Understand the force direction of metal expansion joint, and reduce pits in pipeline design
Two days ago, I met a customer and called in a hurry: "The external pressure single axial expansion joint installed on the steam pipeline of our factory has only been running for three months, and the bellows is bulging, and two tie rod nuts are broken. Is there something wrong with the quality of your product?" I asked him to send a photo to see-yes, the direction is reversed. The arrow pointed east, and he pretended to be west. This is not an isolated case. If you can't figure out the force direction of the metal expansion joint, the equipment will be scrapped at least and the pipeline will crack at worst. Today, let's get this out of the way.
Axial, transverse, angular – what force is the expansion joint carrying?
Absorbs the displacement of the pipe while withstanding the internal pressure thrust. There are three types of displacement:
axial-The pipe is elongated or shortened along the centerline, for example, when the steam pipe is expanded by heat, the expansion joint is compressed;
landscape orientation— — The pipeline is offset left and right or up and down. If the center lines of the two pipes are not aligned, the expansion joint has to "bend over";
angular direction-The pipe is angularly deflected, similar to elbow bending.
These three displacements correspond to different force directions. Many designers only pay attention to the axial compensation amount, but ignore the additional moments generated in the lateral and angular directions. For example: a 30-meter-long steam pipe has a temperature rise of 200℃ and an axial elongation of about 72mm. Everyone can count this. However, if an elbow is installed in the middle, the lateral displacement caused by thermal expansion may reach more than ten millimeters, and the lateral stiffness of your general-purpose corrugated expansion joint is very low, so it can't bear that force at all. As a result, the bellows is unstable. Therefore, when selecting a model, you have to ask yourself first: Which direction of force does this expansion joint have to carry? Single carry axial Or do you have to carry axial + lateral at the same time?
Different structural types of expansion joints, what is the difference in the direction of force?
If this question is not clear, the selection is blind. Let's say one by one:
Universal corrugated expansion joint: The most basic type, can absorb axial, lateral, and angular displacements, but each ability is not outstanding. Suitable for straight pipe sections without special constraints. Note that it cannot withstand internal pressure thrust and must be absorbed by the fixed bracket of the pipe.
External pressure single axial expansion joint: Specialize in axial displacement. Structurally, the bellows is placed outside, and the internal pressure thrust is borne by the shell. The direction of the arrow during installation is the medium flow direction, which is also the direction in which the bellows is allowed to compress. Install backwards? The pressure is directly pressed against the bellows, and it bulges within a week.
Compound hinge transverse expansion joint: Consists of two bellows and a hinge structure that specifically absorbs lateral displacement, with the hinge constraining axial and angular degrees of freedom. Use on pipe sections that need to be "translated", such as smoke ducts.
Straight pipe pressure balanced expansion joint: With its own balance ring or pull rod, it can absorb axial displacement and balance internal pressure thrust, so the pipeline fixing bracket can be made lighter. Common in high temperature steam pipes.
Curved tube pressure balance expansion joint: Used on L-shaped or Z-shaped pipelines, it can absorb both axial and transverse directions, and the internal pressure thrust is self-balanced internally.
AndCompound straight pipe bypass pressure balanced expansion joint、Double hinge expansion joint for air-cooled island vacuum pipelineThe structure of each of these special products determines its stress characteristics. When selecting the model, you can't only look at the compensation amount, but you must find out the allowable displacement direction. Think about it, if it is a double hinge type, and you use it as axial compensation, wouldn't the hinge be hard to carry? It would be weird if it didn't go wrong.
The arrow on the expansion joint is not a decoration. Only by understanding it can it be installed in the right direction
Arrows are printed on the surface of the expansion joints, and "Media Flow Direction" or "Installation Direction" are also written next to them. But many people think that the arrow is just to mark the direction of the fluid-wrong. For axial-type expansion joints, the arrows indicate the direction in which the bellows allows compression, typically coinciding with the flow direction of the medium. Why do you do this? Because the pressure resistance of the bellows is different in two directions: in the compression direction, the corrugated sheet is compressed, and the stress concentration is small; Stretching direction, the corrugated sheet is tensioned, and the weld seam is prone to fatigue cracking. Therefore, in the design, the direction allowed for compression is deliberately corresponded to the flow direction of the medium, so that the internal pressure helps the bellows to "retract" instead of "pull out".
What about the arrows on non-metallic expansion joints, rectangular expansion joints? They are often marked with the direction of pre-stretching at the time of installation. Such asRectangular non-metallic expansion jointIt may be necessary to pre-stretch a certain displacement during installation, with arrows pointing in the direction of actual displacement after installation. If you follow the conventional understanding and think that the arrow is the direction of media flow, the non-metallic loop strip can easily be torn by reverse stretching.
AndLarge tie rod expansion jointThe nut on the tie rod is not screwed to death. It has a pre-adjustment amount, and it should be loosened or locked according to the design displacement during installation. There is a common question in QA "How to adjust the expansion joint tie rod nut"-the answer is to look at the arrow first, confirm the installation direction, and then adjust the nut position according to the displacement. The arrow is installed backwards, and how you adjust it later will be in vain.
The direction of force is wrong, and it is useless to screw the tie rod nut until it breaks
Let's talk about another practical case. The spiral conveying pipeline of a cement plant has low temperature rise but great vibration. SelectedMetal Corrugated Expansion Joints in Cement IndustryIn the general type, the space was found to be narrow during installation, so the worker turned the expansion joint 90 degrees to save trouble and installed it. With the arrow facing up, the medium actually flows horizontally. During operation, the transverse vibration of the pipeline is forcibly absorbed by the bellows into axial deformation, and the tie rod nut should be tight every shift, and finally it will break directly. To measure the fatigue life, only 1/10 of the design life is used. This is a typical example of the mismatch between the force direction and the design working condition.
There is also a common error:Sleeve type pipe expansion jointIt compensates for axial displacement with sleeve sliding, but the sliding direction must be consistent with the pipe axis. If the pipe has a lateral displacement, the sleeve will get stuck, the seal packing is polished, and leaks are a matter of time.
You may ask: Then I just choose an expansion joint that can absorb all directional displacements? Sorry, universal expansion joints do not exist at this time. Each type has its best force direction, which should be selected according to the stress analysis results of the pipe system. Even if you chooseCompound hinge transverse expansion jointDon't forget that its hinge can only withstand lateral forces, and the axial forces are responsible for another fixed bracket. The direction is wrong, the bracket is made for nothing, and the nut is screwed for nothing.
From the actual failure cases: when the selection of the type does not calculate the right direction, how serious the consequence
I was involved in the failure analysis of a power station project. The main pipeline is high temperature and high pressure steam, with a design temperature of 540℃ and a pressure of 10MPa.Corrugated expansion joint for power plant industryThe straight pipe pressure balanced type in. Two fixing brackets are installed at both ends of the expansion joint, which is reasonably fine. After one year of operation, the balance ring of one of the expansion joints deforms and the tie rod bends. Removed for inspection, it was found that the bellows had obvious wear in the transverse direction.
After checking the design documents, we know that only axial thermal expansion was considered in the initial stress calculation, and the transverse reaction force generated by one elbow was ignored. Although the value is only 5mm, the bellows of pressure balance expansion joint is the main component of axial force, and the transverse stiffness is very low. After repeated cycling of 5mm, the bellows will be cracked. What's more troublesome is that the deformation of the balance ring can't balance the internal pressure thrust, resulting in the additional thrust of the fixed bracket and cracks in the bracket welds. The whole pipe system spent an extra 200,000 yuan for repair, and it lost more after a week of downtime.
What does this case say? When selecting the type, "calculating the right direction" is not just to know the axial, transverse and angular directions in general, but to calculate the magnitude, frequency and direction vectors of each displacement, and then compare them with each otherForce direction of metal expansion jointCharacteristics table, pick out the one that matches best. If you are really unsure, chooseCompound straight pipe bypass pressure balanced expansion jointOrCurved tube pressure balance expansion jointThis kind of multi-directional performance balanced product is a little more expensive, but safety comes first.
In short, the metal expansion joint looked like a tin bellows, but it was actually a precision force-bearing component. It takes half an hour to find out the direction of the force before installation, so as to save tens of thousands of rework later. Don't stick arrows randomly, don't twist nuts randomly, and if the direction is right, the pipeline will run safely for 20 years.