That thing about the flue of induced draft fan: high temperature, corrosion and vibration are many
Everyone who engages in boilers knows that the environment in the flue of the induced draft fan is really not ordinary. Think about it, the flue gas temperature is always two or three hundred degrees, and some working conditions have to bear instantaneous overtemperature. If the expansion joint is not selected well, it will crack for you in a few months. In addition, the flue gas contains sulfur and humidity, which is extremely corrosive, especially in the later stage of desulfurization. As soon as the acid dew point is low and the condensed water comes out, it is a terrible one. There is also the vibration of the induced draft fan itself, the displacement caused by the thermal expansion and contraction of the pipe-these factors together, and the requirements for the expansion joint are no joke.
What about that? When choosing a model, you have to take these "enemies" into account. When the temperature is high, it depends on the temperature resistance grade of the material; If the corrosion is heavy, either add anti-corrosion lining or choose corrosion-resistant stainless steel; If the vibration is large and the displacement is fierce, it is necessary to calculate the compensation amount, not just by feeling. To put it bluntly, you have to find out the temper of your flue first when choosing expansion joints, otherwise it will be useless no matter how much money you spend.
Choose metallic or non-metallic? The key depends on the working conditions and budget
It's a cliche, but a lot of people get stuck here. Metal expansion joints, as we often sayUniversal corrugated expansion joint、High temperature axial expansion jointThese have the advantages of high pressure resistance, long service life, and can withstand large displacement, especially suitable for high temperature and high pressure occasions. Like the main flue of power station boiler and the outlet of induced draft fan, they can often be seen. However, the shortcomings of metal are also obvious: fear of corrosion and stress concentration, high centering accuracy is required during installation, and the price is relatively expensive.
What about non-metallic expansion joints? OurNon-metallic expansion joint (fabric fiber expansion joint)、Rectangular non-metallic expansion jointAndrubber compensatorThis kind has many advantages: corrosion resistance, noise and vibration reduction, easy installation, and much cheaper. It is especially suitable for the environment of low temperature and high corrosion of desulfurization flue. But the weakness is also obvious-the temperature resistance is limited, generally no more than 300℃, and the life is relatively short, especially the aging problem of rubber material.
So how to choose? My personal advice is: If you have a sufficient budget and the working conditions are harsh (high temperature + high pressure + frequent start-stop), don't hesitate to put on metal, such asCorrugated expansion joint for power station industryOrMetal Corrugated Expansion Joints in Cement Industry。 But if the flue temperature is not high, mainly corrosion and vibration problems, and you are more concerned about the cost, then feel free to choose non-metallic ones. Of course, there are compromises: metal expansion joints lined with PTFE or rubber, such asPTFE-lined hose、PTFE compensator, both ends, but the price has also gone up.
Rectangular or circular? Don't underestimate the influence of flue shape on expansion joint
And guess what? When many people choose expansion joints, they only look at the material and size, regardless of whether the flue is round or square. As a result, it was found that either the compensation amount was insufficient, or the stress concentration pulled the flange apart.
A circular flue is generally equipped withUniversal corrugated expansion jointOrExternal pressure single axial expansion jointThat's enough, because the circular pipe itself is stressed evenly, and the expansion joint is easy to match. But the rectangular flue is troublesome-its four corners are unevenly stressed, and it is usually very large in size. For example, the rectangular flue at the outlet of the induced draft fan of a power plant is often one or two meters wide. You have to use it at this timeMetal rectangular expansion jointOrRectangular non-metallic expansion joint。 Metal rectangular expansion joints can be specially designed for the displacement of each face, such as single wave, multi-wave, and with guide tubes. The non-metallic rectangular expansion joint is more flexible, can be customized to any size, and is lightweight and easy to install.
The guide tube of rectangular expansion joint is very important. We often talk to customers about itSpecific Function of Expansion Joint Guide Tube-It can guide the flue gas through smoothly and avoid vortex washing the ripples. So don't save the deflector just to save that little money. The gain is not worth the loss.
This installation step is wrong, no matter how good the expansion joint is, it will be useless
Alas, after working for so many years, I have seen too many cases where the installation went wrong. Obviously, there is no problem with the product, but it leaked and cracked within a few days of installing it, and finally it couldn't be cleared.
First of all,Arrow direction of expansion jointNever do it the other way around. Generally, the arrows are written "flow direction", which must be consistent with the flow direction of flue gas. Install it backwards, and the deflector becomes a ride, and the ripples are quickly washed away.
Secondly, when installingDo you need to remove the screw of the expansion joint？ We have been asked this question countless times. The answer is: the tie rod screw for transportation must be loosened before installation, otherwise the expansion joint will not work. But if it is a product with a tie rod, such asLarge tie rod expansion jointOrCompound hinge transverse expansion jointThose tie rod nuts are fixed, and they should be adjusted according to the design requirements after installation. How to adjust it specifically, you can checkHow to adjust the tie rod nut of expansion jointThat Q&A.
Install without forcibly stretching or compressing the expansion joint to align the pipe. You pull hard, and the bellows are prestressed, and it will collapse when it heats up. The correct thing to do is to adjust the pipe bracket so that the expansion joint is in place in the free state.
Where to look for daily inspections? The Three Most Prone Where to Go Wrong
- Corrugated surface: Whether it's metallic or non-metallic, look for cracks, bulges, corrosion pits. Metal expansion joints pay special attention to the vicinity of the weld, where a lot of cracks start.
- Flange connection: Is there an air leak? Are the bolts loose? The flange bead of non-metallic expansion joint is prone to rust and will loosen over time.
- guide tube: Although it is inside, if you have the conditions to look through the observation port and the guide tube is washed and perforated, the smoke will directly hit the ripples, and the life will drop from a cliff.
Also don't forget to checkexpansion joint tie rodThe state of. If the tie rod is bent, it means that the displacement of the pipe exceeds the design value, so you have to find the reason. Don't just change the tie rod.
With baffle doors and metal hoses, the whole system is reliable
Expansion joints are not alone. If you have a flapper door in your flue, likeFlue gas baffle door、Desulfurization flue gas baffle doorOrDouble-sealed single-axis circular baffle doorWhen they turn on and off, they produce impact forces, and the expansion joint must be able to hold it. If the baffle door is not sealed well, the cold air leaking in makes the local temperature of the flue plummet, and the expansion joint will easily "catch a cold"-the corrosion will accelerate.
Also, the vibration of the pipeline at the inlet and outlet of the induced draft fan is often very large, so it can be matched withMetal hoseOrPTFE-lined hoseTo dampen the vibration. Don't underestimate the metal hose. When it matches well with the expansion joint, the stress distribution throughout the flue system will be much more uniform.
In short, when choosing the flue expansion joint of the boiler induced draft fan, don't just stare at the price. Find out the working conditions first: How high is the temperature? Is the corrosion strong? Is the vibration big? Is the flue round or square? Then match materials and structures according to these. When installing, follow the rules, take a second look every day, and finally don't forget to include the baffle door and hose. With this set, your home's flue system can run safely for at least several years.