Why must non-metallic compensators be equipped with thermal insulation? — — Without looking at the temperature drop curve, design is empty talk
To be honest, after more than ten years of pipeline compensation design, I have seen too many cases of "saving the insulation layer and burning the whole compensator". Non-metallic compensators (which we often call fabric fiber expansion joints) rely on flexible bands to absorb heat displacement, but the band materials-fluororubber, silicone, PTFE-have an upper temperature resistance limit there. Medium temperature 600℃? Direct carbonization of the ring band. You may ask: Then why do manufacturers have heat insulation layers?
Simply put, the function of the insulation layer is to "cut off" the transmission path of high-temperature flue gas to the circular belt. According to the calculation of heat conduction, when no insulation layer is added, the surface temperature of the band ≈ the medium temperature ×0.8 (empirical coefficient). With sufficient thickness of insulation layer, the temperature of the belt can be reduced to less than 30% of the medium temperature. The key is to look at the temperature drop curve: every 10mm increase in the insulation layer, the temperature of the ring belt drops by 8~12℃ on average. If you don't take the temperature drop curve against the table when designing, you are gambling on luck. If you win the bet, you can save a few hundred dollars. If you lose the bet, you can replace a non-metallic expansion joint, and the cost will be doubled ten times.
Typical structure of thermal insulation layer: from inner lining to external application, each layer has active
The thermal insulation layer of non-metallic compensator is not just pasted with a quilt. It is a multi-layer composite structure, and each layer has a clear division of labor:
- Inner layer (thermal insulation): Usually, ceramic fiber felt or aluminum silicate needle blanket is used, and the thickness is selected according to the medium temperature-25mm for below 300℃, 50mm for below 500℃, and above 80mm for below 800℃. This layer is directly in contact with smoke, so it must be able to withstand the erosion of air flow.
- Intermediate layer (vapor barrier layer): In many working conditions, the flue gas contains acidic components (such as wet flue gas after desulfurization). If water vapor penetrates into the insulation layer, the ring belt buckle will be corroded after condensation. Therefore, a layer of fluororubber or polytetrafluoroethylene film should be sandwiched in the middle, which is both vapor insulating and corrosion resistant. This station is in the rectangular non-metallic expansion joint matched by the desulfurization flue gas baffle door, and this vapor barrier layer is standard.
- Outer layer (protective layer): The insulation cotton should be wrapped with a layer of stainless steel wire mesh or glass fiber cloth to prevent it from being crushed by bolts during installation, and also to prevent birds pecking and rats biting. Some power plants will apply a flexible silicone coating to the outermost layer to increase airtightness.
This structure is not dead. Two days ago, I met a customer of a cement factory. The medium had a high dust content, and the inner ceramic fiber felt was worn out in three months. Later, we replaced him with zirconium-containing ceramic fiber + stainless steel wire mesh reinforcement, and the life span was directly pulled to two years. Therefore, when selecting a model, the flue gas composition is more critical than you think.
The easiest pits to step on during model selection: medium temperature, flue gas composition and installation space, none of which can be leaked
Just look at the temperature and not the wind speed. The medium temperature is 500℃, the wind speed is 15m/s, and the same insulation layer thickness, the ring belt temperature is 30℃ higher than that at the wind speed of 5m/s. Because the high-temperature airflow will take away the heat from the surface of the insulation layer and accelerate heat conduction. This problem is especially prominent in corrugated expansion joints used in power station industry.
The sulfur content of the flue gas is not counted. Although the temperature of the flue gas after wet desulfurization drops to 50~70℃, it contains a large amount of sulfuric acid condensate. Once the insulation layer absorbs moisture, the ring belt buckle will corrode and rot for half a year. The perfluorine vapor barrier layer must be selected, and the outer surface of the insulation layer should be hydrophobic coating. This station has taken this as a standard configuration in the matching of desulfurization flue gas baffle door and circular baffle door (double seal).
Not enough space for installation. In many renovation projects, the pipeline spacing is only 200mm, and the thermal insulation layer of non-metallic compensator must be within 150mm. What to do? Change to a higher grade insulation material-nano aerogel felt, the thermal conductivity is only 1/3 of that of ceramic fiber, and the thickness can be compressed by half. The price is doubling the price, but it's better than not fitting in.
Who does it work with? Interface fitting of metal expansion joint, baffle door and rectangular expansion joint
Non-metallic compensators rarely work alone, and they are often used in series with metal rectangular expansion joints, flue gas baffle doors and electric plug-in plate insulation doors. Two details should be paid attention to in interface cooperation:
- Flange spacing: The insulation layer of the non-metallic compensator will take up the axial length. For example, the design compensation amount is ±50mm and the thickness of the insulation layer is 80mm, so the flange spacing should be at least 130mm. If the site space is crushed, it is necessary to study whether the insulation layer can be made detachable-but only if it is convenient to shut down and maintain.
- Height of guide tube: The function of the guide tube of the expansion joint is to guide the flue gas through smoothly and avoid direct washing of the ring belt. However, after adding the insulation layer, the inner diameter of the guide tube should be enlarged correspondingly, otherwise the insulation layer will be scratched by the edge of the guide tube. When the metal rectangular expansion joint is matched, a gap of at least 5mm is left between the outer wall of the guide tube and the inner wall of the heat insulation layer.
In practical projects, baffle door + non-metallic compensator is the standard combination of desulfurization island. The flue gas baffle door is used to cut off the airflow, and the non-metallic compensator absorbs the thermal expansion and contraction of the pipe. If the insulation layer is not done well, after the baffle door is closed, the cold air flows back into the compensator, and the condensed water flows along the insulation layer to the ring belt, resulting in corrosion problems one after another.
Life and maintenance: If the insulation layer is broken, don't hold it hard. It is more cost-effective to replace one section than the whole one.
The heat insulation layer of non-metallic compensator itself belongs to consumable parts, and its normal life is 3~5 years, depending on the working conditions. The most common bad is the inner ceramic fiber: long-term high temperature will make the fiber brittle and break into powder, and the heat insulation effect will decline by a cliff. The damaged vapor barrier is more concealed-it can't be seen on the surface, but when opened, the cotton layer is full of water.
What about that? Don't hold on. If the surface temperature of the ring belt is found to rise abnormally (for example, 30℃ higher than the design value), or the outer protective layer is bulging or dripping, stop the machine immediately for inspection. The cost of replacing a section of insulation layer is only about 1/5 of that of replacing the whole non-metallic expansion joint. Moreover, many manufacturers (including us) now provide "on-site replacement service of insulation layer". After disassembling the flange, take out the old insulation layer, plug it into the new insulation block pre-pressed, and then restore the outer protective layer, which can be done in one day.
Don't think about "taking another overhaul cycle". After the failure of the insulation layer, the high temperature is directly transmitted to the ring belt. Once the ring belt of the fabric fiber expansion joint is burned through, the high-temperature smoke in the pipeline leaks out, which can scald the inspection personnel and burn the company camp. Not worth it, really not worth it.