Increasing stiffness is not metaphysics-first understand where the stiffness of the compensator comes from
Compensator stiffness increases, which is quite common in piping systems. Don't be in a hurry to change parts or add gaskets. You have to understand one thing first: what exactly is stiffness? To put it bluntly, stiffness is the ability of the compensator to resist deformation. If you press a bellows with your hand, it means that it can't be pressed, it means that it has a large stiffness, and if it is deflated when pressed, it means that it has a small stiffness. But the compensator is not for you to press by hand, it is used in the pipe to absorb thermal displacement and vibration. Once the stiffness becomes larger, the displacement that should be absorbed can't be absorbed, and the force will all be transmitted to the pipeline, the pipe frame, and even the equipment interface.
Where does that stiffness come from? The core is in the bellows itself. What we always sayUniversal corrugated expansion jointIts bellows are made of very thin stainless steel sheets (usually 304 or 316L) that have been hydraulically or mechanically molded. The parameters of wall thickness, wave height, wave pitch and layer number of bellows directly determine its axial stiffness and transverse stiffness. For example, if the wall thickness is increased by 0.2 mm, the axial stiffness may increase by 30% to 50%. This is not a blind guess. There is a formula in GB/T 12777 that can calculate that the stiffness of the bellows is proportional to the cubic of the wall thickness. Therefore, increasing stiffness is never metaphysics, and there must be changes in physical parameters behind it.
What factors will "push" the stiffness up? Check one by one from materials, structures to working conditions
We'll sieve them one by one. Look firstMaterials。 I met a customer two days ago and said that they had used it for half a yearrubber compensatorIt is getting harder and harder, and it can't be pulled at all. When I asked, the medium temperature soared from 80℃ at the time of design to 120℃, and the vulcanization system in the rubber continued to cross-link at overtemperature. The hardness dried from 60 Shore A to 80 Shore A, and the stiffness naturally doubled. The rubber material has an aging phenomenon, but the heat accelerates it. Replace withTetrafluorine compensatorWill it be good? Does PTFE soften in high temperatures? In fact, PTFE also has a "cold flow" problem, but the stiffness change is generally not as obvious as that of rubber.
Look againStructure。 You useCompound hinge transverse expansion jointTwo bellows plus an intermediate connector, the longer the intermediate connector, the smaller the overall transverse stiffness. But if you make the middle pipe short to save money, or the worker locks the tie rod nut during installation (FAQ), the stiffness will explode directly. AndExternal pressure single type axial type expansion joint, its external pressure will squeeze the bellows, the higher the working pressure, the axial stiffness will increase. This phenomenon is called "pressure-enhanced stiffness", which is usually considered in design, but if the actual working pressure exceeds the design value, it is inevitable that the stiffness exceeds the standard.
AndWorking condition。 Are there water hammers or frequent pressure fluctuations in the plumbing system? I've seen a steam pipe that usesDirect buried (fully buried) type expansion jointThe compaction degree of soil backfill is too great, which crushes the expansion joint directly, and the bellows can't move at all. And there's something more covert – corrosion. The chloride ions in the medium lead to pitting of the bellows, and the local wall thickness decreases. It is reasonable to say that the stiffness should be smaller, but the pitting area will cause stress concentration and lead to plastic deformation of the bellows, which will make the stiffness abnormal. Therefore, to judge that the stiffness becomes larger, it is unreliable to rely on the feel and feeling alone, so it can be measured.
What happens to the piping system after the stiffness becomes larger? Don't wait for it to leak before you regret it
The consequences of stiffness are chain-like. First, the thermal displacement compensation of the pipeline is insufficient. When you designed, you calculated that the thermal elongation of a certain section of pipeline was 50mm. As a result, the compensator can only provide a displacement of 30mm. Where did the remaining 20mm go? Hard eaten by the pipe. The pipeline itself will also have elastic deformation, but if it exceeds the elastic limit, plastic deformation will occur. If the pipe wall stress exceeds the standard, the weld will suffer first.
Secondly, the forces on the fixed bracket and the pipe frame increased sharply. I've seen a case where aCorrugated expansion joint for power station industryWhen installed on the main steam pipe, the axial stiffness changed from the design value of 200 N/mm to 350 N/mm. As a result, the strength of the fixed bracket was not enough, which directly led to the deformation of the bracket and the pipe offset. During the repair, it was found that the expansion joint itself was not broken, but the two sidesDesulfurization flue gas baffle doorDue to the excessive displacement, it was torn apart, and the construction period of replacing a baffle door was three days, resulting in a loss of more than 100,000 yuan.
The scariest thing is actually the sharp shortening of fatigue life. The increase of stiffness will increase the amplitude of stress experienced by the bellows in each cycle, and the number of fatigue times may decrease from 10,000 times as designed to one or two thousand times. Moreover, when the stiffness becomes larger, the compensator is often accompanied by local stress concentration. Once the crack appears, the medium leakage cannot escape. You said you would regret it after it was leaked. Which one is cost-effective: pipeline parking, safety risks and environmental fines?
Treatment Plan: Changing, Adjusting or Recalculating? Discuss by situation
That has become more stiff, what should I do? In three steps. The first step,Check Installation Status。 Many problems are that the tie rod nut is not loosened during installation (such asFunction of expansion joint tie rodIt is pre-stretched or fixed for transport, which must be loosened before running), or the pre-displacement is not done correctly. Loosen the tie rod nut to the design clearance, and the stiffness immediately returns to normal-I looked at it for a cement factory before, and they usedMetal corrugated expansion joint in cement industry, the stiffness of the tie rod is doubled due to locking, and the problem is solved after loosening.
In the second step,Review operating condition parameters。 If the medium temperature, pressure, and displacement do exceed the design value, it can only reduce the actual deviation or replace the product with a higher specification. Like fromUniversal corrugated expansion joint is replaced withHigh temperature axial type expansion joint, the latter material is higher temperature resistant, the structural design also allows higher working pressure, and the stiffness design margin is greater. If space is constrained, considerRotational compensator, whose principle is to absorb displacement by rotation, has almost zero stiffness, and is completely untroubled by the increase of axial stiffness.
The third step,Recalculate。 Especially for the old system that has been installed, you can entrust the manufacturer to re-check the stress of the whole pipe system according to the actual stiffness value. If the calculation results show that the pipe stress is within the allowable range, it is not necessary to change the stiffness. But remember not to pat your head, you have to haveStiffness and Calculation Formula of BellowsDo support, combineInformation such as the weight table of the metal expansion joint to confirm the feasibility of replacement.
How to avoid excessive stiffness from the root? Just be careful when choosing a model
The source is in selection and procurement. Many designers want to save trouble and directly apply the standard model. You choose oneUniversal corrugated expansion joint, the default wall thickness is 1.5mm, but your pipeline pressure is only 0.25MPa, in fact, 0.8mm wall thickness is enough, and the stiffness can be less than half. Therefore, when selecting a model, you mustCompensator stiffness becomes larger? This awareness is pre-set-ask the manufacturer forThe stiffness curve corresponding to the model and size of the expansion joint, instead of just looking at the interface size.
Another, consider the type of compensator more. For large displacement occasions,Non-metallic expansion joints (fabric fiber expansion joints) orThe axial stiffness of rubber PTFE compensators is usually much smaller than that of metal bellows, and it can absorb multi-directional displacements. Of course, the pressure and temperature resistance are limited, so don't use it in the wrong place.
Another point, construction and installation specifications and post-maintenance. The fixing bracket must be firm, the spacing of the guide bracket should be according to the drawing, and the pipe must not have additional cold tightness. Periodic inspections during operation, esp.Compensators near equipment such as circular baffle doors (double seals) should be detected as soon as possible once abnormal stiffness is found. I often say that it costs a small amount to buy a compensator, but it costs a lot of money to stop and repair the pipeline once.