Why Inspection Specifications Can't Be Just Decoration-A Lesson from a Field Accident
Let's tell you the truth first. Last year, there was a chemical plant steam pipeline, which used oursHigh temperature axial expansion jointIt exploded less than three months after installing it. Check the reason. The 304 stainless steel written in the material report is actually made of 201 material-there is more than one grade difference between the inside and the outside. The question is, who signed this material report? The quality inspector in the factory said "I've seen it", but didn't do spectral detection at all. Do you say this is a matter of norms or people?
Well, to put it bluntly,Specification for inspection of metal corrugated expansion jointsIf it only stays on paper, it is no different from waste paper. An expansion joint seems simple, and any fork in bellows, flange, guide tube and tie rod nut may cause leakage or even burst. Today, we will break up and crush the inspection specifications one by one from the materials to the factory, so that some people will try and make mistakes on the spot.
Material inspection: bellows, flange, guide tube, which link is easiest to overturn?
Material inspection is the first step and where the probability of rollover is highest. Many people think that bellows, stainless steel welded pipe is done in one roll. Wrong! The bellows must be made of special plates, and the grain size, chemical composition and mechanical properties must be benchmarked one by one. Such asCorrugated expansion joint for power station industryWith high working pressure and rapid temperature change, the fatigue life of the material will be halved directly once corners are cut. A batch purchased by the last customerDirect buried (fully buried) type expansion jointThe wall thickness tolerance of the bellows exceeded the standard by 0.3mm, and as a result, water seepage began less than half a year after it was buried.
What about flanges and deflectors? The sealing surface of the flange must be flat, and the thickness direction of the guide tube depends on the medium flow rate. LikeDesulfurization flue gas baffle doorThe matching expansion joint contains sulfur in the flue gas, and the material of the guide tube must be corrosion-resistant. Ordinary carbon steel can't last for two years. How to check? Use ultrasonic thickness gauge for sampling inspection, at least 3 points per batch, and return the goods directly if it is less than 10% of the design thickness. Don't be too troublesome, one rollover is enough for you to lose half a year's profit.
Critical checkpoints in the manufacturing process: welding, dimensional tolerances, stiffness, and fatigue life
After the materials have passed the test, the manufacturing link is the real skill. Let's talk about welding first: the butt welds of bellows and flanges and the longitudinal seams of guide tubes must be nondestructive tested. X-ray and penetration testing are used in our factory, and the proportion of spot check is not less than 20%. And guess what? Once a spot check found that there were micro-cracks in a weld, and the master said that the welding wire was damp. Change the wire quickly, rework and reweld-if this is installedDouble hinge expansion joint of air-cooled island vacuum pipeOn, the vacuum is pumped, and the crack directly turns into an air leakage channel.
For dimensional tolerance, don't think it doesn't matter if it's a few millimeters different. The length, wave pitch and coaxiality of the expansion joint are directly related to the force distribution. Such asExternal pressure single axial expansion jointIf the wave pitch deviation exceeds 1mm, the stiffness will deviate by more than 10%. Check it outStiffness and Calculation Formula of BellowsIt will be understood that once the stiffness changes, the compensation amount will deviate, and the pipeline stress will all run to the support and hanger. What about that? Each node is measured with a vernier caliper, measured again after welding, and a report is issued.
Not to mention stiffness and fatigue life. We doCompound hinge transverse expansion jointWhen, the chief engineer required that one piece should be drawn for fatigue bench test in each batch. The standard is to cycle 5,000 times without leakage. In fact, we do 10,000 times before releasing it. Some people think that the cost is high, but if the life span is insufficient, the owner has to change it every two or three years, which is the real cost.
Pressure Testing and Sealability Testing: Water or Air Pressure? How to judge qualified?
This one is the easiest to exploit. Many small factories save trouble, directly pump air pressure, and leave the factory when you have nothing to do. But the air pressure test is risky, and once it bursts, the debris splashes and hurts people. According to the national standard GB/T 12777,Universal corrugated expansion jointGive priority to the hydraulic pressure test, the test pressure is 1.5 times the design pressure, and the pressure is kept for 10 minutes. Then under what circumstances do air pressure? Air pressure is used only when the piping system does not allow residual liquid, such as oxygen piping, and safety precautions must be in place.
During the hydraulic test, all welds and sealing surfaces shall not have leakage or sweating phenomenon. After pressure holding and pressure relief, measure the deformation of the bellows, and the deformation rate is not more than 1%. If you find that a certain wave crest has expanded, it means that the stress is concentrated, and you should quickly repair it. We've come across a group ofLarge diameter thick wall expansion jointThe waveform suddenly collapsed under water pressure. It was checked that the wall thickness was uneven. The manufacturer leveled it with repair welding, and the result was that the strength was not enough. Just scrap and redo-this kind of thing can't be compromised.
Final inspection before leaving the factory: appearance, logo, packaging, and the often overlooked tie rod nut
Appearance inspection looks simple, but it can actually sieve out many low-level errors. There must be no scratches, pits or rust on the surface of the bellows. The flange end face should be clean and the bolt holes should be aligned. Identification must be clear: model, nominal diameter, design pressure, displacement direction arrow, date of manufacture. I've seen some factoriesExpansion joint arrow directionThe printing was reversed, and as a result, the displacement direction was opposite to the thermal expansion direction of the pipeline after installation, and the bellows was abruptly cracked.
Packaging is also critical, especially for long-distance shipping. Bellows are easy to bump, so they should be fixed with wooden boxes, and protective boards should be added to the flange surface. But the most overlooked ones are tie rod nuts and screws. After receiving the goods, many customers found that the tie rod nut was loose and even the screw rod fell off. Why is this happening, you ask? I didn't do a tightening inspection before leaving the factory. according toHow to adjust the tie rod nut of expansion jointBefore shipment, the tie rod nut must be screwed to the pre-tensioned position and fixed with a locking nut. At the same time, the warning word "No disassembly of the tie rod" should be marked outside the package. If there is no bid, the construction team will dismantle it as soon as it is happy, and the expansion joint will lose the installation pre-compensation function.
Common illegal operations and misunderstandings: What happens if the expansion joint arrow is installed in the reverse direction? Should the screw be removed or not?
The direction of the arrow was said earlier, so dig deeper. Does the arrow of the expansion joint indicate the direction of media flow or the direction of displacement? The correct answer is the direction of displacement! However, on-site installers often understand that the medium flow direction, and the pipeline with large airflow is installed along the airflow. As a result, the bellows is subjected to reverse extrusion during thermal expansion, and its life is directly reduced by half. The correct way to do this is: According to the calculation of thermal expansion of the pipe, the arrow points in the same direction as the direction of thermal elongation. If you are not sure, ask the design institute to confirm it. Don't guess.
And should the screw be disassembled or not? Many articles say "it must be removed after installation", wrong! It depends on the structure. ForStraight pipe pressure balanced expansion joint, the tie rod screw is used to constrain the initial displacement and must be removed after installation, otherwise the expansion joint will not work properly. But forCompound straight pipe bypass pressure balanced expansion jointSome tie rods are permanent structural parts, and their removal will lead to overall failure. So, never one size fits all. Check the product manual at the time of acceptance, if not, ask the manufacturer. We all ship with a "Installation Precautions" card that clearly states the removal conditions of the screw. But many on-site workers didn't even look at it, and just clicked the wrench-tsk, wait for something to go wrong later.
Finally, the inspection specifications are not for quality inspectors to see alone. Purchasing should be understood, technology should be understood, and installation team should be trained. Missing a detail in each link may become the trigger of on-site accidents. Today's talk, I hope to help you step on a few pits less.