Metal Expansion Joint Function: Three Cores, None Less
When talking about the function of metal expansion joint, many people's first reaction is "it can be expanded and contracted". Yeah, but not entirely. To put it bluntly, the core functions of this thing are three things: absorbing the displacement caused by thermal expansion and contraction, compensating for the deviation caused by installation error, and isolating the vibration and noise transmitted by mechanical equipment. The "flaws" of stress, displacement and vibration in the pipeline system are all resolved by this "flexible joint".
Think about it, a steam pipe several hundred meters long, rising from normal temperature to 500℃, has to stretch several millimeters per meter. If there were no expansion joints, the pipe would carry itself hard, and the consequence would be either a cracked flange or a twisted bracket. The function of the metal expansion joint is to let the pipe have a place to "breathe a breath" when heated-the bellows compresses or stretches, eats the displacement, and the stress is naturally released.
Installation deviations. How can the on-site construction be so perfect? It is common for the pipeline to be misaligned and the elevation of the bracket to be a few millimeters different. The function of metal expansion joint here is equivalent to a "fault-tolerant mechanism", which digests the deviations accumulated during installation. Otherwise, if you hold the pipe against each other, and let go after welding, the stress will all be held in the weld seam, and something will happen sooner or later.
As for vibration and noise reduction, not to mention. When pumps, fans, compressors and other equipment are turned on, the vibration is transmitted along the pipeline, which not only disturbs the people with noise, but also may loosen the instrument joint. The bellows itself in the function of metal expansion joint is a flexible body, which can effectively cut off the vibration transmission path, similar to a "soft connection". Especially in scenes that are sensitive to vibration, such as air-cooled islands and vacuum pipelines, double-hinge expansion joints and double-hinge transverse expansion joints are specialized in this.
Different working conditions, the functional emphasis of metal expansion joint is very different
Don't think that a general-purpose corrugated expansion energy-saving package conquers the world. Under different working conditions, the emphasis of the functions of metal expansion joints is completely different. For example, the high-temperature steam pipeline at the boiler outlet of the power station has a temperature of five or six hundred degrees, and the pressure is not low. At this time, it is necessary to put on a high-temperature axial expansion joint or a straight tube pressure balance expansion joint, which mainly absorbs axial thermal displacement. Why? Because the steam pipeline goes straight, has a large span, and the thermal elongation is concentrated in the axial direction, an axial type can do it.
Another example is the flue gas desulfurization system, which is called a harsh environment: low temperature, high humidity and acid corrosive medium. You put regular metal expansion joints in it, and it rust through in a few months. What to do? Upper non-metallic expansion joint or PTFE-lined hose. The function of metal expansion joint is no longer the first place in "absorbing displacement" here, but "corrosion resistance and sealing" has become the key. Rectangular non-metallic expansion joints are widely used in the inlet and outlet of desulfurization tower, and the fabric fiber composite layer can not only withstand micro-pressure, but also withstand acid-alkali erosion.
There is also the kiln tail flue gas of cement plant, which is high in temperature and dusty. Someone took a metal rectangular expansion joint to deal with it, but it was perforated in half a year-why? No deflector! High-speed dusty smoke directly washes the bellows, and no matter how thick the stainless steel is, it can't bear it. So you see, the function of the metal expansion joint depends not only on the bellows itself, but also on the internal auxiliary structure.
The internal structure determines the functional upper limit of the metal expansion joint
The bellows is the "heart" of the expansion joint, which is true. But a heart alone is not enough, but a skeleton and protection. The deflector is a key player-it is mounted on the inner wall of the bellows, guiding the medium to the center to flow through, avoiding high-speed fluid directly washing the bellows, and reducing flow resistance. And guess what? Many customers only look for the item of "stretching like a spring" and ignore the role of the guide tube. As a result, there were particulate matter in the medium, and the bellows wore out quickly, and then the expansion joint was scolded for poor quality-in fact, it was very wronged.
Tie rods and limit nuts are another set of logic. If the universal corrugated expansion joint does not add a tie rod, it will twist and twist like a snake under the action of pressure, and become unstable in the lateral direction. Installing the tie rod and locking it with a nut, the expansion joint can be controlled to absorb only axial displacement, and the lateral and angular displacement is limited. This is the secret of the "directional play" of the function of metal expansion joints. There is also cold-tight pre-stretching-intentionally stretching a section during installation, and after the pipe heats up, the expansion joint just returns to the neutral position, and the life can be doubled.
So don't get the structure complicated. External pressure single axial type, double hinge transverse type, straight pipe pressure balance type, curved pipe pressure balance type... Each design corresponds to a specific force mode. If you choose the wrong one, the metal expansion joint function is equal to nothing.
If the selection is not done correctly, the function of metal expansion joint will be in vain
Two days ago, I encountered a cement factory project. They used a metal rectangular expansion joint to treat the flue gas at the kiln tail. As a result, they perforated it in half a year because they didn't consider the anti-wear design of the guide tube. At the scene, the inside of the bellows was ground by dust like a knife. In fact, when selecting the model, just put the hard indicators of medium temperature, pressure, corrosiveness, compensation amount and fatigue life on the table, and then take the seat.
High-temperature steam pipeline, with large compensation, uses straight pipe pressure balance expansion joint, which can absorb axial displacement and balance pressure thrust at the same time, so as to save the pipe support from being made like a bunker. Smoke duct system, rectangular section, with metal rectangular expansion joint or rectangular non-metallic expansion joint, low pressure, medium-low temperature, emphasis on sealing and corrosion protection. The pump outlet pipeline has obvious vibration, and the double hinge transverse type or rubber compensator is used to absorb the transverse displacement and reduce vibration.
There are also vacuum pipes, such as air-cooled island systems, with air-cooled island vacuum pipe double-hinged expansion joints or vacuum-specific hoses-these products have been specifically verified for tightness and stability under negative pressure. You say make do with a generic model? When the vacuum is pumped, the bellows is deflated, and its function is completely wasted.
The function of metal expansion joint does not exist in isolation
The function of metal expansion joint must be matched with the arrangement of pipe support, cold tightening pre-stretching and installation direction, so as to truly realize the effect of "compensation without force, vibration reduction without force transmission". Otherwise, no matter how expensive the expansion joint is, it will be useless.
The direction of the arrow on the expansion joint is not randomly printed. It represents the direction of media flow. Install backwards, the guide tube loses its function, and the high-speed fluid directly rushes the root of the bellows, and the life is directly discounted by 50%. Another example is the tie rod nut. After installation, it should be adjusted to the specified preloading force, not just screwed to death-refer to the question and answer on "How to adjust the tie rod nut of the expansion joint" on our station, which is very detailed.
Pipe supports are also key. The main fixing bracket near the expansion joint must be firm, otherwise the pipeline displacement will be eaten by the bracket, and the expansion joint will be idle. The distance of the guide bracket should also be according to the design. If the distance is too large, the pipeline will be unstable. To put it bluntly, the whole system is an ecology, and the metal expansion joint function is only one part of it.
Therefore, when engaging in engineering selection, don't just stare at the expansion joint parameter table. Consider the working conditions, pipeline layout, bracket type and installation requirements, and then choose the corresponding products-whether it is a general-purpose corrugated expansion joint or a compound hinge transverse type, whether it is a high-temperature axial type or a straight pipe pressure balance type, the difference is big. We have more than 30 kinds of product information on our station, each of which corresponds to specific application scenarios, and can be used when you take it.