What we often say is "General Technical Conditions for Metal Expansion Joints", which actually refers to the national standard GB/T 12777 "General Technical Conditions for Metal Bellows Expansion Joints". This standard covers almost 90% of metal expansion joints on the market, fromUniversal corrugated expansion jointToHigh temperature axial expansion jointAnd then toCorrugated expansion joint for power station industry、Large diameter thick wall expansion jointYou can't get around it. But to be honest, many purchasing and technical personnel are dizzy when they get the standards-dozens of pages of technical requirements, which ones must be stared at when selecting models? Don't worry, let's break them up.
Core parameters: Start at these five numbers first when selecting
I suggest you take a small notebook to write down the core parameters in the standard:Design pressure, design temperature, compensation amount (axial, transverse, angular), fatigue life, and bellows stiffness。 The first two determine what material you should choose – is it a 304 or 316L, or a heat-resistant Incoloy 825? The amount of compensation is directly linked to the fatigue life: How much displacement is the thermal expansion and contraction of your pipe, and how many cycles are required? Don't just look at the "maximum compensation amount" on the sample, which is measured at room temperature and no pressure. Under actual working conditions, the pressure will reduce the compensation ability of the bellows, and too high temperature will make the fatigue life fall by a cliff.
For example, we have encountered customers takingHigh temperature axial expansion jointWhen used in steam pipelines, only the axial compensation amount is large enough, and the fatigue life attenuation at high temperature is not considered. As a result, it leaks in two years. Tsk, this is not a trick. Therefore, when selecting the model, we must compare the fatigue life curve in GB/T 12777 and convert it according to the actual working conditions.
Stiffness: Don't ignore structural differences
Let's talk about stiffness. The rigidity of the bellows determines the amount of force on the constraint point of the pipe (such as the fixed bracket). The stiffness calculation formula and test method are given in the standard, but many people overlook one detail:Straight pipe pressure balanced expansion jointAndThe stiffness characteristics of the transverse expansion joint of compound hinge are completely different-the former absorbs the internal pressure thrust by its own structure, and the thrust to the bracket is small; The latter withstands lateral displacement by the hinge and is much more stiff. When selecting, if you only look at the end thrust and don't distinguish between types, the bracket design will definitely deviate.
Deflector tube. The standard requires that the thickness of the guide tube should not be less than the thickness of the bellows, but when the actual flow rate is high or there is particulate medium, you must strengthen or replace itPTFE-lined hoseThat kind of anti-corrosion and wear-resistant structure (refer to ourPTFE-lined hoseAndPTFE compensator）。 And guess what? Some on-site drawings save trouble, and the guide tube is installed backwards or the thickness is not enough. As a result, the medium directly washes the bellows, and it is perforated in half a year.
Inspection rules: Factory testing is not a formality
Let's get back to the testing rules in the standards. Before leaving the factory, we must do the withstand pressure test and air tightness test. Everyone knows this. But what many people don't know is that the standard also has clear requirements for the surface quality of corrugated pipe after forming-no cracks, folds and rust are allowed. And guess what? Once we went to a power plant site, the other party brought it outMetal rectangular expansion jointThere is an obvious biting edge at the weld, and the other party said "it does not affect the use". Dug out the standard terms and let them return the goods directly.
There is also how to adjust the tie rod nut of the expansion joint. In fact, it is not described in detail in the standard, but our FAQ makes it very clear: the transport screw must be removed after installation, otherwise the compensator becomes a rigid body and cannot compensate. This pit, I'm inManual plug-in insulation doorAndElectric plug-in insulation doorI have seen it many times at the installation site. If the transport screw is not disassembled, the expansion joint will be wasted-what a rough job.
Practical selection: don't rush to turn over the sample
Finally, let's talk about something practical. If you have a selection need at hand, don't rush through the samples yet. The first step is to determine the medium, temperature, pressure and displacement of the pipeline; In the second step, according to GB/T 12777, the number of allowable cycles and safety factor under the corresponding design conditions are found; The third step is to select the structure according to the displacement type: large axial displacement? useExternal pressure single axial expansion jointOrDirect buried (fully buried) type expansion joint。 Large lateral displacement? useCompound hinge transverse expansion jointOrCurved tube pressure balance expansion joint。 Combined displacement ConsiderationCompound straight pipe bypass pressure balanced expansion joint。
The product list on our website basically covers these types, from general-purpose to power station-specific and cement industry-specific, with corresponding standard compliance instructions. Remember one sentence: Standards are dead, working conditions are alive. Don't use the general-purpose model for high-temperature, large-caliber work, and don't take itrubber compensatorTo replace metal expansion joints-the two standards are different, and the applicable temperature and pressure are completely different. Two days ago, another customer asked, "Is the expansion joint the same as the compensator?" I replied in the question and answer: One thing, but the industry has different habits. ButMetal expansion jointAndCorrugated expansion jointDifferences need to be noted-corrugated expansion joints specifically refer to bellows as the type of elastic element, while metal expansion joints also includeSleeve type pipe expansion joint、Rotary compensatorAnd other non-bellows structures (such as oursSleeve type pipe expansion jointAndRotary compensator）。 These products are also subject to the general technical conditions of metal expansion joints, but the assessment focus is different.
One small detail: Don't put the direction of the arrow in reverse
Oh yeah, there's another detail: the standard clearly states the direction of the arrow for the metal expansion joint-which represents the direction of the medium flow, usually in line with the direction of the guide tube. When installing, the arrow points to the flow direction. Once installed backwards, the guide tube becomes a throttle piece, which increases pressure loss at least and causes vibration at worst. Is that the truth? Think about it, when the flow rate of the medium is high, if the guide tube is installed backwards to directly resist the flow direction, vortex will be generated in the bellows. Can the life be good? So be sure to check the direction of the arrow before installation, don't make low-level mistakes.