Why should non-metallic expansion joints talk about heat insulation alone: It's completely different from metal expansion joints
The metal expansion joint is a metal shell, and the insulation layer is attached to the outside, mainly to reduce heat dissipation and prevent scalding. But the non-metallic expansion joint (fabric fiber expansion joint) is different-its skin itself is a flexible composite material, which is laminated by fluororubber, silicone, glass fiber cloth, ceramic fiber, etc. If you wrap it with a layer of stiff insulating cotton, will it still stretch freely? Can't. What's more troublesome is that once the heat insulation of the non-metallic expansion joint is not done correctly, the skin temperature directly soars, the internal condensate can't be drained, and the whole compensator will be scrapped in advance.
To put it bluntly, the insulation of non-metallic expansion joints is not "wearing more clothes", but to control the skin temperature, prevent condensation and stress concentration on the premise of keeping flexibility. It's a lot more troublesome than a metal expansion joint.
How insulation affects the life of non-metallic expansion joints: A chain reaction from skin temperature to condensation corrosion
I have seen a power plant project where the rectangular non-metallic expansion joint on the flue gas pipe had its skin cracked after less than half a year of operation. When disassembled, the insulation layer is directly attached to the outside of the skin, which is thick enough, but there is no air layer left. Results the surface temperature of the skin exceeded the design value, the aging of the fluororubber layer accelerated, and the surface was full of tortoise cracks.
More common is condensation corrosion. When the flue gas temperature is low, water vapor passes through the insulation layer and condenses on the inner wall of the skin. If the insulation layer is not provided with a drainage hole, the condensed water will accumulate in the trough. The skin of non-metallic expansion joints is not afraid of water, but metal frames, platens and fasteners are afraid. Acidic condensate seeped into the gaps of the bolts, and corroded the 316L bolts beyond recognition in a few months. If you think about it, the skin is still intact, the flange is rotten first, and the compensator is still leaking.
Therefore, the design of the insulation layer directly affects the temperature field and humidity field, which determine the life of the skin and metal parts. If thermal insulation is not done well, it is not "poor effect", but it is directly fatal.
How to choose insulation materials: ceramic fiber, rock wool or aerogel? Scoring by working condition
When choosing insulation materials, first look at the flue gas temperature and medium corrosiveness.
- Ceramic fiber: High temperature resistance (above 600℃), low thermal conductivity, good flexibility, suitable for high temperature flue gas. However, it is afraid of water, and the structure collapses after being damp, and the heat insulation performance drops sharply. It is fine to use it in the dry high temperature section.
- rock wool: Cheap, temperature resistance about 400℃, better tensile strength than ceramic fiber, but high water absorption rate. It is used in the low temperature section or in the part where a certain support strength is required. If the medium contains sulfur, the chloride ions in the rock wool may cause stress corrosion to the stainless steel, so the chlorine content should be measured.
- Aerogel felt: The thermal insulation performance is several times that of traditional materials, the thickness can be reduced by half, and it is hydrophobic. But expensive. Used in occasions where the space is limited or high energy saving is required, such as the flue at the inlet of desulfurization tower, where the temperature is not low and the space is tight, aerogel can solve the problem.
Flue gas temperature ≤200℃, non-corrosive working condition, rock wool is enough; 200~400℃, slight corrosion, ceramic fiber felt + waterproof layer; Above 400℃ or ultra-low emission requirements, aerogel felt or composite structure. Selecting materials is not the more expensive the better, but the matching working conditions.
Several pits in the design of thermal insulation structure: the boundary between inner insulation and outer insulation, the function of the guide tube, the thickness and density of thermal insulation
Inside insulation and outside insulation. Inner insulation is that the insulation layer is on the inside of the skin, directly contacting the smoke, and the external insulation is on the outside of the skin. Internal insulation can more effectively reduce the temperature of the skin, but the insulation material has to directly bear the smoke erosion and medium corrosion, and it has to be fixed with high-temperature resistant adhesive, which is difficult to construct, and the skin has to be disassembled during maintenance to see the state of the insulation layer. The external insulation construction is convenient and easy to maintain, but the skin temperature is still on the high side, and if the external insulation layer is too heavy, it will pull the skin and affect the displacement absorption.
Where's the border? Usually, when the flue gas temperature exceeds 300℃, or the resistance temperature of the skin material is close to the upper limit, internal insulation should be used. Below 200℃, external insulation and air layer can be done. In the intermediate temperature zone, many manufacturers choose the combination of inside and outside-a thin layer of thermal insulation on the inside, and thermal insulation cotton on the outside, taking into account both the effect and maintenance.
Many people ignore the deflector when preserving heat. The guide tube is installed in the pipeline, facing the flow direction of the medium. Its function is to prevent the high-speed airflow from directly washing the skin, and second, to make the medium slow down and equalize the flow before entering the expansion joint. In the design of heat insulation, there should be enough expansion gap between the guide tube and the insulation layer, otherwise the expansion of the guide tube in the hot state will break the insulation layer, and the debris will directly be blocked in the corrugation. The gap size should be calculated according to the material of the guide tube and the temperature rise, so you can't pat your head.
The insulation thickness and density should also be controlled. The thickness is not enough, and the skin temperature exceeds the standard; If the thickness is too large, the external insulation will drag the skin down by its own weight, especially the non-metallic expansion joint on the vertical pipeline, and gravity will change the stress state. What about density? If the density of rock wool is too low, it is easy to be compressed and deformed, while if the density is too high and hard, it will affect the corrugation deformation. Generally, the compressive strength is not less than 40kPa, and the thickness is calculated according to thermal engineering.
The most overlooked insulation details during on-site installation and overhaul
I met a customer two days ago. Only three months after the non-metallic expansion joint was installed, the outer insulation layer cracked. In the past, installers used lighters to light insulation cotton to test the flammability-do you think this operation is irritating? The edge of the insulation cotton is directly burned, and the whole insulation layer is redone.
There are several details of on-site installation, which are easier to overturn than material selection:
- The insulation layer cannot be seamed through.The seams must be staggered, or filled with insulation cotton of the same material, otherwise heat will rush out of the gaps, and the skin will overheat locally, forming "hot spots".
- Fixtures for external insulation cannot be welded to the skin.The non-metal skin is a flexible body. If you weld a steel nail to it, it will tear as soon as it stretches and contracts. To be attached to the flange or frame with hoops or wire mesh.
- An air layer must be left between the insulation layer and the skin.At least 10~20mm, let the air flow take away the water vapor. Otherwise, after the heat insulation cotton absorbs moisture and sticks to the skin, the water will not evaporate, which will accelerate the corrosion.
- The drain hole must not be blocked.Drainage holes must be opened at the lowest point of the inner insulation structure, and the holes must be confirmed to be unblocked during maintenance. Many expansion joints are "drowned", not heat-dead.
Key Points of Daily Inspection of Non-metallic Expansion Joints from Insulation Failure Cases
In a waste incineration power plant, the non-metallic expansion joint of the flue has been running for one year, and there is local icing on the outer surface of the skin (in winter), and rust-yellow water stains appear on the lower part of the compensator. Disassembly and inspection found that the insulation layer was seriously damp, the density changed from the nominal 100kg/m³ to 130kg/m³, and the inside was full of condensed water. A large hole was washed out of the insulation layer above the guide tube, and the flue gas directly heated the skin, and the temperature exceeded 120℃, while the design temperature was only 80℃. Although the surface of the skin is not broken, the silicone rubber layer has hardened and will break as soon as it is broken.
For daily inspection, don't just look at whether there are holes in the skin. You should follow this list:
- Touch the external insulation surface, is the temperature difference big? If it is partially hot, it means that the insulation layer has fallen off or the thickness is insufficient.
- Is there a "fizz" leak? Especially at the junction of flange and insulation.
- See if the insulation layer is damp and discolored, and whether there are white or yellow water stains.
- When the seasons change, focus on checking the condensation and condensation. In the morning of winter, scan it with an infrared thermal imaging camera, and the abnormal temperature area is clear at a glance.
- Every time the machine is shut down for maintenance, disassemble the insulation layer to check whether there is rust and micro-crack at the junction between the skin and the metal frame.
The matter of insulation of non-metallic expansion joints, to put it bluntly, means "details determine life". The materials are selected correctly, the structure is done correctly, the installation is not fooled, and the inspection can be kept an eye on. It is normal for a non-metallic compensator to last seven or eight years. However, which link is lazy, skin aging, frame corrosion, and guide tube washing out are all chain reactions. Don't wait for the smoke to remember to check the insulation. At that time, it will not be the insulation layer.