The metal expansion joint, to put it bluntly, is a flexible joint, which absorbs the thermal expansion and contraction of the pipe by the deformation of the bellows. But if you think it can carry any working conditions, it will have to roll over sooner or later. Two days ago, a customer asked me why the expansion joint he installed leaked after only three months. When I removed it, the bellows cracked like a cracked riverbed. What's the problem? When you select a model, you just stare at the compensation amount, and everything else is ignored. Let's break it down today,Disadvantages of Metal Expansion Joints?Where the hell is hidden.
1. Fatigue life: Bellows are not perpetual motion machines, and the number of cycles is the hard threshold
Many people think that the expansion joint can be used for a lifetime, so they dream. Every time the bellows expands or contracts, a micro-damage accumulates inside the material. The fatigue life written in the design standard is usually between a thousand and several thousand cycles-this is not blindly determined, it is measured. If you think about it, steam pipes start and stop once a day, 365 times a year, and thousands of times in three years. If the circulation frequency is high and the displacement amplitude is large, the life time will be cut in half directly.
What about that? Not for you not to use it, but to figure it out. First calculate the total thermal displacement of the pipeline, and then see whether the working condition belongs to low frequency or high frequency. Like those used in the power station industryCorrugated expansion joint for power station industryThe number of cycles requirements must be specified when designing. Don't buy the general-purpose corrugated expansion joint to make do, the fatigue life is not matched, and it is too late to cry later.
2. Media compatibility: There are boundaries in corrosion resistance, so don't use it as a panacea
Stainless steel bellows corrosion resistant? That depends on the medium. Working conditions with high chloride ion content, such as desulfurization flue gas and seawater, 316L will still pitting for you. Two days ago, a case in a chemical plant, usedMetal hoseThrough hydrochloric acid, it leaked within a few months. ChangePTFE-lined hoseOrPolytetrafluoroethylene compensator, instead, lasted for more than two years.
Before the selection, you must get the media composition list, and the temperature, concentration and flow rate must not be less. High-temperature flue gas contains sulfur, ordinary stainless steel can't bear it, have toHigh temperature axial expansion jointOr non-metallicNon-metallic expansion joint (fabric fiber expansion joint)。 Don't gamble on experience, the corrosion boundary is much narrower than you think.
3. Installation and maintenance: pull rod, guide tube, pre-stretching, one step wrong, one step wrong
Installing this thing is really not just a few screws. Let me ask you, what is the tie rod on the expansion joint for? Many workers removed the tie rod directly at the scene, thinking it was a transportation fixture. As a result, as soon as the pipe heated up, the bellows was pulled to the head and torn directly. The correct way is: after installation, adjust the tie rod nut so that the bellows is in the designed position-pre-stretched or pre-compressed, according to the cold tightness requirements of the pipe system.
Let's talk about the guide tube, which not only prevents scour, but also affects the flow direction of the medium. If it is installed backwards or missed, the high-speed fluid rushes directly into the bellows, vibrates and erodes, and the hole will be perforated in less than a month. We have a client usingMetal Corrugated Expansion Joints in Cement IndustryBecause of the reverse installation of the guide tube, three were replaced in three months.
As forCorrect installation method of expansion joint of large tie rodThe tie rod absorbs the internal pressure thrust. During installation, the nuts at both ends of the tie rod should be tightened in place, and the bellows should not be pressurized-this work has to be done step by step according to the instructions, and don't take it for granted.
4. Cost account: Is one-time purchase cheap? Later replacement and downtime losses are deadly
The metal expansion joint itself is not expensive, and a general-purpose corrugated expansion joint may only cost a few thousand yuan. But what about a change? Pipeline venting, thermal insulation dismantling, replacement operation, re-pressurization, plus the loss of production shutdown, six figures per minute. Especially in chemical industry, power plants and cement lines, it is hundreds of thousands to stop a day.
So don't just focus on the money of expansion joints when choosing a model. Working conditions are bad, such as high temperature and high pressure, strong corrosion, rather buyExternal pressure single axial expansion jointOrStraight pipe pressure balanced expansion jointThis structure is more reliable. Although it is twice as expensive, it can be used for five years without accidents. Rubber compensators are cheap, right? However, as soon as the medium temperature exceeds 80℃, it will immediately become soft. You said chooserubber compensatorOr metal? Depending on the working conditions, calculate the general ledger.
5. Special working conditions do not work: vacuum, buried ground, large displacement scenes have to be requested to be smart
Metal expansion joints are not a panacea. Under the vacuum condition, the bellows has a large external pressure and is easily unstable. We have dedicatedSpecial hose for vacuumThe structure is different and can resist external pressure. You take an ordinary metal expansion joint to vacuum, and the bellows are like a flattened can, which is directly scrapped.
What about buried pipes? Buried directly in the soil, corrosion, soil pressure and uneven settlement, the metal expansion joint can't bear it at all. UsefulDirect buried (fully buried) type expansion jointOr simply use a sleeve typeSleeve type pipe expansion joint。 There are also large displacement scenarios-several hundred millimeters of lateral displacement, or axial displacement beyond the bellows limit, then you have to considerCompound hinge transverse expansion jointOrRotary compensatorDon't push on the universal model.
Alas, in the final analysis, choosing an expansion joint is not a set of formulas, but a solution of equations. Media, temperature, pressure, displacement, life, cost, every variable has to be counted. Picked right, it is a buffer for the plumbing system; Wrong choice, it's a ticking time bomb.