Find out first: Are diameter and caliber the same thing?
Many people think that the pipeline of DN100 is equipped with the expansion joint of DN100, and they can be bought and installed. If you really do this, there will be a problem nine times out of ten. The diameter (DN) is the nominal diameter, and the pipe diameter is the actual inner diameter. The two basically correspond to the standard parts, but the structure of the expansion joint-the guide tube, the wall thickness of the bellows, and the flange sealing surface-will eat a part of the circulation area. For example,Corrugated expansion joint for power plant industryIf it is equipped with a larger guide tube, the actual flow capacity will be different from that of a straight tube. You think the DN200 tube with the DN200 expansion joint can pass the same flow? As soon as the guide tube shrinks in diameter, the flow rate directly soars. So, don't use DN as the inner diameter.
The core logic of pairing: flow rate, pressure, displacement, one cannot be less
If the diameter is large, the medium flow rate will of course be low, but the equipment cost will rise upside down, and the flange, bellows and guide tube will all have to be enlarged by one. What about choosing small? If the flow rate is too fast, it will erode the bellows, and in severe cases, it will directly cause vibration. Last year, the steam pipeline of a chemical plant was equipped with DN250 expansion joint in order to save money. As a result, the medium flow rate rose from 15m/s to 22m/s, and the bellows were worn out in three months-replacing it once is enough for you to buy three pairs.
The pressure level and the amount of displacement will also, in turn, constrain the diameter.High temperature axial expansion jointThicker bellows are required, and the actual inner diameter will shrink under the same DN, so it must be recalculated according to the effective cross-sectional area. For example, for the same DN200, the inner diameter of the normal temperature type may be 198mm, while the inner diameter of the high temperature type may only be 190mm due to the increase of wall thickness, and the flow area difference is 8%. If you don't recalculate, the flow rate will be biased.
Difference in diameter selection of expansion joints made of different materials
Metal expansion joints (e.g.Universal corrugated expansion joint、Large diameter thick wall expansion joint) andrubber compensator、Non-metallic expansion joints (fabric fiber expansion joints)The matching method is completely different. Metal parts are rigid, and the diameter usually goes in standard series, but if you useCompound hinge transverse expansion jointIn this structure with hinges, if the diameter is large, extra bending moment will be generated at the flange connection, and the hinge is easy to deflect over the limit.
The rubber compensator has good elasticity but limited pressure resistance, and its large diameter is easy to bulge-especially in large diameter and low pressure conditions. The rubber layer expands outward after being subjected to internal pressure. The larger the diameter, the higher the risk of bulging. What about non-metallic expansion joints? Such asRectangular non-metallic expansion jointIt is mostly used for rectangular smoke ducts, and the diameter needs to be matched with the hole spacing of flange bolts, so the round pipe DN cannot be copied. If you take the round pipe formula of DN1000 to calculate the rectangular flue, the bolt holes don't match, and the site has to be re-drilled.
Common mistake: What happens if the diameter is matched small? What's the pit if it's big?
Two days ago, a customer equipped DN200 pipeline with DN150 expansion joint, and the medium flow rate doubled, and the bellows were worn out in three months-as mentioned earlier. On the other hand, the diameter is larger, although the safety factor is higher, butCompound hinge transverse expansion jointIn this type of hinged structure, additional bending moment will be generated at the flange connection, which can easily lead to deflection exceeding limit. In addition, if the flange is large, it may make the flange wrong-if you take the flange of DN200 to the bolt hole of DN250, it will have to be reprocessed on the spot, and the construction period and cost will all be included.
The gap between the guide tube and the bellows. After the diameter is enlarged, if the gap between the outer diameter of the guide tube and the inner diameter of the bellows is not well controlled, the medium will form vortex in the gap, which will accelerate corrosion. EspeciallyDesulfurization flue gas baffle doorThis kind of used in wet flue gas environment, the corrosion margin is not enough, and it will be perforated in half a year.
Practical Suggestions: Three Steps of Model Selection
The first step is to confirm the actual inner diameter and wall thickness of the pipeline, and calculate the effective circulation area. Don't just look at DN, take a caliper to measure the actual inner diameter of the pipe-especially old pipes, which may have scale or corrosion on the inner wall, and the actual diameter is a section smaller than the nameplate.
In the second step, according to the medium (steam, flue gas, corrosive liquid) and temperature and pressure, find the corresponding type of expansion joint from the product information. For example, a wet smoke environment, withDesulfurization flue gas baffle doorOrNon-metallic expansion joint (fabric fiber expansion joint)The diameter should leave a corrosion margin; For steam linesHigh temperature axial expansion jointThe effective cross-sectional area should be recalculated after the wall thickness is thickened.
The third step is to check the path comparison table provided by the manufacturer. This site hasMetal hose size comparison tableAndMetal expansion joint weight tableCheck flange standards (HG/T 20592, GB/T 9119, etc.). Finally, it is best to let the manufacturer calculate the flow field with finite element-don't save this step. Many manufacturers provide it for free. You only need to send the media parameters and pipe dimensions. They produce a calculation book, which is much more accurate than patting your head yourself.
If you useDirect buried (fully buried) type expansion jointOrDouble hinge expansion joint for air-cooled island vacuum pipelineFor this kind of special structure, the stability of external pressure should also be considered, and the relationship between diameter and wall thickness is more complicated. Go directly to the technical manufacturer for the type selection calculation book-don't be too troublesome. The cost of replacing the expansion joint once is broken is enough for you to buy three pairs.
sum up
Choose the right diameter and double the life of the equipment. The diameter matching is not simple 1:1, and needs to consider the flow rate, medium, structure and installation comprehensively. If you are unsure, flip through this siteMetal hose size comparison tableOrMetal expansion joint weight tableOr directly find the manufacturer to issue a calculation book. Remember: the cost of replacing an expansion joint is enough for you to buy three right ones.