The material is foundation: Don't be fooled by the word "stainless steel"
Two days ago, I met a customer who was doing a chemical project. He told me with a quotation sheet, "They are all 304 stainless steel. Why is your house so expensive?" I saw that the opposite quotation was 304, and the 316L we used was not the same thing at all.
As the core component of the expansion joint, the corrugated pipe material determines the bottom line of corrosion resistance and life. Ordinary 304 is enough in freshwater and weak corrosion conditions, but in chloride-containing environments, such as coastal power plants, desulfurization systems and chemical plants, 316L is the lowest configuration, containing 2%-3% molybdenum, and its pitting resistance has obviously increased to a higher level. Further up, there are high-nickel alloys such as 254SMO and C-276, which can deal with strong acids and high-concentration chloride ions. When it comes to choosing materials, it is not the expensive one, but the right one. But the premise is-you have to know which one the supplier is giving you.
Many manufacturers' quotations say "stainless steel corrugated pipe", and the contract does not write the grade. When stress corrosion cracks, they will tear. So the first thing to look at the quotation list is to look at the material column and write clearly whether it is 316L or 254SMO, not the general "stainless steel" three words.
The design is the skeleton: wave height, wave pitch, layer number, wall thickness, and each set of parameters has special attention
Is it done if the material is right? Far from over. The corrugated pipe is designed like a building block. The wave height, the width of the wave pitch, the number of layers and the thickness of the wall directly determine the compensation amount and fatigue life.
For example, with the same material and the same caliber, the wave height increases, the single wave compensation increases, but the stress also increases, and the fatigue life decreases. Do you say it should be increased or decreased? There is no standard answer. How much displacement is needed in the working condition, how high the system pressure, how much the temperature is, and what the medium is-all these conditions are put out to calculate the reasonable wave height and wave distance.
The number of layers and wall thickness are also a contradiction. The wall thickness increases and the pressure resistance is improved, but the stiffness becomes larger and the compensation amount becomes smaller. At this time, a multi-layer thin-walled structure, such as four layers of 0.5mm superimposed, can not only withstand pressure, but also be more flexible than a single layer of 2mm. The design ideas of expansion joints of carbon steel and stainless steel are different, and the creep characteristics of materials must be considered in high temperature conditions. At the end of the day, design is calculated, not patted on the head.
High temperature operating conditionDon't force the universal corrugated expansion joint. The general design temperature is generally below 400℃. If it exceeds this number, you have to consider the high-temperature axial expansion joint. The materials, structures and heat treatment processes are different. This is not a problem that can be solved by changing a thicker steel plate.
The process is the face: what is the difference between hydroforming and roll forming?
No matter how good the design drawings are, it is useless to pull the hips in the production process. There are two main types of bellows forming: hydroforming and roll forming.
Hydroforming relies on liquid pressure to expand the tube blank out of corrugations. The corrugations have full shape, uniform wall thickness and smooth surface, which is suitable for large displacement and high pressure occasions. Roll forming is cold pressed by mechanical rollers, which has high efficiency and low cost. However, the wall thickness at the fillet of the corrugation may be obvious, and the fatigue life may be discounted accordingly.
How can you tell at a glance? Look at the fullness of the ripples and the surface texture. The hydraulic corrugation has sharp edges and angles, natural transition, and uniform wall thickness transition on both sides; Sometimes the corrugated root is thick and the top is thin in roll forming. Open the weld seam again to see-whether the longitudinal weld seam is fully welded, whether there is biting edge, and whether the surplus height is polished smoothly, which intuitively reflects the skill and quality inspection standards of welders.
Heat treatment is the most easily overlooked step. After the bellows is formed, there is residual stress inside, so it is installed directly without stress annealing, and the risk of stress corrosion cracking is doubled. Some small factories skip this process to save costs, and it can't be seen at all on the outside, but it is like a time bomb, which may crack for you after two or three months of operation.
Selection is the direction: if you choose the wrong type, no matter how good the material is, you can't bear it
The material, design and craftsmanship have all passed, and the last step is to select the type. This step is wrong, and all the previous efforts are in vain.
What direction is the pipe displacement? Axial expansion and contraction are mainly usedUniversal corrugated expansion joint； If you need to absorb lateral displacement, selectCompound hinge transverse expansion joint； Angular displacement is mainly, using hinge type; High pipeline pressure requires a large amount of compensation. Straight pipe pressure balance type or curved pipe pressure balance type can help. They can balance the blind plate force and make the fixed bracket less difficult. Large diameter smoke duct, withMetal rectangular expansion joint； Ultra-high temperature flue gas pipeline should be combined with non-metallic expansion joints for compensation.
Let's just say that choosing a model is like choosing a car. You have to tow goods to buy a pickup truck, and you have to have an entire sports car. No matter how good the engine is, it will be useless. The expansion joint is also the same-the working condition is the highest command, the type is wrong, and it is useless to pile the material to 254SMO.
Inspection is the bottom line: pressure resistance, airtightness, fatigue, none of which can be missed
Factory inspection is the most easily exploited link. The quotation says "Production according to national standard", but when it is delivered, there is not even a complete inspection report. Do you dare to use this?
Resistance voltage testTo verify the pressure bearing capacity, the test pressure is generally 1.5 times the design pressure;Airtightness testCheck whether there is any leakage in the weld and bellows. Brush soapy water again during the pressure holding period, and bubbling means leakage;Fatigue testSimulate repeated expansion and contraction under actual working conditions to see how many cycles it can carry. For pipelines with large displacement and high frequency, the fatigue test data is particularly critical.
It should be noted that some manufacturers take a material book as a universal shield, saying that all the tests have been done but no reports have been made-in this case, you have to be careful. The material certification can only show that the chemical composition of raw materials is qualified, but it can't prove the molding quality, welding quality and fatigue performance. Each batch of expansion joints should have corresponding factory inspection records, and suppliers are required to provide them with the goods. This is the bottom line.
To put it bluntly, the high quality of metal expansion joints is not the number on the quotation, but the five links of material, design, technology, selection and inspection can all stand up. Every link has to spend cost, time and thought. Next time you compare prices, ask the above questions one by one. If the other party is vague, then you should know it in your heart-you can't save money.