It is also a corrugated expansion joint. Why is the height difference of the wave sidewall so much? Find out exactly where this parameter refers first
What is the sidewall height of expansion joint wave? To be honest, there is a problem with this question itself-the height of the wave sidewall is not a fixed value, it follows the working conditions. Don't rush to ask "how much" first, find out which paragraph it refers to.
The waveform of the corrugated expansion joint, you can understand it as a wave. The hypotenuse between the peak and the trough, that is, the vertical distance from the bottom of the trough to the top of the peak, is called wave height in the industry, but when the engineering drawings are actually processed and inspected, it is actually the "wave sidewall height"-it refers to the vertical projection size of the inclined wall on the side of the bellows. Many people confuse it with wave height, which is not exactly the same thing. When you measure with a caliper, you measure the height from the trough to the peak, but the height of the wave sidewall should also consider the transition radius between the peak arc and the trough arc. The measured value will be slightly different from the wave height.
This size directly determines the stiffness, pressure resistance and compensation amount of the expansion joint. The same nominal diameter, high wave side wall height, good flexibility, large compensation, but the pressure resistance will decrease; On the other hand, if the wave side wall height is low, the pressure bearing capacity is strong, but the compensation capacity is limited. So do you say that this parameter is heavy or not? When you can't figure out the selection, most of the problems will occur when you install the pressure test later.
Is there a fixed standard for wave sidewall height? From GB/T 12777 to the actual product, see how it is determined
Some people think that the national standard will specify an exact wave sidewall height for each path, which is overthinking. GB/T 12777 General Technical Conditions for Expansion Joints of Metal Corrugated Pipe mainly stipulates the design, manufacture, inspection and acceptance requirements of corrugated pipe, and gives framework things such as design calculation method, material selection principle and fatigue life test method, which will not determine the height of wave sidewall alone.
There is a parameter table of bellows section dimensions in the standard, which lists the recommended values such as wave height, wave pitch, wave number and wall thickness, but these are design references, not mandatory standards. The same corrugated expansion joint of DN200 is used in low-pressure and large-compensation applications and high-pressure and small-compensation applications, and the wave side wall height is likely to be more than double. For this reason, you can't find a "standard answer" in the national standard. What about that? It has to be pushed from the design source.
There are four key factors affecting the height of wave sidewall: pressure, caliber, material and compensation amount
Let's talk about how the height of the wave sidewall was "forced" out in actual design. Four factors are indispensable:
Stress.This is the first deciding factor. The higher the internal pressure, the greater the circumferential stress on the bellows wall. If the height of the sidewall of the wave is too high, the stress is easy to concentrate at the trough, and the pressure resistance is immediately discounted. Therefore, under high-pressure working conditions, the height of the wave side wall is usually pressed relatively low, the wave shape is flat, and multi-layer thin walls are used to make up the flexibility. On the other hand, in the working conditions of low pressure and large displacement, such as non-metallic expansion joints or large-diameter metallic expansion joints for smoke ducts, the height of wave sidewall can be made higher, and the waveform can be stretched, so that the compensation amount can go up.
Caliber.The larger the diameter, the larger the pressure bearing area, and the greater the total thrust under the same pressure. The design requirements of the expansion joints of DN100 and DN2000 are completely different from the same order of magnitude. Large diameter expansion joint should control the ratio of wave side wall height to avoid excessive plastic deformation of wave trough. For example, the rectangular expansion joint on the smoke duct of power station, which has a super large diameter, is often iterated by finite element analysis, instead of copying the empirical value of small diameter.
Materials.The yield strength and fatigue characteristics of austenitic stainless steels (e.g. 304, 316L) determine how much bending strain the bellows can withstand. The material strength is high, which can make the height of the wave sidewall larger, and the wave shape can hold it anyway; If the material is low in strength or thin in thickness, the situation is the other way around. There is also the problem of material creep under high temperature working conditions. As soon as the temperature rises, the allowable stress of the material drops, and the height of the wave side wall has to be lowered accordingly, otherwise the bellows is easy to become unstable under high temperature.
The amount of compensation.The larger the displacement to be absorbed, the larger the deformation of each wave, and the height of the wave sidewall should naturally be made higher. However, the fatigue life of bellows is directly linked to the displacement of single wave, and it is not because you can increase the compensation infinitely by pulling the height of the wave sidewall hard. Axial type, transverse type and hinge type, each type has different requirements for the height of wave sidewall. Under the working condition of transverse displacement, the height of wave sidewall has a great influence on the shear deformation of corrugated pipe, so it should be specially checked during design.
The height of the wave side wall is not determined by patting the head: what data are mainly looked at in design calculation?
Then how exactly do the designers set this size? The core path is as follows: first, according to the pipeline stress analysis, the displacement to be absorbed-axial displacement, lateral displacement and angular displacement is obtained, and then the materials and layers are selected in combination with the design pressure, design temperature and medium corrosion margin, and then the geometric parameters of the bellows are iteratively calculated by the calculation method recommended by EJMA standard or GB/T 12777. The wave sidewall height is not calculated separately, it is a related dimension that follows the wave height, wave distance and wave number.
One is the compressive strength check, to ensure that the bellows will not yield or instability under the most severe working conditions; The other is the fatigue life check, to ensure that penetrating cracks will not occur within the expected number of cycles. If you find that the sidewall height of the expansion knuckle wave reported by the two manufacturers is much different under the same working conditions, don't rush to compare the price. Let them take out the design calculation book first. If the wave sidewall height deviates too far from the conventional value, either the stiffness and fatigue life are not up to standard, or there is another mystery in the material or wall thickness.
How to confirm the wave sidewall height with the manufacturer when selecting the model? What to pay attention to in field measurement and drawing verification
If your project has entered the procurement stage, when confirming the height of the corrugated sidewall with the manufacturer, the safest way is to ask the other party to mark the expansion drawing of the corrugated pipe on the drawing, instead of simply quoting a number. The wave height, wave distance, wave side wall height, number of layers and wall thickness of a single layer should be marked on the drawing. These data are meaningful when put together. If you ask the height of a wave sidewall separately, and the manufacturer gives a number, you can't see whether it is reasonable or unreasonable.
For on-site measurement, just use a vernier caliper or depth gauge to directly measure the distance from the trough to the peak of the bellows. But one thing to remind: Many bellows have protective sleeves or insulation layers on the surface, so you have to remove the outer sheath and measure it again. The measured value is allowed to deviate from the value marked on the drawing. GB/T 12777 has requirements for the dimensional tolerance of corrugated pipe. Generally, the tolerance of wave height is about ±3%, and the wall thickness tolerance is according to the material standard. If the measured deviation exceeds 5%, we should be alert to manufacturing quality problems.
In addition, attention should be paid to distinguish whether it is a single layer or a multilayer structure during measurement. The wave sidewall height of multi-layer bellows is the total height. What you measure from the outside is the outermost contour, but there may be gaps or misalignments between each layer. This should be confirmed with the manufacturer. Two years ago, there was a customer, and the height of the wave sidewall measured by a caliper was 3mm smaller than the drawing. It was not said that the manufacturer cut corners. As a result, it was found that the outer protective ring blocked part of the measurement position. This kind of low-level misunderstanding, it is good to talk about it, but I am afraid that it will not talk about it.
At the end of the day, what is the expansion knuckle wave sidewall height? This question should not be answered by a "standard answer", but by your working condition data. Report the parameters of pressure, temperature, medium, displacement and number of cycles to the manufacturer, and ask them to take out the design calculation book. You will naturally know what range the reasonable wave sidewall height should fall in.