Type selection is not cabbage selection: working condition matching is the last word
To be honest, I've seen too many people buy expansion joints like cabbage-look at the size, ask the price and place an order. And the result? After less than three months, the bellows cracked, the guide tube fell off, and the construction period was delayed. The cost of disassembly and assembly alone was enough to buy a new one. Metal expansion joint uses this matter, the first pit is on the type selection.
Expansion joints are not universal parts, and there are corresponding products for each working condition. For example, for high-temperature steam pipes, do you make do with general-purpose corrugated expansion joints? When the temperature goes up, the bellows instantly becomes unstable. The corrugated expansion joint and high-temperature axial expansion joint in power station industry are the correct solution. Another example is the desulfurization flue gas system. For corrosive media, you have to choose PTFE hose or PTFE compensator, otherwise chloride ions will bite through the bellows in minutes. There is also the high dust environment of the cement industry, and the metal corrugated expansion joint of the cement industry has wear-resistant guide tubes, which ordinary goods can't bear at all.
Then how to choose? First, the parameters of medium temperature, pressure, displacement, pipe diameter and connection mode are clearly sorted out. Don't slap your head from experience – guess what? Many rollover cases are shot by slapping the head. For example, straight tube pressure balance expansion joint and curved tube pressure balance expansion joint look like each other but have completely different application scenarios. The former is used to absorb axial displacement at straight pipe sections, and the latter is used to absorb lateral displacement at elbows. Wrong choice, wrong force, two or three years to scrap.
Don't rush to get started before installation: how many pits have you stepped on in the direction of the deflector tube, the tie rod nut and the pre-stretch?
When the equipment arrived at the scene, some people wanted to save trouble and welded it directly out of the box. Alas, if you skip this step, you will suffer a big loss later. During the use of metal expansion joints, the inspection and adjustment before installation can determine the life of 70%.
Let's talk about the direction of the guide tube first. The role of the guide tube is to direct the flow direction of the medium and protect the bellows from impact. In many cases, the guide tube is installed backwards, and the medium directly washes the bellows, which is perforated in a few months. Remember: The guide tube arrow points in the direction of the media flow. If you are confused, turn through the article "The Specific Function of Expansion Joint Guide Tube" on our station, and the picture is very clear.
Let's talk about the tie rod nut. The role of the tie rod is to limit the unintended expansion and contraction of the expansion joint during transportation and installation, while withstanding the internal pressure thrust. But many masters don't know-after the installation is completed, the tie rod nut needs to be adjusted according to the design requirements, not screwed to death or completely loose. How to adjust the expansion joint tie rod nut? It is usually divided into three steps: first, confirm that the expansion joint is at its natural length; In the second step, the nut is rotated to a designated scale according to the compensation amount; Step 3: Lock the loose nut. Don't weld to death when you come up. It's equivalent to turning the expansion joint into a rigid part. How can you absorb thermal expansion and contraction?
And pre-stretch. For situations where large axial displacements need to be absorbed, such as high-temperature steam pipes, the expansion joint needs to be pre-stretched during installation. The stretch amount is generally about half of the total compensation amount. Most people don't do this operation because it is troublesome. As a result, the bellows fatigues and breaks quickly after operation.
Dead point during installation: the fixing bracket is not done properly, the torsional stress is not eliminated, and the expansion joint is no matter how good it is
The fixing bracket was not made according to the specification, and the torsional stress of the pipe system was not eliminated.
The fixed bracket is the anchoring point of the expansion joint system. If the fixing bracket is not strong enough or in the wrong position, the thrust generated by the thermal expansion of the pipe can put extra load on the expansion joint and even press the bellows into a twist. All pressure balanced expansion joints (such as straight pipe pressure balanced expansion joints and double straight pipe bypass pressure balanced expansion joints) have particularly strict requirements for fixed brackets, because their working principle is to rely on tie rods or balance pieces to resist pressure thrust. Once the bracket fails, the entire compensator group is directly scrapped.
Torsional stress is more subtle. Many times when installing pipes, because the flange holes are misaligned, workers hard break the pipes to align the bolts. This twisting force is transmitted directly to the bellows, creating torsional stress. The bellows was originally designed to withstand only axial and lateral displacements. If you add a torsional load to it, the weld will crack first. How to avoid it? When installing, ensure that the pipeline is coaxial and the flanges are aligned parallel. Don't use "brutal installation". For large-diameter pipelines, large-diameter thick-walled expansion joints or double-hinge transverse expansion joints are recommended. They have a certain tolerance to torsion, but don't expect to cure all diseases.
Don't just focus on appearance in routine maintenance: displacement, corrosion points and fatigue cracks are the key to life
Many inspectors think it's okay when they see that the expansion joint shell has no paint or air leakage. Tsk, this is the same as buying a coffin for a pearl. Hidden dangers in the use of metal expansion joints are often inside and out of sight.
The displacement should be measured periodically. After several months of operation, the actual displacement may exceed the design value because of support settlement or pipe stress release. Take a ruler to measure the compression or stretch length of the bellows and compare it with the initial installation record. Out of scope? Hurry up and find the reason to adjust.
Corrosion spots are the easiest to overlook. For stainless steel bellows, chloride ion stress corrosion cracking is the number one killer. If you see rust spots, blackening and pitting pits on the surface, don't think it is just a surface problem. Cut a bit for metallographic analysis, or check for microcracks with penetration testing. Corrugated expansion joints used in power station industry often contact steam condensate. Although the concentration of chloride ions in water is low, long-term accumulation still causes problems.
Fatigue cracks are more deadly. Every time the bellows expands or contracts, it experiences a low-cycle fatigue. When the number of cycles reaches the design life, cracks begin to initiate. This thing doesn't say hello in advance, and can only rely on regular non-destructive testing-magnetic particle testing or ultrasonic phased array. Don't think it is troublesome. If you really wait for the leak, the loss of shutdown will be enough for you to do dozens of tests. Refer to the article "Service Life of Expansion Joints" on our station, which contains detailed fatigue life estimation methods.
Real rollover case: improper selection + installation error = scrapping in three months, and the construction period will be paid if the money is spent
Last year, a steam system in a chemical plant used a general-purpose corrugated expansion joint. Design temperature 350℃, actual operating temperature 380 ℃. If it is 30 degrees higher, the high-temperature axial expansion joint should be used when selecting the model. There was no pre-stretching during installation, and the tie rod nut was directly welded to death. The first month of operation was fine, the second month the steam flow fluctuated and the bellows made a "quack" sound. In the morning of the third month, a 4cm-long crack at the weld of the bellows was found, and steam was directly injected. The whole system was out of operation for a week, and eight expansion joints were replaced. Plus the repair cost and lost work cost, the loss was more than 200,000 yuan.
I have repeatedly told the client about this case. The use of metal expansion joints is not a child's play, and every link hides pits. Selection should be based on the benchmark working conditions-high temperature axial expansion joint, corrosion lining PTFE hose, large displacement transverse hinge or sleeve pipe expansion joint. Installation should be according to the regulations-direction of the guide tube, adjustment of the tie rod nut, pre-stretching, fixing the bracket, and eliminating torsion, none of which can be missed. Maintenance should focus on the key points-displacement, corrosion, fatigue cracks, don't just look at the surface.
Don't wait until something happens to regret it. After all, the expansion joint is the "soft joint" of the pipeline system. If it goes wrong, the whole system will have to lie down.