First, bellows, the heart of the expansion joint, how to withstand thermal expansion and contraction?
Bellows is the most critical deformation element in metal expansion joints, none of them. Usually, stainless steel sheet is used, after hydraulic or mechanical forming, and corrugations are pressed one by one. The parameters of wave peak, wave pitch and number of layers directly determine the compensation amount-how much it can pull in the axial direction, how much it can bias in the lateral direction, and how much pressure it can hold.
The general-purpose corrugated expansion joint absorbs displacement by single-layer or multi-layer corrugated pipe, which has simple structure and is suitable for conventional pipelines. But if you run into a high-pressure steam pipe, you have to use an external pressure single axial expansion joint-its bellows are placed in an external medium and have a higher pressure capacity. To put it bluntly, the corrugated shape and wall thickness of the bellows are its "muscles", and the fatigue life must be calculated when designing. Metal expansion joints cannot be used as permanent parts, and usually have a life cycle of more than 100,000 times. This number should be remembered when selecting the model.
Will the bellows be damaged by the medium? Which brings us to the next part.
Second, end connection and guide tube-how to weld the expansion joint to the pipeline without letting the medium damage the inside?
The pipe and flange are the interfaces connecting the pipeline, and the material must match the pipeline-carbon steel to carbon steel, alloy steel to alloy steel, don't match randomly. The guide tube (also called lining tube) is installed on the inner wall of the bellows, and it has two functions: first, it reduces the direct erosion of the bellows by high-speed flowing medium (steam and flue gas); The second is to reduce the eddy current of the medium and reduce the flow resistance.
Corrugated expansion joints used in power station industry and metal corrugated expansion joints in cement industry are usually equipped with guide tubes because of the great impact of dust or high-temperature gas. The thickness and length of the guide tube are determined according to the diameter of the pipe and the flow rate of the medium, and not all pipes can be added casually. For example, the expansion joint matched with the desulfurization flue gas baffle door is highly corrosive in the medium containing sulfide, and the material of the guide tube should be stainless steel or lined with PTFE. Two days ago, I met a customer and asked about the specific function of the expansion joint guide. In fact, I was afraid that the bellows would be worn out.
3. Tie rod, hinge and limiting device-the expansion joint is not a rubber band, and the displacement direction must be controlled
The expansion joint cannot be moved, and must absorb the displacement according to the design direction. The core difference between tie rod expansion joints and hinge expansion joints (such as double hinge transverse expansion joints and double hinge expansion joints) lies in the restraint direction. The tie rod is used to absorb the axial displacement while limiting the lateral displacement to prevent the bellows from becoming unstable; The hinge allows angular or lateral displacement in a particular plane.
The high temperature axial type expansion joint has a guide ring inside to ensure that the bellows only expands and contracts along the axis. After installation, the tie rod nut needs to be adjusted to the predetermined length-two days ago, a customer asked how to adjust the tie rod nut of the expansion joint. In fact, it is to keep the bellows in the designed cold and tight position to avoid exceeding the allowable range during operation. Think about it, if the tie rod is not adjusted properly, and the bellows is directly pushed to death as soon as it expands thermally, wouldn't it be wasted?
4. Different types of structural differences-from single axial direction to pressure balance, what are the doors?
Metal expansion joint compensators are not a "one mold" thing. For example, the straight pipe pressure balance expansion joint eliminates the blind plate force through two sets of bellows and balance rings, which is suitable for large-diameter pipelines; While the curved tube pressure balance type expansion joint is used at the bend. The double straight pipe bypass pressure balance expansion joint is suitable for the space constrained.
The structure of sleeve pipe expansion joint (also called packing box expansion joint) is completely different-it relies on the sealing packing and the sliding of inner and outer sleeves to compensate the displacement, but the pressure and temperature resistance are limited, so it is only used for low-pressure and low-temperature pipelines. There are also rotary compensators, directly buried expansion joints, large-diameter thick-walled expansion joints... Each structure has applicable scenarios. Which you choose depends on the direction of the pipe, pressure rating, displacement type, and site space. Don't think that the more complicated the structure, the better. There is no need to use the pressure balance type for the general-purpose corrugated expansion energy-saving thing.
5. How Structural Design Affects Actual Type Selection-Inferring Structural Parameters Back According to Working Conditions
When selecting a metal expansion joint compensator, you must first find out the pipeline medium (temperature, pressure, corrosiveness), displacement (axial, transverse, angular) and connection mode (welding or flange). For example, the expansion joint matched with the desulfurization flue gas baffle door requires corrosion resistance and sulfide resistance, usually with stainless steel or PTFE-lined hose. On the air-cooled island vacuum pipeline, more dual-hinge expansion joints are used to absorb multi-directional displacement.
The more complex the structure is, the better-large-diameter thick-walled expansion joints are used in high-pressure steam lines, but the general-purpose type for small-diameter water pipes is enough. Remember: structural rationality directly affects the life. Before selection, it is best to check the fatigue times according to the stiffness calculation formula of the bellows, so as to avoid the problem of "leaking after two or three years of installation".
Finally, the structure of the metal expansion joint compensator is disassembled here, and the core is one sentence: the bellows determines the compensation ability, the guide tube protects the interior, the tie rod hinge controls the direction, and different structures are selected for different working conditions. After understanding these, go to the product manual and it will be clear at a glance.