Do Half-Arc Metal Expansion Joints Really Fit Your Pipes? Five Questions Self-Test First
Choose the expansion joint. It's simple to say, but it's really difficult to say. Especially the semi-arc metal expansion joint. In the past two years, many people have asked about it, but few people really understand it. Someone placed an order with the words "high pressure resistance", and turned back to the installation site with a dumbfounded face. Don't worry, answer the following five questions before deciding whether you want to use it or not.
1. What exactly does "half-arc" mean? Crest or section?
Many people heard "semi-arc" for the first time, and subconsciously thought that the pipeline section was made into a semi-circle. Wrong. The corrugated shape of the semi-arc metal expansion joint-the circle of the wave crest-is not a complete U-shape or an omega-shape, but a transitional profile in between. To put it bluntly, looking from the side of the ripple, the crest of the wave carries an arc transition, like a half arc lying on the trough, so it is called "half arc".
This shape has nothing to do with the pipe section, which is still round. If you take a ruler to measure the height and pitch of the wave crest, you will find that it is obviously different from ordinary U-shaped ripples. When selecting, don't just look at the picture, you have to look at the waveform code in the technical parameters.
Second, why is semi-arc corrugation more pressure-resistant than ordinary U-shaped?
There's a stress distribution problem here. At the transition between crest and trough of U-shaped corrugation, the curvature radius is small, and the stress concentration is particularly obvious. Once the pressure came up, those few points were the first to be unable to bear. The radius of curvature in the transition zone of the semi-arc corrugation is larger, the stress is dispersed widely, and the internal pressure can be withstood at the same wall thickness is higher.
Let's use the data to speak. It is also a stainless steel corrugated pipe with DN300, 5 waves and a wall thickness of 1.5mm. The design pressure of U-shaped corrugation is about 1.0MPa, and the half-arc can reach 1.6MPa or even higher. What about stiffness? The axial stiffness of a half-arc corrugation is typically 10 to 20 percent lower than that of a U-shape, meaning that there is less thrust on the pipe support when absorbing displacement. Do you think this thing is more suitable for high pressure pipelines? However, it should be noted that low stiffness does not mean that it can be used casually, and the algorithm of fatigue life is different.
Third, the selection can't only look at the compensation amount: how to weigh pressure, temperature and fatigue life?
"Can you make me a 500mm one?" Of course, the amount of compensation is important, but only focusing on the amount of compensation, sooner or later, the car will flip over. The compensation ability of semi-arc metal expansion joint is proportional to the wave number, but the more wave number, the lower the column instability pressure-this is a law that structural mechanics can't avoid.
How to weigh pressure, temperature and fatigue life? Give you an idea: first set the design pressure and temperature, determine the materials and the number of layers; Then calculate the thermal displacement and installation error, and fix the wave number; Finally, the fatigue life is checked. For example, a semi-arc metal expansion joint is used on the steam pipeline of the power station, with a working pressure of 2.5MPa and a temperature of 400℃. If it is made of 304L material, the single wave compensation amount can reach about 15mm, and the service life can reach about 1000 cycles according to GB/T 12777 standard. If it is changed to 316L, the temperature resistance and intergranular corrosion resistance are better, but the price is about 20% more expensive. How to choose? Look at the corrosion allowance of your pipes.
There is another point that many engineers will ignore-the effect of stiffness on the pipe rack. The stiffness of the semi-arc metal expansion joint is low, the thrust transmitted to the fixed bracket is small, the bracket can be made lighter, and the civil construction cost is saved a lot. This account has to be settled.
4. Several details of the easiest rollover at the installation site
To be honest, no matter how good the expansion joint is, it will still leak if it is installed wrong. Semi-arc metal expansion joint At the installation site, there are three common pits.
First, the direction of the guide tube is reversed.The role of the guide tube is to guide the flow of the medium and reduce vortices and scour. On the horizontal pipeline, the direction of the guide tube is consistent with the flow direction of the medium; On the vertical pipe, the direction of the guide tube should be upward to prevent condensate from accumulating in the corrugations. Just wait for wear and vibration when installed backwards.
Second, the tie rod nut is not correctly adjusted.The tie rod nut is used to limit the displacement when the semi-arc metal expansion joint has a tie rod. The nut is locked when it leaves the factory. After installation, it is necessary to adjust the nut to the appropriate position according to the actual required expansion and contraction amount. Some people try to save trouble directly, the expansion joint becomes a rigid part, the corrugation deformation is too large, and it will crack after a long time. Others have tightened the nut too tightly, completely limiting the displacement of the corrugation, so why do you need an expansion joint?
Third, pre-deformation is not done.When the installation temperature of the pipeline is much different from the design temperature, it must be pre-deformed. For example, the design temperature is 300℃ and the installation temperature is 20℃. The bellows is in a free state when they leave the factory. You have to stretch or compress it for a certain distance before welding it to the pipeline. In this way, the displacement of the bellows is within the safe range during hot operation. How to calculate the amount of pre-deformation? Subtract the natural compensation amount per meter of pipeline thermal expansion, and divide the rest by the wave number, which is the size of pre-deformation required for a single wave. This calculation cannot be saved.
V. Semi-arc metal expansion joint vs. non-metal expansion joint: who has the final say?
Some people compare half-arc metal expansion joints with non-metal expansion joints and ask which one is better. The question itself is wrong-there is no better one, only the more appropriate one.
The medium plays a decisive role. If it is smoke, dust, or corrosive gas containing acid and alkali, non-metallic expansion joints (such as fabric fiber expansion joints) are the right solution, because they are corrosion resistant, high temperature resistant, and can absorb multi-dimensional displacement. If you take the metal one to connect the desulfurization flue gas pipeline, it will be corroded in half a year.
In terms of temperature, the energy saving of non-metallic expansion is above 1000℃, and the semi-arc metal expansion joint is generally about 800℃ even if superalloy (such as Inconel 625) is used. However, the pressure-bearing capacity of metal is incomparable to that of non-metal-the design pressure of non-metal expansion joints usually does not exceed 0.1MPa. If you insist on it to bear 1.6MPa, it is a joke with your life.
The displacement type cannot be ignored either. The semi-arc metal expansion joint mainly absorbs axial displacement, and the transverse and angular displacement is realized by hinge type and compound hinge transverse type. What about non-metallic expansion joints? They are naturally flexible and can move in three directions. So you see, there are many non-metals in low-pressure and large displacement occasions such as air ducts and flues; Steam, high-pressure medium pipeline, or semi-arc metal expansion joint is reliable.
Finally, let me give you a judgment logic-look at the medium and pressure first, then talk about temperature and displacement. The medium is non-corrosive, the pressure exceeds 0.1MPa, and the axial displacement is the main one. The semi-arc metal expansion joint is preferred; On the contrary, low pressure, high temperature, corrosive media, non-metallic expansion joint is the best choice.
At the end of the day, expansion joints are not standard pieces, and each one is custom made. Next time you select a model, go through these five questions and talk to the manufacturer. No one can fool you.