When eight out of ten people in the dry flue gas pipeline industry choose non-metallic high-temperature expansion joints, the first sentence is "how many degrees can they carry". Of course, the temperature resistance number is important, but if you only stare at this number, sooner or later, the installation and operation will go wrong.
The fundamental difference between non-metallic high temperature expansion joints and metallic expansion joints is not in who is more temperature resistant, but in the compensation mechanism. The metal corrugated expansion joint absorbs the displacement by the deformation of the corrugated shell, and has great stiffness and large reaction force; The non-metallic expansion joint is deformed by fabric fiber and flexible insulation layer, which has low stiffness and much less load requirement for pipe support. Especially in high-temperature smoke duct such as large cross-section, low pressure and complicated displacement direction, metal expansion joints are often heavy and expensive, and are prone to fatigue cracking. Non-metallic, a loop with thermal insulation cotton can absorb axial, lateral and angular displacement at the same time, which is why it is preferred by power plants and cement plants.
The upper temperature limit is not a number printed on the nameplate
The upper temperature resistance limit of non-metallic expansion joints is determined by three things: the innermost temperature resistant material, the design of the intermediate insulation layer, and the overall structural form. Missing one, the nominal temperature is armchair.
The innermost layer directly contacts the smoke, generally using glass fiber cloth, ceramic fiber cloth or high silicon oxide cloth. Ceramic fiber can carry above 1000℃, but it is afraid of high-speed airflow scour; Glass fiber is cheap, but the strength drops sharply above 600℃. The intermediate insulation layer is the real technical work. If the thickness is not enough, the surface temperature will penetrate into the outer fluororubber or silicone coating. As soon as the coating ages, the whole ring belt will be brittle. The structural form is equally critical, single layer or multi-layer, whether wire mesh is set to reinforce, and whether there is an outer guard plate, all directly affect the actual operating temperature.
Don't be fooled by the "instantaneous temperature resistance of 1200℃" on the sample. That number refers to short-term thermal shock. Under continuous operation conditions, the allowable temperature is often discounted by 30% or even half. The flue gas of your project is 850℃ for a long time. When selecting the model, you have to check the configuration of the insulation layer according to the continuous temperature resistance of at least 900℃.
Don't just focus on temperature: There are four parameters waiting for you
Pressure, displacement, medium corrosiveness and guide tube configuration are the four places that really open the gap.
Pressure, the pressure bearing by non-metallic expansion joints is generally not high, generally from a few kPa to tens of kPa. However, there is a negative pressure condition in the flue gas pipeline. The negative pressure will make the ring belt sink inward and the heat insulation cotton will be sucked away, so the negative pressure support ring or reinforcing rib must be considered in the design.
The displacement should be calculated accurately. The thermal displacement of smoke duct is often not in a single direction. You have to mention all three-way displacements (axial, transverse and angular) to the manufacturer. Some design institutes save trouble and only give a comprehensive displacement. As a result, the expansion joint twists during actual operation, and the local stress of the ring belt is too large, which will tear in a few months.
Media corrosiveness is easily overlooked. There are SO₂ and HCl in the desulfurization flue gas, and there is a lot of water vapor after wet desulfurization, which will corrode the outer rubber coating. At this time, you have to choose a corrosion-resistant fluororubber band, or add a layer of PTFE lining, which is similar to the ideas of "non-metallic expansion joint (fabric fiber expansion joint)" and "PTFE compensator" in our station, but the structure is more aimed at high-temperature flue gas.
Don't save the guide tube. The wind speed in the high-temperature flue is more than ten meters per second. If there is no guide tube, the airflow directly washes the heat insulation cotton, and no matter how good the ceramic fiber is, it can't bear it. Especially for flue gas with particulate dust, the guide tube must extend a certain distance to both sides of the expansion joint, otherwise the wear is more fatal than the temperature.
Installation and maintenance misunderstandings, I will give you one by one
Pre-stretched. Non-metallic expansion joints generally leave the factory with tie rods or positioning pins, which are used for transportation and installation. It must be loosened after installation to allow the loop strap to deform freely. Some installation teams plan to save trouble and don't dismantle it. As a result, once the pipeline expands thermally, the force is all held on the expansion joint, and the ring belt is torn or a trivial matter, so it will be troublesome to destroy the equipment.
Direction markers. Non-metallic expansion joints usually have flow direction arrows on them, which refer to the direction of media flow. This stuff is not randomly drawn. Because the direction of the guide tube and the overlapping direction of the inner and outer layers are all related to the flow direction, if the guide tube is installed backwards for a ride, the resistance will be greatly increased, and the ring belt will be easily lifted up. Two days ago, a buddy called and said that they had installed it backwards on the spot, dismantled it and reinstalled it, and the construction period was delayed for a week.
Emergency treatment after the fabric layer is broken. What should I do if the ring belt is scratched by a foreign object in the running? Take the glass glue and wipe it? Don't make a fuss. The correct emergency approach is to temporarily seal the break with high-temperature-resistant tape, then cover the outer surface with a ceramic fiber cloth and tie it tightly with stainless steel clamps. Replace the entire loop belt when the machine is shut down for maintenance. Some units use metal patches to weld directly on the frame of the expansion joint, and as a result, the flexible structure is welded to death, which is equivalent to no expansion joint installed, so it is strange that it does not crack.
Real Cases: Lessons from Cement, Power and Desulfurization Projects
The inlet of the waste heat boiler at the kiln head of a cement plant is a non-metallic high-temperature expansion joint, with a design temperature of 950℃ and an actual operating temperature of about 880℃. After half a year, a large area of cracks appeared on the surface of the ring belt. When removed, the insulation layer is not thick enough, and the external surface temperature is as high as 150℃, which exceeds the tolerance limit of the coating. Later, it was replaced with double-layer insulation cotton and air insulation layer, and the external surface temperature dropped to 80℃, and there was no problem.
In the power project, the metal rectangular expansion joint was originally used on the primary air duct of the thermal power unit, which had loud vibration and noise, and the bracket was often loose. After replacing the rectangular non-metallic expansion joint, the vibration is significantly reduced. However, after three months of trial operation, it was found that the insulation cotton inside the expansion joint was blown out of a pit-because the guide tube was made short, the air leakage formed a vortex, hollowing out the cotton bit by bit. After lengthening the guide tube, the problem is solved.
Desulfurization projects are more typical. The expansion joint behind the original flue gas baffle door is belted with ordinary silica gel coating. As soon as the sulfide is eroded, it will be powdered in half a year. After changing to fluororubber ring tape and lining with polytetrafluoroethylene, it took more than two years, and the surface only slightly discolored. Therefore, when selecting a model, don't just ask "whether it is resistant to high temperature". Corrosive media is more terrible than high temperature.
To put it bluntly, the non-metallic high-temperature expansion joint is a systematic project. Temperature is just the ticket, pressure, displacement, corrosion, installation details, each can determine its longevity. No matter how beautiful the selection table you get, it's better to explain the working conditions clearly with the manufacturer and let experienced technicians help you make a plan. Don't be afraid of trouble. Once this thing goes wrong, the money for shutting down the furnace will be enough for you to buy dozens of expansion joints.