When the metal expansion joint is found to be bulging, twisted and cracked, the first reaction is mostly to scold the manufacturer for cutting corners. But have you ever thought that in many cases, this pot really shouldn't be borne by quality-misaligned selection, pitted installation and excessive working conditions, which are more common than material problems. Today, let's break it up and talk about the abnormal deformation of the metal expansion joint.
First distinguish: which deformations are considered normal work and which ones are really wrong?
The expansion joint itself absorbs the thermal displacement of the pipe by corrugation deformation. The corrugation is compressed, stretched and deflected, which is its job. It is normal that the corrugated surface has uniform undulations when you touch it with your hands. For example, for the high-temperature axial expansion joint on the steam pipeline of the power station, the wave distance from cold state to hot state changes by one or two millimeters, which is too normal.
But if you see the following situations, you have to be vigilant: local bulging, corrugation instability, edge wrinkles, weld cracking. The bulge is not uniform deformation, but a local bulge of a certain trough, indicating that the local stress is exceeded; Ripple instability is when the whole wave is crooked like a twist; Not to mention weld cracking, direct media leakage. The key to distinguishing is one sentence: uniform deformation is work, and local deformation is injury.
Misselection: The Biggest Stealth Killer
Not to scare you, I've seen too many people on the spot who use the universal corrugated expansion joint as a jack-of-all-world balm. The design of the universal corrugated expansion joint mainly absorbs axial displacement, but you bias it to absorb lateral displacement-the bellows is in an overload state as soon as it is started, so it is strange that it does not deform. What is even more outrageous is that the axial expansion joint is mounted on the pipe section with an elbow. During thermal expansion and contraction, the lateral force generated by the elbow is fully pressed on the corrugation. How long do you say it can last?
The waste heat power generation pipeline of cement industry and the flue gas pipeline of desulfurization system have a winding layout, with lateral displacement and angular displacement accounting for the majority. In this case, a double hinge transverse type expansion joint or a universal hinge type expansion joint should be used, but an axial type hard joint should not be used. High-temperature axial expansion joints are often used in the main steam pipeline of power stations, but the premise is that the pipe system goes straight, the guide brackets are arranged in place, and the displacement is indeed mainly axial. This matter of type selection is really not something that can be done by copying the work of the factory next door.
Pits in the installation process: cold tightening, guide bracket, pre-displacement
I met a customer two days ago, saying that the expansion joint leaked three days after it was installed. When I went to the scene, I found that the transportation protection tie rods were not dismantled, and the bellows were locked. As soon as the pipeline expanded, the force was all held on the weld-can this not leak? The top three pits for installing this piece are: the cold tightening is not done or reversed, the distance between the guide bracket is wrong, and the hanger is used as a fixed bracket.
What is cold tightness? That is, the expansion joint is pre-elongated or compressed for a section in the cold state, which is equivalent to saving a little reverse displacement in advance, so that the actual displacement of the bellows is small in the hot state. If the direction of cold tightness was reversed, it would be a bad favor. Instead, the thermal displacement that was supposed to be compensated was superimposed. The distance of the guide bracket is also particular. If the pipe is too far apart, it will deflect laterally under its own weight and thermal expansion, and twist the bellows into a twist. If the tie rod nut should be loose, it is called locking; If it's not supposed to be loose, it's called failure. It's full of details here, outside and inside.
The operating conditions are worse than you think
The design pressure is 1.0MPa, and the actual operation rushes to 1.3MPa for you from time to time; The design temperature is 350℃, and the operator fires hard to 400℃ in order to increase the output. The pressure fluctuation exceeds the limit, and each wave of the bellows is making reciprocating motion beyond the design value, and the fatigue life decreases exponentially. The chloride ions and sulfide in the medium are the natural enemies of the corrugated pipe. The stainless steel corrugated pipe undergoes stress corrosion cracking in the chloride ion environment. The surface looks good, but the interior is brittle like a biscuit. Not to mention systems with frequent water hammering phenomena, the instant impact force can directly flatten the bellows.
Pay special attention to the expansion joints operating under high temperature conditions, such as straight tube pressure balance expansion joints and high temperature axial expansion joints of power station boilers. These guys bear the triple superposition of temperature + pressure + displacement. During design, the fatigue life is calculated according to the number of cycles. Once the operating conditions exceed the limit, the actual life may be less than one-fifth of the design value.
Inferring the reason from the deformation shape: What do bulging, twisting and pulling explain?
The deformed form is the fingerprint of the accident scene. The wave pitch of the bellows generally becomes larger, especially on the side that is axially stretched, which indicates that the thermal expansion of the pipe exceeds the design value-either the cold tightening is not done, or the expansion section is small. The trough bulges outward, which is the result of the deformation of the trough brace because the internal pressure exceeds the limit, and the pressure is higher than the design value for a long time. The overall torsion indicates that there is torsional load in the pipeline, which is mostly due to unreasonable arrangement of guide bracket or additional external force in the pipeline system. Local small cracks appear on the surface of corrugated pipe, and chloride ion stress corrosion is preferentially suspected-the crack trend is perpendicular to the corrugation direction, which is different from fatigue cracks.
When shutting down for maintenance, take a flashlight to shine on the trough and weld heat-affected zone. These details can help you avoid many detours. Remember one sentence: 90% of the metal expansion joint is abnormally deformed because the three things of type selection, installation and working conditions are not done correctly, and the remaining 10% is material problems. Next time such a thing happens, it is not too late to go through these three items first, and then go to the manufacturer to theorize.