Why is carbon steel the "king of cost performance" of pipeline compensation? — — Talk about the logic behind material selection
After doing pipeline compensation for so many years, the most contacted one is the carbon steel metal expansion joint bellows. Why, you ask? To put it bluntly, it is real. Carbon steel, which has enough strength, low cost and good weldability, is the absolute main force under non-corrosive working conditions. Think about it, a steam line, the temperature is around 400℃ and the pressure is 1.6MPa, you use stainless steel? Lined with PTFE? Not to mention doubling the cost, it may not be able to bear it. Carbon steel is in this range, just stuck at the balance point between performance and price. Two days ago, there was a customer who insisted on stainless steel on the whole line. I asked him, "Your pipeline is run by ordinary steam, and there are no chloride ions. Carbon steel can be used for ten or eight years. Why can't it match the budget?" He thought for a while and changed it back. This is the logic of carbon steel: just use enough, don't waste it.
How does the bellows "move"? Understand structure, stiffness, and the role of the guide tube
The core of the bellows is to absorb the displacement by the deformation of the peaks and valleys. You think of it as a spring — but a hollow spring. The greater the stiffness, the stronger the ability to resist deformation, but the amount of compensation is small; The stiffness is small, the compensation amount is large, but the pressure bearing capacity is weak. So you have to calculate when designing, not pat your head. The stiffness formula is available in the FAQ of our station. If you are interested, you can turn it over. There is also a guide tube, which has a simple function: guide the medium to the internal flow channel of the bellows to prevent high-speed fluid from directly washing the wall of the bellows. If you forget to install the deflector when you install it, or install it in the opposite direction (the arrow is facing the flow direction of the medium), then the bellows life will be directly discounted by 50%. Don't ask me how I know. Only after suffering a loss at the scene can I have a long memory.
Look at these three points first: temperature, pressure and displacement-don't let the parameters fool you
What are you most afraid of when choosing? Afraid of placing an order only after looking at the pipe caliber. Temperature determines the material grade. Carbon steel metal expansion joint bellows are generally suitable for-20℃ to 150℃. Beyond this range, you have to consider alloy steel or stainless steel. What about the stress? The pressure grade is PN0.6 to PN6.3. If it is high, you have to add multiple layers of corrugation or increase the wall thickness. The displacement is more critical-axial, transverse and angular. Each displacement has different effects on the fatigue life of bellows. For example: For a pipeline that needs to absorb 100mm axial displacement, you bought a general-purpose corrugated expansion joint. As a result, the installation space was not enough, and it was hard broken into lateral displacement, and the corrugated pipe cracked in a few days. Therefore, before selecting, be sure to list the three parameters clearly, don't rely on guessing.
4 details that are most prone to rollover at the installation site: Is the tie rod adjusted or not? Can the screw be removed or not?
Alas, when it comes to installation, rollover cases I can talk all day. First detail: the tie rod. Many expansion joints come out of the factory with tie rods, which are used for transportation and fixation, not for you to be permanently supported! After installation in place, the tie rod nut must be loosened to allow the bellows to expand and contract freely. You're not loose? That's equivalent to welding a hard connection. What's the use of it? Second detail: screw. Some models of expansion joints have limit screws, which are used to prevent excessive stretching or compression, and are usually removed after installation. However, some working conditions require the limiting function to be retained. At this time, you have to see the drawings. Third detail: Directions. The arrow of the guide tube must point to the flow direction of the medium, and if it is installed backwards, it will wait for leakage. Fourth detail: pre-stretched or pre-compressed. For low-temperature pipelines or occasions where large compensation is required, the pre-stretching amount should be calculated according to the ambient temperature during installation, otherwise the bellows may be pulled out after operation. For details, you can refer to the article "How to Adjust the Tie Rod Nut of the Expansion Joint" in our station, which is written clearly.
Carbon Steel vs Stainless Steel vs PTFE Liner: How to Choose under Different Working Conditions?
You've certainly encountered this kind of tangle. Don't worry, I'll help you figure it out. Carbon steel metal expansion joint bellows: suitable for dry steam, hot water, oil, compressed air and other non-corrosive media. Limit temperature 150℃, medium-low pressure. The price is cheap, and it doesn't hurt to replace it if it is broken. Stainless steel (304/316L): Suitable for slight corrosion or cleaning, such as food, medicine, chemical industry. The temperature resistance can reach 600℃ (high temperature type), but the cost increases by 30%-50%. Lined with PTFE: Professional to deal with strong acid, strong alkali and high corrosive media. For example, the one in our stationPTFE-lined hoseAndPTFE compensator, lined with PTFE, the corrosion resistance is first-class, but the temperature resistance does not exceed 200℃ (PTFE limit), and it cannot be used for high pressure. Which one to choose? Look at the media, the temperature, and the budget. There is no universal material, only the most suitable scheme.
3 Practical Lessons for Extending Service Life-From Field Feedback
Check the bellows surface for scratches and pits before installation. Even a small pit can develop into a crack under the stress cycle. Article 2: Regularly check whether the tie rod nut is loose (if not removed) or whether the guide tube is displaced during operation. Article 3: When doing hydraulic pressure test, the pressure should not exceed 1.5 times of the design pressure, and the pressure should be increased slowly, otherwise the bellows may produce plastic deformation. These three points seem simple, but there are really not many that can be done. Especially the last one, many operator diagrams save trouble, directly hit the test pressure, and as a result, the bellows bulged. You say it was wrong or not? Therefore, it is slow when it should be slow, and thin when it should be thin. Carbon steel metal expansion joint bellows are used well, and there will be no problem for ten or eight years; If you don't use it well, it will leak in half a year.