Let's start with the structure: What is the role of the medium film?
The middle piece of the metal expansion joint, to put it bluntly, is the annular partition plate in the middle of the bellows. If you take apart a multi-wave bellows, you will see a flat ring plate between the crest and trough, which is the middle piece. It strings each wave together, like a bamboo skewer for candy haws — no, more like a slab between floors, separating the upper and lower waves and connecting them again.
This thing would be interesting when paired with the deflector. The guide tube guides the medium flow inside to reduce scour and vortex, and the middle piece fixes the corrugations from the outside to prevent the waves from pushing each other and instability. Some expansion joints you don't see the middle piece, such as single-wave expansion joints and small-diameter general-purpose corrugated expansion joints, which have less wave number and enough overall stiffness, so they don't need additional strengthening. However, when there are many waves, such as high-temperature axial expansion joints or corrugated expansion joints used in power station industry, the middle piece is standard. Otherwise, when the bellows is subjected to internal pressure, each wave will drum out and squeeze each other, and it will be fatigued and cracked in a few strokes.
How to choose the material: This layer of partition can't be put on by just taking a piece of steel plate
Stainless steel, carbon steel, alloy steel. In the general corrugated expansion joint, the bellows is 304 or 316L stainless steel, and the middle piece also follows the material of the bellows, keeping the same potential and avoiding electrochemical corrosion. If you try to save trouble with a Q235 carbon steel medium piece for the 316L bellows, the weld will rust through in less than a few months. This is not alarmist. When different metals come into contact in the electrolyte environment, the potential difference directly accelerates the corrosion.
Temperature-resistant piece, within 300℃ under normal working conditions, 304 stainless steel medium sheet is fine; If it exceeds 400°C, it is necessary to switch to 321 or 347 steel grades containing stabilizing elements, or directly use nickel-based alloys such as INCONEL 625. In terms of pressure resistance, the thickness of the middle piece directly determines how much internal pressure it can hold-the expansion joint of the same DN500, the middle piece is 6mm thick and 10mm thick, and the pressure resistance grade is completely two concepts. The dust and abrasion in the metal corrugated expansion joint of the cement industry are large, and the surface of the middle piece should be wear-resistant. Otherwise, the flue gas rushes with particulate matter, and the edge of the middle piece will be ground out first, and the bellows will suffer.
What can go wrong if you choose the wrong material? Let's talk about a real case: the chlorine pipeline of a chemical plant, Hastelloy used for bellows, used 304 stainless steel cheaply in the middle picture. As a result, the chloride ion stress corrosion cracked. In less than three months, the middle picture was broken in half, the chlorine in the pipeline leaked, and the whole workshop was evacuated urgently. The little money saved from materials is enough to compensate for ten sets of equipment.
Influence of Medium Sheet on Expansion Joint Performance: How Much Can One Diaphragm Change?
The middle piece is equivalent to adding circumferential reinforcing ribs to the bellows. For every 2mm increase in thickness, the axial stiffness may increase by 15% to 20%. When the stiffness is large, the ability of the expansion joint to absorb displacement decreases, and the compensation amount decreases accordingly. You want to thicken the middle piece to improve the strength, but as a result, the thermal expansion compensation of the pipe is not enough, and the pipe frame is crooked. This account is not cost-effective.
Let's talk about fatigue life. The connection mode between the middle piece and the bellows is usually fillet weld or full-penetration structure-fillet weld is low in cost but has a large stress concentration, and the fatigue life may only be 60% of that of full-penetration structure. I have seen a flue gas pipe project. The fillet weld in the middle piece was cracked. When I removed it and looked at the cross section, it was all fatigue patterns, and there were annual rings like trees. Each pattern represented a heat cycle. The operator said that the pipeline started and stopped twice a day, 730 cycles a year, and the design life requirement was 5,000 times. As a result, it cracked 800 times. The problem was that the weld structure was not designed according to the fatigue conditions.
There's also the problem of opening holes. There are often liquid-phobic holes or exhaust holes on the middle piece, and the aperture diameter and opening position are particular. When the hole is opened wider, the local stress concentration increases sharply; The hole opened directly below the trough, which happened to be the area with the greatest stress, which was equivalent to laying a mine for yourself.
Failure case of middle piece in actual working conditions: How can the middle piece of flue gas pipeline in power plant crack?
Last year, I met a power plant customer. The flue gas pipeline exported from the desulfurization tower used a corrugated expansion joint used in the power station industry. After less than half a year of operation, the middle piece cracked. The crack starts at the edge of the inner ring and extends radially outward, typical fatigue crack. Take it for testing, the metallographic structure is fine, the material is right, and the chemical composition meets the standard. So what's the problem?
Later, I turned through the installation records and found that the middle piece and the bellows were concentric when they left the factory. During installation, the pipes were forcibly matched, and they were hard pulled by two millimeters, and the middle piece suffered an additional bending moment. In addition, the flue gas temperature fluctuates greatly, and it starts in the morning and stops at night. The thermal stress of the middle film superimposes the mechanical stress, and the fatigue life is directly reduced by half. This pot has to be installed with a square back, but the designer is also responsible-the thickness of the middle piece is only calculated according to the design pressure, and the additional load caused by the installation deviation is not calculated. How to avoid it? Two methods: First, use temporary brackets to adjust the pipeline during installation, and it is strictly forbidden to pull and pull; Second, in the design stage, the middle piece + bellows assembly is required to be pre-displaced to offset the installation error. Many domestic design institutes don't do this step, all depending on the feeling of the on-site workers. It is luck if there is no accident, and it is an accident if there is an accident.
Selection suggestion: Ask these parameters clearly, the manufacturer can't fool you
General-purpose corrugated expansion joint and high-temperature axial expansion joint, the middle piece configuration is completely different from the same level. Most of the general-purpose types are not equipped with medium films, with less wavenumber and low medium temperature; High-temperature axial type products have a working temperature of five to six hundred degrees and high pressure. The middle piece should not only be matched, but also be matched with a double layer-a layer of heat insulation gasket sandwiched between the two layers of middle pieces to reduce the conduction of heat to the root of the bellows. Straight pipe pressure balance type expansion joint and curved pipe pressure balance type expansion joint, the position and structure of the middle piece are different, they have to bear the blind plate force generated by internal pressure, the middle piece has to be designed as a reinforced type, and the thickness and connection mode are specially considered.
Don't just stare at the price when purchasing. These parameters must be asked clearly:
- Is the material of the medium sheet consistent with the material of the bellows? If different, are there quarantine measures?
- What is the thickness of the medium sheet? Is there a strength calculation book under design pressure?
- Is the connection mode between the middle piece and the bellows fillet weld or full penetration? Full penetration is recommended under fatigue conditions.
- Is there a liquid phobic hole/vent hole on the middle piece? How do aperture sizes and locations avoid high stress zones?
- Is there a cooling or insulation design for the medium plate under high temperature operating conditions?
Some manufacturers only write "containing medium film" on their quotations, but don't write the thickness and material, so the water in the middle is deep. You send screenshots of the above five questions to the sales. If the other party can't answer them, change them as soon as possible.
The thicker the medium film is not the better, and it is not as long as it does not crack. Expansion joints have fatigue life requirements. As a load-bearing component, the design goal of the middle piece should be not to crack within the design life, instead of pursuing infinite life. You design by 5000 cycles, and it can guarantee 5000 times without cracking; If you expect it to last a lifetime, you have to afford it. Anyone who engages in engineering understands whether this is rough or not.