First, the expansion joint is not a lifetime piece, so why must there be a replacement standard?
Metal expansion joints on pipes-like the one we stand onUniversal corrugated expansion joint、High temperature axial expansion joint、Corrugated expansion joint for power station industry— — Undertake the task of absorbing thermal displacement and reducing vibration and noise. However, the corrugated pipe will suffer from fatigue failure sooner or later if it bears alternating stress, medium corrosion and temperature shock for a long time. Without clear replacement criteria, either excessive replacement is wasted for nothing, or wait until a leak or even a pipe burst is remedied – the loss is not the money of an expansion joint. Two days ago, I met a power plant customer. Because I didn't change it according to the standard, the high-temperature axial expansion joint on the steam pipeline suddenly cracked, and the whole line stopped for two days. Do you think it's a loss?
Second, how to judge the change on the spot? Look at these 5 hard indicators
Judging whether you should change or not, don't rely on feeling, it depends on the actual phenomenon. If any of the following 5 indicators appears, change them directly.
- Appearance distortion:The bellows appear obvious plastic deformation, bulging, instability, or uneven corrugation spacing. Especially for the expansion joint of the large tie rod, if the tie rod is crooked, it means that the structure can't bear it.
- Corrosion perforation:Particularly used in operating conditions containing corrosive media, such asDesulfurization flue gas baffle door、Metal Corrugated Expansion Joints in Cement Industry。 There are spots, thinning or even local perforations on the surface. Don't hesitate at this time, the holes will only get bigger and bigger.
- Crack:If there is intergranular corrosion or stress corrosion crack in stainless steel bellows, even if it is only as thin as a hair, it must be replaced. Cracks can propagate rapidly, especially in high temperature and pressure environments.
- Leakage:Dynamic and static seal failure, media leakage. This article doesn't need to be explained. If you leak it, it will be wasted.
- Fatigue life to limit:When designing, it is generally calculated according to a certain number of cycles (such as 1000 times). After reaching the design number of cycles, it is recommended to replace even if the appearance is ok. Why? Because microcracks have accumulated inside the material, it may break brittle at any time. And guess what? The design fatigue life curve provided by some manufacturers is calculated according to normal temperature. When the actual working temperature is high, the number of cycles will be greatly reduced.
3. What is the difference in replacement cycle under different working conditions?
The same metal expansion joint, placed on a circulating water pipe at normal temperature and low pressure, may be used for 10 years without any problem; However, if it is used in high-temperature steam pipes (refer to our station's Q&A about what category steam pipes belong to) or smoke ducts of power stations, it may not be able to bear it in 3-5 years. Such asHigh temperature axial expansion jointWhen the working temperature exceeds 400℃, the creep and oxidation of the material will accelerate the failure, and the replacement frequency is twice as high as that of normal temperature. Another exampleDirect buried (fully buried) type expansion jointIt is impossible to visually inspect it when buried in the soil. It is recommended to replace it preventively in advance according to the design life-don't wait until you dig it out to find that it has rotten. AndCorrugated expansion joint for power station industryIn the flue gas desulfurization system, the medium contains sulfur and chlorine, which is extremely corrosive, and its service life is often only half of the design value. Is it time to check it regularly? The answer is that the inspection cycle must be regular or even shortened.
4. The easiest pit to step on in the replacement operation
Before dismantling old parts, it must be confirmed that the pipeline has been relieved and cooled, and safety is the first. Needless to say, there are always people who are eager for quick success and accidents.
When installing a new expansion joint, the adjustment of the tie rod nut (refer to our station's question and answer on how to adjust the tie rod nut of the expansion joint) should strictly follow the instructions, and the amount of pre-stretching or pre-compression cannot be wrong, otherwise it will have deviation stress when installed, and the life will be directly halved. The direction of the guide tube (do you remember the specific function of the expansion joint guide tube?) should be directed toward the medium flow direction, and cannot be installed backwards. Install the guide tube backwards, and the medium directly washes the bellows, which is perforated within a few months.
There is also a large tie rod expansion joint, and the fixed bracket and guide bracket setting emphasized in the correct installation method cannot be saved in one step. There was a chemical plant before, installingCompound hinge transverse expansion jointWhen the guide bracket was missed, the pipe system was unstable, and the whole expansion joint was twisted into a twist. The hard lesson.
5. Where does the replacement standard come from? What specifications are referenced?
At present, metal expansion joints mainly follow GB/T 12777 General Technical Conditions for Metal Bellows Expansion Joints and JB/T 12235-2015 in China (although this standard is aimed at non-metallic expansion joints, the selection and design of metals is often referred to). Industry internal regulations such as DL/T 5121 "Technical Regulations for Design of Flue Wind Pulverized Coal Pipeline in Thermal Power Plant" also have suggestions on the replacement cycle of corrugated expansion joints used in power station industry. More direct method: let the manufacturer provide the design fatigue life curve of this model, and convert it according to the actual working conditions. For example, you useStraight pipe pressure balanced expansion jointThe number of cycles given by the manufacturer is 2,000 times, but it actually starts and stops once a day, 365 times a year and more than 1,800 times in five years. It should be changed in the seventh year. Don't wait until Year 8.
Expansion joint, if you change it early, it will only cost more money, but if you change it late, it may kill you. Standards are not decorations, but the accumulation of experience of real money. I hope that all field engineers will take this standard to compare with those "old guys" on their pipelines, and change them if they should. Don't hesitate.