What exactly does the metal mesh in the expansion joint look like?
As soon as many people hear about expansion joints, they think of that ring of shiny bellows. To be honest, metal bellows are indeed the main character, but this thing we're talking about today- -Expansion joint metal meshHe was the guy who was hiding behind him. You go and remove a non-metallic expansion joint that has been used for three to five years, such as the one in our stationNon-metallic expansion joint (fabric fiber expansion joint), orrubber compensatorThe first thing revealed was not rubber or PTFE, but the ashy metal net that rustled to the touch. It is usually a mesh structure woven of stainless steel wire or galvanized wire, attached to the outside of a rubber or PTFE liner, like a tight piece of armor. If you think about it, without this mesh, the pressure in the pipe would bulge as soon as the rubber layer alone.
Many people think that metal mesh is only used to prevent tearing. In fact, its function is far more than that. Let's talk slowly later.
The material and weaving process are exquisite
The material of the metal mesh directly determines how long the expansion energy saving will live in what environment. For example, those used in desulfurization systemsDesulfurization flue gas baffle doorOr chemical pipeline, with acid in the medium, then you have to go to stainless steel wire mesh. 304 is the starting point, and 316L is reliable. If it is a room temperature air duct, such as the rubber compensator of the ventilation system, galvanized wire mesh is enough, which is cheap and cost-effective.
From our stationLarge diameter thick wall expansion jointThe number of metal mesh layers is often three, and the number of meshes per inch is controlled between 20 and 30 meshes. Why? If the density is too low, the rubber will be ejected out of a bag as soon as the medium is flushed; The density is too high, and the flexibility is gone. As soon as the pipe expands and contracts, the metal mesh rips itself first. This balance depends entirely on experience, and it can't be used just by making a net.
Many manufacturers will cut corners in this piece-use thin filaments and thin nets to save costs. And the result? It leaked less than half a year after installation. Two years ago, the author went to a cement factory for after-sales work. The general-purpose corrugated expansion joint they bought had only one layer of metal mesh, less than 0.3mm wire diameter and only 12 mesh. When I took it apart, the net had broken in three places. So next time you select a model, don't just stare at the ripple height, and ask the manufacturer "what silk, several layers and how many meshes do you use for this net".
What kind of work does metal mesh carry?
Expansion joint metal meshIt plays three roles in the structure-compression, wear and fatigue resistance.
Let's start with stress resistance.When there is pressure inside the bellows, the metal mesh is pulled like a hoop to prevent the bellows from bulging and deforming in the radial direction. Especially high-voltage scenarios, such asDouble hinge expansion joint for air-cooled island vacuum pipelineThe internal vacuum degree plus the external atmospheric pressure, if there is no metal mesh to cover the bottom, the bellows will be deflated in minutes.
Let's talk about wear resistance.Pipelines in cement industry and electric power industry often carry particulate dust. The particles in the airflow directly scour the rubber layer, and they can wear through in a few minutes. The role of the metal mesh is to act like a shield, bouncing the particles back. From our stationMetal Corrugated Expansion Joints in Cement Industry, the inner layer is specially added with double-layer metal mesh, and no matter how much dust is, it can last for two years.
Finally, there is anti-fatigue.The expansion joint expands and contracts thermally every day, the metal mesh bears most of the cyclic stress, and the bellows body is easier. You can imagine: Without this network, ourHow many years does general-purpose corrugated expansion save energy? I'll tell you, you have to change it in half a year at most. In fact, many failure cases are that the metal mesh breaks by fatigue first, and then the rubber layer is torn.
How to choose a network in different working conditions?
The choice of network is inseparable from the three elements of working conditions: temperature, medium and pressure.
High temperature scenario:For example, those used in the power station industryCorrugated expansion joint for power station industryThe medium temperature exceeds 400 °C. At this time, the heat resistance of ordinary stainless steel wire mesh is not enough, so heat-resistant stainless steel woven mesh has to be used, and the wire diameter should be thickened to more than 0.5mm. At the same time, we should also pay attention to the bonding process between the net and the glue layer. The glue layer is easy to age and fall off at high temperature, and if the metal net is not fixed well, it will become a source of wear.
Corrosive environment:Like the flue gas of desulfurization system, containing sulfuric acid and hydrochloric acid, it must be 316L or even Hastelloy wire mesh. Simultaneous cooperationPTFE-lined hoseOrPTFE compensator, PTFE layer is corrosion resistant, and metal mesh provides strength. Last year we dispensed with a chemical plantRubber PTFE compensatorThe customer designated Hastelloy net. Although it is expensive, it saves money if you don't have to change it for five years.
High pressure pipeline:Such asDouble hinge expansion joint for air-cooled island vacuum pipelineAlthough the pressure is not high, the vacuum degree is large, and the metal mesh has to bear the shrinkage force generated by negative pressure. At this time, double-layer twill woven net has to be used, and the tensile strength is at least 800MPa. The wrong choice can lead to air leakage or pipe bursting. Last year, a cement factory customer used a non-metallic expansion joint of single-layer plain mesh for cheap, which only lasted three months before bulging and scrapping-you see, the money saved was not enough to pay for a shutdown loss.
What if the metal mesh fails?
Localized corrosion perforation and fatigue fracture.
Corrosion perforation mostly occurs at welded joints, because the welding heat affected zone changes the intergranular structure of stainless steel, and the corrosion resistance decreases. Special reminder: Never think about welding and repairing metal mesh on the spot! As soon as the whole mesh is welded, it is annealed, and the strength is directly lost by half. If a small break is found, you can use the same material wire mesh patch and high-temperature glue for temporary emergency, but it is best to replace the whole expansion joint directly. Temporary repairs can only last for about a month, which is not a long-term solution.
The fracture of fatigue fracture presents a distinct shell pattern, which is the result of long-term vibration or frequent displacement accumulation. The preventive method is actually very simple: when installing, ensure that the guide tube is facing the medium flow direction-pay attention to the direction of the arrow on the expansion joint, that is, the medium flow direction indication. If the arrow is right, the metal mesh will not be directly scoured; The arrow is installed backwards, and the particles hit the net directly, which will wear out in half a year.
Routine maintenance is also important. Check the surface of the expansion joint regularly for rust spots or bulges. Bulge indicates that the metal mesh has been partially broken or deformed, and must be stopped for treatment. Also, rightDirect buried (fully buried) type expansion jointSuch things that cannot be directly visualized can be combined with pressure monitoring or acoustic emission detection for early warning.
Choose the right metal mesh, and the expansion joint can really "expand". To put it bluntly, metal mesh is the most inconspicuous but deadliest layer in the expansion joint. The product information in our site, whetherMetal rectangular expansion jointStillrubber compensatorAs long as there are non-metallic parts, they basically rely on this layer of net to cover the bottom.