1. How terrible is the low temperature? — — Starting from the brittleness of materials
Have you broken branches in the northern winter? It breaks as soon as you break it, crisp and neat. The metal expansion joint at low temperature also has a virtue-304 stainless steel, which originally has good toughness, may drop from 100J to below 20J when it reaches below-40℃, and it will break brittle directly without even making a discount. This is not alarmist. GB/T 150 standard has rigid requirements for the impact power of low-temperature materials, but many engineering purchasers just stare at the price and ignore the brittleness.
Last year, the air-cooled island pipeline of an LNG receiving station, equipped with ordinary austenitic stainless steel expansion joints, cracked three ripples in the first winter of operation. At the scene, the crack passes straight from the trough to the peak, which is a typical low-temperature brittle fracture. Check that the design temperature is only-45℃, but the material used is 304, the carbon content is on the high side, and there is no low-temperature impact test. And the result? One exchange, along with the loss of shutdown, a small two million went in.
How exactly is cryogenic defined? In China's low-temperature pressure vessel specification, lower than-20℃ is called low temperature. However, in pipeline systems, -40℃ and-196℃ are also common. The lower the temperature, the more difficult the dislocation movement in the crystal structure of the material, and the plastic deformation ability decreases by a cliff. Therefore, the key is one sentence: when choosing low-temperature metal expansion joints, the first step is the low-temperature toughness of the material.
Second, material selection is not a pat on the head: 304, 316L or Inco Nickel?
Two days ago, a buddy who was doing cold storage project called me and asked me, their pipeline temperature was-35℃ and the pressure was 0.6MPa. Can you use 304? I say yes, but there are conditions: it must be ultra-low carbon 304L, and it must be done a low temperature impact test at-40℃, and the impact power is not less than 27J (transverse specimen according to ASTM A262). If the supplier can't come up with the report, then don't use it.
- 304/304L: Suitable for working conditions above-196℃, but 304L is safer than 304, with carbon content ≤0.03%, avoiding intergranular corrosion and embrittlement. It is recommended for low and medium temperature scenarios such as air-cooled islands and cold storage.
- 316L: It has more molybdenum than 304L, and is more resistant to pitting in chloride-containing environments (such as coastal air-cooled islands). The low-temperature toughness is also not bad, and the impact power can usually reach more than 60J at-196℃.
- Inconel 625/718: Expensive, but expensive for a reason. In the environment of liquid nitrogen and liquid oxygen at-196 ℃ or even lower, ordinary stainless steel will be brittle like glass, and inko nickel can still maintain toughness. Moreover, it resists hydrogen embrittlement and fatigue, and is suitable for such fatal places as LNG liquefaction stations and low-temperature vacuum pipelines.
Where is the pit of material selection? One is that the supplier pretends to take 304 as 304L-the carbon content is 0.01% different, and the low-temperature performance is a bit worse. The other is to only look at the material grade, not the heat treatment state. Austenitic stainless steel with insufficient solution treatment has more carbide precipitation on grain boundary and is still brittle at low temperature. My suggestion: When ordering, specify material standards (e.g. ASTM A240), upper carbon content and low temperature impact test requirements directly and write them in the contract.
3. What pits are hidden in the structural design? Wave height, guide tube and tie rod are all indispensable
The material is right, and the structural design is still finished when it rolls over. There are three main points in the structural design of low-temperature metal expansion joints:
Wave height and wave distance
The compensation amount of bellows at low temperature depends on the wave deformation, but the larger the wave height, the more concentrated the stress at the wave peak. Experienced designers will control the wave height between 0.15 and 0.25 times the nominal diameter, and verify the fatigue life through finite element analysis. Remember one principle: it is better to have a few more waves than to stretch the height of a single wave too far.
guide tube
The role of the guide tube is not just to guide the flow. You can see the detailed explanation of the role of the guide tube on this site in the question and answer. Let me emphasize one more point here: the low-temperature fluid has erosion and temperature difference impact on the inner wall of the guide tube. If the guide tube is not thick enough or not washed out, it can wear out in a few months. After all, the flow rate of low-temperature medium often exceeds 10m/s. Under this working condition, the guide tube must be made of a plate thicker than the bellows, and the material is preferably the same as the bellows but not less than 304L.
Tie rod and limit structure
The 30-meter-long carbon steel pipe dropped from normal temperature to-100℃, and the shrinkage amount exceeded 40mm. If the expansion joint is not limited by a suitable tie rod or hinge structure, the bellows can be straightened or even torn. The common solution is to use a compound hinge transverse expansion joint (this site has such products) or an external pressure single axial expansion joint, with a tie rod to bear the internal pressure thrust, and the bellows is only responsible for compensating the displacement. Note: The nut of the tie rod cannot be locked after installation, so the cold shrinkage allowance must be reserved. How to adjust it? Refer to the question and answer of this site "How to adjust the tie rod nut of expansion joint".
4. You must have encountered these scenarios: air-cooled island, LNG, cold storage pipeline
Let's talk about a few actual scenarios, and you will understand them at a glance.
Air-cooled island vacuum pipe: Temperature-40°C to-50°C, pressure close to negative pressure. At this time, you have to use air-cooled island vacuum pipe double hinge expansion joint (yes, this site has this product). The double hinge structure can absorb lateral displacement and avoid bellows instability under vacuum. The material is generally 304L or 316L, but the key depends on whether there is condensed water inside the pipe-if so, 316L is more stable.
LNG discharge arm and pipelineAt the liquid nitrogen temperature of-196℃, ordinary stainless steel is basically useless. I have seen the composite structure of Inco Nickel 625 bellows +316L flange for later modification, which is expensive, but it has been used for ten years without any problems. Note that LNG pipeline should also consider the design of cold insulation layer and pre-cooling conditions, and accurate calculation of cold tightness of expansion joint.
Cold storage pipeline: Around-30℃, ammonia refrigeration system. Ammonia has corrosion on copper and has little effect on stainless steel. However, cold storage pipelines often have hot and cold alternating. At this time, directly buried (fully buried) expansion joints or external pressure single axial expansion joints are more suitable, which can separate the bellows from the soil and reduce corrosion.
5. When installing, shake your hand, and the back is all dry-cold tightness and protection points
Many people think that the expansion joint can be installed to tighten the screws, which is a big mistake. Installation under low temperature conditions must be pre-stretched-the expansion joint is pre-stretched for a length during installation to offset cold shrinkage. How to determine the amount of stretch? Calculated by about half of the cold shrinkage at the design temperature of the pipeline. For example, if you cold shrink 40mm, pull 20mm. But you can't move the hard sleeve dead, it depends on the actual compensation amount of the expansion joint, and don't pull the bellows too hard to be scrapped.
Cryogenic pipes usually have a cold insulation layer, and the cold insulation layer cannot be pressed on the bellows! The cold insulation layer must be set up with support rings or brackets to allow the bellows to deform freely. If the cold insulation layer is directly wrapped on the surface of the bellows, the hard shell after icing limits the deformation, and the expansion joint will fail.
Let's talk about the tie rod and the limit bolt. After installation, the tie rod bolts for temporary transportation should be adjusted to working condition-the nut should be loosened, leaving a compensation gap. Many on-site construction teams try to save trouble, and regard the transportation tie rod as a working tie rod without dismantling or adjusting it. As a result, as soon as the pipeline shrank, the bellows was pulled to death and directly torn.
Six, summary: Don't wait for the pipe to burst before regretting, low-temperature expansion joint selection in three steps
After long-winded, I'll draw a key point for you:
- fixed working condition: What's the lowest temperature? How much pressure? Is the medium corroded or not?
- Selection of material + structureSelect 304L, 316L or Inco nickel according to the temperature; According to the displacement type, choose general type, double hinge type or external pressure single type; Don't forget to add deflectors and qualified tie rod limits.
- Cold tight + tight installation: Find the manufacturer or design institute to calculate the cold tightness, adjust the tie rod when installing, and never press the bellows in the cold insulation layer.
Is that clear enough? Next time if you encounter low-temperature metal expansion joint selection, compare these three steps directly. Still not sure? Look through the products in the product catalog of this site, such as the double hinge expansion joint of air-cooled island vacuum pipeline and the external pressure single axial expansion joint, each of which has detailed parameters. Don't wait for the pipe to burst before slapping your thigh. Compared with the maintenance cost, the selection fee is a drop in the bucket.