Metal Expansion Joint Butting, These 3 Details Determine Pipe Life
The butting of the metal expansion joint seems simple-just screw the two flanges together or weld them twice. However, the veterans who have worked in pipeline engineering for several years are all clear: more than 60% of the late leakage faults are in the docking link. The core function of the expansion joint is to absorb displacement. If you don't align the interface, install the guide tube backwards, or select the type at all and don't match the actual working conditions, then no matter how good the bellows is, it will be for nothing.
I met a customer two days ago who used our power station projectUniversal corrugated expansion jointAs a result, there was not enough pre-stretching during installation, and the weld cracked after three days of trial operation. This is not a product problem, but the docking process has not kept up. So today, let's break it up and break it up, and what points should we pay attention to when docking.
The first level: the wrong choice, the rest is all in vain
According to the structure, metal expansion joints are divided into axial type, transverse type, hinge type, pressure balance type and direct burial type. Before docking, you first have to find out the direction of displacement of the pipeline system. For example, the steam pipeline mainly expands and contracts in the axial direction, so it is preferredHigh temperature axial expansion jointOrUniversal corrugated expansion joint； If a large diameter flue is limited in space but has a lateral displacement, useCompound hinge transverse expansion jointOrCurved tube pressure balance expansion joint。 If you choose the wrong interface form, you can't save it if you add the limit rod in the later period.
Let's talk about the deflector. Expansion joints with guide barrels have arrows on the barrel-the direction of the arrows must point to the direction of the medium flow. What happens if you put it backwards? The medium directly washes the inner wall of the bellows and wears out in a few months. This detail is clearly explained in "The Specific Function of Expansion Joint Guide Tube", so I won't say much.
The second level: the hard indicators in the docking process
Flange connection should pay attention to the uniform bolt pre-tightening force, divided into 2-3 diagonal tightening, the torque value refer to the product manual. For welding connection, a welding groove is usually reserved at the end of the expansion joint, but the arc must not fall directly on the bellows during welding-high temperature will cause local annealing and reduce elasticity. Some projects need on-site cutting to adjust the length. After cutting, stainless steel must be polished to remove the oxide layer and passivation treatment must be done again.
Let's talk about the pull rod. In the installation stage, the nut on the tie rod is in the pre-tensioned state, and the pipeline system can only be loosened to the design position after the pressure test is qualified. Then "How to adjust the expansion joint tie rod nut"? It is adjusted according to the cold tightness value required by the design, and it is not screwed to death or completely loose. After adjustment, the nut position should be locked to prevent loosening during operation.
The third level: 90% of newbies have stepped on these pits
The most common pits in actual docking are three.
The first one: the reserved amount of pipeline cold drawing is not counted correctly.Metal expansion joints usually have pre-stretching or pre-compression strokes when they leave the factory, and the correction value should be calculated according to the ambient temperature and the design temperature during installation. For example, in winter installation in the north, the ambient temperature is-10℃ and the design temperature is 200℃. You have to calculate the pre-stretching amount according to the temperature difference, otherwise the bellows will not be in place after heating up.
Second: There is no fixing bracket when multiple expansion joints are used together.A string in a pipeUniversal corrugated expansion jointAndSleeve type pipe expansion jointThe guide bracket must be added in the middle, otherwise the lateral instability will take minutes. And guess what? In a chemical factory, because this bracket was saved, the expansion joint was directly twisted into a twist during pressure test.
Third: Ignore the dead weight of large-diameter pipes.LikeLarge diameter thick wall expansion jointIt is very heavy in itself, so it should be supported by temporary support when docking, and then withdrawn after the pipeline is fixed. Otherwise gravity will crush the bellows and cause permanent deformation.
How to dock under complex working conditions?
When encountering complex working conditions, such as desulfurization flue gas system, large temperature fluctuations and corrosive media, the metal expansion joint docking often has to followNon-metallic expansion joint (fabric fiber expansion joint)Orrubber compensatorUse it in conjunction. A non-metal expansion joint is used for absorbing three-dimensional displacement at the flue gas inlet, a metal expansion joint is used for bearing pressure at the outlet section, and the sealing gasket at the butt interface is made of PTFE material. And the cement industry, in the high-temperature dust environment,Metal Corrugated Expansion Joints in Cement IndustryWhen docking, pay attention to adding wear-resistant bushings on the inside of the bellows, and the thickness of the guide tube should be thickened. Air-cooled islands and vacuum pipes in the power station industry are often usedDouble hinge expansion joint for air-cooled island vacuum pipelineWhen docking, the vacuum degree is high, and radiographic flaw detection must be done for welding.
Is there a solution to solve all the problems at once? No. Every working condition has its temper, and it can only prescribe the right medicine.
Acceptance: Don't rush to package insulation
Don't rush to fill the insulation after the docking is complete. Do the pressure test first. If the medium is gas, do the airtightness test, and the liquid do the hydraulic pressure test. Observe whether there is any abnormal deformation of the bellows, whether the position of the tie rod nut is correct, and whether the guide tube is offset. The service life of metal expansion joints largely depends on the initial stress state at the time of butting-if it is adjusted, it will last for ten or eight years. If you can't adjust it properly, it will be scrapped in one year.
Of course, regular inspections are also indispensable, especiallyHigh temperature axial expansion jointAndDirect buried (fully buried) type expansion jointThe buried part is difficult to observe, so the displacement of the monitoring point can be preset. AndExternal pressure single axial expansion jointIts external pressure structure is sensitive to the installation direction, and it must be confirmed that the medium flow direction is from the inside to the outside when docking.
In the final analysis, metal expansion joint docking is not a sophisticated technology, but every step is like a lifeblood. You pay attention to it, and it is responsible for the pipeline. When it comes to engineering, the details are the lifespan, and there is no negotiation.