When should metal expansion joints be scrapped? Don't wait until the pipe cracks to regret it
Last winter, a main steam pipeline of a thermal power company in the north suddenly burst, and high-temperature steam gushed out. Fortunately, no one came close at that time, but the whole pipeline was out of operation for three days, and the emergency repair cost hundreds of thousands. After checking, the problem was the one in the middle of the pipeexpansion jointUpper-The bellows has long had visible cracks with the naked eye, but the inspectors felt that it could still be used, so they dragged it out. As a result, the bellows was completely torn and the pipe was forcibly broken.
And guess what? This expansion joint has only been used for less than four years, which is far from the manufacturer's nominal "design life of ten years". However, the design life is the theoretical value under ideal working conditions. When running on the actual site, it is not unusual that the life is discounted by 30% when high temperature, corrosion and vibration take turns. So,Scrap metal expansion jointI can't wait for it to have an accident before thinking about it.
Three axes of scrapping judgment: leakage, deformation and displacement attenuation, none of which can be ignored
How to determine whether a metal expansion joint should be replaced? Don't believe in your eyes, let alone "feeling can still be used". Three hard targets, any of them will be scrapped decisively.
Number one, leaks.Whether it is a media leak or an abnormal pressure gauge reading, it means that the bellows or weld has failed. The most common phenomenon is white smoke, dripping water above the pipeline, or smelling chemicals close to it. Don't underestimate leakage-it's just a hole the size of a needle at the beginning, but the corrosive medium will drill in like a bug and can quickly tear a large hole.
Second, deformation.The deformation here does not refer to a slight change in shape, but rather to the appearance of significant asymmetry, arching, or local depressions in the bellows. Especially under high temperature, metal creep will lead to wave crest collapse. You take the ruler against it. If the difference between the left and right wave distance is more than 10%, just change it. Don't hesitate. Two days ago, a customer of a cement factory sent a photo. The corrugation of their expansion joint was crooked like a twist, and he asked if it could be repaired-if it couldn't be repaired, the internal metal structure had been fatigued and failed.
Third, the amount of displacement is attenuated.The core function of the expansion joint is to compensate the thermal displacement of the pipeline. If you find that it can't be twisted, stuck, or the actual displacement is more than 30% less than the design value, it means that the bellows inside has lost its elasticity. If you don't believe me, compare the stiffness of new pieces with old pieces-the old ones are obviously stiff, which is metal fatigue. Don't have to wait for it to break completely before regretting it.
Difference in life of metal expansion joints under different working conditions-high temperature, corrosion and vibration, which is the most fatal?
The expansion joint of the same model may last 5 years when placed on the steam pipe and 2 years when placed on the flue gas pipe. Order of influencing factors:High Temperature> Corrosion> Vibration。
Heat is the number one killer. When stainless steel is above 300℃, the creep rate increases exponentially. You go look at those power station boilersHigh temperature axial expansion jointThe design life is usually calculated directly by the number of thermal cycles, not by years. If the actual operating temperature is 30℃ higher than the design temperature, the life is directly cut in half. Therefore, when selecting a model, don't try to be cheap. Be sure to provide the manufacturer with the highest temperature and fluctuation frequency of the medium, and it is best to report even the lowest temperature in winter.
Corrosion second. Sulfide in desulfurization flue gas and chloride ions in chemical pipelines will cause stress corrosion and cracking of stainless steel. At this time, don't use ordinary 304, you have to choose 316L or duplex steel, or even go onPTFE-lined hoseOrPTFE compensator— — Fluoroplastics are corrosion resistant, but their pressure and temperature resistance are limited. How to weigh it needs to be communicated with the manufacturer.
Vibration ranks third, but it's not to be underestimated. The pipeline near the pump outlet and the compressor has a high vibration frequency, and the expansion joint bellows is prone to fatigue. Recommended choice for this occasionCompound hinge transverse expansion jointOrStraight pipe pressure balanced expansion jointThey can absorb displacement in multiple directions while reducing impact on the bellows.
What to do with scrapped metal expansion joints? Recycling Value and Compliance Disassembly Guide
Don't throw away the scrap expansion joints that have been removed. First of all, check the material: stainless steel can be sold as scrap iron for recycling, and one ton can be worth thousands of pieces, but the premise is to ensure that there are no residual hazardous chemicals inside. If the chemical pipeline is removed, it must be cleaned and neutralized first, and then the residual concentration is measured with a gas detector, which can only be cut if it meets the safety standards. Otherwise, when the welding torch touches, the flammable gas remaining inside explodes, which is no joke.
The first step is to vent and relieve pressure and remove the flange bolts; The second step is to carry out blind plate plugging on the connection port (to avoid dripping of residual medium); In the third step, the whole is hoisted to the designated area, and the bellows section and flange section are separated by plasma cutting. The flanges can be reused, and the bellows are sent directly to the waste yard. If it isRectangular non-metallic expansion jointOrNon-metallic expansion joint (fabric fiber expansion joint)The glass fiber and silicone cloth inside should be disassembled and sorted. The silicone cloth can be incinerated to generate electricity, but the fiber dust needs a dust mask.
Don't buy blindly for replacement: which products of this site correspond to the three elements of pressure, temperature and displacement?
When choosing a new expansion joint, don't ask "Give me the same" with a photo of the old part-the old part was probably selected wrong in the first place. Three core parameters must be defined:Design pressure, design temperature, compensated displacement(Including axial, lateral, angular displacement).
Steam pipeline pressure 1.6MPa, temperature 350℃, axial displacement 80mm, optionalUniversal corrugated expansion jointOrExternal pressure single axial expansion joint。 If the pipe layout is compact and space is constrained, considerDirect buried (fully buried) type expansion joint。 If the medium is corrosive, such as desulfurization flue gas, look firstDesulfurization flue gas baffle doorCooperatePTFE-lined hoseCombination scheme of-baffle door for isolation, hose for compensation.
Air-cooled island vacuum system, requiringDouble hinge expansion joint for air-cooled island vacuum pipelineThis product can withstand vacuum negative pressure and high temperature, and ordinary expansion joints are deflated within a few months of being installed. For another example, large-diameter thick-walled pipeline, chooseLarge diameter thick wall expansion jointThe welding process and wall thickness are different from the standard parts.
You can check the detailed parameter page of each product on this site, such asCorrugated expansion joint for power station industryAndMetal Corrugated Expansion Joints in Cement IndustryAll have made enhanced designs for their respective industries. Bottom line: Don't save the half hour of model selection, changing the wrong model is more dangerous than not changing.
How many installation maintenance pits have you stepped on? Three practical suggestions for extending life
Screw the tie rod nut of the expansion joint to death. Many masters think that "tightening is safe". As a result, the expansion joint becomes a rigid connection. Once the pipe is heated, there is nowhere to go, so the bellows can only be cracked abruptly. Remember:tie rodThe function is to prevent the bellows from over-stretching, not for fixing. After installation, use a wrench to loosen the nut, leave a gap of 1~2mm, and allow free displacement. For specific adjustment methods, please refer to the article "How to adjust the tie rod nut of the expansion joint" in the FAQ of this site.
In terms of maintenance, doing three things can extend life by more than 30%:
- Check the deflector regularly.The function of the guide tube is to guide the flow direction of the medium and reduce erosion. If the guide tube is punched through, the medium directly hits the bellows, and perforation will soon appear. Every time you shut down for maintenance, take a peek through the flange gap or reach in with an endoscope to check.
- Don't wrap the insulation too dead.In some projects, in order to save energy, the expansion joint is also covered with thick heat insulation. As a result, condensed water accumulates at the root of the corrugation, which accelerates corrosion. The correct thing to do is to leave an inspection opening alone, or choose a model with insulation.
- Record the service time of each expansion joint.Establish a ledger, each expansion joint is numbered when it leaves the factory, labeled, and written with the installation date. When it reaches 70% of the design life, it begins to focus on it, and the displacement is measured every six months. Don't wait for it to call the alarm-it often leaks when it does.
After all, the expansion joint is a "consumable", just like a tire. Change when you need to, and don't feel bad about that little money. One cracked pipe is enough for you to buy ten new expansion joints.