Non-metallic expansion joints, to put it bluntly, are a layer of fabric fibers with rubber or PTFE lining, and then pressed into a corrugated shape. Compared with the metal corrugated expansion joint, the biggest difference is that it is born with corrosion and high temperature resistance, but its pressure bearing capacity is weak. The metal relies on stainless steel bellows to carry hard, and the non-metal relies on flexible fabric to block soft. The structure determines that it is most suitable for absorbing multi-directional displacements-lateral, angular and axial, and the installation space requirements are low. However, remember that it is afraid of high pressure, and the design pressure generally does not exceed 0.1MPa. If it exceeds, you have to add a metal frame or directly use metal.
Thermal power plants: standard in desulfurization flue
Power plant flue duct systems, especially the front and rear of desulfurization flue gas baffle doors and dust collector inlets and outlets, those with low temperature corrosive media (such as sulfur-containing wet flue gas) and large diameter rectangular ducts, non-metallic expansion joints (especiallyRectangular non-metallic expansion joint) Almost standard. Why? Because the metal bellows is prone to stress corrosion cracking here, the weld leaks as soon as it cracks. The non-metallic fabric layer and rubber lining can hold acid solution with a PH value of 2-3, and the temperature below 150℃ can hold it. Moreover, the rectangular pipe has a large cross-section, and the metal expansion joint is either expensive or insufficient compensation, so it is enough to press a few pleats casually for non-metallic ones.
For example, the front and back of the desulfurization flue gas baffle door, cooperate with the station'sDesulfurization flue gas baffle doorAndElectric plug-in insulation doorWith non-metallic expansion joints to block vibration and displacement, the effect is much longer lasting than that of metal. Measured data: The desulfurization flue of a 600MW unit was originally made of stainless steel metal expansion joint, and it was perforated in less than two years; Replace withRectangular non-metallic expansion jointIt has been running for four years without any problems.
Cement Industry: The Nemesis of High Temperature Dusty Gas
The raw mill, grate cooler and preheater in the cement production line are characterized by three words: heat, dust and grinding. The gas temperature is 200-400℃, and the dust concentration is as high as 100g/Nm³. The bellows welds of metal expansion joints are particularly prone to cracking in this environment-thermal fatigue plus abrasive scour, and they will be scrapped in one year. WhileFabric fiber expansion joint(That is, this station'sNon-metallic expansion joint) But it can be carried, because the fabric layer itself is heat-resistant, and the lining is made of polytetrafluoroethylene or silicone rubber. When dust is washed, the surface is smooth and does not accumulate dust, and it has good flexibility, so it absorbs heat expansion and does not produce stress concentration.
However, it should be noted that metal bellows in the cement industry are not useless. For example, the outlet of the high-temperature fan, the pressure is slightly higher, it is suitable for this stationMetal Corrugated Expansion Joints in Cement Industry。 The best solution is a "non-metal + metal" combination: non-metal for high-temperature and low-pressure sections, and metal for high-pressure and medium-temperature sections. For example: the outlet pipe of the grate cooler in a cement plant was originally made of metal bellows, which cracked three times in two years; Later, it was used for the inlet of the grate coolerHigh temperature axial expansion joint(Metal), non-metallic expansion joint for export, which has not been replaced for four years now.
Chemical and metallurgy: Acid, alkali and steam can be switched at will
In chemical plants, corrosion resistance is the first priority. Concentrated sulfuric acid, hydrofluoric acid, steam…Each medium has its own non-metallic expansion joint. Such asPTFE compensatorThe lining is pure PTFE, resistant to strong acids and alkali, the temperature can reach 200 ℃, and the flexibility is good, which can absorb the displacement in multiple directions. AndRubber PTFE compensator is to combine rubber and PTFE together, which is corrosion-resistant and cheap, and is suitable for acid pipelines with low pressure.
Rubber compensators are more grounded-they are everywhere in water treatment and circulating water systems. It can isolate vibration and noise, and it is easy to install. For example, the water pump outlet pipe can absorb vibration with a rubber compensator and protect the valves and equipment behind it. However, rubber is afraid of oil and high temperature. If the medium contains oil or the temperature exceeds 80℃, it has to be replaced with PTFE.
Common pits in model selection: installation space, oil with water, pressure fluctuation
Although non-metallic expansion joints are easy to use, many people step on pits. I summarize the three most typical minefields:
- Limited installation space: In some sites, the pipe spacing is particularly small, and the non-metallic expansion joint needs a certain expansion space. If the reserve is not enough, the fiber will be stretched and torn. The solution is to increase the compensation amount when selecting the type, or use the structure with the guide tube. This site'srubber compensatorAndRectangular non-metallic expansion joints all support custom lengths, but mounting dimensions must be provided in advance.
- Medium oily with water: Rubber lining (including butyl rubber and neoprene rubber) will swell when exposed to oil. Although PTFE lining is oil-resistant, the interface seal is not good or leaks. What is even more pitful is that the wet flue gas condensate water accumulates at the bottom of the corrugation, and long-term soaking will soak the fabric layer. Countermeasures: When selecting the lining material, it is necessary to clarify the composition of the medium. If it contains oil, add PTFE, and if it contains water, add hydrophobic structure (such as opening a drainage hole at the bottom of the expansion joint).
- Pressure fluctuation: The pressure limit of non-metallic expansion joints is usually only 0.1MPa. If the system has frequent pressure fluctuations (such as blower start and stop), the non-metallic fabric will be bulged or even burst by impact. In this case, either metal is used (such as this station'sUniversal corrugated expansion joint), or add stainless steel mesh sleeve to reinforce the non-metal periphery.
What if I run into these pits? This site'sPTFE compensatorAndThe rubber PTFE compensator is specially designed for oil with water. The PTFE layer can resist corrosion, and the rubber layer can buffer pressure fluctuation. If the space is too small, you can still use itSleeve type pipe expansion jointSubstitution (of metals). In short, give the working condition parameters before selecting, don't rely on feeling.
Nonmetals save more money than metals? The conclusion is not necessarily
The purchase price of a rectangular non-metallic expansion joint with a diameter of 2 meters may only be 60%-70% of the equivalent functional metal expansion joint. However, the life of non-metallic expansion joints is usually 3-5 years, and the life of metallic expansion joints can reach 10 years. The cost alone is not necessarily cost-effective. But why are design institutes and owners increasingly inclined to the "non-metal + metal" combination scheme?
Easy maintenance and replacement。 The fabric ring belt of the non-metallic expansion joint can be replaced separately, without removing the flange, and can be done after two days of shutdown. Once the metal expansion joint leaks, it is often necessary to cut the pipe and replace it with a new one, and shut down for a week to start. In addition, the non-metallic expansion joint has very little thrust on the pipeline, which can save the cost of reinforcing the bracket. Comprehensive calculation shows that the whole life cycle cost is low.
Do you dare to use all non-metals in your working conditions? If the pipeline pressure does not exceed 0.05MPa, the temperature is lower than 400℃, and there are no hard particles in the medium to be directly washed, it can be completely unified with non-metals. Otherwise, let's honestly match the metal.