Metal expansion joint perforation: the pits you stepped on in those years and the cure

Who exactly is the culprit of the piercing? — — Triangular relationship between corrosion, stress and manufacturing defects

I have been engaged in expansion joints for more than ten years. Every time I hear a customer say "it is pierced again", my first reaction is not to sigh, but to ask about the working conditions first. And guess what? Nine times out of ten, there are three reasons: corrosion, stress fatigue, or illness after leaving the factory.

Let's start with corrosion. This is the most common and the most injustice. Chloride ions in the medium? The stainless steel bellows is just like paper paste, pitting and intergranular corrosion take turns. Especially in the pipeline behind the flue gas baffle door in the desulfurization system, the residual acid after wet desulfurization is directly on the corrugation, so it is strange that it is not perforated. Corrosion is not done in a day, it is like an ant gnawing on a bone, biting in little by little from the outer wall or the inner wall. Two days ago, there was a power plant customer who used usCorrugated expansion joint for power station industryIt stands to reason that the corrosion resistance grade is high enough, and the result is that it leaks after less than two years of operation. When disassembled, the inner wall is full of spots-the temperature fluctuation of the medium leads to condensation, and the acidic condensate is soaked for a long time.

Let's talk about stress. Many people think that expansion joints are soft connections, so they can be installed casually. Wrong. The axial forces generated by hot and cold expansion of the pipe, the bending moments generated by lateral displacement, plus the preload forced to center during installation-these stresses are superimposed on the corrugations, and each trough is the stress concentration point. Over time, microcracks appear, then expand, and finally perforate. Especially the kindHigh temperature axial expansion joint, the creep strength of the material decreases at high temperature, and the stress problem is more prominent.

Finally, there are manufacturing defects. To put it bluntly, it is cutting corners. The wall thickness of the bellows is insufficient, the wave pitch is uneven, and the weld seam does not penetrate-these "internal injuries" that exist from the factory will explode sooner or later. A small factory produces itUniversal corrugated expansion jointIt looked pretty well-mannered, but it leaked when it was suppressed. When cut, the thickness at the crest is 0.3mm thinner than that at the trough. Who will perforate if this is not perforated?

Hidden dangers invisible to the naked eye: how to accurately diagnose the initial symptoms of perforation?

By the time I saw the media streaming out, it was already too late. The real master, is to pull out the piercing before it happens. What about that? By "seeing, hearing, asking and seeing"-no, it is a combination of several detection methods.

Visual + penetration detection. Disassemble the insulation layer and look at the ripples for rust, discoloration or slight bulging. Take a flashlight from the side, spray coloring penetrant on suspicious spots, and wait a few minutes to see it again. A red line comes out, that's a crack. This method is simple, but you can only check the surface.

Ultrasonic thickness measurement. The wall thickness of bellows is inherently uneven, so it must be measured wave by wave with a twin crystal probe. When measuring, pay attention to recording the peaks and troughs separately. If a certain trough is found to be more than 15% thinner than an adjacent trough, it is not far from a perforation. We've encountered it on siteDirect buried (fully buried) type expansion jointIt is impossible to see it buried underground, all depending on regular thickness measurement. Once, it was measured that there was only 1.2mm left in a certain trough (the original design was 2.0mm), so I changed it quickly. It would definitely leak two months later.

Acoustic emission. For high-pressure pipelines, this is the most reliable. The sensor is attached to the tubes at both ends of the expansion joint, and the acoustic emission signal is collected during operation. Once there is microcrack propagation, the signal characteristic frequency will change. The system alarms automatically. Of course, this thing is expensive, and it is generally used in high-risk situations such as petrochemical industry and air separation units. Such asDouble hinge expansion joint for air-cooled island vacuum pipelineOnce the perforation of the vacuum system has a huge impact, install a set of acoustic emission monitoring, value.

Don't ask me which method is best. Different working conditions, different choices. But there is an iron rule: Don't wait until the leak is visible to the naked eye before you start, when the maintenance cost will at least triple.

Selection error is the biggest cost: Which expansion joint should be used in different working conditions?

Two days ago, I met a buddy who made pipes, and said with a sad face that the expansion joint installed on their steam line had exploded again. I asked him what model he installed, and he said, "Just the most common bellows". Tsk, that's the problem. The steam pipe has high temperature, high pressure, and water hammer impact. You useUniversal corrugated expansion jointAren't you looking for death?

Different working conditions, one word difference in type selection, five years difference in life. I'll give you a look at the common scenario:

• High temperature and high pressure steam pipeline: must be used withHigh temperature axial expansion jointOrExternal pressure single axial expansion joint。 The external pressure structure makes the bellows under pressure on the outside and tension on the inside, which has stronger fatigue resistance. In addition, remember to add a guide tube to protect the inner wall from being washed by high-speed steam.
• Chemical pipeline with corrosive media: PreferredPTFE-lined hoseOrPTFE compensatorThe tetrafluorine layer separates the medium from the metal. However, pay attention to the temperature, PTFE can only carry within 200℃. Use it when the temperature is higherLarge diameter thick wall expansion jointMatched with corrosion resistant alloys (e.g. Hastelloy).
• Large displacement, large angle pipeline: Don't expect to carry it hard on bellows alone. useCompound hinge transverse expansion jointOrCurved tube pressure balance expansion jointThese structures can spread the displacement across the two sets of ripples and also balance the pressure and thrust. At the beginning, a chemical plant had to use a single expansion joint to absorb lateral displacement, and it cracked in a few months.
• Underground directly buried pipeline: HonestlyDirect buried (fully buried) type expansion jointWith its own anti-corrosion layer and guide tube, the outer wall has also been strengthened. Ordinary expansion joints are buried and perforated in less than three years.
• Flue gas pipeline (desulfurization, dust removal): High temperature, dusty, and corrosive gases. At this timeNon-metallic expansion joint (fabric fiber expansion joint)More suitable, temperature-resistant, corrosion-resistant and cheap. If you have to put metal on it, you have to use itRectangular non-metallic expansion jointWith a metal frame, or simply useMetal Corrugated Expansion Joints in Cement Industry(They are designed with wear in mind).

If you choose the right model, the rest of the things will be less than half. If you are not sure, go directly to the manufacturer to mention the working condition parameters and ask them to produce a calculation book. Don't pat yourself on the head.

Those fatal details that are easy to overlook in installation and operation and maintenance (with real cases)

There is a saying that "three-point selection, seven-point installation". Not an exaggeration at all. You have stepped on at least two of the following pits.

The first pit: cold tightness is not done right.Expansion joints typically leave the factory pre-compressed or pre-stretched with the goal of keeping it at optimal stress at operating temperatures. However, many people at the scene didn't understand, so they directly screwed the tie rod nut to the end and even removed the tie rod. And the result? As soon as the pipe is hot, the bellows is directly pulled to the limit, and the wave crest is torn. There was a case in a coking plant last year where they installedRotary compensatorWhen I didn't adjust the preload according to the instructions, it was perforated less than a month after turning on. Our people went to the scene to see that the position of the tie rod nut was 30mm different from the design value.

The second pit: the guide bracket is not installed.The function of the guide bracket of the expansion joint is to ensure that the pipe only expands and contracts in the axial direction without deflection. Some construction teams save trouble and save the guide bracket. Results the corrugated pipe was subjected to bending stress outside the design, which accelerated fatigue. We have counted that perforations caused by missing guide brackets account for 12% of the total number of failures.

The third pit: the insulation is wrapped too dead.The working part of the expansion joint (bellows) usually does not allow the pack to be insulated because of heat dissipation. Some sites are afraid of heat dissipation loss, so insulation cotton is stuffed into the corrugated gap. This can cause heat to build up and the material to overheat and fail. Moreover, the insulation cotton will accelerate corrosion after absorbing water.

Fourth Pit: The drain is blocked.Many expansion joints are designed with drainage holes at the bottom to drain condensate or rinse fluid. But no one pays attention to the drainage holes in daily inspections. After plugging, the liquid accumulates in the low-lying corrugation for a long time, and the electrochemical corrosion is aggravated. The probability of piercing is doubled. It happened on a blast furnace gas pipeline in a steel plant.Double-sealed single-axis circular baffle doorThe expansion joint at the back, the drain outlet was pasted to death by dust, and it was worn in half a year.

Don't be in a hurry to change if it is perforated: Comparison between on-site emergency repair and long-term transformation scheme

Pierced, first reaction is to change for a new piece? Wait a minute. If it's just a needle tip-sized hole, or downtime doesn't allow, there are quick emergency solutions on site.

Emergency repair scheme:

• Metal patch + clamp: Cut a stainless steel sheet into a circle two turns larger than the perforation, pad it with a high-temperature-resistant sealing gasket (such as a graphite gasket), and then press it tightly with a Huff clamp on the outside. This program can last from days to weeks, but it must be run in a depressurized manner. Suitable for temporary treatment before scheduled maintenance.
• On-site cold welding of polymer composites: There are specialized metal repairing agents on the market, which are rubbed on holes like putty and can resist several MPa pressure after curing. However, the surface should be clean and dry, and the medium temperature should not exceed 150℃. We treated a cement plantMetal Corrugated Expansion Joints in Cement IndustryThe perforation was wrapped with repairing agent and fiberglass cloth, and it lasted three months until the overhaul.
• Hoop welding: If the perforation is in the trough position, you can make a special curved hoop and weld it directly on the straight edges on both sides of the trough. Be careful not to weld on the corrugation itself, otherwise the stress concentration is more severe. This method is relatively reliable, but it requires good welder skills.

Long-term transformation plan:

Emergency is an emergency after all. If we want to cure it, we have to change it from the design. Something like:

• If it's a corrosion problem, upgrade the material. PutUniversal corrugated expansion jointReplace withPTFE-lined hoseOr high nickel alloy as a whole. The cost is a bit higher, but the lifespan has changed from half a year to five years.
• If it is stress fatigue, change the structure. For example, increase the wave number to reduce the single wave displacement, or change it toCompound hinge transverse expansion jointShare the stress. Another option is to add tie rods or limits to prevent excessive axial compression.
• If the medium contains particle wear, add wear-resistant liner or guide tube. a great manyCorrugated expansion joint for power station industryWill add ceramic fiber bushing inside, the effect is good.

How to choose? Calculate an account: emergency cost (materials + labor + shutdown loss) vs long-term renovation cost. If the same location is perforated more than twice a year, decisively go to the long-term protocol.

Eliminate from the source: Are material upgrading and structural optimization worth investing in?

Finally, this question is often tortured by customers' souls: "Boss, it costs twice as much to change good materials in this working condition. Is it cost-effective?"

Settle the general ledger. Direct loss of perforation of an expansion joint: shutdown of production, media leakage may cause safety accidents, labor and spare parts cost of replacement, and environmental fines. These add up, often several times the price of the expansion joint itself. Especially at some key stations, such asDesulfurization flue gas baffle doorThe front and rear expansion joints, once perforated, the whole desulfurization system has to stop. The downtime loss of a 300,000 unit is hundreds of thousands a day. Is it worth spending twice the material cost at this time?

-Conventional stainless steel 304→316L→254SMO→Hastelloy C276. The higher the concentration of chloride ions, the higher the content of nickel and molybdenum.
-High temperature operating conditions: the upper limit of the allowable temperature of the ordinary 304 is about 800°C, but the creep strength is low. Switch to Inconel 625 or GH3030, which can carry above 1000℃.
-Wear-resistant conditions: the inner wall is sprayed with tungsten carbide or ceramic lining plate is added.

-Increase the wavenumber or use multiple layers of corrugation. Multilayer wave is equivalent to multiple sheets stacked, with lower total stiffness and higher fatigue life.
-Switch to weldless construction. Such as Special hose for vacuumWith a monolithic molding process, the weakest link is eliminated without a weld.
-Install deflectors and drains. In addition to protecting the inner wall of the corrugation, the guide tube can also guide the flow direction of the medium to avoid local erosion.

Truth be told, the return on investment cycle is usually no longer than two years. Those customers who blindly want to cheap end up spending double tuition fees.

Perforation of metal expansion joint is not a natural disaster, but a man-made disaster. As long as we understand the three points of corrosion, stress and manufacturing, choose the right model, install it in the right position, and don't save money, it can serve safely for more than ten years. Do you think that's the case?