"How much does an expansion joint cost a meter?"-Don't ask for a price in a hurry. Metal expansion joints are never universal parts that can be "installed". Before the selection, I can't figure out the use restrictions. It is a trivial matter to buy it back and not install it. During operation, the bellows cracks, the medium leaks and the pipeline is unstable, which is the real trouble. Let's make it clear: the use limit of metal expansion joints, to put it bluntly, there are five red lines, and which one you step on is a joke about the life of the equipment.
Medium and temperature: Not all fluids can be crammed into bellows
The bellows is the "heart" of the expansion joint, and its material directly determines what medium you can use. Don't think that stainless steel bellows is universal-304L can't last a few months in front of dilute sulfuric acid, and 316L still cracks under stress corrosion in high-temperature chloride ion environment. When selecting, you have to give all three parameters of media composition, concentration and temperature at once, which is the most basic.
The temperature thing, just give a "high temperature" is not OK. At the same 800℃, the requirements of flue gas pipes and steam pipes for bellows are completely different. Why can't corrugated expansion joints for power station industry and metal corrugated expansion joints for cement industry be interchanged? It is because the fluctuation curves of smoke composition, dust content and temperature are very different. If there are corrosive components in the medium, remember to consider anti-corrosion schemes such as PTFE-lined metal hose or PTFE compensator, but the upper temperature resistance limit of PTFE-lined is there, and it will not work if it exceeds 180℃. At this time, you have to change the high-temperature alloy bellows, and the cost will go up.
Pressure vs. Vacuum: Thicker Bellows Are Not Safer
If the pressure is high, then use thick-walled bellows. Quite the opposite is true. The pressure resistance of bellows comes from the wave design and multi-layer structure, rather than the pure stack wall thickness. As the thickness goes up, the flexibility goes down, and the ability to compensate for displacement is weakened. More troublesome, thick-walled bellows are more sensitive to cyclic fatigue-this account is not cost-effective.
Vacuum. The vacuum line looks "no pressure", but in fact the external pressure will make the bellows unstable. At this time, you have to choose vacuum special hose or external pressure single axial expansion joint, and the design logic is completely different from the positive pressure working condition. The directly buried (fully buried) expansion joint is used in buried pipelines. Besides the pressure, it also depends on the soil load. It is not just a general-purpose corrugated expansion joint that can be topped.
Displacement Types and Constraints: Tie rods and hinges are not furnishings
The expansion joint is divided into axial type, transverse type and angular type according to the displacement type, each of which has corresponding constraint structure. The universal corrugated expansion joint mainly absorbs axial displacement. If you force it to absorb transverse displacement, the bending moment bearing by the root of the corrugated pipe will exceed the limit in minutes. At this time, the scheme has to be switched: the double hinge transverse expansion joint or the double straight pipe bypass pressure balance expansion joint is the one to do this job.
The restraints such as tie rods, hinges and universal hinges must be adjusted in place as required after installation. Two days ago, I encountered a scene, and all the locking nuts of the expansion joint of the big tie rod were screwed to death, and the displacement couldn't move at all. The thermal expansion stress of the pipeline was all suppressed on the fixed bracket-then what do you need the expansion joint for? The role of the tie rod is to restrain the bellows from bearing the internal pressure thrust, not to limit its compensated displacement. After installation, regardless of adjustment, it is equivalent to destroying this structure.
Fatigue life and number of cycles: Don't use expansion joints as permanent parts
It is a consumable with a lifetime. With each expansion and contraction, the bellows undergoes a stress cycle, and at a certain number of times, fatigue cracks will appear. The design life is usually 1000 cycles-sounds like a lot? If the pipeline starts and stops once a day, it will be more than 700 cycles a year, and it will reach its life in two years.
Therefore, when selecting the model, it is necessary to clarify the number of cycles of working conditions. For example, double hinge expansion joint of air-cooled island vacuum pipeline and high-temperature axial expansion joint, the fatigue life will be checked according to high-frequency cycles during design. There is also a large-diameter thick-walled expansion joint, which looks strong. The thickness of the single layer of the bellows is large, and the cycle life is lower than that of the multi-layer thin-walled structure. One more step can save the downtime cost of subsequent replacement.
Installation and maintenance: changing pipeline direction on site is the biggest pit
The position of the expansion joint in the pipeline is determined according to the results of pipeline stress analysis in the design stage. In order to avoid beams, columns or equipment, the pipeline was turned around and a support and hanger were added, and the whole stress distribution was completely changed. The actual displacement direction of the expansion joint does not match the design value, and the bellows soon has problems.
The direction of the guide tube should be consistent with the flow direction of the medium. The pre-displacement for pre-cooling or pre-heating must be carried out according to the drawings. The outside of the bellows should not be wrapped with an insulation layer (it should also be wrapped on the outside of the shell). The daily inspection after operation focuses on whether there is abnormal deformation on the surface of the bellows and whether the tie rod nut is loose. The service life of the expansion joint depends on the right selection, correct installation and diligent maintenance. None of these three things can be settled.
In the final analysis, the use limitation of metal expansion joints is the above five red lines. Spend half an hour before the selection to sort out the six parameters of medium, temperature, pressure, displacement, number of cycles and installation conditions, so that you can reduce ten trips to the site later. Only when the data is in place and the working conditions are clearly explained can the supplier give you a reliable plan.