1. What is the performance of excessive expansion and contraction?
To put it bluntly, the expansion joint extends or compresses more than its original design limit during operation. You may see bellows pulled straight like springs, or in turn pressed into piles, or even locally bulging, jamming between ripples. Two days ago, a steam pipeline purchaser came to me, saying that the expansion joint on their site cracked after less than half a year. When I removed it, I saw that the corrugation spacing had long been uneven, which was typical for over-range use. This phenomenon is especially common in high-temperature axial expansion joints and general-purpose corrugated expansion joints, because many people think that "the bigger the better", and it will be done if the compensation amount is left with a margin. As a result, it will be bad if the margin is too large.
Second, why is there too much expansion and contraction?
Type selection, installation and working conditions.
The most common thing in model selection is the error of compensation estimation. For example, the problem of cold pre-tension/pre-compression is ignored in the calculation of pipe thermal expansion, or the displacement between multiple fixed brackets is superimposed incorrectly. For example: a 300-meter-long steam pipeline, with a design temperature of 250℃ and a theoretical thermal elongation of about 1.2 meters, someone directly selected an expansion joint with a compensation amount of 1.5 meters, thinking that everything would be fine. However, there are elbows in the actual pipeline direction, and the fixed brackets are not arranged according to the specifications. As a result, the displacement of the expansion joint is far more than axial expansion and contraction, but also lateral and angular displacements, and the compensation amount is simply not enough.
In terms of installation, it is a big problem that the tie rod nut of the expansion joint is not adjusted according to the factory requirements. Expansion joints usually come out of the factory with limit screws or tie rods to prevent excessive expansion and contraction during transportation and installation. Some on-site drawings save trouble, and directly screw off the tie rod or screw the nut to death, resulting in the expansion joint either stuck and unable to work, or losing the constraint to expand and contract freely. You can find "How to adjust the tie rod nut of the expansion joint" and "Does the screw of the expansion joint need to be disassembled" in the FAQ of this site. The answer is clear: the screw must be accurately adjusted according to the pre-stretching amount during installation, and cannot be messed around.
Changes in working conditions cannot be ignored. The pipeline originally designed at normal temperature suddenly experiences sudden temperature rise or pressure fluctuation, or the corrosion of the medium causes the wall thickness of the bellows to decrease and the stiffness to decrease, which will make the expansion and contraction out of control. Products such as metal corrugated expansion joints in cement industry and corrugated expansion joints for power station industry are designed for specific working conditions. If a medium or temperature range is changed, the performance curve will change completely.
3. How serious are the consequences of excessive expansion and contraction?
To put it lightly, the local stress of the expansion joint bellows is concentrated, and its life plummets-it was originally a product designed for 10 years, but fatigue cracks may appear in half a year. To put it seriously, the rupture of the bellows leads to media leakage, steam burns people, toxic gases leak out, and even fires. There is something more hidden: when the expansion joint expands and contracts too much, it will transmit huge thrust to the adjacent pipe supports and equipment, ranging from loose valve flange to overall displacement of pipeline and cracking of equipment foundation. We have customer feedback that a double hinge transverse expansion joint directly broke the smoke baffle door next to it because the transverse displacement exceeded the limit, and the maintenance cost was five times more expensive than the expansion joint itself. So don't underestimate this "excessive expansion", it is a ticking time bomb in the system.
Fourth, how to solve this problem from the root?
The first step is to re-check the selection parameters. Don't just look at the nominal diameter and pressure grade. The displacement must include axial, transverse and angular displacement, and cold pre-tension should be considered. The general corrugated expansion joint of this station mainly copes with axial displacement. If there is a large transverse displacement in the pipeline, it is necessary to replace the transverse expansion joint of compound hinge or the pressure balance expansion joint of curved pipe. High-temperature environments (such as power stations and cement industries) should use high-temperature axial expansion joints or large-diameter thick-walled expansion joints, and the material and number of corrugated layers should be recalculated.
The second step is to strictly follow the drawings during installation. The direction of the arrow on the expansion joint refers to the direction of the medium flow, so don't install it backwards. The tie rod nut must be adjusted according to the calculated pre-stretching amount after the pipeline system is installed and the fixed bracket is welded. The function of the guide tube is to protect the inner wall of the bellows from high-speed media. If you buy an expansion joint with a guide tube but install the direction of the guide tube in reverse direction, it is only a matter of time before it expands and contracts too much.
The third step is to check and maintain regularly. Inspect at least once a year to see whether there are corrosion spots, tension marks and bulges on the surface of the bellows, and measure whether the actual expansion and contraction amount is within the design range. For non-metallic expansion joints, attention should also be paid to the degree of aging of fabric fibers. If you find abnormal expansion and contraction, don't rush to change it-first check whether the pipe fixing bracket is loose and whether the guide bracket is failing. Many times the problem is with the pipe support system, and the expansion joint is just the scapegoat.
V. A real case of model selection
Last year, there was a customer of desulfurization and denitrification project, and the site space was limited, so he could only use single-axis double baffle door with expansion joint. At first, they chose the compound straight pipe bypass pressure balance expansion joint, and the calculated result was that the axial compensation amount was enough, but the lateral displacement caused by flue bending was ignored. During the trial operation, the expansion joint was directly crooked, and the tie rod was bent. Later, we helped him change to an external pressure single axial expansion joint plus a guide bracket, and re-selected the larger specification according to the actual displacement, and the problem was solved.
First, don't be superstitious about calculation books. On-site measured data is the gold standard. Second, in the same set of pipelines, the cooperation between the expansion joint, the baffle door and the hose is also very critical. For example, if the sealing surface of the desulfurization flue gas baffle door is stressed too much, the leakage rate will immediately soar. At this time, it is necessary to consider installing metal hoses or rubber compensators upstream and downstream of the baffle door to isolate the stress.
VI. To sum up: the essence of excessive expansion and contraction is that there is a problem with system matching, not that the expansion joint itself is unqualified.
It takes half an hour to calculate the displacement when selecting the model, look at the instructions twice when installing, and measure it regularly during maintenance-if these three points are achieved, there will be no problem for the metal expansion joint to use for ten or eight years. If you are not sure which type of product to choose, or if you encounter a project with complicated working conditions, talk to the manufacturer directly. There are more than 30 kinds of expansion joint and baffle door products in this station, from general-purpose type to high-temperature axial type, from direct buried type to vacuum special hose. Each product has detailed technical parameters and application instructions, so it will not deviate according to the selection. Remember: the expansion joint expands and contracts too much, don't rush to throw the pot to the quality, first check whether the system itself is reasonable.