Core Challenges to Expansion Joints in High Temperature Flue Gas Conditions: Temperature, Corrosion and Pressure
Friends who design flue gas pipes all know that high-temperature flue gas is not easy to serve. The temperature is always five or six hundred degrees, and it is common to meet the tail of cement kiln and power station boiler. Think about it, when the metal expansion joint is directly exposed in this environment, the wall thickness of the bellows is only a few millimeters, and the creep strength of the material decreases rapidly due to heat radiation and convection heat transfer. What's more troublesome is the flue gas components-sulfide, chloride, water vapor, and the corrosion rate doubles at high temperatures. The pressure is not high, generally from several kPa to tens of kPa, but with the addition of alternating thermal stress, the fatigue life becomes a big problem. Therefore, three parameters must be found out before selection: the maximum temperature of the medium, the concentration of corrosive components, and the displacement of the pipeline (including installation error).
Common Failure Modes and Causes of Metal Expansion Joints in Flue Gas System
The most typical failure ever encountered is a cracking of the bellows root. And guess what? Nine times out of ten, the design of the guide tube is unreasonable. The guide tube is too short or not installed, and the high-temperature flue gas directly washes the bellows trough. Local overheating and flow-induced vibration will lead to cracks. There is also a corrosion perforation, especially in the desulfurization flue gas system, wherein although the flue gas temperature drops to about 150 DEG C, the acid dew point corrosion is extremely strong,High temperature flue gas metal expansion jointIf you don't make lining or choose corrosion-resistant alloys (such as 316L and 254SMO), it will leak in half a year. There is also instability, which mostly occurs when the axial expansion joint is used to absorb lateral displacement, and the bellows bulges laterally. Designers should remember: axial type can only absorb axial displacement, lateral displacement must be usedCompound hinge transverse expansion jointOrCurved tube pressure balance expansion joint。
Selection Logic of High Temperature Axial Expansion Joint and Complex Hinge Expansion Joint in Flue Gas Pipeline
Axial elongation caused by temperature change of straight pipe section, selectHigh temperature axial expansion jointMost direct. However, there is a premise: the pipeline must have more than two fixed brackets, and the guide brackets on both sides of the expansion joint are properly spaced apart, otherwise the bellows will be unstable. A few days ago, in a power station project, there was an L-shaped turn in the pipeline direction, and the displacement directions of the horizontal section and the vertical section were different. As a result, the hard axial type bulged after three months of installation. Later replaced withThe compound hinge transverse expansion joint, the hinge structure restrains the axial force, only let the bellows bear the angular displacement, the problem is solved. To put it bluntly, the selection logic is one: first calculate the displacement direction and size, and then match the expansion joint type. Straight line walk axial, turn walk hinge, space limited walk pressure balance type.
Guide tube, thermal insulation layer and temperature resistant materials: Key structural design of high temperature flue gas expansion joint
The guide tube is not simply adding a sleeve. It has to withstand high temperature erosion, and the material is usually heat-resistant stainless steel (such as 310S) with a thickness of 4~6mm. There must be enough clearance between the guide tube and the bellows to prevent thermal expansion from squeezing the wave crest. More importantly, the front end of the guide tube should extend at least 50mm out of the bellows end, so that the mainstream of high-temperature flue gas does not directly hit the trough.Specific Function of Expansion Joint Guide TubeAs mentioned in the Q&A of this site, it can also reduce flow resistance and noise. What about insulation? There are two types: internal insulation (filling ceramic fiber cotton between the guide tube and the bellows) and external insulation (laying an insulation layer outside the bellows). When the temperature exceeds 450℃, it is recommended to directly choose one with heat insulation layerHigh temperature axial expansion joint, the manufacturer prefabricates it after leaving the factory, saving trouble on the spot. In terms of temperature-resistant materials, ordinary 304 can only be used at about 450℃, 321 or 316L can reach 550℃, and Inconel 625 or Haynes 230 can be used above 800℃, but the price has doubled several times, depending on the project budget.
Typical configuration case of expansion joint of high temperature flue gas pipeline in power station and cement industry
The flue of power station boiler, from economizer to air preheater and then to dust collector, has a medium temperature dropped from 400℃ to 160℃, with long pipelines and many lifting points. Common configuration isCorrugated expansion joint for power station industryIt is divided into axial type, double hinge type and pressure balance type. For example, the outlet flue of the air preheater of a 2×300MW unit has a horizontal section length of 25 meters and a thermal expansion of about 120mm. Two flues are arrangedHigh temperature axial expansion jointWith a guide bracket in the middle, it has been running for 6 years without trouble. The cement industry is more demanding-the smoke temperature at the head and tail of the kiln is high and the dust content is large. The kiln tail pipe is usually usedMetal Corrugated Expansion Joints in Cement Industry, with wear-resistant guide tube and anti-dust accumulation design. Last week, a cement factory reported that the original common expansion joint was worn out in one year, and it was replaced with the one lined with wear-resistant ceramic sheet recommended by usHigh temperature flue gas metal expansion jointAfter that, the life span is extended to more than three years. Tsk, choosing the right product saves worry and effort.
Installation, Pre-stretching and Operation and Maintenance: Make High Temperature Flue Gas Metal Expansion Joints Run More Stably
Check the direction of the arrow on the expansion joint, which represents the flow direction of the medium. If the guide tube is installed backwards, it will corrode. Pre-stretching is a door technical work-installed at ambient temperature, the expansion amount of the pipe is positive when the operating temperature is high, so the axial expansion joint needs to be pre-stretched when installed, and the stretching amount is equal to half of the designed thermal expansion amount. How to tune it? The adjustment of the tie rod nut is particular: first loosen the locking nut, and then twist the adjustment nut symmetrically, each time twisting 1/4 turns alternately to prevent deflection. In terms of operation and maintenance, the inspection focuses on whether there are cracks and corrosion points on the surface of the bellows, and whether there are wear marks at the outlet of the guide tube. In addition,Function of expansion joint tie rodIt is limit and transportation protection. Before trial operation, the tie rod nut should be adjusted in place according to the actual displacement. Don't forget to remove the limit bolt. Two days ago, I met a customer, who had been installed for three years without maintenance. The bellows accumulated serious dust, and uneven heating led to local creep failure.