Standards can't be used, when should I take the customized route?
Two days ago, a customer who made steam pipelines called and asked, "Can your general-purpose corrugated expansion joint, DN500, be directly used in the working conditions of 480℃ and 2.5MPa?" I asked him to send the pipeline drawings. Good guy, the axial displacement requires 80mm, and the lateral offset is 15mm. Standards? That's really hard to handle.
When is it time to go custom? To put it bluntly, there are three situations: First, the displacement of the standard product is not enough. For example, the general-purpose corrugated expansion joint can only compensate the axial displacement of 30mm at the maximum, and you want 50mm here; Second, the temperature and pressure of the medium exceed the allowable range of the standard product. For example, although the high-temperature axial expansion joint can withstand 600℃, if your pipeline is a medium containing chloride ions, the standard stainless steel bellows may crack in two years; Third, the installation space is limited, or the pipeline is complicated, so special structures such as compound hinge transverse type and straight pipe pressure balance type are needed. If you force the standard, it can be a leak or a torn pipe-anyone can settle the bill.
The first level of customization: caliber, pressure, temperature and displacement, none of which can be missed
Doing round metal expansion joint customization, the first step is to smash these four parameters to death. The caliber is easy to say, report according to the inner diameter or outer diameter of the pipe; The pressure is divided into design pressure and test pressure. Don't just say "2.5MPa", it has to be clear whether it is gauge pressure or absolute pressure, and whether it is at normal temperature or high temperature. Because as soon as the temperature rises, the allowable stress of the material falls-the strength of carbon steel at 400℃ is only half that of normal temperature.
The displacement is more complex. There's axial compression, axial tension, lateral displacement, angular displacement, and you have to figure out the actual line thermal expansion. Doesn't count? Ask the design institute for a stress analysis report. For example, a 30-meter-long carbon steel pipe, with a temperature rise of 200℃, has a thermal expansion of 30×12.2×10⁻⁶ ×200≈73mm. You said that you ordered a general-purpose corrugated expansion joint, and the maximum compensation amount was only 60mm. Isn't that digging a hole for yourself?
There's also a pit: Is the temperature fluctuating or constant? If it is a steam pipeline that starts and stops frequently, the bellows will bear alternating stress. At this time, the number of corrugated layers and wave height have to be redesigned. Don't expect to get through with standards.
Structural design is not a pat on the head: guide tube, tie rod and hinge, where and how to match them?
Structural design is the core of round metal expansion joint customization. You think the bellows will be done as soon as they are welded? Too naive.
Let's start guide tube。 This thing has one function-diversion. When the medium flow rate is high and there are particles, there is no guide tube, and the bellows directly face the scouring, which will wear out in a few months. The material of the guide tube is generally one grade higher than that of the bellows, and the inner diameter should be reserved for gaps to prevent thermal expansion from sticking. And guess what? I've seen people weld the deflector to the connector, and as a result, the bellows are squeezed and deformed as soon as it heats up. Tsk, low level error.
Again tie rod。 The tie rod is not a show-off, it is used to withstand pressure thrust. If the pipe is a fixed bracket, you install a universal corrugated expansion joint instead of a tie rod, and the expansion joint will spread itself as soon as the pressure comes up. Common structures are: large tie rod expansion joint, small tie rod expansion joint. Large tie rods are generally used in occasions where large blind plate forces are needed, and the length of tie rods should be accurately calculated, otherwise the displacement direction cannot be limited.
hingeWhat? For absorbing angular displacement. For example, the double hinge transverse expansion joint, with a hinge in the middle of two bellows, can make the pipe swing in the same plane. Where your tube turns, if there is not enough space, you can consider the curved tube pressure balance expansion joint, which can absorb axial and lateral displacement by itself, without adding a fixing bracket. But the price... Tsk, double the start.
If the material is not selected correctly, it will crack after two years-how to choose carbon steel, stainless steel and nickel-based alloy
Material selection is the easiest link in the customization of round metal expansion joints. Many purchasing managers only look at the price, carbon steel is the cheapest, 304 stainless steel is a little more expensive, 316L is more expensive, and nickel-based alloys (Inconel 625, Hastelloy C276) are ridiculously expensive. But cheap doesn't necessarily save money.
Carbon steelIt can only be used in a dry environment ≤425℃. Once there is moisture or moisture, the corrosion rate is visible to the naked eye. Many corrugated expansion joints used in the power station industry are carbon steel, but people use overheated steam, and they have regular anti-corrosion maintenance. You a cement industry smoke exhaust pipe, the flue gas contains sulfur, use carbon steel? Piercing in two years.
Stainless steel304 and 316L. 304 stress corrosion cracking is a high probability event in the environment where the chloride ion concentration exceeds 100ppm. 316L contains molybdenum and has a slightly better resistance to chloride ions, but it is also not suitable for seawater or chemical media. At this point, we have to increase the weight —Nickel-based alloy。 For example, high-temperature axial expansion joint is used in flue gas desulfurization system. Although the temperature is not high, the medium contains a large amount of chloride ions and sulfuric acid, which must be Inconel 625. Don't try to save money, the money saved will be paid to the maintenance cost in the future.
Don't wait for installation to find problems: 3 key points for factory acceptance and on-site installation
After the customization is finished and the goods arrive at the scene, do you think everything will be fine? Three things must be checked and accepted before leaving the factory:
First, Hydraulic test。 Push water pressure according to 1.5 times the design pressure, and hold pressure for 30 minutes to see if there is any leakage in the weld. Don't just look at the watch, take a flashlight to shine a circle, especially the circumferential welding seam of the bellows and the connecting pipe and the positioning welding of the guide tube. I once saw a double straight pipe bypass pressure balanced expansion joint, and the bellows bulged during water pressure-the manufacturer pressed the wave pitch wrong, and the wall thickness was uneven.
Second, displacement test。 If possible, do a pre-stretch or pre-compression to check whether the tie rod nut can be adjusted smoothly. How to adjust the expansion joint tie rod nut? First loosen the locking nut, rotate the adjustment nut to make the bellows reach the predetermined compression amount, and then lock it. This has to be confirmed before installation, otherwise you will be in a hurry at the site.
Third, On-site installation。 Three pits: First, don't use wire rope to directly strangle the bellows when hoisting-put rubber or cloth strips on it; Second, the fixed bracket must be welded firmly, and the sliding bracket cannot be stuck; Third, don't let the sundries fall into the bellows trough when transporting. Once the stones are stuck in that place, they will snap during operation, and the bellows will soon break from fatigue. By the way, remember to remove the tie rod or support for transportation after the installation is completed-someone installed a directly buried expansion joint and forgot to remove the limit, and it collapsed directly as soon as the temperature rose. Do you think it was wrong?