The expansion joint in the high-temperature pipeline is most afraid of not thermal expansion and contraction, but poor thermal insulation. Think about it, a high-temperature axial expansion joint has a design life of 10 years, but as a result, due to the failure of the insulation layer, the local overtemperature of the bellows leads to fatigue cracks, and it is scrapped in two years-such cases abound in power stations and cement industries. What exactly is the problem to solve for high temperature metal expansion joint insulation? To put it bluntly, there are two things: one is to prevent condensation corrosion caused by external cold air or moisture intrusion, and the other is to prevent the bellows from deforming freely due to hard contact between the insulation layer and the expansion joint body. If you step on one of these two pits, the equipment life will be directly discounted.
The core design logic of insulation layer: move away from displacement, insulation in place
The expansion joint is different from the straight pipe section in that it absorbs displacement itself, so the insulation structure must be "let go". The common practice is to wrap a layer of soft insulation material (such as ceramic fiber blanket) on the outside of the bellows, and then wrap it with a metal shell. But many people overlook one detail: the thickness and compression ratio of the insulation material. The higher the temperature, the easier the material settles. For example, for power station pipelines whose flue gas temperature exceeds 600℃, if ordinary rock wool is selected, the thickness will collapse by 30% after half a year, and the thermal insulation performance will be useless. At this time, it is time to put on the composite structure-the inner layer is made of high-temperature resistant aluminum silicate fiber, the outer layer is made of aerogel felt, and the guide tube design of "non-metallic expansion joint (fabric fiber expansion joint)" or "high-temperature axial expansion joint" of this station can effectively insulate heat.
What is the role of the deflector itself? It blocks the high-temperature medium from flushing the bellows directly, but the gap between the deflector and the bellows must be filled with a flexible insulation layer, otherwise heat will be transferred to the bellows surface through radiation. I recommend leaving at least 20-30mm for this gap, and plugging it with a ceramic fiber blanket but not crushing it-it must allow the bellows to expand and contract freely. And guess what? In order to save trouble at some sites, the insulation material is directly tied in the trough with iron wire. The bellows can't move, and the displacement is fully pressed on the end weld. It is only a matter of time before the weld cracks.
Connection method: Don't let the insulation layer become a "shackle"
Another place that is prone to rollover is the way the insulation layer is connected to the expansion joint body. Someone directly stuffed the insulation material into the bellows trough and tied it to death with iron wire-this was over, the bellows couldn't move, and the displacement was all pressed onto the end weld. The correct approach is to reserve the expansion gap: the inner diameter of the insulation layer should be 20-30mm larger than the outer diameter of the bellows, or the layered slidable structure should be adopted. When designing the "high-temperature axial expansion joint" and "external pressure single axial expansion joint" of this station, special attention should be paid to the heat insulation distance of the guide tube-the guide tube itself can block the high-temperature medium from directly washing the bellows, but the gap between the guide tube and the bellows must be filled with a flexible insulation layer, otherwise the heat will be transferred to the bellows surface through radiation.
For horizontally mounted expansion joints, the self-weight of the insulation can cause sinking, so you have to set up load-bearing brackets below. The riser is more troublesome. If the insulation layer is too thick and heavy, the bottom will be seriously compressed and deformed, and it will collapse over time. I met a customer in a cement factory who wrapped a 200mm thick aluminum silicate blanket for the "metal corrugated expansion joint in cement industry". As a result, the pipe support was compressed and deformed-the bulk density and compressive strength of the insulation material must match the working conditions, and the thicker the better.
Material selection: Don't be superstitious about thickness, waterproof is more important than anything else
In material selection, don't be superstitious about "the thicker the better". Two days ago, I met a customer from a cement factory, and wrapped a 200mm thick aluminum silicate blanket for the "metal corrugated expansion joint of cement industry". As a result, the pipe support was compressed and deformed. The bulk density and compressive strength of insulation materials should match the working conditions. For risers, excessive self-weight of the insulation layer will lead to compression deformation of the lower part; For horizontal pipes, load-bearing brackets are required. In addition, the waterproof layer is equally critical-the expansion joint arranged in the open air. Once the rainwater penetrates into the insulation layer, it quickly vaporizes and expands at high temperature, which can cause the insulation to fail or cause the external pressure of the bellows to be unstable. The thermal insulation design of "double-sealed single-axis circular baffle door" and "desulfurization flue gas baffle door" in this station specifically considers the waterproof vapor barrier layer, which can be used for reference.
Flue gas baffle door this equipment, the internal medium temperature is high, the external environment is humid, once the insulation layer enters water, the pressure after vaporization can bulge the shell. Therefore, it is necessary to add a layer of aluminum foil vapor barrier, or use closed-cell foam glass such an impervious material. One more word here: Don't use ordinary rubber and plastic sponge. It ages quickly at high temperature and will be powdered in two years.
Acceptance criteria and routine maintenance: Is the shell hot when touched by hand?
Acceptance criteria for insulation of high-temperature metal expansion joints, many projects have been completed, and the shell is too hot to touch by hand-indicating that the thermal conductivity is not up to standard. Industry specifications require that the outer surface temperature of the insulation layer should not exceed 50℃ (at room temperature of 25℃), which can be known by an infrared thermometer. In more severe working conditions, such as the high vacuum environment of "double hinge expansion joint of air-cooled island vacuum pipeline", the insulation layer should not only insulate heat, but also avoid the precipitated gas from polluting the vacuum. That one has to be a metal reflective screen and multi-layer vacuum insulation board.
Regularly check whether the insulation layer is cracked or fallen off, especially the peak positions where the expansion joint displacement is the largest-that place is most prone to problems. In addition, the lap seam of the metal shell should face down to prevent rainwater from flowing back. If you find that the shell is corroded, replace it in time. Don't wait for the insulation layer to be soaked in water before treating it, it will be too late.
In the final analysis, the insulation of high-temperature metal expansion joints is not so mysterious, but the details determine success or failure. When designing, spend more time on gap reservation and material matching, and check frequently during operation. It is not a problem for an expansion joint to last ten or eight years. Otherwise, no matter how expensive the expansion joint is, it will be for nothing.