Find out first: You are choosing expansion joints for square flues, not round pipes
The rectangular metal expansion joint looks like a relative to the round expansion joint, but the idea of selecting materials is completely different. The expansion joint of circular tube bears axial force and circumferential stress. What about rectangular section? There is stress concentration at the four corners, unbalanced force on the plane, and dust accumulation, corrosion and turbulence in the flue gas pipeline, so the working condition is much worse than expected.
The inlet of desulfurization tower of power station, the outlet of induced draft fan and the flue gas pipeline at the tail of cement kiln are all rectangular in cross section. The medium temperature may vary from 100℃ to 800℃, and when the dust content is high, it can reach 20g/m³, and the sulfur dioxide concentration can vary from thousands of mg/m³. Do you take the hard sleeve of the selection table for the round expansion joint? It won't take long to leak. The first step of selection is to list the medium temperature, dust content and corrosive gas composition clearly, and then talk about the material.
Bellows material: Don't ask "304 is OK" as soon as you come up
Bellows are the core of rectangular metal expansion joints and are the most prone to problems. If the material is selected wrong, the bellows will crack at least, and the whole flue system will shut down at worst. First, give a basic boundary: 304 and 316L can cope with the working conditions below 300℃, without chloride ions and low sulfur; From 300℃ to 500℃, 321 or 316Ti must be considered, which has better intergranular corrosion resistance; Above 500℃, or if the flue gas contains sulfur and chlorine, don't hesitate to go directly to nickel-based alloys such as Inconel 625.
Sulfur-containing flue gas is the typical killer. Although the original flue gas temperature of desulfurization tower is not high, sulfuric acid dew point corrosion is severe. The corrosion rate of 316L in dilute sulfuric acid above 60℃ is very fast. At this time, sulfuric acid corrosion-resistant materials or thickened bellows must be used as the sacrificial layer. We have compared a power station project: in the same position, the 316L bellows pitted after half a year, and after being replaced with Inconel 625, it was still running for more than three years.
Note that the corrosion resistance limit of austenitic stainless steel is bounded. The higher the temperature, the greater the concentration of chloride ions, and the faster the stress corrosion cracking. There is often condensed water in the flue gas pipeline, and chloride ions are enriched in the bellows trough. In this environment, 304 is basically unsolved. Nickel-based alloys are more expensive, but the labor and downtime cost of replacing the expansion joint is an order of magnitude higher than that material price difference.
Frame and deflector: save this part of money, and the back is full of pits
Many people pay full attention to the bellows, thinking that the frame is just a few steel plates welded into a shelf, which can be used casually. Wrong. The frame of the rectangular expansion joint has to bear the internal pressure thrust. Even if the flue gas pressure is only 5kPa, the thrust generated on the large section is considerable. Carbon steels Q235B and Q345R are the most used, but carbon steels have creep problems under high temperature conditions. When the temperature exceeds 400℃, the allowable stress of Q235B drops very rapidly. At this time, the frame is either thickened, heat-resistant steel is selected, or the metal temperature is lowered by lining with heat insulation layer.
Don't ignore the deflector. The role of the guide tube is to guide the flow and protect the bellows, and it is not directly subjected to pressure, but the erosion and corrosion are all on it. In many projects, the material of the guide tube is out of sync with the bellows. The bellows has been upgraded, and the guide tube is still ordinary stainless steel. As a result, the guide tube is perforated first, and the high-temperature flue gas directly washes the bellows, which leaks before the maintenance cycle.
Should the deflector be made of the same material as the bellows? Not necessarily. If the dust content of the flue gas is high, the deflector should consider wear resistance, such as using 16Mn or thicker plate; If corrosion is predominant, the deflector must not be at least one grade lower than the bellows. The deflector breaks fast, and the bellows will suffer. Don't save this money.
How do you match non-metal and metal? Distinguish what you want before purchasing
Non-metallic expansion joints are also common in rectangular flues. For example, rectangular non-metallic expansion joints, with skin and insulation cotton to absorb displacement, low cost, high temperature resistance, suitable for low pressure and large displacement occasions. However, if the medium pressure is high and the temperature fluctuates greatly, the metal rectangular expansion joint is a more reliable choice.
Metal frame plus non-metal skin. This structure is completely different from the material system of pure metal rectangular expansion joint. The skin is flexible material, and the frame is only shaped and does not bear elastic compensation. Before purchasing, you should know in your mind, which one are you asking? Take the requirement of non-metal skin to set metal bellows, or vice versa, will go wrong.
Let's just say that if you need pressure resistance, erosion resistance and long life, choose a metal rectangular expansion joint; If cheap, large compensation, and high temperature resistance are pursued, non-metallic may be more suitable. But don't think that "half metal and half non-metal" can take advantage of both ends. The junction is often a leakage point.
Real selection case: What is changed is not the bellows, but the whole system thinking
In the first case, the rectangular expansion joint at the outlet of the induced draft fan of a power station, the original design of 316L bellows, leaked in half a year. After analysis, the flue gas temperature fluctuates high and low, and the sulfur content exceeds the standard. 316L can't bear the double blow of alternating thermal stress and sulfur corrosion. After replacing the whole with Inconel 625, the life span is directly pulled to three years. The material money is expensive, but the expansion joint is changed twice a year less, and the maintenance workload is much smaller.
In the second case, the rectangular expansion joint at the tail of the cement kiln and the corrugated pipe are made of heat-resistant alloy, but the guide tube is still ordinary 304. Results After more than one year of operation, the guide tube was perforated, the high-temperature dusty flue gas was directly sprayed to the inner wall of the bellows, the bellows was partially overburned, and the whole expansion joint was scrapped. The reason is that the material of the guide tube has not been upgraded synchronously-the selection can't just look at the bellows, but the frame, the guide tube and the heat insulation layer are a system.
So don't just look at the plan saying "304 stainless steel" and think it's done. Ask clearly: What material is the frame made of? How thick is the deflector? How is the insulation made? How to deal with welds? These details add up to the whole picture of the material of the rectangular metal expansion joint.