Choose metal expansion joints, eight out of ten people come up and ask for the price, and the remaining two ask for the diameter. However, what really makes the expansion joint scrapped in advance on the pipeline is often that the material specifications are not right. Two days ago, I met a customer. The steam pipe used 304 bellows, which cracked in three months. Later, after checking, the chloride ion exceeded the standard, and 304 couldn't bear it at all. You say it was wrong or not?
It is also 304, why do some expansion joints use 316L? Matching Logic of Material and Working Condition
304 is a general-purpose stainless steel, and its corrosion resistance is enough in ordinary environments. However, the wall thickness of the expansion joint bellows is usually only 0.5mm to 2mm, which is much thinner than that of the pipe. At the same corrosion rate, the bellows is perforated first. With molybdenum added to 316L, the resistance to chloride ion pitting and crevice corrosion is significantly stronger. Chemical, desulfurization, coastal high humidity environment, don't hesitate to go directly to 316L. If the medium contains high concentrations of chloride ions, such as seawater heat exchange pipelines, even consider 254SMO or nickel-based alloys. The more expensive the material is, the better it is, the better it matches.
Bellows, connectors, flanges and guide tubes: How to match the material specifications of each component
Many people think that the material of the expansion joint is the material of corrugated pipe, ignoring the pipe and flange. If you think about it, no matter how resistant the corrugated pipe is to corrosion, if the joint and flange are made of ordinary carbon steel, the welds and flange surfaces will still rust. It is not that all components have to be made of the same material, but they have to be matched in layers according to the medium and working conditions. For example, the medium temperature is not high but the corrosion is strong. 316L is used for bellows, 304L can be used for takeover, and carbon steel galvanized for flanges can also be used. But the deflector must be said alone-it is in direct contact with the high-speed flowing medium, and the wear and erosion are more severe than the bellows body. For expansion joints that have been worn in wear-resistant conditions, the material of the guide tube should be either thickened, cemented carbide surfaced, or simply duplex steel. Don't save money on the deflector. If it breaks, the bellows will suffer.
Diameter, pressure, temperature, compensation amount: the four most overlooked pits in specification parameters
The diameter is easy to understand, and the same inner diameter of the pipe is done? Not right. The diameter of the expansion joint refers to the inner diameter of the bellows, but the wave root diameter of the bellows is smaller than the diameter, and the medium flow rate will change. When you select the model, quote DN500. The actual wave height of the bellows is not enough, the flow area is small, the flow rate is fast, and the erosion is aggravated.
Pressure parameters are also prone to rollover. Many people only report work pressure, not test pressure. The compressive strength of bellows is calculated according to the design pressure, but the hydraulic test pressure is usually 1.5 times the working pressure. If you only select the type according to the working pressure, the bellows may be plastically deformed during the test. Not to mention temperature, which directly affects the material permissible stress. 304 at 300℃ and 304 at normal temperature have a large difference in allowable stress. With the same amount of compensation, the fatigue life of bellows may drop to one tenth of that of normal temperature at high temperature. The compensation amount depends on whether it is axial, transverse or angular. When the displacement is combined in multiple directions, it is not a simple addition, but a composite displacement. If one of these four parameters is reported wrong, the expansion joint will be installed in vain.
From general-purpose type to high-temperature axial type: Differences in material specifications of expansion joints with different structures
Different structures, the emphasis of material specifications is different. Universal corrugated expansion joint is a standard single-layer or multi-layer corrugated pipe, usually made of 304 or 316L, which is suitable for ordinary thermal pipelines. However, the working temperature of high-temperature axial expansion joint may be above 600℃. At this time, the bellows must be made of high-temperature alloys such as Inconel 625 or GH4169, and the connectors and flanges must also be made of heat-resistant steel. The expansion joint of cement industry and power station industry has a large dust content in flue gas. Besides the temperature resistance of the material, the wear-resistant lining should also be considered. External pressure single axial type expansion joint, bellows external pressure force, high stability requirements, material strength should be enough, otherwise it is easy to become unstable.
There is also a straight pipe pressure balance expansion joint. The bellows has to bear the internal pressure thrust, and the material and wall thickness have to be added. You can't take a general-purpose 304 bellows to replace the high-temperature axial Inconel bellows. The structure is wrong, and the material and strength are orders of magnitude worse. Before choosing the type, determine the structural form, and then talk about the material specifications. Don't reverse the order.
Selection habit of old engineers: calculate life first, then talk about material specifications
Material specification is the means, life is the end. Bellows are fatigue components, and their design life is generally calculated according to the number of cycles, such as 1000 or 5000 times. The material determines the allowable stress, the allowable stress determines the wave height and wave pitch, and the wave height and wave pitch affect the fatigue life. You choose 316L not because it is more expensive than 304, but because under the same working conditions, the fatigue life of 316L may be twice that of 304. After all, the labor cost and shutdown loss of replacing the expansion joint are much greater than saving that little material cost.
So how to choose the material specification of metal expansion joint? First list all working conditions: medium, temperature, pressure, displacement, number of cycles, environmental corrosion factors. Determine the structural form, and finally calculate the life. If you can't calculate it, just change the material, add the wall thickness, or change the multi-layer structure. If you are really unsure, just ask the manufacturer for a calculation book, and don't pat your head yourself.