What exactly is cross-sectional area? Don't confuse it with the path
Many novices in pipeline design equate the cross-sectional area of the expansion joint to the circular area corresponding to the inner diameter of the pipeline-a big mistake. The effective cross-sectional area of bellows directly determines the thrust generated by the expansion joint under the action of internal pressure, and this thrust will affect the selection of fixed bracket and guide bracket. You are too small, and the bracket is deformed; It's too big, a waste of steel.
The corrugation shape, wave height and wave pitch of the bellows will shift the effective cross-sectional area. For example, the actual effective cross-sectional area of a DN800 bellows may be several percentage points larger than π × (800/2) ². Simple estimation: effective cross-sectional area ≈ π × (mean radius) ², and the mean radius takes the middle value of the crest and trough. For example, the crest radius is 410mm, the trough radius is 390mm, the average radius is 400mm, and the cross-sectional area is ≈ 3.14×0.4² =0.5024m². But if directly calculated according to the inner diameter of the pipe of 400mm, the cross-sectional area is the same-this is just a coincidence. Switch to a bellows with a larger wave height, and the deviation will come out.
How does cross-sectional area affect stiffness and compensation capacity?
The larger the cross-sectional area, the smaller the stiffness? Wrong, it's a positive correlation. The larger the cross-sectional area, the larger the axial force generated under the same pressure, but the single-wave stiffness does not necessarily increase linearly. Here, the plate and shell theory of material mechanics is involved. Simply put, the wall thickness, wave height and wave pitch of corrugated pipe together determine the stiffness, and the cross-sectional area is only one of the variables.
Take this site'sUniversal corrugated expansion jointAndHigh temperature axial expansion jointIn contrast, with the same diameter DN300, the wall thickness of the high-temperature type increases from 2mm to 3mm because of heat resistance, and the cross-sectional area changes little (the wave height is almost unchanged), but the single-wave stiffness directly doubles. What about the amount of compensation? For bellows with large cross-sectional area, if you want to maintain the same compensation amount, either increase the wave number or adjust the wave height. For example, commonly used in the cement industryMetal Corrugated Expansion Joints in Cement IndustryThe diameter is 800mm, the wave height is 70mm, and the cross-sectional area is 15% larger than that of the ordinary type. The compensation amount is enough, but the axial thrust also goes up-the rollover case will be talked about later.
Cross-sectional area trade-offs under different working conditions-you can't want both
Two days ago, I met a customer in the cement industry and chose one with a diameter of 800mmMetal Corrugated Expansion Joints in Cement IndustryAs a result, the on-site pressure test found that the bracket was deformed. After checking, the cross-sectional area is calculated as small, and the actual thrust is 30% larger than the design value. Why? He calculated the cross-sectional area directly according to the nominal diameter, without considering the wave height and pitch of the ripples. The average diameter of the actual bellows is nearly 20mm larger than the nominal diameter, and the thrust naturally exceeds the standard.
Desulfurization and denitrification pipelines are more headache. useDesulfurization flue gas baffle doorWhen matched with the expansion joint, the medium contains corrosive gas, which needs to be lined with PTFE or thickened wall, and the cross-sectional area will become larger. However, what should be considered at this time is the influence of internal pressure thrust on the sealing performance of the flue gas baffle door-the thrust is too large, the baffle door is not closed tightly, and the leakage rate exceeds the standard. We came across a power plant,Desulfurization flue gas baffle doorThe thrust of the expansion joint was neglected during the selection, and the sealing surface of the baffle door was compressed and deformed. Finally, the double seal was replacedRound Flap Door (Double Seal)Only solved.
Used in power station industryCorrugated expansion joint for power station industryUsually faced with high temperature and high pressure, the design of cross-sectional area should take into account the fatigue life. The cross-sectional area is larger, the stress concentration is more serious, and the number of cycles decreases. Is that the smaller the better? Neither is it. The cross-sectional area is too small, the compensation amount is not enough, and the thermal expansion of the pipe will break the bellows. There is a balance point that usually needs to be verified by fatigue testing.
Cross-sectional area selection rollover case: a real lesson
The steam pipeline of a power plant was usedStraight pipe pressure balanced expansion jointThe design of the pressure balance chamber is calculated based on the cross-sectional area of the bellows. As a result, the manufacturer took the average value of peak and trough wrong, and the cross-sectional area of pressure balancing chamber was calculated by 8%, resulting in insufficient balancing force and extra displacement of pipeline. Finally, the limit tie rod had to be installed, which cost more than 100,000 yuan in renovation fees.
This kind of problem occurs inCompound hinge transverse expansion jointAndCurved tube pressure balance expansion jointAlso common on. The hinge-type expansion joint is subjected to the blind plate force by the hinge, which is the cross-sectional area multiplied by the internal pressure. You have calculated the cross-sectional area wrong, and the hinge is stressed by force deviation. The deflection angle is wrong at least, and the hinge breaks at worst. There was a chemical factory the year before lastCompound hinge transverse expansion jointBecause the cross-sectional area is too small, the hinge pin is cut off and the whole pipeline is scrapped.
How to quickly judge the cross-sectional area from the product data?
Don't guess. The products of this site are such asUniversal corrugated expansion joint、External pressure single axial expansion joint、Large diameter thick wall expansion jointThe effective cross-sectional area or the average diameter of the bellows are generally marked in the parameter table. If not, it can be estimated using the nominal path plus twice the wave height-but this is only an estimate, with an accuracy of ±5%.
Metal rectangular expansion jointAndNon-metallic expansion jointThe cross-sectional area of is calculated differently. The rectangle should be calculated according to the equivalent diameter, and the formula is √ (4× length × width/π). Don't directly set the formula of circular tube. Non-metallic expansion joints are more troublesome, the elastic deformation of fabric fibers is large, and the effective cross-sectional area will change with pressure. You have to find the manufacturer for measured data.
Really unsure? Ask the manufacturer directly for bellows drawings. Regular manufacturers will provide bellows waveform diagrams and geometric parameter tables. Look at the radius of the crest and trough, and calculate it by yourself. It is more reliable than anything else.