The density and depth of ripples is not a question of whether they look good or not
Let's start with the ripples. If you find any expansion joint, the first thing you see will definitely be the circles of ripples. But the layman looks at the excitement, and the insider looks at the doorway-the ripples are dense and shallow, and the sparse and deep, and the uses are completely different.
Wave height and wave distance determine the amount of compensation. The higher the wave height and the larger the wave pitch, the greater the axial displacement that a single ripple can absorb. Conversely, the wave pitch is small and the corrugation is dense, which is usually designed to withstand higher pressure because the stress concentration at the wave root is small and it is not easy to crack by fatigue. The number of layers is also hidden in the appearance: multi-layer corrugations look rounder than single-layer corrugations, and the delamination can be vaguely seen on the corrugated sidewalls. General corrugated expansion joints generally have moderate wave height and uniform wave pitch, while axial expansion joints may have more wave numbers and deeper corrugations. If you see a double hinge transverse expansion joint, there will be two corrugated sections in the middle, connected by a connector in the middle, and there are hinge structures at both ends. The appearance is by no means a bare tube.
Therefore, to judge what an expansion joint is used for, first glance at the ripples: sparse, deep, with little wave number, mostly low voltage and large compensation; Dense, shallow, with many wavenumbers, mostly high voltage and small compensation. This rule holds under most working conditions.
Surface treatment and welds, all written on the face
The appearance, material and surface treatment of metal expansion joints can't deceive people. The brushed surface has directional texture, which looks low-key. It is often used in food, medicine and other pipelines with requirements for cleanliness. The polished surface is bright enough to be used as a mirror, mainly for anti-adhesion and easy cleaning. The sandblasted surface is matte silver gray, with large roughness, which can increase coating adhesion, and is generally used in occasions where external corrosion protection is required. After pickling and passivation, the surface is white, with a uniform oxide film, and the corrosion resistance is the best, which is seen more in chemical plants.
Look at the weld again. The welds on the bellows are divided into straight seams and ring seams. Straight seams go in the direction of the corrugation and usually appear at sheet splicing; The ring seam is the connecting weld between the bellows and the end connection. Experts look at two things when looking at welds: one is that the fish scale patterns are uneven, and the other is whether there are biting edges and pores. The fish scale pattern is fine and even, indicating that the welding parameters are stable and the welder's skill is passed. The fatigue life of the expansion joint is suspended if the weld is different in width or if the edge is dented. You can feel whether the weld is smooth or not with your hands-a good weld is polished gently and does not hurt your hands.
Accessory Details: Tie rods, deflectors, hinges all have statements
Just looking at the ripples is not enough, but interfaces and accessories are where design skills are reflected.
The guide tube is installed inside the expansion joint to reduce the erosion of the corrugation by the medium. How do you see if the direction of the deflector is right from the appearance? Look at the direction of the inlet-the guide tube usually has a bell mouth at one end, and the bell mouth should face the direction of the medium flow. If the flanges at both ends can visually see that the guide tube is obviously skewed to one side, or the media flow direction mark is opposite to the actual installation direction, it is installed backwards. The inverted guide tube not only fails to protect, but also increases the flow resistance. After a long time, the inner wall of the corrugation will be pierced first.
The position of the tie rod nut is also particular. The role of the tie rod is to restrain the bellows and prevent irregular deformation. Where the nut is screwed directly reflects whether the pre-deformation is in place. If the product has been pre-stretched or pre-compressed before it leaves the specified clearance between the nut and the tie rod. You measure the spacing between the inner sides of the nuts at both ends, and then compare the pre-deformation amount on the drawing to judge whether there is loosening during transportation. A few days ago, a customer said that the on-site expansion joint tie rod nut was all stuck dead, and it was found that the locking nut slipped because of the transportation bump-this abnormal appearance is often a signal that it needs to be reset before installation.
The appearance of accessories such as hinges and universal rings mainly depends on whether the rotation is flexible. Use a hand pull to the hinge. If there is jamming or abnormal noise, it means that the pin shaft is poorly lubricated or has been rusted. The transverse expansion joint of compound hinge depends on the hinge to bear the internal pressure thrust. If the hinge does not move smoothly, the transverse displacement cannot be absorbed.
The same expansion joint, the power station and the cement plant look different
Take the corrugated expansion joint used in power station industry and the metal corrugated expansion joint in cement industry together. The most obvious difference lies in the "coat".
Corrugated expansion joints in power station industry, especially those used in smoke ducts, often have a heavy thermal insulation shell in appearance, and then a layer of iron or aluminum sheet is wrapped on the outside. Because the temperature of the smoke duct of the power station is high and the temperature difference is large, the thermal insulation shell protects the bellows from overheating on the one hand, and prevents people from burning on the other hand. However, many of the metal corrugated expansion joints in the cement industry are directly exposed, and the surface is grey and dark. This is because the cement working condition is dust and abrasion is strong, and the bare wave is convenient to clean up the dust. The surface layer is generally made of wear-resistant spray coating instead of insulation layer.
Looking at the high-temperature axial expansion joint again, the outer layer is not bare metal, but usually has an independent insulation jacket, which is tied with stainless steel belt. When you remove the insulation jacket, you can see that the surface of the bellows inside is blue and purple, which is a normal trace of high-temperature oxidation. As for the directly buried (fully buried) expansion joint, the appearance is completely different-the surface is smooth, with neither thermal insulation shell nor exposed tie rod hinge, because it has to be buried directly in the soil, relying on the outer anti-corrosion layer and the guide sleeve on the end pipe.
With thermal insulation shell, high temperature; Bare, straight buried; Sandblasted matte, wear resistant; Polished and shiny, high cleanliness requirements.
Nameplate is the real "ID card"
Having said all that, the most overlooked thing is actually the nameplate. For expansion joints produced by regular manufacturers, the nameplate must indicate the material, design pressure, compensation amount, factory date and standard number. This information is not decorative and is the basis for subsequent installation, maintenance and verification.
Expansion joints without nameplates or incomplete nameplate information can't be used no matter how beautiful they look. Why? Because there is no design pressure and compensation amount, you cannot tell if this expansion joint is suitable for the current piping system. It had an accident when it was installed, and it couldn't even be traced back. The first thing to do during on-site acceptance is to look at the nameplate and certificate, and check whether the parameters are consistent with the design drawings. By the way, the way nameplates are fixed is also particular-riveting is more reliable than sticking, and those with clear handwriting and deep steel stamps are more durable than inkjet printing.
The layman looks at the ripples, the insider looks at the doorways. The exterior of the metal expansion joint hides the design ability, manufacturing level and sense of responsibility of a factory. Next time you see the expansion joint, don't rush to take photos and send them to your circle of friends to compare whether it looks good or not. First, squat down to see the ripples, touch the weld and turn over the nameplate. If you can understand this, you are in the business.