Look at the structure first: the bellows is the core "spring" of the expansion joint
When you stare at a disassembled metal expansion joint, your first reaction must be: How can this thing "expand"? In fact, the secret is all in the middle circles of bellows. To put it bluntly, a metal expansion joint is a section of thin-walled tube with corrugations, with joints or flanges welded at both ends. The bellows is the equivalent of a metal spring that can be telescoped — but not a coil spring, but an annular ripple that runs in circles. These ripples are elastic in the axial direction, and when subjected to tension or pressure, the peaks and valleys of the ripples will open or close, resulting in displacement.
How much can the bellows stand? It depends on its stiffness. The smaller the stiffness, the easier it is to deform, and the greater the amount of compensation; But the stiffness is too small to hold up internal pressure. Therefore, it is necessary to look at the number of layers, wall thickness, wave height and wave pitch of bellows when selecting the type. For example, the general corrugated expansion joint we often say is generally a single-layer or multi-layer stainless steel bellows, while the high-temperature axial expansion joint under high-temperature working conditions has to be made of heat-resistant alloy and multi-layer structure.
Axial expansion: How does the bellows "expand and contract" when the pipe expands and contracts thermally?
The most common scenario is a steam pipe. When the pipe rises from cold to several hundred degrees, the length will be stretched by several centimeters. At this time, an axial expansion joint is installed in the middle of the pipeline, and the bellows is compressed-the distance between peaks and valleys is reduced, absorbing the elongation of the pipeline. In turn, if the pipe cools and shrinks, the bellows stretches. So you see, how do metal expansion joints expand? In fact, it is more accurate to say "compensate for displacement". The bellows itself does not necessarily really "expand", but eats the thermal expansion and contraction of the pipe through elastic deformation.
The amount of compensation. How many millimeters of axial displacement an expansion energy saver can absorb depends on the wave number of the bellows and the amount of single wave compensation. For example, external pressure single axial expansion joint, because the outside of the bellows bears the medium pressure, the stability is better, and the single wave compensation can be larger, which is suitable for long-distance pipelines. However, the directly buried (fully buried) expansion joint should consider external pressure and soil corrosion, and its structure is more complicated.
Lateral and Angular Displacement: Can Bellows Still "Walk Bending"?
The pipe is not a straight line, and turning, misalignment, and vibration will all produce lateral and angular displacements. Axial compression alone is not enough at this time. What to do? Let the bellows "bend". For example, the double hinge transverse expansion joint uses two sets of bellows to add a hinge structure in the middle, which can absorb transverse displacement and angular displacement. Imagine that two sets of bellows, one bent and the other extended, can make the pipe "drift" in the horizontal direction with the rotation of the hinge.
There is also a curved tube pressure balance type expansion joint, which is specially used in the curved tube part with pressure thrust. It adds a balanced bellows inside to offset the blind plate force and avoid excessive thrust on the pipe support. Two days ago, a customer asked me, saying that the bracket was always broken at the corner of the hot air duct in their factory. When asked, the blind plate force was not counted. Later, the pressure balance expansion joint of the curved tube was replaced, and the problem was directly solved.
Select different "expansion methods" for different working conditions: high temperature type, pressure balance type, double hinge type... Don't choose the wrong one
Choosing the wrong expansion joint is equal to laying a mine. Simply a few points:
- High temperature flue gas pipeline(For example, power stations and cement industries) -use high-temperature axial expansion joints or non-metallic expansion joints. Non-metallic fabric fibers can carry hundreds of degrees, and there is no internal pressure thrust, but the strength is not as strong as metal.
- Steam Supervisor(High pressure, high temperature) -Straight pipe pressure balanced expansion joint or double straight pipe bypass pressure balanced expansion joint. They absorb axial displacement while balancing pressure thrust and do not require a primary fixing bracket.
- Air-cooled island vacuum pipe— — The double hinge expansion joint of air-cooled island vacuum pipeline is used, and the sealing requirement in vacuum environment is extremely high, and the double hinge structure can absorb complex displacement.
- Large diameter thick-walled pipe(For example, desulfurization flue) -large-diameter thick-walled expansion joint, strengthened wall thickness, and guide tube has to be added to prevent scour.
- Insulated parts requiring frequent overhaul— — With manual plug-in type insulation door or electric plug-in type insulation door, use expansion joint to compensate pipeline displacement, and then use baffle door to cut off the medium.
You see, the "expansion method" is different for each working condition. When selecting the model, you have to compare the model and size table of the expansion joint, calculate the compensation amount, pressure and temperature, and don't just look at the price.
"Expansion" doorways in installation and use: the key points of deflectors, tie rods, pre-stretching
Only when the expansion joint is installed correctly can it "expand" normally. Article 1:guide tube。 The guide tube is installed on the inner wall of the bellows, and its function is to guide the flow medium to avoid the direct washing of the bellows by high-speed fluid. Especially in the flue gas pipeline with particulate matter or high flow rate, the bellows will wear out in a few months without the guide tube. The specific function can refer to the question and answer in our station.
tie rod。 Many expansion joints come out of the factory with tie rods or transport screws, which is to prevent the bellows from being crushed or pulled during transportation and installation. How do you adjust the tie rod nut after installing it in place? It is generally required to loosen the nut and allow the tie rod to float freely, but in some cases it is necessary to adjust the tie rod length to pre-compress or pre-stretch the expansion joint. For example, when the temperature of the pipeline is lower than the operating temperature, it has to be pre-stretched. After the temperature rises, the expansion joint just returns to the free length, and the compensation efficiency is the highest. Does the screw of the expansion joint need to be removed? The transportation screw must be disassembled, and the pull rod depends on the situation, and some are reserved as limits.
Pre-stretching。 It's a practical trick. For example, the design compensation amount of an axial expansion joint is 50mm. If the installation temperature is 20℃, the operating temperature is 200℃, and the pipeline is extended by 80mm, the expansion joint has to be pre-stretched by 30mm, installed on the pipeline and then welded. This operates without exceeding the compression limit of the bellows. This is often done in the cement industry and power station industry with corrugated expansion joints.
Don't reverse the installation direction of the expansion joint. The direction of the arrow of the expansion joint refers to the flow direction of the medium. If it is installed backwards, the guide tube will not play the role of diversion, but will become a throttling piece, and the pressure loss will increase greatly. Many on-site accidents are caused by reverse directions.