Why do you have to know the expansion joint weight? Don't wait for your purchase to roll over to regret it
Two days ago, I met a customer and ordered a batch of large-diameter thick-walled expansion joints. As a result, during the installation, it was found that the hoisting equipment was selected small-30% overweight. Not to mention the temporary addition of money to adjust the crane on site, the construction period was delayed for three days. This kind of thing is really not unusual in the engineering circle. The weight of the metal expansion joint is not "about the same" data, it directly determines the transportation cost, installation plan, bracket load, and even flange bolt specifications. If you engage in design or procurement, you can't calculate the weight, from the light to the supplement freight, to the worst, the stress of the whole pipeline system exceeds the standard, and the bellows fatigue in advance. Therefore, whether you are a newcomer or a veteran, it is a basic skill to thoroughly understand the formula for calculating the weight of metal expansion joints.
Where does the weight come from? Algorithm of disassembling bellows, flange and guide tube
The weight of the metal expansion joint is, to put it bluntly, three big pieces:Bellows、flange、guide tube。 Sometimes there are tie rods, nuts and external pressure sleeves, but the main body is just these three things. Let's dismantle them one by one.
Bellows weight: Density by volume. Bellows are annular folds pressed out by thin-walled cylinders, and the volume is not easy to calculate directly? There is an engineering simplification formula:W wave = (π × Dm × t × L × ρ) /1000000, where Dm is the average diameter (inner diameter + wall thickness), t is the wall thickness, L is the total length of the bellows (excluding straight edge segments), and ρ is the material density (about 7930 kg/m³ for 304 stainless steel, 7850 for carbon steel). In actual calculation, the expansion coefficient of peak and trough should also be considered, which is usually 1.1~1.25. This coefficient depends on the wave height and wave distance. If you choose a large coefficient, it will be heavy. If you choose a small coefficient, you will find that it is not enough when you install it.
Flange weight: Don't be fooled by the flange's disc. Calculate according to the volume of the ring-the area of the outer circle minus the area of the inner circle, then multiply the thickness, multiply the density. Formula:W = (π/4) × (D outer ²-D inner ²) × h × ρ. Note that the inner diameter of the flange should be calculated according to the nominal diameter of the pipe, not the inner diameter of the bellows. Many novices get confused here, and the result is a big difference in flange weight.
Weight of guide tubeIt is a thin-walled cylinder, usually tapered or straight. The straight tube is simplest:W derivative = π × D derivative × t derivative × L derivative × ρ。 Take the average diameter of the tapered guide tube. The function of the guide tube is to protect the bellows from being washed by high-speed fluid, but with this piece, the weight is at least 5% ~10% more. For example, when we make metal corrugated expansion joints in the cement industry, the guide tube is often a must, so don't miss the calculation when calculating the weight.
What is the difference between different models? Take general-purpose type, high-temperature axial type and large-diameter thick-wall type as examples
The same expansion joint of DN500 can be twice the difference in weight between models. Believe it or not?
- Universal corrugated expansion joint: The most standard structure, single layer corrugated pipe + flat welded flange, the weight is mainly concentrated in flange. DN500 PN10 universal type, 304 material, bellows length 400mm, calculated to be about 80~90kg.
- High temperature axial expansion joint: In order to withstand the high temperature above 700℃, the bellows has to use a double-layer or even three-layer structure, with a heat insulation layer sandwiched in the middle, and the guide tube is also lengthened. Similarly, DN500, the weight jumps directly to 130~150kg. Why? Because the wall thickness is increased from 1.5 mm to 3 mm, the number of layers is doubled, and the density is unchanged but the volume is larger.
- Large diameter thick wall expansion jointFor example, above DN2000, the wall thickness of bellows may reach 6~8mm, and the flange thickness is even more amazing. A while ago, there was a desulfurization flue gas baffle door project, and the single weight of the supporting large-diameter thick-walled expansion joint exceeded 2 tons. Calculate this kind of large piece, the deployment coefficient of the bellows has to be measured and corrected, otherwise the error of manual calculation will make you doubt your life.
You see that? If you want to conquer the world with a universal weight gauge, you will flip over sooner or later. It must be calculated item by item for the specific structure.
Calculating weight can inverse material thickness and stiffness? This technique is used by veterans
Here is a practical reverse thinking-if you have the weight and dimensions of an expansion joint on hand, you can estimate the wall thickness and number of layers of the bellows in turn. Because the weight ratio of flange and guide tube is basically fixed, subtract these two pieces from the total weight, and the rest is the weight of bellows. Then according to the outer diameter, wave number and length of the bellows, the wall thickness is inversely solved. This technique is especially useful when surveying old equipment or replacing spare parts. For example, if you have an external pressure single axial expansion joint, the nameplate is gone, and you don't know the parameters, so weigh it-assuming that the total weight is 85kg, and the flange and guide tube are estimated to be 20kg, then the bellows weighs 65kg, with a known outer diameter of 600mm, 8 wave numbers and a total length of 350mm. By substituting it into the formula, the wall thickness can be calculated to be about 2mm. Then the stiffness can be inverted by the formula in the FAQ of "Stiffness and Calculation Formula of Bellows". Is that the truth?
Too much trouble with your hands? I'll give you a practical case and follow it through
Okay, just talk and don't practice fake hands. We're really one. Suppose you need to purchase aHigh temperature axial expansion joint, nominal diameter DN400, design pressure 1.0MPa, material SUS304. Known parameters: the inner diameter of the bellows is 406mm, the wall thickness is 2.5mm, the wave height is 35mm, the wave pitch is 25mm, the wave number is 10, the length of each straight edge section is 30mm, and the total length is 530mm. Flange model: DN400 PN10 flat flange, outer diameter 565mm, inner diameter 410mm, thickness 28mm. With guide tube: inner diameter 400mm, wall thickness 2mm, length 200mm. Count the total weight.
- Bellows weight: Average diameter Dm = (406+2.5) =408.5 mm. Effective length of bellows L =530-30×2=470 mm. Bellows volume = π ×408.5×2.5×470×1.18 (take the deployment factor as 1.18) ≈ 2,226,374 mm³ =0.002226 m³. Weight =0.002226×7930 ≈ 17.65 kg.
- Flange weight: Area = π/4× (565²-410²) =0.7854× (319,225-168,100) =0.7854×151,125=118,697 mm² =0.1187 m². Volume =0.1187×0.028=0.003324 m³. Weight =0.003324×7930 ≈ 26.36 kg. Note that this is the weight of a single flange, there are two flanges, so ×2=52.72 kg.
- Weight of guide tube: The average diameter is 400mm (ignoring the wall thickness). Area = π ×400×200=251,327 mm² =0.2513 m². Volume =0.2513×2=0.5026×10⁻³ m³? Wait, unit conversion: area mm² multiplied by wall thickness mm to get mm³, and then convert to m³. Direct calculation: π ×400×2×200=502,654 mm³ =0.0005027 m³. Weight =0.0005027×7930 ≈ 3.99 kg.
- Total weight=17.65+52.72+3.99 ≈ 74.36 kg. Add tie rods and nuts (about 1~2kg) as appropriate, and the final report is about 76kg.
You see, there is a basis for every step. If the site size allows, you can also use this result to back-calculate the material thickness to confirm whether the design margin is sufficient.
Tsk, it does take a bit of effort to calculate by hand, but I understand it. After changing the model and material in the future, I can calculate it by mouth in three minutes. Faster than taking an Excel and knocking it hard. Of course, if you really don't want to calculate manually every time, most general-purpose, high-temperature axial and large-diameter thick-walled expansion joints in the product information page of this site have weight reference tables attached, but remember: the reference table is a typical structure, and for special configurations (such as adding guide tubes and multi-layer bellows), you must ask the manufacturer for the final data.
Calculate the weight clearly, purchase without rollover, install without scratching. The formula for calculating the weight of metal expansion joint is really not difficult, but the difficulty is whether you are willing to calculate it again.