Two days ago, I met a friend who made desulfurization pipelines. When I came up, I asked, "Can non-metallic expansion joints really withstand high-temperature and high-pressure pipelines?" His doubts were not unreasonable. In many people's impression, non-metal = cloth bag, which breaks when poked. In fact, high-pressure non-metallic expansion joints that are really used in the right place have been running in power plants, cement plants and chemical plants for seven or eight years.
Today, I will explain this thing thoroughly around the five most deadly questions.
1. Under high-pressure working conditions, why can non-metallic expansion joints replace metal parts?
Let's start with the principle of bearing pressure. The metal expansion joint is deformed by the corrugated thin wall, and the pressure bearing depends on the elastic limit of the material; Nonmetallic expansion joints are different in that they rely on the "flexible restraint" of multiple layers of composite fabric and rubber-the pressure acts on the fabric and is evenly dispersed throughout the circumference, rather than concentrated on a crest. Therefore, as long as the structural design is reasonable, non-metallic parts can run stably in the range of 0.1MPa to 2.5MPa, and individual special designs can achieve higher levels.
In terms of temperature resistance, the hard strength of non-metals is high temperature resistance: the ceramic fiber layer can carry more than 1000°C (with heat insulation layer), while the ordinary metal bellows has to be lined or switched to high nickel alloy above 600°C, and the cost doubles several times. Not to mention the compensation ability, non-metallic expansion saves energy and absorbs multi-dimensional displacement (axial, transverse and angular), which is especially suitable for large-size rectangular pipes-such as flues and air ducts. If metal parts want to make such a large compensation, the volume and cost are scary.
Then, "Are non-metals easier to leak"? Haha, this question has to be asked the other way around: Which qualified non-metallic expansion joint have you seen is "air leak" leak broken? Non-metallic failures are mostly tearing, aging, and abrasion, not "leakage"-because its sealing surface is continuous and integral, unlike metal bellows with weld seams. The real weakness lies in the wrong selection or installation, which has nothing to do with the material itself.
2. One layer of cloth and one layer of glue? Understand the structural doorway
The layman sees it as "a layer of cloth and a layer of glue", while the insider sees a set of systematic engineering. The typical structure of high-pressure non-metallic expansion joints is roughly divided into four parts:
- Fabric fiber layer (skeleton): Main stress layer, common glass fiber cloth, ceramic fiber cloth, and even stainless steel wire mesh reinforcement layer. It determines the strength and upper temperature resistance limit of the expansion joint.
- Rubber/PTFE composite layer (sealing layer): Responsible for air tightness and corrosion resistance. Fluorine rubber is suitable for high temperature and acid resistance, and tetrafluorine (polytetrafluoroethylene) is suitable for strong corrosion, but the elasticity is weak. In high-pressure designs, rubber and PTFE are often used on top of each other-such as the "rubber PTFE compensator" in the station.
- Metal frame and platen: At both ends of the non-metal ring band, clamp and secure to the pipe flange with a metal frame. Under high pressure conditions, the bolt pre-tightening and pressure plate stiffness of this part directly determine whether the seal can withstand pressure pulsation.
- Guide tube (liner tube)A lot of people ignore this stuff. In the dust-containing airflow, the guide tube can protect the ring belt from being worn by particulate matter; In high-speed airflow, it also prevents swirling currents from impacting the inner walls. When selecting the model, be sure to confirm the material and direction of the guide tube.
Therefore, "high pressure" is not to thicken the cloth a little, but to calculate the force, temperature gradient and corrosion allowance of each layer of materials to make a combination. Why can't it be carried hard by a single layer of material? Because a layer wants to be high temperature resistant, stretch resistant, corrosion resistant and sealed, no material can be all-inclusive. Combined structure is the key to high voltage of non-metallic expansion energy saving.
3. Settle accounts first in type selection: pressure, temperature, displacement and medium
If one of these four parameters is missing, it is easy to cause accidents. According to the requirements of national standard JB/T 12235-2015 "Non-metallic expansion joint", the selection must provide: design pressure (not working pressure!), design temperature, displacement in each direction (axial/transverse/angular direction), medium composition (dust concentration, corrosive gas).
Many users report "high voltage", but it is not really high voltage. For example, the pressure in the flue gas pipeline is only 0.05MPa (that is, the slight positive pressure after the fan overcomes the resistance), and the vibration and pulse force are not small-in this case, taking pressure as the main index and ignoring the fatigue life means abandoning the basics and chasing the last. On the other hand, some compressed air pipelines are nominal to 1.6MPa, and the actual operating pressure fluctuates greatly and starts and stops frequently. If you only choose according to the steady-state pressure, the ring belt will be quickly broken by the "pump". Therefore, be sure to write clearly when reporting parameters: -Maximum operating pressure and pressure pulsation amplitude -Maximum instantaneous temperature and duration (e.g. blow pipe condition) -Dust content, particle size and flow rate in the medium -Bracket arrangement of the pipe (determines how much displacement the expansion joint actually bears)
Don't bother. The conversion of these two numbers is wrong, from leakage to shutdown, to bursting and injuring people.
4. When a bolt is twisted during installation, the high-pressure expansion joint may be scrapped in half a year
If you choose the right model, it will still be useless if the installation is not strong. A few details, each of which is a lesson learned by the accident:
Butt tube fixed.The non-metallic expansion joint itself does not absorb the blind plate force generated by the internal pressure of the pipeline, so the pipeline at both ends must be provided with fixed brackets or guide brackets, and it must not be allowed to "borrow force". Under high pressure conditions, the blind plate force is amazing, the pipe frame is not done well, and it is only a matter of time before the expansion joint is pulled off.
Cold tight pre-displacement.When the ambient temperature is greatly different from the operating temperature at the time of installation, pre-displacement is required-the expansion joint is pre-biased by a quantity in the opposite direction of displacement. In this way, the actual displacement can really fall within the design range during operation. Without preloading, it is equal to wasting half of the compensation ability. Don't reverse the specific direction. The manufacturer's drawings are clearly marked. Look twice before installing.
Inner liner barrel direction.The opening direction of the guide tube must follow the direction of the medium flow. If it is flipped, the airflow directly washes away the corrugation or fabric layer, and a hole will be grinded for you in a few months.
Finally, don't try to save trouble with the air pressure test after installation. When testing the pressure with compressed air, the boosting pressure must be kept in grades, and it cannot be hit in place at once; After the test, the pressure should be completely relieved, and then the pipeline should be purged with nitrogen to remove the residual moisture and impurities. Otherwise, once the wet medium enters, the life of the PTFE layer and the rubber layer will be greatly reduced.
5. It leaked after three years of use. When I disassembled it, I saw that the problem lies in "low-voltage selection"
Share two real troubleshooting cases, both of which are typical examples of "laying mines during selection and exploding during operation".
Case 1: The flue gas pipeline pulse pressure is underestimated.
The non-metallic expansion joint used in the flue gas pipeline of the sintering machine head in a steel mill leaks frequently after two years of operation. Disassembly and inspection, the loop belt is intact, but the bolts on the metal pressure plate are all loose, and the pressure plate cuts out a cut in the loop belt. Check the design parameters: the design pressure is 0.15MPa. When the induced draft fan is started in actual working conditions, the system has a low-frequency pulsation of 1.2Hz, and the instantaneous peak pressure reaches 0.35MPa. Under this cyclic load, the bolt gradually relaxes, the pressure plate loses the clamping force, and the edge of the ring belt slips and wears until it breaks. Solution: Check the fatigue according to the pulsation peak value again, use the double-row bolt pressure plate instead, and add the limit tie rod outside the expansion joint, so that the problem can be solved.
Case 2: A rubber PTFE compensator is used in an overtemperature position.
Hot air ducts in a chemical plant, the medium temperature often fluctuates between 180°C and 220°C. When purchasing, I saved trouble, so I directly used the standard model "rubber PTFE compensator" in the station-the normal temperature resistance range of this product is-20°C to 120°C, and the PTFE layer begins to soften at 180°C, and the rubber layer can't bear it. As a result, after less than one year of use, the inner rubber was coked, PTFE cracked, the medium penetrated into the fabric layer, and the whole circle was scrapped. Later, it was replaced with a specially designed "non-metallic expansion joint (fabric fiber expansion joint)", which was coated with polytetrafluoroethylene film and ceramic fiber insulation layer. The temperature was matched to 250°C, and it has been in good condition so far.
To put it bluntly, the core of high-pressure non-metallic expansion joint selection is to match the working conditions and accurately calculate the four numbers of pressure, temperature, displacement and medium. Don't be cheap, don't try to save trouble, give all the data, and the manufacturer can give you a durable and economical solution. If you are not sure, find an experienced manufacturer to send the working condition table and let them help you calculate it-it is much more reliable than patting your head in the office.