Start with a power plant pipeline leak: How deadly is corrosion?
Two days ago, I met a friend from the operation and maintenance of a power plant, and talked about an unplanned shutdown of their plant last year-the reason is very simple.Corrugated expansion joint for power station industryCorrosion perforation occurred. And guess what? A pinhole with a diameter of only two millimeters stopped the entire production line for three days, causing the direct economic loss to exceed two million. Hey, this kind of thing is not uncommon in our industrial circle.
Seriously,Anti-corrosion of metal expansion jointThis matter is often regarded as a simple job of "painting a layer of paint" in many projects. But wait until the leak is repaired, disassembled, welded, re-purchased, and the cost is doubled several times. The safety risk is really scary. Corrosion does not come slowly, it is an invisible killer-at first it just thins, then suddenly perforates, and high-temperature and high-pressure medium is sprayed out, and the consequences are unimaginable.
Several "Dead Methods" of Metal Expansion Joint Corrosion
To understandAnti-corrosion of metal expansion jointFirst, we have to find out how corrosion tosses metal. There are three common ways:
Chemical corrosion.To put it bluntly, it is a direct chemical reaction between metal and medium. For example, in the flue gas desulfurization system,Desulfurization flue gas baffle doorThe corrosion rate can be dozens of times faster after the passivation film on the surface of stainless steel is destroyed by long-term contact with the acid condensate with the bellows. Think about it, that stuff sometimes has a pH level below 2, like drinking vinegar.
Electrochemical corrosion.This one is more cunning. If there are impurities, welds, or overlapping of different materials on the metal surface, micro-batteries will be formed. Such asPTFE-lined hoseIf there is a potential difference between the metal joint and the bellows, an "electrochemical party" should be held directly in a humid environment. Especially in environments with many chloride ions, such as chemical plants and seawater cooling systems, pitting and stress corrosion cracking appear as soon as they appear.
Scour corrosion.When the medium flow rate is high or contains solid particles, the metal surface protective film is washed away, exposing a fresh surface, and corrosion follows. InHigh temperature axial expansion jointThe inner wall of the guide tube is the most typical problem. The guide tube is used to protect the bellows, but if the guide tube itself is not handled properly, the impact area will be like a knife scraping as soon as the flow rate rises.
Anti-corrosion is not as simple as applying a coat of paint
Many purchasing managers think that anti-corrosion is to paint two coats of epoxy paint, and then everything will be fine. Tsk, it's not that simple.Anti-corrosion of metal expansion jointIt is a systematic project, and we have to start from four dimensions:
Material selectionIs the first line of defense. Ordinary 304 stainless steel can not last two months under chlorine-containing conditions, but 316L or even super austenitic stainless steel, or something likeCorrosion-resistant alloy is directly used like the general-purpose corrugated expansion joint. To the extreme,PTFE compensatorFully lined construction, directly separating the metal from the medium.
liningIt's the second one. LikePTFE-lined hoseThe PTFE layer wraps the metal bellows tightly, and its chemical inertness is extremely strong, which can basically hold all strong acids and bases. However, there is a pain point in the lining-machining defects or cracking during thermal expansion and contraction, which will accelerate crevice corrosion.
CoatingLike paint, thermal spray ceramics, metal coating, suitable for some low temperature, corrosive conditions. However, the coating is most afraid of bumps and aging. At the entrance of the desulfurization tower of the power plant, manyFlue gas baffle doorIt is resisted by glass flake coating, but it has to be re-maintained after three to five years.
Structural designOften the easiest to overlook. Such asSpecific Function of Expansion Joint Guide TubeIt is mentioned that the guide tube can avoid the direct flushing of the bellows by high-speed media. There are also details such as drainage holes and reinforcing rings, which can reduce corrosion dead corners. If condensed water accumulates in the structure, it is equivalent to soaking the metal in corrosive fluid every day.
Anti-corrosion strategies under different working conditions
The working conditions are different,Anti-corrosion of metal expansion jointThe strategy is completely different. Take three typical scenarios:
Flue Gas Desulfurization (FGD) System。 The temperature is not high, but the humidity is high, the acid condensate is contained, and the chloride ion is enriched. Suggested choiceNon-metallic expansion joint (fabric fiber expansion joint)If it is not metal, it has to be lined with high nickel alloy + PTFE, or simplyrubber compensatorAcid and alkali resistant. Moreover, the liquid drainage port must be designed, otherwise the corrosion rate will double if the liquid accumulates too much.
High temperature steam pipelineFor example, thermal power plant main steam. The temperature is above 500℃, and the pressure is more than ten MPa. At this time, the corrosion is mainly high-temperature oxidation and steam erosion. The material must be a heat-resistant alloy, such as Inconel 625. OurCorrugated expansion joint for power station industryCan handle this kind of working condition. Also, pay attention when installingArrow direction of expansion jointThe flow direction of the medium must be corrected, otherwise the guide tube will fail and the corrosion will be aggravated.
Chemical media, what strong acid strong base organic solvent, varied. At this timePTFE-lined hoseAndPTFE compensatorIt's the main force. However, pay attention to the medium temperature. The upper limit of PTFE temperature resistance is about 200℃. If it exceeds, it has to be changedRubber PTFE compensatorOr metal lining. In addition, the flow rate and particle content of the medium should also be considered. If necessary, thicken the guide tube or adoptLarge diameter thick wall expansion joint。
Don't wait for a leak before repairing: Practical experience of anti-corrosion inspection and maintenance
You may ask, how do you know how corroded the expansion joint is? You can't disassemble it and install it again. There are some ways:
Measure the thickness periodically.The peak and trough of bellows were measured by ultrasonic thickness meter, focusing on the outlet area of the guide tube and near the weld seam. If the thinning exceeds 20% of the design thickness, it is time to consider replacement.
Penetration detection.For the suspected parts of microcracks, such as the welding heat-affected zone, the coloring penetrates and the cracks appear immediately. EspeciallyCompound hinge transverse expansion jointThis complex structure, with many welds, is more prone to hidden dangers.
Check the lining and coating.Open the manhole or look through the viewportPTFE-lined hoseIs the lining bulging, peeling, or cracking. Once the lining is broken, the medium comes into direct contact with the metal and corrodes faster than if it is unlined — because the gaps corrode.
Maintain records.Don't be too troublesome, establish a ledger for each expansion joint to record the installation time, media parameters and detection data. It is much more cost-effective to find a problem in advance than to wait for a leak and then stop the network.
Choose metal expansion joint or non-metal expansion joint? Comparative decision-making from the perspective of anti-corrosion
Finally, let's talk about this cliche.Anti-corrosion of metal expansion jointDone well, it can handle many working conditions, but some places are naturally more suitable for non-metals.
Non-metallic expansion joint (fabric fiber expansion joint)The advantages are: overall corrosion resistance (because it is a corrosion-resistant material itself), low cost, easy installation, and can absorb multi-dimensional displacement. However, the weakness is also obvious-the pressure and temperature resistance are limited (generally no more than 0.1MPa and 600℃), and it is easy to age and wear.
Metal expansion jointThe strengths are high temperature and pressure resistance, long life and high reliability. However, anti-corrosion needs careful design, and materials, linings, coatings and structures are indispensable. LikeStraight pipe pressure balanced expansion jointThis kind of pressure-bearing member, once corroded failure, the consequences are very serious.
In occasions with high temperature and pressure and demanding sealing requirements (such as main steam and chemical high-pressure pipelines in power plants), choose metals decisively, but the anticorrosion scheme should be meticulous. With low temperature and pressure, strong corrosiveness and complicated working conditions (such as flue gas desulfurization and sewage treatment), non-metals are often more worry-free. What if you want to be resistant to both temperature and corrosion? Then go onPTFE-lined hoseOrRubber PTFE compensatorEach takes his or her strengths.
After all,Anti-corrosion of metal expansion jointThe core of this matter is not to choose expensive materials, but to thoroughly understand the working conditions, match the scheme and take precautions. Don't wait until you miss it before shooting your thigh-the cost of that day is enough for you to buy several good units.