How to set and select the expansion joint of metal bellows? Don't wait for the pipe to break before you regret it
Two days ago, I met a friend of a power station project, saying that the expansion joint cracked just half a year after a main steam pipeline in their factory was put into operation. After removing it, it was found that the actual displacement of the high-temperature section was not calculated at all during the selection, and a general-purpose corrugated expansion joint was used. Tsk, this is not saving money, it is clearly laying mines for yourself. Today, let's make it clear how to set up and choose this thing.
1. Before setting up the expansion joint, find out what the pipeline should compensate for
Many people asked "How to choose the expansion joint" as soon as they came up, but after asking, they found that they didn't even understand the compensation object. Pipelines need to deal with nothing more than three things:Thermal displacement、Vibration、Installation error。 But these three things have to be seen separately-thermal displacement is the main cause, accounting for more than 90% of the working conditions. For example, the temperature difference of steam pipes from cold to hot is two to three hundred degrees, and the axial elongation can reach several centimeters or even more than ten centimeters. That's when you have to goAxial expansion jointOrExternal pressure single axial expansion jointThey specialize in this straight-line expansion.
What about vibration? Pump outlet, compressor inlet and outlet pipeline, high-frequency vibration will shake the bellows root out of fatigue crack. This situation is more suitable forMetal hoseOrCompound hinge transverse expansion jointTo absorb the vibration instead of carrying it hard. The installation error is better understood-a few millimeters away from the field welding butt, fine-tune it with the expansion joint to save you from re-cutting the pipe. But don't design the installation error as the main compensation amount, that is to dig a hole for yourself.
Find out what is moving and how in the pipe system first. Is it running in a straight line or swinging up and down? Is it long-term slow displacement or instantaneous impact? Get it backwards, and the rest is all for nothing.
2. Three hard parameters that cannot be avoided in model selection: pressure, temperature and displacement
These three parameters are the bottom line. Don't place an order if one is less. I said it firstPressure rating: The design pressure of the pipeline is 2.5MPa. If you choose a 1.6MPa expansion joint, the on-site pressure will collapse. For example, the main steam pipeline of the power station has high pressure and high temperature, so it can be usedCorrugated expansion joint for power station industryIts pressure-resistant layer and corrugated structure have been strengthened. The air duct pressure in the cement industry is low, but there is much dust and large temperature fluctuation, so you have to go upMetal Corrugated Expansion Joints in Cement IndustryThe guide tube should be thickened and wear-proof.
AgainTemperature range: The upper temperature resistance limit of the corrugated pipe material must be higher than the maximum operating temperature of the pipe. Ordinary austenitic stainless steel 304 is sensitized above 450 ℃, which is prone to intergranular corrosion. If the temperature of the flue gas pipe rushes to 800℃, then useHigh temperature axial expansion jointInner lining with heat insulation layer, outer wall plus heat sink. The temperature is not right, and the bellows will oxidize into slag in a few months.
And finallydisplacement amount: Many design institutes give a general "compensation amount", but is it axial, lateral or angular displacement? It has to be specific to millimeters and angles. For example, a 30-meter-long pipeline, with a temperature rise of 200℃ and an axial elongation of about 72mm, then the expansion joint you choose has enough single-wave compensation ability. If it is not enough, go to multi-wave structure orDouble straight pipe bypass pressure balance type expansion joint. Blind guess displacement, nine out of ten will have an accident.
3. Different industries have huge differences, so don't use general welding to die all working conditions
The universal corrugated expansion joint is indeed cheap, but covering everything is equal to less than covering everything. The pressure level of the main steam pipeline, reheating pipeline and water supply pipeline in the power station ranges from several MPa to more than ten MPa, and the medium temperature ranges from normal temperature to 600℃. The bellows material must be Incoloy 800 or Hastelloy. And you take a general-purpose type made of 304 material to weld, which is called "sending a head".
What about the cement industry? The air duct is full of high-temperature dusty gas, and the bellows are most afraid of wear and dust accumulation. SoMetal Corrugated Expansion Joints in Cement IndustryIt must be equipped with a wear-resistant guide tube, and the direction of the guide tube should be facing the flow direction of the medium-if it is wrong, the dust will directly wash the root of the corrugation, and the hole will be perforated in two months. The chemical industry is more troublesome. Acids, alkalis, organic solvents and corrosive media can be usedPTFE-lined hoseOrPTFE compensatorOrdinary stainless steel can't handle it.
To put it bluntly, there is no cure-all. For flue gas pipelineNon-metallic expansion joint(fabric fiber expansion joint) can be done, but you can try the power station steam pipe? The non-metallic ones blow away directly.
4. Several details of installation settings that are prone to rollover
Will the tie rod be removed or not?Many expansion joints leave the factory with transport rods or adjustment rods-note that the transport rods must be removed after installation, otherwise the expansion joints are locked and cannot compensate for displacement at all. However, some tie rods with limit function (such as large tie rod expansion joint) can't be removed, and only the nut position can be adjusted. How to distinguish? Read the product manual, or ask the manufacturer directly. If you disassemble it wrongly, you can leak or pull off the pipe at worst.
Where is the deflector?The function of the guide tube is to protect the inner wall of the bellows from the medium scour and reduce the flow resistance. Install so that the bell mouth of the deflector is directed in the direction of media flow. In reverse, the medium will directly impact the bellows root, forming vortices and erosion. There was a chemical project before, and the deflector was installed backwards, and it leaked in three months. When welding the end pipe, pay attention to the arrow on the outer wall of the expansion joint. The direction of the arrow is the flow direction of the medium.
The direction of the arrow is particular.The arrow printed on the housing of the expansion joint not only indicates the flow direction, but also indicates the installation direction. Such asSingle-axis double-flapper doorOrRound flapper doorAn arrow pointing to the closing direction or the medium flow direction. Don't take it for granted, just go with the arrows. In addition, when the expansion joint is installed, it is strictly forbidden to forcibly stretch or compress to make up the nozzle distance-that is called "pre-deformation", which must be strictly adjusted according to the designed cold tightness value. How to tune it? Refer to the standard procedure for expansion joint tie rod nut adjustment.
5. Choosing the right is only the first step, daily use and life management are the real way to save money
Expansion joints are not installed once and for all. And guess what? Many factories don't change expansion joints until they leak, and there is not even a patrol inspection in the middle. Bellows belong to pressure pipe components, and their fatigue life is limited. according toService life of expansion jointFrequently asked questions and answers, the number of design cycles under normal working conditions is usually 1000~5000 times, depending on the displacement amplitude and pressure. However, if you have frequent start-and-stop on site and drastic temperature fluctuations, the actual life will be greatly reduced.
What to do? Develop several habits: First, visually inspect at least once a year to see if there are cracks, corrosion pits and deformations on the surface of the bellows. Second, check that the tie rod and limit device are loose-the nut is loose and the displacement is out of control. Third, for the expansion joint with the guide tube, regularly use an endoscope to see the wear of the guide tube. If the wear exceeds half of the wall thickness, replace it directly.
Don't wait until it leaks before replacing it. Regular replacements are much less costly than accident repairs. For example, the expansion joint on high-temperature pipeline, which is rotated one batch in 3~5 years, is far more cost-effective than the shutdown loss caused by sudden pipe burst. Also, in spare parts management, it is best to use the same model in the same working condition to avoid confusion on the spot. Power stations, cement and chemical industries, the models of expansion joints corresponding to each working condition are available in our station-fromDirect buried (fully buried) type expansion jointToCurved tube pressure balance expansion jointAnd then toSleeve type pipe expansion jointIf you choose the right one and use it well, the pipeline will run for decades without fault.
Don't wait for the pipe to break before you regret it. The expansion joint is a small thing, which costs a small amount of money and guarantees a large system. One more step before setting up, ask more when selecting, take a second look when installing, and the maintenance cost saved later is enough for you to drink good tea for ten years.