After the on-site installation, it was found that the metal expansion joint had been forgotten to be installed. The pipe has been welded and the hanger has been fixed. You can't tear down the whole pipe, can you? He asked me if I could just cut a cut in the pipe, stick the expansion joint in and weld it.
To be honest, this kind of thing is not unusual at construction sites-the drawings have been changed, the installation sequence has been messed up, or the list has simply been missed. Today's article is dedicated to talking about:The metal expansion joint is missing. Can it be cut and repaired? What can I do to keep it from rolling over?
1. How did the missed installation happen? — — Several common rollover scenarios on the scene
There are various reasons for missing expansion joints, but they can be summarized in three categories:
- Drawing changes were not disclosed: The design was changed to a scheme with expansion joint, and the construction team still worked according to the old drawings. Only after the pipe was welded did it find that something was missing.
- Expansion joint arrived too late: In order to catch up with the construction deadline, the straight pipe section was welded first, thinking of opening and reassembling it later. As a result, it was dragged until it was pressed.
- Pure blindness: Take a real case-the installer of a steam pipeline in a power plantUniversal corrugated expansion jointThinking of it as an ordinary flange joint, he threw it aside and butt-welded the pipe.
In either case, the result is the same: the pipe has formed a rigid connection, and a section must be broken to fit the expansion joint in.
2. Can you cut it? Weigh the risks first-the location of bellows, guide tubes, and welds determine the cutting depth
Don't rush on the cutting machine. Find out what type of expansion joint you are missing first. If it isCorrugated expansion joint for power station industryOrHigh temperature axial expansion jointThere are multiple corrugations inside, and a flow guide tube. When cutting, once the bellows body is cut or the guide tube is injured, it is not a problem of repair welding-the whole expansion joint is scrapped.
- Is the expansion joint already attached to the pipe?If you just miss the entire expansion joint (such as one that should have been installedCompound hinge transverse expansion joint, the result is not installed), that cut is a straight pipe section, the risk is relatively low.
- Medium and pressure of pipeline: Steam, high-temperature flue gas, corrosive media, the welding requirements are completely different.
- Structural form of expansion joint: Expansion joint with guide tube, the guide tube is usually on the medium flow side, and the knife should be cut from the other side when cutting.
Do not blindly cut the knife until you are unsure of the internal structure。 It is best to find the drawings of the model or ask the manufacturer directly.
3. Cut open practical operation: where to cut? How to cut? How much allowance is left?
Suppose you confirm that the missing location is a straight pipe, and the pipe itself is made of carbon steel or stainless steel. Then the operation steps:
- Measurement positioning: The total length of the expansion joint plus the welding allowance at both ends (generally 5-10mm at each end). Like aUniversal corrugated expansion jointWith a length of 300mm, plus weld allowance at both ends, it is necessary to cut off about 320mm of pipe segments.
- Cutting method: Plasma or mechanical cutting, gas cutting is prohibited (unless it is a thick-walled carbon steel pipe and subsequent machining is required). The cutting surface must be perpendicular to the pipe axis with a deviation not exceeding 1mm.
- Bevel treatment: After cutting, make V-shaped grooves at both ends, with blunt edges of 1-2mm and gaps of 2-3mm. It is recommended to use mechanical beveling for stainless steel pipes to avoid carbon pollution.
Fix the pipe before cuttingOtherwise, the tube will bounce off after cutting. Especially for high-temperature pipes, the nozzle displacement after stress release can be frightening.
Four, welding this step is the most fatal-welding material, preheating, interlayer temperature, preventing bellows from overburning
What are you most afraid of when welding? I am afraid that your bellows will be baked after welding. The wall thickness of the bellows inside many expansion joints is only 0.3-0.5mm, which is close to the straight pipe section. If the welding heat is transmitted, the bellows will directly anneal or even burn through.
- Welding material selection: After the pipe material is determined, the welding materials must match. For example, 304 stainless steel pipe with ER308 welding wire, carbon steel with ER50-6. It should be noted that there are some expansion joints in this station such asHigh temperature axial expansion jointInconel alloy may be used, and the welding material should be selected separately.
- preheating: The thickness of carbon steel pipe exceeding 20mm needs to be preheated at 150-200℃. Stainless steel is generally not preheated, but must be preheated when the ambient temperature is below 0℃.
- Interlayer temperature: The stainless steel is controlled below 150 °C to prevent sensitization. Carbon steel does not exceed 300°C.
- Anti-overheating measures: Wrap wet asbestos cloth around the expansion joint body, or blow compressed air toward the bellows side to cool it. When welding, try to use small current, fast welding, multi-layer and multi-pass.
In addition,Specific Function of Expansion Joint Guide TubeIs to protect the bellows from high-speed media scour. If the guide tube is misaligned with the inner wall of the pipe during welding, it will cause the medium to directly impact the bellows, and the life of this expansion joint will be greatly reduced.
5. Post-welding inspection and pressure test-no leakage does not mean that it can carry displacement
Don't be in a hurry to shout OK when the welding is done. First do the appearance inspection: the weld surface has no cracks, no pores, and the biting depth does not exceed 0.5mm.
Then doStress testing: Hydraulic pressure test is carried out at 1.5 times of the design pressure, and the pressure is held for 30 minutes. But hydraulic testing carries risks – for bellows, the stress caused by hydraulic pressure may exceed the design value. It would be safer toAir pressure test(1.1 times the design pressure, use compressed air or nitrogen), but pay attention to the safe distance.
Most easily overlooked isDisplacement compensation capability test。 After the expansion joint is installed, it must be checked whether it can still expand and contract normally. On the spot, you can use a jack or manual pull rod to simulate the thermal displacement of the pipeline to see whether the bellows is stuck and whether the guide tube collides. If the expansion joint tie rod is screwed to death during welding, or the bellows is skewed due to welding deformation, then the expansion joint will be wasted.
6. What should I do if I can't save it? — — Replace the expansion joint with a new one or reinstall it?
- The bellows has been burned out during welding
- The pipe is too misaligned and the bellows is twisted after being forcibly installed
- The medium is corrosive, and the heat treatment cannot be guaranteed by field welding
Don't take it hard at this time. The best way to save money isReassemble a section of short section + flange: Cut off a section of the original welded pipe, weld flanges at both ends, and connect them with a metal expansion joint with flanges in the middle-such asUniversal corrugated expansion jointThere's a flange. The cost does not increase much, but the on-site welding risk is avoided.
If the pipe has been completely welded and there is no margin, it is only necessary to remove the entire pipe and repurchase the pipe section with the expansion joint. Don't feel like a waste. Compared with the pipe cracking and leaking in the future, this little loss is nothing.
How to weld the metal expansion joint when it is missing?There is no unified answer to this question, but as long as you figure out the structure, choose the right process, and take precautions, you can save it in most cases. If it doesn't work, it is better to admit defeat and replace it with a new one than to have an accident.