Two days ago, I met a cement factory purchaser, and complained to me that the expansion joint they just changed took less than three months, and the bellows leaked. When I removed it, the inner layer burned like a honeycomb. He said: "I thought the inner layer was just a layer of iron, as long as it could be carried." Alas, that sounds familiar. Many people think that the inner layer of the metal expansion joint is a dispensable supporting role. The result? The pipeline system was scrapped in advance, and all the money spent was wasted. Today, let's talk about how deadly this "little thing" hidden inside is.
The inner layer is not a sheet of iron, it carries the life of the whole pipe
Guess which is the most overlooked but most deadly part of a plumbing system? Is the inner layer of the metal expansion joint. It contacts the media directly and resists scour, corrosion, and high temperatures. If there is no inner layer, or the wrong material is chosen, the bellows will streak naked in the harsh environment-as soon as the corrosive gas rushes, it will perforate in a few days; When the high-temperature flue gas is baked, the metal fatigue is accelerated; When high-speed dust is grinded, the weld seam is directly cracked. To put it bluntly, the inner layer is the real "handle", and the bellows is like its bodyguard. Once the inner layer fails, the whole expansion joint is scrap iron.
A steam pipe, temperature 400℃, pressure 1.6MPa. If the inner layer is made of ordinary carbon steel, it is a matter of time before the oxide scale falls off and thins. And if the inner layer is made of heat-resistant alloy (such as Inconel 625), the life can be extended by 3-5 times. Here's the gap.
Deflectors, liners, wear coatings-what do they tube?
Many people conflate these three concepts, but their roles are completely different:
- guide tube: Like a "traffic police" in a pipe, guiding the medium to flow smoothly through the inside of the bellows, reducing vortex and erosion. This site'sexpansion joint guide tubeIt is clearly written in the product description-it also prevents the media from directly impacting the bellows. The guide tube is usually made of stainless steel or heat-resistant steel, with a thickness of 3-6mm. The key depends on the flow rate and particle content.
- inner liner: This is the workhorse of antisepsis. Such asPTFE-lined hoseOrPTFE compensatorResist strong acids and bases by the chemical inertness of PTFE. The inner liner is generally 3-5mm thick, but its weakness is that it is not resistant to high temperatures (the upper limit of long-term use is about 200℃). Therefore, another material should be selected under high temperature working conditions.
- Wear resistant coating: For high velocity gas streams or slurries containing hard particles. Spray ceramic or tungsten carbide coating directly on the inner wall of the guide tube or bellows, which has high hardness and wear resistance. Used in the cement industryMetal Corrugated Expansion Joints in Cement IndustryMany come with wear-resistant coatings.
Guide tube + inner lining, guide tube + wear-resistant coating, or simply three layers superimposed. When selecting the model, don't just stare at the bellows, the inner configuration is the key.
High temperature flue gas, corrosive medium, high speed dust, three kinds of working conditions and three kinds of inner layer beating methods
Different working conditions, the design ideas of the inner layer are very different. Let's say one by one:
High temperature flue gas (e.g. power plants, boilers)
The flue gas temperature is frequently 600-900 DEG C, and it also contains sulfide. At this time, the inner layer of the metal expansion joint must be made of heat-resistant alloys, such as 321 stainless steel, 310S stainless steel, or even nickel-based alloys. The guide tube should be made into a double-layer structure, with an air insulation layer in the middle, or filled with ceramic fibers. This site's High temperature axial expansion jointIt is dedicated to this, and the inner layer can withstand the short-term impact of 1000℃. Remember: Never use a regular 304, it will brittle quickly in high temperatures.
Corrosive media (e.g. chemical, desulfurization)
Sulfuric acid, hydrochloric acid, hydrofluoric acid... This thing bites when it hits metal. Lined with tetrafluorine (PTFE) is the mainstream solution, such as PTFE-lined hoseAndPTFE compensator, first-class corrosion resistance. But there is a pit: PTFE is afraid of negative pressure, so under negative pressure conditions, steel wire skeleton has to be added or FEP/PFA has to be replaced. In addition, if the temperature exceeds 200℃, don't think about PTFE. Consider high nickel alloy or Hastelloy as the inner layer.
High-speed dust (e.g. cement, steel)
The air duct on the cement production line has high dust concentration and fast flow rate, and the ordinary guide tube is worn out in half a year. How to fight? The guide tube is made of wear-resistant steel (such as NM400), the inner wall is welded with tungsten carbide coating, or the replaceable wear-resistant bushing is directly installed. This site's Metal Corrugated Expansion Joints in Cement IndustryIt comes standard with thickened deflectors and wear-resistant coating-this is not icing on the cake, but life-saving.
A case: The inner layer of a cement customer burned through. What is the problem?
When I mentioned that cement customer at the beginning, I carefully asked their working conditions: the temperature design value of the flue gas at the outlet of the preheater is 800℃, but the actual fluctuation is 950℃. The expansion joint guide tube they bought is made of 304 stainless steel with a thickness of 4mm. And guess what? Three months later, the guide tube burned through, the high-temperature flue gas directly eroded the bellows, and the whole pipe burst.
What's the problem? Two: First, the material selection is wrong. 304 oxidizes extremely fast above 800°C and must be used with 310S or Inconel 600. Second, the thickness and structure of the guide tube are neglected. 4mm is too thin, at least 6mm, and it is best to add heat insulation to the inner wall of the guide tube. The client later changedHigh temperature axial expansion jointThe inner layer is insulated with 310S plus ceramic fiber, and there has been no problem for two years. The lesson is one sentence: the material and thickness of the inner layer must match the actual peak temperature, and don't hold on to the design value.
The two easiest pits to fall into when selecting the inner layer
The first pit: only look at the thickness of the outer wall and ignore the inner material.
Many purchasers take the product atlas and ask, "How thick is this bellows?" Thickness is important, but the material and structure of the inner layer are the ones that determine the longevity. It is also 3mm thick, and the temperature difference between 304 and 310S is twice the same. Whether there is a deflector and a liner, you must ask clearly.
The second pit: the gap between the inner layer and the bellows is left too large or too small.
If the gap is too large, the medium generates vortex in the gap, which accelerates local wear; The gap is too small, and the inner layer and bellows are extruded and deformed after thermal expansion. There is usually a 3-10mm gap between the guide tube and the bellows (calculated according to the diameter and temperature). This detail is not mentioned by many manufacturers. Have you also encountered abnormal noise and vibration after installation? Eighty percent of the gap was miscalculated.
Corrosion protection of flange sealing surfaces. Many people do the inner layer well, but the flange surface forgets the lining, and as a result, corrosion drills through the flange seam. Therefore, the inner layer must extend to the sealing surface of the flange and cover the whole.
Take the inner layer into account, and the expansion joint is selected in place
To say a thousand words, choosing a metal expansion joint is not just choosing a bellows model. Temperature, pressure, media composition, particle content, flow rate-these parameters directly determine what material, thickness and structure the inner layer is made of. Products on this site, such asMetal Corrugated Expansion Joints in Cement Industry、High temperature axial expansion joint、PTFE-lined hoseThe inner layer configuration of each series is different, which is customized for different working conditions.
What to save? Don't save the inner layer. If the inner layer is selected correctly, the expansion joint will last for ten years without any problem; If you choose the wrong one, it will be scrapped in half a year, and the maintenance fee will be enough to buy three new ones. Is the plumbing system in your hands worth the risk?