What Expansion Joints Do Boiler Flues Use? From working conditions to type selection, explain it thoroughly
Two days ago, a buddy from a power plant called and asked, "What kind of expansion joint should the boiler flue use? Our original rubber compensator leaked after less than half a year of use. Let's replace the metal hose, and the weld cracked again. Give me a trick." This question is not new, but it does have to be talked about at the root. The working conditions of the boiler flue, to put it bluntly, are high temperature, corrosion, displacement and vibration stacked together. How can ordinary rubber compensators hold it? Rubber is basically soft and mushy when it exceeds 150℃; Although metal hoses have high temperature resistance, fatigue cracking is common under frequent thermal cycles. What happens if you choose the wrong one? From smoke leakage to flue deformation and cracking equipment-this is no joke. I've seen cases where the entire flue at the inlet of desulfurization tower was torn askew.
So exactly what expansion joint does the boiler flue use? The answer is no "jack of all balm". The two most common families: non-metallic expansion joints (also called fabric fiber expansion joints) and metallic rectangular expansion joints. Each has its own temper, and it depends on the working conditions to choose.
Let's talk about non-metallic expansion joints first-the "frequent visitors" of boiler flues
From our stationNon-metallic expansion joint (fabric fiber expansion joint)It is very widely used in boiler flues. It has a skill: high temperature resistance (aluminum silicate fiber felt base, no problem above 400℃), corrosion resistance (sulfur flue gas? fluororubber or PTFE coating), and it can absorb multi-directional displacement-axial expansion and contraction, lateral misalignment, angular deflection, all of which can be eaten. It is also lightweight to install, and one person can carry a section. Price? It's a cut cheaper than the metal ones. However, it has a weakness: its weak pressure capacity. Under negative pressure conditions (such as the inlet of induced draft fan), it is easy to deflate, so an internal support ring has to be added. In addition, it is afraid of sharp objects and open flames. When the welding slag splashes on it during installation, it will burn a hole directly, which will be in vain.
Let's talk about the metal rectangular expansion joint-the tough guy of large-diameter square flue
If it is a large-diameter square flue (such as the horizontal flue behind the desulfurization tower), the non-metal often can't hold the pressure, so you have to go upMetal rectangular expansion joint。 This thing has high pressure resistance, strong structural strength, and can resist high temperature corrosion with stainless steel 321 or 310S. But there are two things that must be paid attention to: First, it is expensive, heavy and laborious to hoist; Second, the guide tube can't be less-the specific function of the expansion joint guide tube is to protect the bellows from high-speed dusty smoke, and the direction of the arrow must be consistent with the flow direction of the medium during installation (as specifically mentioned in FAQ, "the arrow direction of the expansion joint refers to the flow direction"). If it is installed backwards, the guide tube will not only not work, but also easily vibrate and crack.
In addition, circular flues (such as the pipe from the boiler outlet to the economizer) are commonly usedHigh temperature axial expansion joint, it specializes in absorbing axial displacement and has a compact structure. If the lateral displacement needs to be absorbed simultaneously, thenCompound hinge transverse expansion jointOrCurved tube pressure balance expansion jointMight be more appropriate. When selecting the type, the displacement must be calculated clearly: it is normal for the boiler to start and stop once, and the flue to expand and contract by several centimeters in the axial direction. If the compensation ability is not enough, the pipe will be deadpulled, and the weld will have problems sooner or later.
Three hard indicators: temperature, pressure and displacement
Selection without looking at the parameters is a rogue. Temperature over 400℃? The temperature-resistant layer of non-metal must be aluminum silicate fiber felt, and the metal must be stainless steel 321 or 310S. Ordinary 304 can't withstand sulfur corrosion at all. What about the stress? Most of the boiler flue is slightly positive or negative pressure, and non-metal is enough. However, the positive pressure at the outlet of the induced draft fan is high, so it must be metal. The displacement must be calculated honestly-how much is the axial, transverse and angular direction? The safety factor should be amplified according to the actual displacement in the manufacturer's design. There are also media: sulfur-containing flue gas is highly corrosive, corrosion-resistant alloy should be selected for metals (such as ND steel resistant to sulfuric acid dew point corrosion), and fluororubber or PTFE coating should be used for non-metals. Don't be cheap. Later air leak repair is much more expensive than buying a good expansion joint.
Don't forget the smoke baffle doors – they're a pair
The expansion joint alone is not enough, the flue gas baffle door must be equipped in the boiler flue system. The expansion joint absorbs the displacement, and the baffle door is used to cut off or adjust the smoke. The cooperation of the two can ensure the sealing and safety. For example, the inlet of the desulfurization tower: the high-temperature flue gas first passes through the non-metallic expansion joint, and then entersDesulfurization flue gas baffle door。 The expansion joint absorbs the thermal displacement of the pipe so that the baffle door can be closed tightly. If you useRound Flap Door (Double Seal)OrManual plug-in insulation doorThe same reason. The baffle door is not closed tightly, the smoke flows back, the pressure of the whole system fluctuates, and the expansion joint also suffers.
Two days ago, I met a customer. The flue system was equipped with an expansion joint but no baffle door. As a result, when the boiler was shut down for maintenance, the non-metallic expansion joint was deflated by cold air backflow. Do you think it's a loss or not? Therefore, the baffle door and expansion joint have to be considered overall in the planning stage.
If you don't pay attention to the installation and maintenance, no matter how good the expansion joint is, it will be useless
The screw of the expansion joint is pre-tightened when it leaves the factory and must be loosened after installation (refer to FAQ for detailsDo you need to remove the screw of the expansion joint）。 If it is not loose, the expansion joint is locked and cannot compensate for the displacement. How to adjust the tie rod nut? It can't be screwed to death, so you have to set aside a compensation margin-generally, there is a recommended value in the product manual, so just adjust it. In addition, the direction of the guide tube must coincide with the flow direction of the medium (the arrow is the flow direction). Non-metallic expansion joints are also afraid of welding slag and sharp scratches, so protective cloth must be laid during installation. For metal, attention should be paid to prevent stress concentration-especially when the boiler starts and stops frequently. Fatigue cracks often start from the heat-affected zone of the weld, and the state of the bellows should be checked every six months.
Look at the appearance, listen to the sound, measure the temperature. Local overheating of the expansion joint surface? Probably the insulation came off. Hear the wind whistle? There is a high probability that it missed. Don't delay, a small leak becomes a major overhaul.
To conclude
The expansion joint is selected for the boiler flue, and there is no "universal" model. Non-metal is economical and flexible, and the metal skin is solid and pressure-resistant. Which one to use depends on your flue temperature, pressure, corrosiveness, space size and budget. If you are not sure, find the manufacturer to calculate the working condition parameters-all products in our station (fromNon-metallic expansion joint (fabric fiber expansion joint)ToHigh temperature axial expansion jointAnd then toDesulfurization flue gas baffle door、Round Flap Door (Double Seal)、Manual plug-in insulation dooretc.) can be accurately matched. Don't make do, it's no joke if this thing goes wrong.