Is the expansion allowance of metal expansion joint the more the better or just right?
The word expansion margin sounds professional, but when it is disassembled, it means "keeping the surplus expansion amount". Many engineers habitually add 20% safety factor when selecting models, and as a result, they either can't be installed or scrapped in advance. The expansion margin is not determined by patting the head, it directly determines whether the bellows can fill the design life under the working conditions of high temperature, high pressure and multi-directional displacement. Think about it, after the steam pipe heats up, it elongates by tens of millimeters. If the margin is small, the bellows will be directly cracked; Too much, excessive pre-compression, accelerated metal fatigue. So this thing has to pick the details.
Three "troublemakers" are making trouble behind their backs
influenceExpansion allowance of metal expansion jointThe three biggest factors: operating temperature, medium pressure, and the cold tightness at the end of the pipe. The temperature is easy to understand. The linear expansion coefficient multiplied by the temperature difference is the theoretical elongation. However, pressure will trigger "pressure thrust", which will cause additional deformation of bellows, especially high-pressure pipes. This force can eat a large part of the balance. Cold tightening is more subtle – deliberately pre-stretched or pre-compressed during installation, with the aim of keeping the expansion joint in the middle position at operating temperatures and avoiding unidirectional limits. However, many project maps save trouble, and the amount of cold tightness is given casually. As a result, the margin all deviates. And guess what? Two days ago, I saw a steam pipeline project. The design temperature was 350℃, and the cold tightness was 30% according to experience. During actual operation, the bellows was directly pushed to death. When it was removed, the corrugation spacing was all crushed.
The margin sensitivity of different types of expansion joints is very different
Different types of metal expansion joints have completely different responses to margin. Let's sayUniversal corrugated expansion joint, mainly absorbs axial displacement, and the margin directly corresponds to the number of ripples, and one more ripples will compensate. ButCompound hinge transverse expansion jointOrThe coupling effect of lateral displacement and angular displacement should also be considered for straight pipe pressure balanced expansion joint. For example, the commonMetal Corrugated Expansion Joints in Cement IndustryThere is a lot of dust in the pipeline. If the guide tube is not well designed, the dust accumulation will jam the bellows, and the actual usable margin will be compressed by more than 30%. Let's talk about the power station industry,High temperature axial expansion jointUsed in the main steam pipe, the temperature is five to six hundred degrees, and the bellows is often made of Inconel 625, but the balance is 10% more, and the fatigue life may double-at the cost of soaring costs. Tsk, is it worth the money? Depends on the project life cycle.
Don't just focus on thermal expansion, installation errors and foundation settlement are the invisible killers
Many people only count thermal expansion and forget about installation errors and foundation settlement. I met a customer two days ago. The pipe gallery is 200 meters long, and I used severalCompound straight pipe bypass pressure balanced expansion jointIt stands to reason that the margin is enough. As a result, it leaked half a year later. When I disassembled it, the bellows was stretched to the limit. Check the reason, it turned out that the middle bracket settled by 5mm, swallowing all the balance. Therefore, when calculating the margin, it is necessary to list the displacement tolerance of the pipe support, the thrust of the equipment nozzle, and even the ambient temperature fluctuation. Don't just stare at the design temperature. What about that? It is recommended to allow a margin of 10%-15% for unforeseen displacement during calculation, but not more than 20%, otherwise the bellows is precompressed too hard, and the fatigue life will decrease.
Doorway with pre-compression installation and tie rod adjustment
In actual selection, some manufacturers will recommend the "pre-compression installation method": for example, if the axial compensation amount of 50mm is calculated, then choose the expansion joint with a rated compensation amount of 80mm, and pre-compress 30mm during installation. This not only leaves a margin, but also gives the bellows an "intermediate state". But notice,External pressure single axial expansion jointAnddirectly buried expansion jointBecause the structure is different, the pre-compression operation is not the same, you need to read the manufacturer's installation manual. The adjustment of the tie rod nut is also a doorway-the function of the expansion joint tie rod is to limit excessive displacement. If it is too tight, it will cancel the margin. If it is too loose, it will not protect the pipeline. The correct way is: loosen the locking nut after installation, let the pull rod slide freely, only play a limiting role. How exactly to adjust? You can refer to the FAQ "How to adjust the expansion joint tie rod nut" on our website.
There is no universal formula, standards are the last word
One thousand words, one thousand,Expansion allowance of metal expansion jointThere is no universal formula. Each project has to be checked according to the actual working conditions, pipe materials, and the type of expansion joint. If you are not sure, go directly to the manufacturer for a detailed calculation book, and ask them to calculate it according to GB/T 12777 or EJMA standard, which is more reliable than patting your head. After all, metal expansion joints are easy to install, but they are difficult to replace. If the pipeline is running with high-temperature steam or corrosive medium, a leak is a shutdown accident. Therefore, don't be too troublesome. It is much more saving to spend half an hour calculating the margin than to change a dozen expansions afterwards.