Can the expansion joint carry bending moment at all? Things the designers didn't tell you
Two days ago, I met a customer, and without covering the heat of the drawings, he threw over: "Can this expansion energy saving withstand bending moment?"-I guess you have been asked a similar question. The answer is not simply "yes" or "can't", but many designers just wait for you to give a quick word. Today, let's puncture this layer of window paper and talk about what bending moment means to the expansion joint, and why this thing is the most common pit in model selection.
Find out first: The job of expansion joint is not to carry bending moment
The original design of the expansion joint is to absorb the axial displacement, transverse displacement and angular displacement caused by thermal expansion and contraction of the pipe. The bellows itself is a flexible, thin-walled structure — think of it like the bellows of an accordion, which stretches with a gentle pull, but if you push it on the side, it immediately deflates. Explained by the concept of stiffness (for details, refer to the question and answer "Stiffness and Calculation Formula of Bellows" of this site), the bending stiffness of bellows is only a fraction or even lower than the axial stiffness. Once the bending moment exceeds the design range, it can lead to instability of the bellows (local bulge or depression), or directly tear the weld in the worst case. Therefore, letting ordinary expansion joints carry bending moments is equivalent to letting gymnasts lift weights-the professional is not right.
Where the hell does that bending moment come from? Pipeline vibrations, wind loads, earthquakes, support settlements, and even installation errors can produce bending moments. Some designers mistakenly believe that as long as an expansion joint is installed, the pipe can "automatically adapt" to all external forces. Alas, this idea is more brittle than paper.
Some expansion joints are inherently bending moment resistant materials
But then again, there are indeed expansion joints on the market that are specifically designed for bending moments. Such asCompound hinge transverse expansion jointIt converts the bending moment into a force couple through two sets of hinges, and the bellows only bear pure lateral displacement;Curved tube pressure balance expansion jointAndThe straight pipe pressure balance expansion joint uses the pressure balance structure to cancel out the internal pressure and thrust, thus allowing the bellows to remain stable when subjected to bending moment. These products have to be treasures when selecting models-you let them carry bending moments, and not only do they not lose their temper, but they do a good job.
In turn, some expansion joints are inherently "soft persimmons":Universal corrugated expansion joint、External pressure single type axial type expansion joints, they have no limiting structure or hinge inside, and the bellows are directly exposed. You let them carry bending moments? That's not trust, that's murder. Remember: Expansion joints without limiting devices can't bear bending moments, even if the designer patts his chest and says "it should be fine".
The Biggest Myth of Field Installation: Expansion Joints Are Not Fixed Brackets
I have seen too many sites where workers use the expansion joint as a fixed bracket-the pipe is not equipped with a guide bracket, and it is all relied on the expansion joint to carry it hard. And the result? The bellows is twisted and twisted, the flange surface is skewed, and finally the medium leaks before going back to the manufacturer. In practice, if there is additional bending moment in the pipeline system (such as vibration, wind load, earthquake), guide brackets or tie rods must be provided on both sides of the expansion joint. This site's question and answer "How to adjust the tie rod nut of expansion joint" and "Correct installation method of large tie rod expansion joint" make it clear: the function of tie rod and screw is to limit excessive displacement and prevent the bellows from being pulled out or crushed to death, instead of bearing bending moment. The tie rod itself is a rigid connection. If the bending moment acts directly on the tie rod, the tie rod will also deform and fail. Therefore, we must distinguish the difference between "limit" and "load-bearing" when installing.
Tsk, at this point, you may ask, "Then I installed the expansion joint with the tie rod, so I don't have to worry about the bending moment?" The answer is: the tie rod can only limit the excess axial displacement, but can't eliminate the direct effect of bending moment on the bellows. Only structures such as compound hinges can convert bending moments into force couples.
Real Case: Lessons from Cement Plant
Last year a cement factory approached us and said they usedMetal Corrugated Expansion Joints in Cement IndustryAs a result, after half a year of operation, the welding seam cracked. When we looked at it in the past, the root of the problem lies in the horizontal pipe-the lateral bending moment was not calculated in the design, and the pipe's own weight plus the wind load twisted the bellows into a twist. Later replaced withCompound hinge transverse expansion jointAnd adding guide brackets to the pipe, the problem was completely solved. This case tells us: When selecting the type, we must submit a complete pipeline stress analysis report, and don't expect the expansion joint to be packed to the end. The displacement, force and moment of each point will be indicated in the stress analysis report, so that the manufacturer can match the appropriate structure accordingly. If you throw over a simplified diagram and say, "You look at it," that's laying a mine for yourself.
To sum up a sentence: Don't pat your head, look for a manufacturer
Can it resist bending moments? Let's see what type of expansion joint it is first. If you are not sure, find the manufacturer, put out the working conditions, stress analysis reports and installation environment of your pipeline, and let professional people do professional things. There are so many models in this product list, from general purpose to pressure balance, each with its own temper-Universal corrugated expansion jointThe softest temper can only walk displacement;Compound hinge transverse expansion jointAndCurved tube pressure balance expansion jointYou can chew hard bones. Remember: the words "probably" and "probably" are ticking time bombs in engineering. You asked me can the expansion joint withstand bending moment?
Look at the model, look at the structure, see if there is a limit device. Don't ask if you can, ask if you should let it carry it first.