How to read axial expansion gauge of metal expansion joint? These 4 numbers are most easily overlooked when selecting a model
Get a metal expansion joint axial expansion gauge, which number do you look at at first glance? Compensation amount? Don't worry, just move your gaze to the far left column —Nominal diameter, wave number, axial compensation, stiffnessThese four numbers are the vital gates that determine whether the pipeline can "breathe" safely. Taking the general corrugated expansion joint and the high-temperature axial expansion joint of this station as examples, there is a design logic behind each value in the table: the larger the nominal diameter, the larger the effective area of the bellows, the greater the blind plate force generated under the same pressure, and the compensation amount is limited. When you get a table, the first thing you look at should be the wave number, not the compensation number. Wavenumber determines the limit of elasticity-if you don't believe me, turn it overExpansion joint model and sizeIn that section, the stiffness decreases with more wave numbers, but the fatigue life may not be better. The more the wave number, the smaller the deformation amount of a single corrugation, which is beneficial to low cycle fatigue; However, if the internal pressure is high and the wave number is large, it is easy to become unstable. When selecting the model, look at the wave number first, and then talk about compensation.
Similarly DN200, why does Factory A give ±50mm, and Factory B only gives ±30mm?
The key to this difference is that the bellowsNumber of layers and material thickness。 This site'sExternal pressure single axial expansion jointWith the multi-layer bellows design, the axial stiffness difference between the single layer thickness of 0.5mm and the double layer of 0.3mm is more than double. There are also pressure levels-the wall thicknesses of bellows of 0.6MPa and 1.6MPa are different, and the compensation amount between the outside and the inside is the design value at room temperature. As soon as the actual working temperature goes up, the compensation ability will be discounted by 20%. Two days ago, I met a customer who took the room temperature meter to set the steam pipe. As a result, it leaked three months after installation. Tsk, this is because I didn't look at the temperature coefficient. When the temperature exceeds 100℃, the elastic modulus of the material decreases, so does the allowable stress, and the compensation amount must be multiplied by the temperature correction coefficient (about 0.8~0.9 for carbon steel and 0.7~0.85 for stainless steel). Don't just stare at the DN200 numbers, find out your pressure level and medium temperature first, otherwise the selection is just a chance.
How do you match the numbers on the table with the field displacement?
The compensation amount on the axial expansion gauge is ±50mm. How long can the pipeline extend on site? First calculate the thermal elongation of the pipe: temperature difference × pipe length × linear expansion coefficient (carbon steel about 0.012mm/m·℃). For example, a 10-meter steam pipe, with a temperature rise of 200℃, has a thermal elongation of 24mm. Is it enough to choose an expansion joint with axial compensation ≥24mm? Wrong. Also considerguide tubeFunction-the guide tube will occupy part of the effective area, especially the axial displacement is limited. The inner diameter of the guide tube is one turn smaller than the inner diameter of the bellows. When the axial compression is excessive, the trough may hit the end of the guide tube, and the actual available compensation amount should deduct the interference space of the guide tube. In additionInstallation pre-stretch amountAlso have to be counted, refer to this siteHow to adjust the tie rod nut of expansion jointIn that question and answer, cold pulling half the thermal displacement is the standard operation. For example, thermal elongation is 24mm, and pre-stretching is 12mm during installation, so that the bellows works in the middle position during operation, and the fatigue life is the longest. Don't be stupid and directly take the compensation amount to benchmark the thermal elongation. If the pre-stretching is not done, the bellows will have problems sooner or later.
The most easily overlooked in model selection: applicable criteria and cycle life
The domestic national standard GB/T 12777 for corrugated expansion joints stipulates the corresponding relationship between test pressure and displacement, but the tables given by many manufacturers are theoretical calculations, without considering the cycle life. Do you choose 10,000 or 1,000 lifespan? Table data is completely different. This site'sCorrugated expansion joint for power station industryAndMetal Corrugated Expansion Joints in Cement IndustryAll of them have been de-rated for high-frequency vibration conditions-the external and internal compensation is only 60% of the standard value. Remember one sentence: the figures on the table are ideal laboratory data, and the actual working conditions should be factored, especially for pipes with vibration and water hammer. The instantaneous impact pressure generated by the water hammer may reach 2~3 times of the working pressure, and the axial displacement of the bellows will instantly exceed the limit at this time. Therefore, when selecting the model, don't only look at the compensation amount at normal temperature. Ask the manufacturer: How many times of fatigue life does this compensation amount correspond to? Is it 1000 times or 10000? Usually, the higher the cycle requirement, the lower the allowable compensation amount, and a discount in half is the norm.
Practical skills: displacement direction marking and critical pressure
When you get the axial expansion gauge, look at the displacement direction mark first. Some manufacturers use "axial" to refer to the compression direction, while some write "stretching". In fact, the standard stipulates that the positive value is stretching and the negative value is compression. This site'sHigh temperature axial expansion jointThe positive and negative directions are clearly marked on the product page, so be sure to distinguish between them when using them with tie rods-if they are installed backwards, the bellows may become unstable during compression. The function of the tie rod is to bear the blind plate force, but it does not limit the axial displacement, so the tie rod nut should be adjusted in place during installation, see the specific operationHow to adjust the tie rod nut of expansion joint。 Also, the "critical pressure" that is usually not indicated in the table. Did you forget it? When the internal pressure exceeds a certain value, the axial compensation amount will drop rapidly. This data has to be asked for the bellows stability calculation book by the manufacturer. The critical pressure depends on the geometric dimensions of the bellows (wave height, wave pitch, wall thickness) and the material yield strength, which is generally obtained by finite element analysis. Don't just look at the watch. Behind the watch is the game of working conditions-pressure, temperature, number of cycles, and corrosiveness of the medium. If any variable changes, the number on the watch becomes waste paper. Therefore, in the next selection, the four numbers (nominal diameter, wave number, axial compensation amount, stiffness) are tied together with the working condition parameters, so that the pipeline can breathe safely.