Why do plumbing systems need the dual characteristics of "shock absorption" and "thin wall"? -Don't let traditional expansion joints hold you back
Two days ago, a friend who was doing a power station project spat with me, saying that the vibration frequency of the outlet pipe of their induced draft fan was ridiculously high, and it used ordinaryUniversal corrugated expansion jointIt cracked in three months. When you remove it, you can see that the fatigue cracks at the root of the corrugation are dense. This is actually a typical problem-many designers are still using the old idea: to resist pressure, thicken; To compensate for the displacement, increase the wave height. And the result? The equipment got heavier, the bracket couldn't stand it, the vibration amplified instead, and finally the bellows shook itself off.
Shock-absorbing thin-walled metal expansion jointThe appearance is for this contradiction. Thin wall is not cutting corners, but by optimizing the wave structure and mechanical properties of materials, on the premise of ensuring the pressure bearing capacity, the wall thickness is reduced. After thinning, the flexibility of the bellows is better, and the absorption capacity of high-frequency vibration is directly doubled. If you think about it, when a steel pipe vibrates at 100Hz, the stress wave transmission speed is fixed, but the geometric nonlinearity of the bellows can dissipate the energy. The thin-walled design improves this dissipation efficiency by at least 30%.
What about shock absorption? Instead of simply relying on rubber damping, it uses the hysteresis loop of the metal bellows itself-to put it bluntly, the metal grains produce internal friction during repeated deformation, and convert the vibration energy into heat energy. Which andrubber compensatorThe principle is different, rubber is viscoelastic damping, suitable for low frequency and large displacement; The metal bellows is more suitable for high-frequency and small-amplitude vibrations, such as the harsh vibration of fans and pump outlets.
The core structure of shock-absorbing thin-walled metal expansion joint: What is the difference between it and ordinary general-purpose corrugated expansion joint?
Many people look at the appearance and think that this is just a bellows. Wrong. commonUniversal corrugated expansion jointThe wall thickness is generally 2-4mm, and the wave pattern is mostly U-shaped or ω-shaped. The main considerations are axial compensation and pressure bearing. WhileShock-absorbing thin-walled metal expansion joint, the wall thickness can be 0.8-1.5mm, and the wave mode adopts multi-layer thin-walled composite structure-for example, two layers of 0.5mm 304 stainless steel sandwiched with a layer of damping net. This damping net is the key. It is equivalent to adding an "internal friction plate" to the bellows. Each time it deforms, slip occurs between layers, eating up the vibration energy.
In addition, the end connecting flange of the common expansion joint is often directly welded on both sides of the bellows, which has great rigidity and concentrated stress. The ends of the shock-absorbing thin walls may be designed as tapered transitions, or may be used withMetal hoseOf the kind of woven mesh sleeve as a transition. Don't underestimate this detail, it can avoid stress peaks at the connection between the bellows and the flange. We have measured that under the same working conditions, the fatigue life of ordinary general-purpose corrugated expansion joint is 150,000 times, while that of shock-absorbing thin-walled structure can reach more than 800,000 times-more than five times the difference.
Another point is that shock-absorbing thin-walled products usually add internalguide tube。 The role of the guide tube is not only to guide the flow (prevent the medium from washing the bellows), but more importantly, it can play the role of "tuning mass damper"-the guide tube itself is a thin-walled cylinder, and there will be relative vibration between it and the bellows, which is equivalent to embedding a vibration absorber in the system. This design is in theThe corrugated expansion joint used in the power station industry has been used very maturely.
Which industries are already inseparable from it? Power stations, cement, desulfurization…one is just needed than the other
Let's start with the power station. The vibration frequency of induced draft fan and blower outlet of thermal power unit is generally 30-80Hz, which belongs to the typical mixed working condition of "low frequency and large amplitude" and "high frequency and small amplitude". traditionalHigh temperature axial expansion joint orExternal pressure single type axial type expansion joint, although high temperature resistance, but vibration absorption capacity is really insufficient. In the flue system of a 600MW unit, after using the shock-absorbing thin-walled structure, the peak vibration value of the pipeline dropped from 12mm/s to 4mm/s, which directly eliminated the quarterly shutdown for maintenance.
The cement industry is even more ruthless. The mill outlet pipe, that vibration is constant and contains a lot of dust particles. The bellows of ordinary expansion joints are quickly worn and perforated.Although the metal corrugated expansion joint in cement industry has been treated with wear resistance, the wall thickness should not be too thick, otherwise the flexibility will not be enough. The shock-absorbing thin-walled bellows adopts thin-walled + wear-resistant ceramic coating, which not only ensures flexibility, but also can withstand particle erosion. One of our customers used 8 sets of preheaters at the outlet, and they haven't been changed for three years. Before, ordinary expansion joints had to be changed every 8 months on average.
What about the desulfurization industry? The flue gas temperature is not high, but it is extremely corrosive, and the vibration comes from the booster fan and slurry circulation pump. A lot of people will useDesulfurization flue gas baffle door to cut off, but the pipe connection still needs an expansion joint. It used to be usedNon-metallic expansion joints (fabric fiber expansion joints), corrosion resistant but short life, generally two years to change. Now change to shock-absorbing thin-walled metal expansion joint, the material is replaced with 316L or duplex steel, the wall thickness is 1.0mm, and the life span easily reaches more than 5 years. Moreover, the metal material is not afraid of ultraviolet rays and ozone, and there is no need to worry about aging for outdoor installation.
Selection and installation: Should the tie rod be disassembled? How to install the deflector? These Details Determine Life
As soon as many people getShock-absorbing thin-walled metal expansion joint, regardless of the three seven twenty-one when installing, putThe expansion joint tie rod was removed directly. It was a fatal mistake. The role of the tie rod is to prevent the bellows from being overloaded during transportation and installation, but not all products require removal after installation. If it isLarge tie rod expansion joint orThe double hinge transverse expansion joint, the tie rod is a permanent structural part, used to bear the thrust of the pipe, and the bellows will be pulled out. For the shock-absorbing thin-walled type, usually the tie rod is temporarily fixed and must be dismantled after installation-but note that the pipe support must be adjusted in place before dismantling, otherwise the bellows will bear additional radial force. aboutHow to adjust the tie rod nut of expansion jointOur FAQ has detailed instructions: first loosen the locking nut, then adjust the length of the tie rod to make the bellows free, and finally lock it. Don't come up and screw with a wrench.
Let's talk about the deflector. The direction of the guide tube must coincide with the flow direction of the medium, and the direction of the arrow is the flow direction. Many on-site workers don't look at the arrows and install them backwards. As a result, the guide tube becomes a "blocking tube", and the medium directly washes the root of the bellows.The specific function of the expansion joint guide tube is not only guiding, but also silencing. Because the thin wall of the shock-absorbing thin-walled expansion joint is thin, it is easy to produce wind sound. If the gap of the guide tube can be controlled at 1-2mm, the airflow noise can be greatly reduced. We suggest that the gap between the guide tube and the inner wall of the bellows should be 0.5%-1% of the diameter of the bellows. If it is too small, it will jam, and if it is too large, it will lose its effect.
And one more thing,The shock absorbing thin-walled metal expansion joint requires very high installation coaxiality. Because of the thin wall, the lateral displacement capacity is limited. If the pipe support is a rigid support, it is best to add a spring hanger near the expansion joint, which can absorb the vertical vibration and let the bellows only bear axial and slight angular displacement.
Common Myth: Thin Wall Equals Not Durable? Shock absorption is soft? Don't be taken off by layman's words
"It's so thin that it will break when you touch it, right?” That's what laymen say the most. But you should know that the fatigue life of metal bellows depends on the stress amplitude, not the wall thickness. Under the same pressure, the bending stress generated by thin walls is less-because thin walls are more likely to deform and the stress distribution is more uniform. For example: 1.0mm thick 304 stainless steel bellows, under the same displacement, its maximum stress is only 60% of that of 2.0mm thick bellows. SoShock-absorbing thin-walled metal expansion joints are not less durable, but more resistant to fatigue. Of course, the premise is that the materials and craftsmanship pass. Those that use cheap 201 stainless steel to pretend to be 304 are really easy to corrode and perforate when they are thin. That's cutting corners, not a design problem.
Another misunderstanding is that "shock absorption is soft, and if soft, it will bear poor pressure". In fact, the pressure-bearing capacity of shock-absorbing thin-walled expansion joint mainly depends on the wave number and layer number of bellows, rather than the wall thickness. For example, the pressure bearing capacity of double-layer 0.8mm stainless steel is comparable to that of single-layer 2.0mm. Moreover, the multi-layer structure has another advantage: once there are micro-cracks in the outer layer, the inner layer can continue to bear pressure, which is equivalent to double insurance. That's why we haveCompound straight pipe bypass pressure balance type expansion joint andStraight pipe pressure balance expansion joints are essentially multi-layer designs, but the shock-absorbing thin wall optimizes the interlayer damping on this basis.
It is considered that the shock absorbing thin wall is only suitable for normal temperature and low pressure. Wrong. InIn the high temperature axial expansion joint, if Inconel 625 or 825 nickel-based alloy is selected, the thin-walled structure can still maintain good elasticity and fatigue resistance at 800℃ environment. On the contrary, thick-walled corrugated pipes are more prone to cracking due to concentrated thermal stress at high temperatures. Therefore, don't be scared by the word "thin", choose the right material and structure, and the shock-absorbing thin-walled metal expansion joint is the optimal solution under high temperature, corrosion and high frequency vibration conditions.
Do pipeline design, if you encounter projects with high frequency vibration, limited space and high weight reduction requirements, don't hesitate to go directlyShock-absorbing thin-walled metal expansion joint. The saved bracket cost and maintenance cost are enough for you to replace several sets of expansion joints.