How to choose a metal expansion joint? Let's make these six categories clear first
Two days ago, a purchaser who was doing a power plant project called and asked, "Just recommend a general model for your metal expansion joint." I laughed when I heard it-how could there be any general model for this thing? If you choose the wrong one, the pipeline will leak or the whole system will shut down. The classification of metal expansion joints really has to be broken apart and crumbled into pieces.
1. According to the structural form, it is divided into circular and rectangular, with one stall for each tube
Looking at the cross-sectional shape, there are two most intuitive ways to divide it: circle and rectangle. Circular expansion joint, the most common one in our house isUniversal corrugated expansion jointAndLarge diameter thick wall expansion jointThe circular pressure bearing capacity is naturally strong and suitable for pipeline systems. What about rectangles? MainlyMetal rectangular expansion jointIt is used in places such as flue duct and inlet and outlet of desulfurization tower. Large cross-section and low pressure, but need to withstand out-of-plane displacement caused by thermal expansion and contraction. Someone asked if I could take a circle instead of a rectangle? Don't trouble, the corrugation of the rectangular expansion joint is specially designed, and the circular one is hard installed. The stress distribution is wrong, and it will crack in a few days.
Second, according to the compensation form: axial, transverse, angular, and pressure balance.
This is the most brain-burning category of engineering selection. Let's startAxial type, includingExternal pressure single axial expansion jointAndHigh temperature axial expansion jointSpecially absorbs the expansion and contraction of the pipe along the axis direction. Think about the steam pipe heating up tens of degrees, stretching out several centimeters in length, all by it.
What about lateral compensation? The pipeline is turned or the space is limited, so it is necessary to useCompound hinge transverse expansion jointOrLarge tie rod expansion joint。 The use of angularDouble hinge expansion joint of air-cooled island vacuum pipe, a single hinge can solve the angular displacement.
Focus onPressure balance type——Straight pipe pressure balanced expansion joint、Curved tube pressure balance expansion jointAndCompound straight pipe bypass pressure balanced expansion joint。 This guy comes with its own balanced bellows, and the blind plate force generated by the internal medium pressure will be digested by itself and will not be transmitted to the equipment. For example, the inlet and outlet of steam turbine and the nozzle of pump must be pressure balanced, otherwise the thrust can break the anchor bolt.
Third, according to industry scenarios: power station, cement, desulfurization... special funds are just needed
Different industries, working conditions vary greatly. In the power station industry, the temperature of the main steam pipeline is five or six hundred degrees, and the pressure is more than ten MPa. We recommendCorrugated expansion joint for power plant industryThe material must be 316L or even Inconel, and the wave number and wall thickness must be calculated according to ASME standard. In the cement industry, the air ducts are full of dust, the abrasives are very worn,Metal Corrugated Expansion Joints in Cement IndustryTo make a wear-resistant guide tube, lined with ceramic or wear-resistant steel plate.
Desulfurization is the most evil-the flue gas contains sulfur, water and chloride ions, which is extremely corrosive. Conventional metal expansion joints can't last for half a year. Must be onDesulfurization flue gas baffle doorcomplementaryMetal rectangular expansion jointOr simply useNon-metallic expansion joint (fabric fiber expansion joint)。 By the way, we still haveFlue baffle doorSeries,Double-sealed single-axis circular baffle door、Single-axis double-flapper doorFor cutting off the airflow, it works best when combined with the expansion joint.
Fourth, according to special working conditions: high temperature, high pressure, vacuum, direct burial, one more tricky than the other
Let's start with the heat. The conventional expansion joint is 300 degrees high, and it must be used if it meets the blast furnace hot blast furnace above 800 degreesHigh temperature axial expansion jointInsulation layer is added to the outer layer of the bellows, or refractory material is filled with a guide tube.
What about high-pressure conditions? Synthetic ammonia, ethylene plant, the pressure is tens of MPa, we supplyLarge diameter thick wall expansion jointThe wall thickness of a single layer can reach 8mm, which is stable under pressure.
Vacuum pipes have more pits-under negative pressure, the bellows may be deflated. At this time to useSpecial hose for vacuumOrDouble hinge expansion joint for air-cooled island vacuum pipeline, corrugated pipe plus inner support ring, or use multi-layer structure to resist instability.
There was also a kind of underworld scene —Direct buried (fully buried) type expansion joint。 The pipeline is buried underground and can't be dug for maintenance. This expansion joint must have its own waterproof, anticorrosion and insulation layer. Our directly buried products, before leaving the factory, do 1.5 times pressure test, plus polyurethane insulation layer and three layers of PE anti-corrosion, and the life guarantee is 15 years.
5. Don't forget the metal hose and sleeve compensator-they also count as "expansion joints"
Many people think that the expansion joint is a bellows, in factMetal hoseAndSleeve type pipe expansion jointAlso belongs to the compensator family. Metal hoses, such asPTFE-lined hoseIt is mainly used in vibration absorption and installation deviation compensation, and is often used in pump inlet and outlet and compressor pipeline. What about the sleeve compensator? The axial displacement is compensated by the sliding seal of the inner and outer tubes. The pressure is high and the temperature is high, but the seal is easy to age. OurRotary compensatorIt is an upgraded version of the sleeve, which can relieve stress by rotating and have a much longer life.
And there's alsorubber compensatorAndPTFE compensatorThe former is good in vibration reduction and the latter is corrosion resistant, which are all good things for specific occasions. Oh, yeah,Rubber PTFE compensatorIt's kind of hybrid, soft and antiseptic.
6. The key to not rolling over in model selection: if one parameter is wrong, the whole pipeline system will suffer
Having said so many classifications, how can you place an order in practice without rolling over? Remember four parameters: amount of compensation (axial/transverse/angular), operating pressure, operating temperature, media corrosivity. Neither is indispensable.
A previous paper mill customer only gave the caliber and compensation amount, but didn't mention that the medium was black liquor (containing sulfur). We did it with carbon steel and the bellows were perforated three months later. Later replaced withPTFE-lined hoseSo far, no problem. Therefore, be sure to talk about the working conditions before placing an order, and don't think "almost OK". Also, pay attention when installingExpansion joint arrow directionIt must be pointed to the medium flow direction, and the tie rod nut should be loosened after the pressure test, otherwise the compensation will not work. How to adjust it specifically depends on ourExpansion joint tie rod nut adjustmentQuestion and answer will get it.
Same caliber and pressure grade, made by different manufacturersMetal expansion joint weight tableThe energy difference is 20% because the wall thickness and wave pitch design are different. So don't just compare the price, ask about the design standards and fatigue life. If you choose the right one, you will use a set for more than ten years; Wrong choice, downtime loss alone is enough to buy several sets.
What happened to the client at the beginning of the article? I recommended him according to his working conditions (secondary air duct of power plant, rectangular section, temperature 350 degrees, slight negative pressure)Metal rectangular expansion jointMatchingFlue gas baffle doorIt has been running for two years and is still in use. This matter of type selection, there is no rush. It is only serious to understand the classification thoroughly and check one parameter at a time.