How does the wave pitch of the bellows change? Full interpretation from mold to application
A few days ago, a customer who made power station pipelines called and asked, "How did you adjust the wave pitch of your bellows? I see 20mm marked on the drawing. Can you change it to 15mm for me?" This problem is actually quite typical-many people think that the wave pitch is a fixed size, and the manufacturer can set as much as he wants. Today, let's break this matter apart and explain it clearly, from mold to manufacturing to type selection, explain it clearly at once.
Wave length is not randomly determined: first understand what this parameter is for
Wave distance, to put it bluntly, is the axial distance between two adjacent peaks (or troughs). It directly determines one of the core capabilities of the bellows-how many waves can be stuffed into a unit length. The more wavenumbers, the greater the amount of compensation; The larger the wave pitch, the higher the stiffness of the individual waves.
But this stuff isn't a patter on the head. A high-temperature axial expansion joint is used in steam pipeline, and a metal corrugated expansion joint in cement industry is used in kiln tail pipeline. The logic of wave pitch is completely different. The former has to bear high temperature and high pressure, and the wave distance is too small to easily concentrate stress; The latter is mainly to absorb thermal displacement, and a larger wave pitch will be more resistant to fatigue.
So you see, the wave pitch is essentially a "performance adjustment knob". Wherever the knob is twisted, the stiffness, compensation amount and fatigue life all change accordingly. Then how is this knob twisted in the manufacturing process?
Manufacturing: How to Control Wave Pitch in Hydroforming and Mechanical Forming
Hydroforming and mechanical forming (also called roll forming). The wave length control logic is completely different.
Hydroforming: The pipe blank is filled with high-pressure oil, and the pipe wall is pushed up by oil pressure to form waves. The wave pitch is determined by the axial spacing of the mold-the groove engraved on the mold, the groove pitch is the wave pitch. Because of the uniform oil pressure and full wave crest, hydroforming is suitable for small wave distance and multi-wave number products, such as universal corrugated expansion joints and metal hoses. If you want to change the wave pitch under this process, you have to change the mold or adjust the spacer ring on the mold. So once shaped, the wave distance is basically locked.
Mechanical forming: Roll the pipe wall by a pair or several pairs of rollers, and gradually press out the corrugations. The wave pitch is controlled by the ratio of the feed speed and the rotation speed of the roller. The advantage is that it can be adjusted flexibly-the same set of molds, adjusting the speed ratio can enlarge or reduce the wave distance. The disadvantage is that the material requirements are high, and the stainless steel thin-walled pipe is easy to wrinkle. Many large-diameter thick-walled expansion joints and external pressure single-type axial expansion joints are mechanically formed, because such products often need customized wave pitches to match specific compensation requirements.
The bellows will have a rebound during the molding process. There will be a difference of 0.3-0.8mm between the actual wave distance and the mold size, and experienced masters will count the rebound amount in advance in the mold design. After you receive the product, measure it, and the wave distance can generally be controlled within ± 0.5mm.
When the wave pitch changes, the performance changes completely: the relationship between stiffness, compensation amount and fatigue life
Let's start with stiffness. The larger the wave pitch, the larger the bending radius of the single-wave metal material, and the stronger the ability to resist deformation – the stiffness is naturally high. The reason is very simple, just like a spring, the larger the lap gap, the harder it is to press. For straight pipe pressure balance expansion joint, which relies on stiffness to balance the thrust, the wave pitch is often designed to be too large. On the other hand, the transverse expansion joint of the compound hinge needs a large amount of compensation, and the wave pitch has to be small, so that the unit length can accommodate more waves.
What about the amount of compensation? Total compensation amount = single wave compensation amount × wave number. When the wave distance is smaller, the wave number increases at the same length, and the total compensation amount increases. However, the amount of single wave compensation is not fixed-the single wave with large wave distance can be pulled more open, so the two should be calculated comprehensively. Take our commonly used bellows as an example: if the wave pitch is changed from 20mm to 16mm, the total compensation can be increased by about 30%, but the fatigue life may be reduced by 15%-20%. How to choose? Depends on the working conditions.
Fatigue life is the most sensitive piece. When bellows expand and contract frequently (such as double hinge expansion joint for air-cooled island vacuum pipe), if the wave pitch is too small, the stress concentration at the wave root will increase, and cracks often start there. I handled a case in which the customer temporarily changed the wave pitch by two millimeters, but it leaked after eight months of use-it was replaced with the original design wave pitch, and it was fine for three years. Therefore, when making non-standard customization, don't just focus on the compensation amount, the fatigue life is the ceiling.
Practical Selection: How to Select Wave Distance under Different Working Conditions
Give you a few specific scenarios, and you will understand by comparing them:
- High temperature and high pressure steam pipeline(Corrugated expansion joint for power station industry): The wave pitch is recommended to be 18-25mm. This range can take into account both stiffness and fatigue life, and it is more stable with multi-layer corrugated structure.
- Corrosive media transport(Lined with PTFE hose and PTFE compensator): The wave pitch should be too large, 22-30mm. Because the lining layer is brittle, the wave distance is too small to break the PTFE layer easily.
- Large displacement and low frequency working condition(Direct buried expansion joint, rotary compensator): The wave pitch can be as small as 12-15mm. It mainly depends on the compensation amount of the wave number pile. Anyway, it only acts dozens of times a year, so fatigue is not a problem.
- Smoke duct system(Non-metallic expansion joint, rectangular non-metallic expansion joint): In fact, metal bellows are not used in this occasion, but if you use metal rectangular expansion joint in the flue, the wave pitch will be larger-more than 30mm, because soot particles are easy to accumulate in the trough, and if the wave pitch is too small, it will fail.
In addition, to be honest, many customers only look at the diameter and pressure when selecting models, and don't mention the wave distance at all. The wave distance column on the sales bill is often blank. But if you think about it, the price difference between the wave pitch of 18mm and 22mm of the same general-purpose corrugated expansion joint can be 20%, and the performance is completely different. Therefore, the next time you inquire, it is best to tell the manufacturer your "displacement + working frequency" directly, and the manufacturer can help you calculate the appropriate wave distance.
How to deal with wave pitch abnormality during installation and maintenance
Finished installing on site and found that something was wrong with the wave distance? Don't panic, first judge whether it is caused by manufacturing deviation or installation stress. Take a ruler and measure the wave distance at three different positions:
- If the deviation is within ±0.5mm, normal, rebound influence.
- If the deviation exceeds 1mm and is concentrated on one side, there is a high probability that there is a problem with the pipeline alignment during installation, and the bellows is pulled off. At this time, you have to loosen the expansion joint tie rod nut and readjust the axial position of the pipe. For specific adjustments, you can turn our previous article "How to Adjust the Expansion Joint Tie Rod Nut".
- If the whole bellows is axially compressed and the wave pitch generally becomes smaller, it means that the cold drawing amount of the pipeline is not done correctly, or the medium temperature exceeds the design value. After parking and cooling down, observe it. If the wave distance can't be restored, you have to change parts-forcibly running with illness will accelerate fatigue cracking.
It is found that the wave pitch is significantly larger during maintenance, usually because plastic deformation (over-stretching) has occurred. This is basically hopeless, just replace it with a new one. Remember to add a limit tie rod (such as the tie rod structure in the expansion joint of the large tie rod) to the bellows when selecting the model next time to prevent accidental over-displacement.
The more accurate the wave distance is, the better. Some high-end customers take vernier calipers for acceptance, and they have to be stuck to a whole millimeter, which is actually unnecessary. As long as within the design tolerance, priority is given to ensuring that the compensation amount and stiffness meet the standard-after all, bellows is a "flexible element", and being too real may turn out good things.
Regarding the wave distance, if you still want to talk about specific working conditions, call our technical department directly. We have more than 20 kinds of drawings of expansion joints, from high-temperature axial type to directly buried type, from metal hose to PTFE compensator, and how to determine the wave pitch has ready-made case reference. Don't worry, don't fool you into choosing the most expensive, only help you choose the most correct one.