Let's be honest first: what is the use of tensile strength?
Not all metal expansion joints live on this parameter. Think about it, a bellows in the pipeline has to absorb thermal displacement, withstand the medium pressure, and occasionally cope with the misoperation during installation-at this time, tensile strength is its "trump card". Two days ago, I met a customer who was doing a power station project. He didn't let go until he stared at the 600MPa marked on the sample. As a result, he chose a bellows with too large wall thickness. The stiffness was too high, but the pipe was broken. High tensile strength does not mean easy use, it has to be looked at in conjunction with fatigue life and stiffness coefficient. If your pipeline system is not under high pressure and mainly resists thermal displacement, then the tensile strength can meet the standard. Leaving too much margin is a waste of cost. CommonUniversal corrugated expansion jointThe material of corrugated pipe is mostly 304 or 316L, and the yield strength is between 200-300MPa, which is enough to cope with most steam pipeline working conditions. However, if you are usingLarge diameter thick wall expansion joint, or to deal with high-temperature and high-pressure media (such as the boiler outlet section of power station), then tensile strength has to be used as the core index to assess-at this time, the candidate 316L or even Incoloy 825 is a routine operation. So figure out what your system wants it to carry before talking about the numbers.
What factors are secretly manipulating tensile strength?
The material is on the surface, but the corrugated structure is behind it. It is also 304 stainless steel, single-layer corrugated pipe and double-layer corrugated pipe, and the tensile strength is one grade worse. The wave height, wave pitch and wall thickness of bellows are just like people's height, weight and skeleton-only when they are well matched can they be durable. Such asHigh temperature axial expansion jointIn order to maintain strength at 800 °C, a multi-layer thin-walled structure can be adopted, each layer is 0.5 mm thick, and the superimposed tensile strength is better than that of a single layer of 2 mm, because the multi-layer design can disperse stress. In addition, the forming process is also very critical: the material at the wave peak of the hydroformed bellows is stretched and thinned, and the local tensile strength will decrease by about 20%; While mechanical rolling, the uniformity of wall thickness is better, and the strength discreteness is small. You go and see thoseMetal Corrugated Expansion Joints in Cement IndustryWhy do you emphasize "thick walls"? Because there is much dust and great temperature fluctuation in the cement production line, once the bellows is cracked, the whole set of equipment has to be stopped. There's another small detail: the deflector. The specific function of the expansion joint guide tube is to protect the inner wall of the bellows from the erosion of high-speed media, but the installation method of the guide tube also affects the tensile strength-the welding is not firm or the guide tube is too rigid, which is equivalent to adding an additional constraint to the bellows, and maybe the stress concentration starts here. Therefore, when selecting the model, don't just look at the material table, ask the manufacturer what structure and shape the bellows is, which is stronger than anything else.
Expansion joints with high tensile strength, must be suitable for you? Not necessarily.
Compound hinge transverse expansion jointIts working principle is that two groups of bellows cooperate with hinges to absorb lateral displacement, and the tensile strength is mainly borne by hinges and tie rods, while the bellows itself does not need too high tensile value. If you have to put a high-strength and high-rigidity bellows on it, the hinges and tie rods become short boards. Another exampleStraight pipe pressure balanced expansion jointIt has a balanced bellows inside, which relies on the self-balancing of pressure at both ends. If the tensile strength is too high, it will make the balanced bellows lose its elastic compensation ability. Then in what scenario must we chase high intensity? When high temperature, high pressure and large displacement occur at the same time. Such asDouble hinge expansion joint for air-cooled island vacuum pipelineThe internal pressure is negative (vacuum), but the self-weight and wind load of the pipeline will produce a great axial tension. At this time, the tensile strength of the bellows must be sufficient, otherwise the bellows will be deflated when vacuuming-this really happened. AndDesulfurization flue gas baffle doorThe matching expansion joint contains sulfide corrosion in the flue gas, and the material is not selected correctly. No matter how high the strength is, it is blind. Bottom line: tensile strength is a necessary condition, but not a sufficient condition. You have to put the temperature, pressure, medium and displacement types on the table and calculate them all before you know what indicators to ask the manufacturer.
How to measure tensile strength? A few pits that the manufacturer won't say.
In the industry, the standard method for measuring tensile strength is to cut splines from bellows for tensile test. But note, is the spline cut from the trough or crest? The material at the crest is stretched, and the measured value will be 10%-15% lower than the trough. Some manufacturers play this trick, specially picking troughs for sampling. The numbers on the report are beautiful, but they may not be able to bear it in practice. Also, bellows have different circumferential and axial tensile strengths-the circumferential direction (perpendicular to the corrugation direction) is usually one section higher than the axial direction because the circumferential direction is not significantly thinned when the corrugation is formed. However, in pipeline systems, axial tension is the most dangerous, because bellows mainly rely on axial expansion and contraction to compensate for displacement. Therefore, when selecting a model, you have to make it clear whether you want axial tensile strength or circumferential tensile strength. Don't be fooled by a report. Be mindful: let the manufacturer provide the stiffness and calculation formula of the bellows (refer to the instructions in the FAQ of this site), and then combine the tensile strength data to calculate the maximum allowable internal pressure and maximum allowable axial force of the bellows-these two values are the parameters that the design institute really wants.
Actual combat selection: three "look" formulas.
Temperature, pressure, medium, amount of displacement, number of cycles. Second, look at the bellows structure: wall thickness, number of layers, wave height and pitch, and forming process. Third, look at the third-party test report-preferably the data of the whole pipe blasting test, not the kind with the spline broken. Take a practical case: Steam pipeline of a chemical plant, DN600, working pressure 1.6MPa, temperature 350℃, selectedUniversal corrugated expansion joint。 According to the conventional design, 304 stainless steel, wall thickness 1.5mm, theoretical tensile strength is sufficient. But after two years of actual operation, the bellows cracked at the crest. Check the reason: the steam flow rate is too high, the guide tube is not installed, and the medium directly impacts the bellows, resulting in local corrosion thinning and tensile strength decreasing. Later switched to one with a guide tubeCorrugated expansion joint for power station industry, the wall thickness has been increased to 2.0mm, and the material has been upgraded to 316L, which is no problem so far. In the final analysis, the matter of tensile strength is a matching problem. If you insist on taking a high-strength product to set low-requirement working conditions, the price is expensive, and the stress concentration at other points of the pipeline may be caused by excessive stiffness. In turn, if the strength is not enough, it will wait for the air leak to burst the pipe. So, don't pat your head yourself, find a professional manufacturer to help you calculate it, which is more reliable than anything else. If you are not sure, you can flip through the FAQ on "Expansion Joint Model and Size" on our site.