually leave the factory with a preset cold tightening, which is calculated based on the thermal displacement of the pipe. If the actual direction of the pipeline is inconsistent with the design during your on-site installation, you have to recalculate the pre-displacement amount, and don't hard install it. The consequence of hard installation is that the actual force direction of the bellows is opposite to the design, and the life span is greatly reduced.
5. What points should be seen in maintenance inspection: bellows surface crack, weld corrosion, reset of displacement indicator, and replacement boundary with rubber compensator and sleeve pipe expansion joint
After the equipment is in operation, maintenance is not just to see if there is any air leakage. Double-section metal expansion joint focuses on inspecting three parts: bellows trough, weld seam and tie rod hinge. Wave troughs are most prone to dust and liquid accumulation, and chloride ion corrosion often starts here. Gently tap the bellows with a small hammer to listen to whether the sound is stuffy, which means there are cracks inside. Permeation detection or ultrasonic detection is used for the weld position, especially when the medium contains sulfur, and the heat affected zone of the weld is prone to stress corrosion cracking.
The displacement indicator is also a good thing. The pointer scale installed on the tie rod or the middle pipe can visually see the actual displacement of the bellows. Record the position of the pointer during each overhaul, and you can know at a glance whether the expansion joint works according to the design. If you find that the reset is abnormal, or the displacement exceeds the design value, don't hesitate to check whether the pipe support is loose.
Then when should I replace it? Replacement can be considered if the bellows have penetrating cracks, the corrosion depth of the weld exceeds 10% of the wall thickness, or the fatigue life reaches 80% of the design times. Replacements do not necessarily use double-section metal expansion joints-if the working conditions change, such as the temperature drops and the displacement becomes smaller, a rubber compensator can do this, and it is not afraid of corrosion. However, the upper limit of temperature resistance of rubber compensator is low, and if it exceeds 150℃, you have to honestly use metal. What about sleeve pipe expansion joints? It is sealed by sliding sleeve, which can absorb large axial displacement, but the seal should be changed regularly, and it is easy to leak when the pressure is high. Therefore, replacing the boundary is very simple: look at the temperature, pressure and medium, which of the three conditions has changed, and then decide whether to continue using the double-section type.