Let's talk about the phenomenon first-what does compensator failure look like?
Who in the dry plumbing business hasn't touched a compensator strike a few times? The most common scenes: cracks and bulges appear on the surface of the bellows, or a hole is simply torn from the weld, and the medium is sprayed all over the workshop. More seriously, the guide tube fell off, the tie rod nut collapsed, and even the whole expansion joint was deformed by the lateral thrust-I tell you, that scene was no different from the scene of a car accident.
Two days ago, a buddy from the cement factory called and said that they had installed a machine on the kiln head pipeMetal Corrugated Expansion Joints in Cement IndustryIn less than three months, fatigue cracks appeared in the bellows. After removing it, the groove has been worn out at the peak position, which is obviously caused by high-frequency and low-amplitude vibration. And even more outrageous: a power station project, on a steam pipelineHigh temperature axial expansion jointBecause the transport screw was not disassembled during installation, it was run directly with the restraint bolt, and as a result, the bellows was forcibly pulled and cracked.
Nine times out of ten, these failure cases are not the problems of the products themselves-the mines buried in design selection and the pits stepped on in on-site installation are the real culprits.
"Mining" in Type Selection and Design
Compensator failure analysis The first step is always to see if the design parameters are correct first. Many purchasing diagrams save trouble, just buy a general model according to the nominal diameter of the pipeline and install it. For example, the steam pipeline, the medium temperature is 400℃, the pressure is 1.6MPa, and the result is a conventional oneUniversal corrugated expansion jointThe wall thickness is not enough, the wave number is not correct, and the compensation amount is not enough, and the deformation exceeds the limit directly when the machine is started.
Calculation of the amount of displacement. Pipe thermal expansion is calculated according to temperature, pipe length and material, but many people only calculate axial displacement, ignoring lateral and angular displacement. EspeciallyCorrugated expansion joint for power station industryThe direction of the pipe system is complicated and there are many turns. If only a single axial type is equipped, the lateral displacement that should be absorbed has no place to go, and the bellows will be stressed by different forces, and the fatigue life will plummet.
There is also a "stealth bomb" by design-media corrosion. The flue gas in the desulfurization system contains sulfide, and the temperature is not high but the humidity is high. If ordinary stainless steel bellows is selected, chloride ion stress corrosion cracking is almost inevitable. This kind of working condition can be usedPTFE compensatorOrLined with PTFE hose to isolate metal from corrosive media. But the price is a little more expensive, and Party A often wants to save this money, and as a result, it spends ten times the money to change the pipe later.
"Man-made knife" in installation and operation and maintenance
The design is fine, but it can still mess up when you get to the scene. The most typical one I've seen: the expansion joint is installed, but the transport screw is not removed. The customer said that "I was afraid of air leakage and kept insurance". As a result, once it was run, the bellows was stretched to the limit and directly plastically deformed and scrapped. Remember, allStraight pipe pressure balanced expansion jointAndExternal pressure single axial expansion jointThe tie rods and nuts on the drawings are marked as "removed after installation", and none of them can be left.
Another fatal operation-forcing counterparts. The pipeline installation error is big. Some construction teams use hoists to hard-pull the pipes to align them, and use the expansion joint as a jack. The bellows is forcibly twisted or stretched, residual stress has been created inside, and it cracks not long after running. In this case, you should addManual plug-in insulation doorOrDouble-sealed single-axis circular baffle doorTo adjust the flow path, don't save that money.
What about the operation and maintenance link? Inspection is a mere formality. Many people take a flashlight and shine it around, and if they see that there is no air leak, it is done. In fact, the micro-cracks, indentations and corrosion pits on the bellows are simply not visible with the naked eye. Wall thickness testing and non-destructive testing must be done regularly. Especially those operating at high temperature and pressureSleeve type pipe expansion jointOrRotary compensatorIf the sealing surface is not replaced in time after it is worn, the leakage accident will occur in minutes.
Different "death methods" are different under different working conditions
There are many strange failure modes, but they can be summarized in several kinds: fatigue failure, corrosion failure, overpressure blasting and instability buckling. Different working conditions, the main reasons are completely different.
Power Plant Steam Pipeline-High-frequency thermal cycling coupled with vibration, is most prone to low-cycle fatigue. We did a diagnosis for a thermal power plant, and they used the pipelinesCompound hinge transverse expansion jointToo many wave numbers lead to insufficient overall stiffness and direct instability under the impact of pipeline water hammer. Later replaced withDouble hinge expansion joint for air-cooled island vacuum pipelineThe problem was solved only after the limit device was installed.
Flue Gas Desulfurization Systems – Corrosion is the number one killer. For the import and export of a desulfurization towerNon-metallic expansion joint (fabric fiber expansion joint)It could have been acid and alkali resistant, but when installing, the orientation of the fabric layer was reversed, and the medium penetrated from the weak side, and it rotted out in three months. On this occasion, apart from choosing the rightDesulfurization flue gas baffle doorAndRubber PTFE compensatorYou must also pay attention to the installation direction sign.
Direct buried pipeline-the groundwater level is high, the soil is corrosive, and ordinary metal bellows can't bear it at all. Must chooseDirect buried (fully buried) type expansion joint, plus anti-corrosion coating and sacrificial anode protection. Last year, a thermal pipe network in the north was replaced by an ordinary type, and it leaked in two years. After excavation, the bellows had been corroded into a honeycomb shape.
How to predict and prevent in advance? — — Closed loop from design to inspection
Don't wait for the explosion to analyze the compensator failure analysis? That's called remedy. The real reliable approach is to form a closed loop:
- Design phase:Get the detailed stress analysis report of the pipe system and confirm the thermal displacement and pipe thrust. Don't just look at the caliber when selecting a model, but match it accuratelyHigh temperature axial expansion jointOrCurved tube pressure balance expansion jointAnd other special models. Media corrosive? UpperPTFE compensatorOrrubber compensator。 Space constrained? useLarge diameter thick wall expansion jointOrUniversal corrugated expansion jointPaired with limit pull rod.
- Installation phase:Strictly follow the installation instructions. The transport screw must be removed, and the direction of the guide tube must be aligned with the flow direction arrow (Specific Function of Expansion Joint Guide TubeIs to protect the bellows from being washed by the medium, and the wrong direction is equal to white installation). Installation error does not allow the use of the expansion joint hard top, the bracket should be adjusted or addedDouble-sealed single-axis circular baffle doorTo cut off the airflow.
- Operation and maintenance stage:Develop inspection cards and record the displacement, surface temperature and vibration values for each time. If abnormal vibration or large displacement is found, give priority to inspect the bracket andMetal hoseIs it loose or not. For critical partsCorrugated expansion joint for power station industry, it is recommended to do a metal fatigue life assessment once a year.
In the final analysis, the compensator is a treasure if it is designed correctly, but a time bomb if the selection or installation is wrong. Don't always wait for something to go wrong before you pat your thigh. Fill in the holes in front of you first, and it is not difficult to run safely for ten or eight years.