1. Why do you have to use non-metallic expansion joints for hot primary air ducts?
How bad is the working condition of hot primary air duct of power station boiler? The temperature is often 350℃ ~450℃, and even exceeds 500℃ for a short time; A large number of unburned pulverized coal particles are sandwiched in the air duct, and the scour speed is more than ten meters per second; In addition, the three-dimensional displacement caused by boiler start-stop and load fluctuation-axial expansion and contraction, lateral deflection and angular displacement are all available. You put metal expansion joints on? In this environment of high temperature + dust washing, the metal bellows will crack or wear out in fatigue within a few months, and then it will be shut down for maintenance, resulting in heavy losses.
Non-metallic expansion joint (to be precise, it is of this siteNon-metallic expansion joint (fabric fiber expansion joint)) was born to deal with this kind of scenario. Its flexible skin can absorb large displacement, the inner layer of heat insulation cotton carries high temperature, and the guide tube guides the air flow to the center to avoid coal powder directly rushing into the skin. To put it bluntly, metal parts are hard, while non-metals are "soft to overcome rigidity"-that's why on the hot primary air pipeline of the power station, everyone chooses non-metals by default.
2. Disassemble to see the structure: skin, thermal insulation cotton, frame, guide tube
This non-metallic expansion joint in the station looks like a large "sandwich biscuit", but in fact, its internal structure is quite specific. The outermost layer is a fabric skin coated with fluororubber or silicone rubber, which is responsible for sealing and resisting the external environment. The thermal insulation cotton in the middle clip is generally made of ceramic fiber or aluminum silicate fiber cotton, and the thickness is determined according to the temperature, which can effectively insulate the heat from being conducted to the frame. Further inside is the metal frame, usually Q235 or weathering steel, on which the skeleton of the whole expansion joint depends. The most important thing is the guide tube-this thing is installed inside the pipe, extends in the direction of the airflow, and directs the high-temperature dusty airflow away from the skin. Think about it, if there is no guide tube, the pulverized coal in the airflow hits the skin directly, which is not the same as sandpaper polishing?
Frequently asked questions and answers in the station about the direction of installation of the deflector"Specific Function of Expansion Joint Guide Tube"It is very clear in: the direction of the arrow is the flow direction of the medium, and it must not be installed backwards. When installed backwards, the airflow flows backwards, and the guide tube itself becomes the windward side, doubling the wear.
3. Compared with metal expansion joints, what are the advantages of non-metal?
Take oursCorrugated expansion joint for power station industrySpeaking, the metal expansion joint is indeed the king in steam pipelines and high-temperature and high-pressure oil pipelines, but when it comes to low-pressure, large-displacement and dusty situations such as hot primary air, it is beyond its power. The advantages of non-metallic expansion joints are endless in one breath:
- Large amount of compensation: The axial displacement of a single non-metallic expansion joint can be ±50mm or even greater, and the lateral displacement can easily absorb 20~30mm. The metal bellows needs the same amount, and multiple layers of corrugation can be obtained, which doubles the cost.
- Corrosion resistance: The skin is made of polymer material, so it is not afraid of acid and alkali flue gas. The low-temperature flue gas pipeline after boiler desulfurization can still be used.
- Vibration and noise reduction: The fabric skin itself has damping, and the vibration of the pipe is transmitted by it first.
The pressure resistance of non-metallic expansion joints is weak, generally not more than 0.1MPa (100kPa). The pressure of hot primary air duct is usually 2~5kPa, which is completely fine. If you use it on compressed air or steam systems, wait for it to burst. Don't make a mistake about the pressure when selecting a model-this is a pit that many newcomers step on.
4. Five pits that are easy to step on in the selection
The first pit: leave too little temperature margin.Some purchases only look at the working temperature of 400℃, so they choose the skin with temperature resistance of 400℃. However, there will be short-term overtemperature in the operation of the boiler, such as the hot air returning after the coal mill trips, and instantly rushing to 500℃. Once the skin is overheated, the silicone rubber is carbonized directly. It is recommended to leave a 20% margin and choose at least the specification with a temperature resistance of 500℃.
Second pit: Install the guide tube in reverse direction.As stated above, the guide tube arrow must follow the direction of the airflow. During on-site installation, some masters welded directly without looking at the marks. As a result, the airflow recoiled, and the skin wore and leaked for half a year.
The third pit: I don't know how to adjust the tie rod.The tie rod on the expansion joint is not used to screw dead and fix it. Its role is to limit overstretching while allowing normal displacement. Inside the reference station"How to adjust the tie rod nut of the expansion joint"Experience: When installing, first loosen the tie rod nut by 2~3 buckles, so that the expansion joint is in a free state; After the pipeline pressure test, adjust the nut to the position corresponding to the design displacement. Don't screw to death, screwing to death is equivalent to locking the expansion joint and losing the compensation function.
Pit 4: Dust scour is ignored.Even with a guide tube, if the pipe elbow is too close to the expansion joint, the airflow deflection will still wash down to the skin. At this time, it is necessary to install a wear-resistant liner plate or extend the guide tube at the inlet side of the expansion joint.
The fifth pit: frame preservation is ignored.Frame in open air environment, long-term moisture, weld first rust through. Make at least hot-dipped zinc or weather-resistant steel.
5. Installation and routine maintenance
"Don't screw the bolts, don't lock the draw rods, and don't crush the heat preservation to death"。 Just tighten the flange connecting bolts according to the torque. Don't apply force to the dead, otherwise it will crush the edge of the skin flange. The adjustment of the tie rod nut, already mentioned above. The insulation layer is wrapped outside the expansion joint, but be careful-once the insulation cotton is damp, the thermal conductivity soars, the heat is directly transferred to the skin, and the life is plummeted. Therefore, the insulation layer must be waterproof, and an inspection port must be left to facilitate regular inspection of the skin state.
Daily maintenance, the experience in the station is to check once a quarter: see if there are cracks, blisters and wear on the skin surface; Press the insulation cotton with your hand to see if it has collapsed; Are the tie rod nuts loose (if loose, refer to"How to adjust the tie rod nut of the expansion joint"Re-tune). In addition, each time the boiler is overhauled, the insulation must be disassembled for inspection, because some hidden dangers are hidden under the insulation layer.
6. Common faults and handling
This is the most common malfunction. If it is a small hole or crack, you can temporarily plug the leak with special repair glue, but it is recommended to replace the skin in the whole circle after shutting down. Never use a welding rod to weld, it will burn out the surrounding skin.
Long-term vibration causes flange bolts to loosen, tighten them in time after discovery, and install spring washers or anti-loosening nuts.
Generally, the thermal expansion of the frame is uneven due to overtemperature, or the guide tube falls off and hits the frame. Treatment method: First check whether the guide tube is in good condition, then correct the frame, and re-reinforce it if necessary.
Don't panic when you encounter these situations, check the national standardsJB/T 12235-2015, which stipulates the technical requirements and inspection methods of non-metallic expansion joints. If you follow the standard, there will be a basis for maintenance.