Let's talk about the conclusion first: Should the metal expansion joint be wrapped with heat insulation?
The answer is-it is needed in most cases, but it depends on the specific working conditions. Two days ago, a purchaser from a power plant asked me that the bellows on their steam pipe cracked after two years of use. Was the insulation not done well? When I heard it, it was probably. The core function of the metal expansion joint is to absorb heat displacement, but it is also a guy who is afraid of cold and heat. Today, let's talk about this logic.
Why do most metal expansion joints have to be insulated? Three hard truths
First, anti-condensate corrosion.The medium temperature in the steam pipeline is high, and the ambient temperature is low. When the temperature difference is large, the surface of the expansion joint will condense. When water bubbles, especially the corrugated roots where stress is concentrated, the corrosion rate doubles up. The power station industry in our station uses corrugated expansion joints and general-purpose corrugated expansion joints. Most of the corrugated pipes are stainless steel, and austenitic stainless steel is the most afraid of chloride ion stress corrosion. A package of insulation layer can suppress the dew point problem.
Second, prevent the medium from freezing and blocking.In northern winter, if the pipeline is water or condensate, once the inside of the expansion joint is frozen, the bellows will directly crack. Think about it. On a day of minus 20 degrees, the water is still flowing in the pipe, the insulation is not done well, and it is frozen. That's no trivial matter. Like the double hinge expansion joint of air-cooled island vacuum pipeline, the medium temperature is low and it belongs to the negative pressure condition. It pays more attention to antifreezing and heat insulation, which is often done with polyurethane foaming.
Third, maintain the thermal compensation effect.Thermal insulation can ensure that the body temperature of the expansion joint is consistent with that of the pipeline, and prevent the deviation of displacement calculation caused by local temperature drop. The compensation amount of the expansion joint design is calculated according to the thermal expansion of the whole pipe system. If the temperature of the expansion joint itself is lower than that of the pipe, the deformation amount of the bellows will be wrong, and the fatigue failure will occur after a long time. It's the same as wearing clothes-your arms are cold, your whole body shrinks, and your movements are unnatural.
But in some cases, thermal insulation is bad
For example, high-temperature axial expansion joints and external pressure single axial expansion joints are designed to allow the outer wall of the bellows to be directly exposed to high-temperature flue gas or steam. If the insulation is blindly wrapped, the bellows will be wrapped dead, which will affect its free expansion and contraction. There are also those expansion joints with guide cylinders (see the article in the station for the specific function of the expansion joint guide cylinder). The guide cylinder itself has separated the high-temperature medium, and the outside does not need additional heat insulation. For example, the metal corrugated expansion joint in the cement industry is used in the kiln tail dust removal pipeline. The medium temperature is high but the dust is large, and the insulation layer is easy to accumulate dust and scale, which increases the difficulty of maintenance-in this case, the pros and cons have to be weighed. Some customers want to save trouble, and all of them cover heat insulation. As a result, the high-temperature axial expansion joint is stuck and can't be displaced. Finally, it has to be disassembled and redone, spending more money and missing work period.
If you decide to do insulation, what should you pay attention to during construction? Two things must be kept an eye on
First, the insulation layer cannot limit the displacement of the expansion joint.The design displacement of the metal expansion joint is realized by the deformation of the bellows. If the insulation material is stuck too dead and stuffed too tightly, it is equivalent to adding a "tightening curse" to it. The correct way to do this is: use elastic insulation felt or rock wool, leave an expansion joint, or make a slidable structure when wrapped with iron sheet. We have seen a project in which the construction team stuffed rock wool tightly. As soon as the expansion joint expanded and contracted, the insulation layer directly broke the iron sheet, and the water vapor went in, and the corrosion was faster than not wrapped.
Second, never let the insulation layer seep water.Once rainwater drills in through the insulation seam, and the water vapor is sealed inside and can't come out, the corrosion rate is faster than without insulation. Although the directly buried (fully buried) expansion joint in the station has an anti-corrosion layer, the insulation layer also needs waterproof treatment. The outer iron sheet should be tightly overlapped with the seams facing down to avoid rainwater backflow. If possible, it is safer to brush another layer of waterproof coating on the outside of the insulation layer.
How to choose insulation scheme for different products? Take an example
The corrugated expansion joint used in the power station industry equipped with the steam pipeline of the power station has high medium temperature and high pressure, so it must be insulated with composite silicate or aerogel felt, plus galvanized iron sheet. However, the double hinge expansion joint of air-cooled island vacuum pipeline has low medium temperature and negative pressure, so it pays more attention to antifreeze and heat insulation, so it can be foamed with polyurethane. Another example is the expansion joint next to the desulfurization flue gas baffle door. The flue gas has high humidity and sulfur. The focus of heat insulation is to prevent condensation corrosion, and anti-corrosion coating has to be added outside the insulation layer. These details are directly related to the life of the equipment. Some factories are cheap, using ordinary glass wool, which collapses within two years and has to be reworked, which is more expensive.
Finally, a common misconception
Since the metal expansion joint needs to be insulated, can I paint a thick layer of paint on the outside of it? No way. Paint is an anti-corrosion coating, not an insulation layer, with high thermal conductivity and thin thickness, which can't stop the heat loss at all. Insulation must be made of materials with low thermal conductivity, and the thickness is calculated according to the design temperature of the pipeline. Others say that the non-metallic expansion joints (fabric fiber expansion joints) we use don't have to be insulated? Non-metal itself has limited temperature resistance, such as rubber compensator and polytetrafluoroethylene compensator. Instead, the medium needs external insulation or protective sleeve when it exceeds the tolerance range. In a word: whether thermal insulation is not thermal insulation depends on the matching relationship between the medium temperature, ambient temperature and the material of the expansion joint. If you are not sure, ask the manufacturer directly for the design parameters, which is more reliable than guessing.