What exactly is a straight pipe metal expansion joint?
Don't hurry to ask for the price yet. Straight pipe metal expansion joint, to put it bluntly, is a corrugated compensator installed on a straight pipe section to absorb axial displacement (that is, the thermal expansion and contraction of the pipe along the length direction). It looks very similar to the general-purpose corrugated expansion joint, but the general-purpose type is usually only used in low pressure, small diameter and simple displacement requirements; External pressure single axial type expansion joint, bellows under external pressure, better stability, suitable for high pressure and large displacement; The straight tube pressure balance type expansion joint has a balance ring inside, which can eat the internal pressure thrust by itself without transmitting it to the fixed bracket.
Which one you choose directly determines how to calculate the force of the pipeline. Two days ago, a customer asked the price of straight pipe metal expansion joint with the drawing of straight pipe pressure balance type. I said that you could use ordinary straight pipe axial type in this working condition. As a result, he insisted on buying it according to the drawing. Finally, all the fixed brackets had to be reinforced, which cost tens of thousands more. So first thing: Make sure that your pipe needs to absorb axial displacement and does not require balanced pressure thrust, then choose a straight pipe metal expansion joint.
Calculate three data before buying
Axial stiffness, effective area, and whether to install the guide tube-these three are the selected foundations. If any one is calculated incorrectly, the expansion joint will not live up to the design life.
Axial stiffnessDetermines the reaction force of the expansion joint on the pipe support. The greater the stiffness, the greater the thrust generated by the same displacement, and the greater the force on the bracket and fixed point. When selecting, don't only look at the displacement, but also the stiffness value, especially for high-temperature pipes with large diameter, where the stiffness is a little worse and the bracket design is completely changed.
Effective areaIt's used to calculate pressure thrust. The formula is simple: pressure thrust = medium pressure × effective area. This force is much greater than the force generated by thermal displacement. If the pipeline is not equipped with the main fixing bracket, the metal expansion joint of the straight pipe will run directly against the pipeline, and this is how the weld cracking accident comes from.
guide tubeDo you want to put it on? Look at media flow rate and particles. The flow rate of steam pipeline is high, and the bellows without guide tube will be washed and thinned; Hot air ducts with dust, the guide tube can also avoid dust accumulation in the trough. However, the installation of the guide tube will increase the internal constraint, and the axial displacement ability will be affected a little. Therefore, the bigger the better, it is necessary to match the working conditions.
Field installation is more deadly than the product itself
And guess what? Many expansion joints are broken, not quality problems, but those screws were not disassembled during installation.
For the convenience of transportation when leaving the factory, both ends of the expansion joint will be locked with transport screws. After installation, they must be loosened or removed, otherwise the expansion joint can't be deformed at all. As soon as the pipe expands, the force is all held on the bellows and bulges directly. There are also tie rod nuts, which are used to limit the amount of displacement, not for you to tighten. Do not move the factory-adjusted position unless you have clear pre-deformation requirements.
When it comes to pre-deformation, many old masters are also prone to mistakes. The pre-deformation is so that the expansion joint is pre-offset by half the thermal displacement in the cold state, so that it works in the middle position in the hot state. If the pre-deformation is not done, the bellows may be overstretched and compressed over the limit, and the fatigue life will be halved. How to do it? Calculate the cold installation length according to the pre-deformation amount given by the manufacturer, don't pat your head.
Different industries, material and structure are two ideas
The steam pipeline of power station can have a temperature of 500℃ and a high pressure. The bellows are generally made of austenitic stainless steel 321 or 347, and the number of layers may be combined with multiple layers, and a guide tube has to be equipped to prevent high-temperature steam from washing. Hot air ducts in cement plants have large temperature fluctuations and dust erosion. The material is more stable with heat-resistant stainless steel 310S. The structure tends to be a straight metal expansion joint with external insulation layer, and the bellows can't directly contact high-temperature flue gas.
The desulfurization flue gas pipeline is completely reversed. The temperature is not high, but the medium is extremely corrosive and it is wet smoke. At this time, focus on corrosion-resistant materials, such as 316L, 317L, and even lined with fluoroplastics. Don't be greedy for too many layers. The more layers, the easier it is to accumulate liquid between layers, which will accelerate corrosion. I have seen many expansion joints at the outlet of desulfurization towers. Use ordinary 304, which leaks in half a year. Choosing materials is not the more expensive the better, but the more matching the better.
Self-check checklist for common faults
Bellows leakage, instability, fatigue fracture-encounter these three faults, don't scold the supplier first, check in order:
- leak: First see whether there is a weld problem, and then see whether the bellows has been corroded by media corrosion. If the material is selected low during the selection, it is the selection and backing the pot; If welding slag splashes and injures the bellows during installation, it is installation mining.
- Instability: Lateral bulging or torsional deformation of the bellows is mostly due to the pressure exceeding the design value or the axial displacement exceeding the limit. Check whether the safety valve takeoff pressure is too high and the displacement of the pipeline fixing bracket is not calculated correctly.
- Fatigue fracture: Bellows crest cracking, focusing on whether the fatigue life calculation covers the actual number of cycles. Many manufacturers start and stop once a day. If you actually cycle every half hour, it will crack.
A steam pipe of a thermal company, the straight pipe metal expansion joint leaked after eight months. When removed, there are a lot of pitting perforations in the bellows trough. The reason is that the steam is of poor quality and contains chloride ions, and the material is 304. Later, it was replaced with 316L, and the condensation slope was increased, and the problem was solved. This is not an installation problem, but the media composition was not asked clearly at the beginning.
Five Questions You Must Ask Suppliers Before Purchasing
Before placing an order, throw these five questions over, and the supplier with vague answers will change them directly.
1. How is fatigue life calculated?According to GB/T 12777 standard or EJMA? What is the number of design cycles? Ask them to come up with a calculation book, and don't just give one conclusion.
2. Do you want to have a lined guide tube?Let the supplier judge based on your media temperature, flow rate, particle content, not you guess it yourself. They're more accurate than you.
3. Who will bear the pressure thrust?If an ordinary straight pipe metal expansion joint is chosen, the pressure thrust must be borne by the pipe fixing bracket. Ask if the effective area is marked in their product parameters so that you can design the bracket.
4. How to do transportation protection?Does the bellows have a protective sleeve? Is the transport screw pre-installed? How to store it after you arrive? These details are not in place, and the goods are injured before they are loaded.
5. What failure modes does the warranty cover?Only guaranteed without leakage? Or guarantee fatigue life? Does corrosion perforation count? Read the terms of the contract clearly, and don't wait for something to find out that the warranty is just leaking.
After asking these five questions, you will basically not be tricked. The technical threshold of straight pipe metal expansion joint is not high, but the pits are all in the details. Understand the displacement type, calculate the stiffness area guide tube, do a good job in on-site installation, and select materials according to working conditions. Only by doing these things in place can the expansion joint work honestly.