Last summer, the desulfurization flue of a coastal power plant was shut down for maintenance. The reason was dumbfounded-the flange gasket was aging, and the bolt preloading force was uneven. The flue gas came out of the gap, and the whole line was forced to land the load. Lao Wang, who is in charge of the maintenance, told me, "The corrugated pipe material cost a lot of money, and the result was planted in the connection method." Indeed, non-metallic expansion joints (fabric fiber expansion joints, rubber compensators, rectangular non-metallic expansion joints) are good at carrying heat expansion and cold contraction in flue ducts and desulfurization systems, but if the connection method is not selected correctly, none of them can run away from air leakage, running and shutdown.
1. Flanged connection: the most common pit, often the deepest
Flange connection is the most commonly used way of non-metallic expansion joints, none of them. Regardless of the siteNon-metallic expansion joint (fabric fiber expansion joint)StillRectangular non-metallic expansion jointThe mating flanges will be equipped when they leave the factory. But when installed on site, three details are prone to rollover.
flange surface flatness— — I have seen the on-site pipe flange tilt up by 3mm after welding, and the workers firmly plugged the asbestos gasket flat. As a result, it leaked one week after it was put into operation. Correct practice: the flatness error of the matching flange is ≤1mm/m, otherwise it should be leveled with sealant or machining.
bolt preload force-Many people think that "the tighter it is screwed, the less it leaks", which may be right for metal pipes, but it is a disaster for non-metal expansion joints. The loop tape (fabric fiber layer) is pressed too tightly by the bolt, the edge stress is concentrated, and it tears in a few months. This site'sRectangular non-metallic expansion jointClearly mark the torque value in the installation manual. For example, the preload force of M16 bolts should be controlled at 80-100N·m. Don't rely on the feel.
Gasket selection— — The flue gas temperature is high and contains sulfur, so ordinary rubber gaskets can't bear it.Rubber PTFE compensatorCommonly used integral flange + PTFE gasket, corrosion resistance and good fit. But note: the gasket should not be too thick, otherwise the bolt will not be compressed firmly; It should not be too thin, otherwise the compensation joint will be easily displaced when vibrating. According to JB/T 12235-2015 standard, the gasket compression ratio is controlled at 25%-30% to be the most stable.
Used in a cement plantNon-metallic expansion joint (fabric fiber expansion joint)Connecting the cyclone outlet, the workers tried to save trouble and screwed all the flange bolts to the maximum torque. As a result, the ring belt tore a hole along the bolt hole, and the smoke was sprayed with dust for half an hour. Later, I changed the preload wrench and never happened again.
2. Connection between clamp and buckle edge: save money? Temperature resistance passes first
Many people like to connect low-pressure large-diameter pipelines with clamps or buckles, which are cheap and convenient to disassemble. But here's the paradox: You think clamps save money and hassle? Wrong, the temperature resistance rating does not match the fastener material, and it will come loose in a few months.
Clamps are mostly used in circular pipes, such as the flue section behind the flue gas baffle door. However, the bolts and spring sheets of the clamps will creep at high temperatures-when the temperature exceeds 200℃, the elastic modulus of ordinary carbon steel buckles decreases by 30%. A stainless steel 304 clamp was used in a desulfurization project. Half a year later, the bolts were loosened due to thermal cycle, and acid fog emerged at the interface. Later, it was replaced with a 310S clamp to stabilize.
This site'sRectangular non-metallic expansion jointThe structure of buckling edge + strip is mostly used: the edge of the expansion joint band is flanged, embedded in the flange groove, and the outside is fixed with an entire strip. This design is particularly friendly to the four corners of a rectangular pipe, and there will be no case where the clamp can't press the R corner. However, if the material of the strip is not selected well, it will also roll over-ordinary galvanized strip will rust and wear in the wet desulfurization environment for three months. It is recommended to use 316L or FRP composite strips.
Have you ever used a failed clamp case on your project? Chat in the comments section.
3. Hot vulcanization bonding and integral molding: Don't be superstitious about the word "sticky"
Rubber non-metallic expansion joints, such as this station'srubber compensatorAndRubber PTFE compensatorThe connection method is commonly bonding or integral vulcanization molding. However, many purchases are biased by sales talk: "We use imported glue, which sticks firmly!"-Not enough bonding strength? That's because you didn't follow the standards.
High temperature flue gas section (> 120℃) must use hot vulcanization process。 The essence of vulcanization is that rubber molecules are crosslinked with reinforcing materials to form chemical bonds, and the peel strength can reach more than 50N/mm. In the normal temperature section (≤80℃), cold adhesion and special primer can be used, and the peel strength can also reach ≥25N/mm as required by JB/T 12235-2015.
I met a customer who used a cold-stick rubber compensator in a hot air duct (180℃) of a power plant, and the bonding surface degummed after two weeks. During the maintenance, it was discovered that the cold adhesive glue softened at high temperature and couldn't hold it at all. So,Not all non-metallic expansion joints can be bonded— — When selecting the model, be sure to ask clearly: What is the medium temperature? Is it frequent start and stop?
Integral molding is to directly mold rubber or PTFE into a compensator with flanges, such as this station'sPTFE compensator, the flange and the corrugated body are formed at one time, there is no bonding interface, and the reliability is higher. But the cost is also high, and it is not suitable for oversized sizes (such as rectangular sections above 5 meters).
4. Special structure: pipe welding and composite seal
In some operating conditions, the standard connection method is simply not sufficient. For example, double seal structure-learn from this siteRound Flap Door (Double Seal)The idea that the non-metallic expansion joint can also be made into two layers of seal inside and outside. The inner layer is covered with fabric fiber skin to absorb displacement, and the outer layer is protected by metal deflector + temperature-resistant sealing strip. This structure is commonly found in high temperature fan inlets and outlets that require zero leakage.
Such asHigh temperature axial expansion jointNon-metal alternative-welded to the pipe with stainless steel through pipe, covered with multiple layers of skin (silicone cloth + ceramic fiber + stainless steel wire mesh), the design logic of layered overlap is simple: each layer is responsible for a function-the innermost layer is insulated, the middle layer is pressurized, and the outer layer is corrosion-resistant. The weld position must be staggered to avoid thermal stress concentration. After one project did so, it was extended from quarterly maintenance to biennial maintenance.
In addition,Double hinge expansion joint for air-cooled island vacuum pipelineAlthough it is mostly used in metal pipes, its hinge compensation concept can also be transplanted to non-metals: the universal joint structure allows the displacement in three directions, and the composite gasket is added at the connecting flange, which has a very stable effect.
To summarize – there is no best way to connect, only the most suitable. If the medium temperature is lower than 150 ℃ and the pressure is slightly positive, the flange connection is sufficient; Large-diameter low-pressure buckling strip; Vulcanization or integral molding for high temperature; If you want zero leakage, go to double seal. This site'sNon-metallic expansion jointCovering fabric fiber, rubber, PTFE and other materials, the connection methods can be customized from flange, clamp to vulcanization bonding. The key is to select according to parameters-the temperature, pressure and displacement are clearly reported, and we will help you match the scheme without running.