Find out how bad it is in the flue first
The flue expansion joint looks humble, but if you choose the wrong one, you can remove the pipe again within a year. Not alarmist. Two days ago, there was a power plant customer. The desulfurization flue was equipped with 304 bellows, which cracked in half a year. Why? The chloride ions in the flue gas exceed the standard, and the stress corrosion crack directly passes from the trough to the peak. Therefore, before talking about which flue expansion joint is durable, first understand the working conditions thoroughly-high temperature, corrosive smoke, pressure fluctuation and installation space limitation. Which is the culprit of early scrapping?
Tempers vary greatly from flue to flue. Wet flue gas after desulfurization in power plant, the temperature is not high, but the acid condensate is extremely corrosive; The high-temperature dry flue gas of cement kiln head can reach above 600℃, and a large amount of dust is washed. The test of the expansion joint in these two working conditions is completely different. If you take the plan to deal with dry smoke to deal with wet smoke, it will be weird if there is no problem.
Metal corrugated expansion joint: it can withstand high temperature and pressure, but it is afraid of two things
This station has corrugated expansion joints for power station industry, high temperature axial expansion joints, metal corrugated expansion joints for cement industry and other products. Stainless steel bellows can really hit under high temperature and pressure, but there are two Achilles' heel: chloride ion stress corrosion and low cycle fatigue. I just said that the power plant case didn't hold it when it was replaced with 316L, and finally it stabilized when it went to 254SMO. If the medium is also high in sulfide, consider duplex stainless steel or nickel-based alloys.
Let's talk about structure selection. Large diameter flue generally uses large diameter thick wall expansion joint or straight pipe pressure balance expansion joint. The former has sufficient wall thickness and strong resistance to internal pressure; The latter has its own pressure balance function, which can absorb axial displacement without producing blind plate force. However, the axial displacement and lateral displacement must be calculated clearly when selecting the model, and the configuration of the tie rod and the guide tube cannot be saved-the role of the guide tube is to guide the airflow and avoid high-speed particles from directly washing the bellows (for details, please refer to the explanation of the role of the guide tube of the expansion joint in our Q&A).
By the way, the expansion joint near the flue baffle door has to consider the pressure shock when opening and closing. At this time, the compound hinge transverse expansion joint or the external pressure single axial expansion joint are more reliable and can bear the impact load and angular displacement.
Non-metallic expansion joint (fabric fiber): high upper temperature resistance limit, but with a pit
The non-metallic expansion joints (fabric fiber expansion joints) and rectangular non-metallic expansion joints of this station have unique advantages in temperature resistance and displacement compensation. The silica glass fiber composite structure can carry above 600℃, and its ability to absorb multi-dimensional displacement is much stronger than that of metal-it can be swung left and right up and down casually, and the installation space is small, which is particularly friendly to projects with compact flue layout.
But where is the pit? Afraid of sharp objects scratching, afraid of local overheating after dust accumulation. Non-metallic expansion joints are most used in cement plants because of large dust, frequent temperature fluctuation, and good flexibility of fabric itself, which is not easy to produce thermal stress. However, if there are sparks or unburned particles in the flue, the life of the nonmetallic fabric will be compromised. In addition, rubber compensators are rarely used on flues and age when the temperature is high, unless it is a low-temperature flue (such as the clean flue gas section after desulfurization).
Do you choose metal or non-metal? Look at these three indicators
Ask which flue expansion joint is durable, and the standard answer will always depend on the working condition. My suggestion is to look at three hard metrics:
- Flue gas temperatureIf it exceeds 400℃ for a long time, the metal corrugated expansion joint will be preferred, but it must be lined with ceramic fiber or guide tube to prevent erosion; If the temperature is lower than 400 °C and the corrosion is not heavy, non-metallic may be considered.
- Medium corrosivity: Contains chloride ion or sulfide, must use corrosion resistant alloy (316L starts, 254SMO or Hastelloy is more stable). In this kind of environment, the sealing layer of non-metal is easily eroded, so fluororubber or PTFE coating should be selected.
- Pipeline displacement mode: Nonmetal is more flexible if it needs to swing at a large angle or absorb multidimensional displacement; With simple large axial displacement and straight pipeline, metal bellows are more efficient.
In addition, the expansion joint used near the flue baffle door has to take into account the pressure shock when opening and closing. At this time, the double hinge transverse expansion joint or the external pressure single axial expansion joint are more reliable and can absorb impact and angular displacement.
Finally: Installation is easier to roll over than material selection
No matter how durable the expansion joint is, it will be wasted if it is not installed correctly. How to adjust the expansion joint tie rod nut, the medium flow direction pointed by the arrow direction, and whether the guide tube is aligned-these details are more prone to problems than the material selection. Before, a customer chose the right 316L corrugated expansion joint for power station industry, but the guide tube was installed backwards during installation, and the airflow directly washed the corrugated pipe, which was perforated in three months. Therefore, it is best to calculate the model and size of the expansion joint clearly in the design stage of the flue system. Don't wait until you install it and find that the angular displacement exceeds the limit before replacing it. The cost is much higher than buying a good product.
There is no universal expansion joint, only the selection of matching working conditions. Find out the three elements of temperature, corrosion and displacement, and then compare the product list of this site-high-temperature axial expansion joint and large-diameter thick-walled expansion joint for metal; For non-metallic, look at non-metallic expansion joints (fabric fiber expansion joints) and rectangular non-metallic expansion joints-essentially no craters. If the working conditions are extreme, such as high temperature, corrosion and vibration at the same time, then find a manufacturer to make non-standard customization, and don't buy general parts for cheap.