Why must the flue inlet be fitted with an expansion joint? Thermal expansion, contraction and vibration are no joke
What is the location of the flue entrance? Connected to the outlet of the boiler or kiln on one side, the temperature is usually seven or eight hundred degrees; The other side is connected to the follow-up dust removal and desulfurization equipment, and the temperature may have dropped by half. Not to mention the large temperature difference, the large flue gas flow rate, the vibration of the pipeline, and the dimensional deviation during the installation of the equipment-these factors come together. If there is no expansion joint, the pipeline will hit hard, and the consequence will be interface cracking, equipment deformation, and even safety accidents.
If you think about it, the thermal expansion coefficient of flue steel is about 12 μ m/ (m·℃). Assuming that the flue inlet temperature rises from normal temperature to 600℃, the elongation of a 10-meter-long pipe is calculated to be nearly 70mm. What is the concept of 70mm? The flange bolt holes can't swallow the amount of allowance. Without the expansion joint, this 70mm displacement all becomes stress, and it is pressed on the equipment interface. Can the weld bear it?
Three core roles: absorbing displacement, reducing stress, maintaining seals — one less won't do
The first action, absorbing displacement. The displacement at the flue entrance is not only axial expansion and contraction, but also lateral deflection and angular displacement. Don't think that the expansion joint is only lengthened and shortened. A good design must be able to absorb deformation in multiple directions at the same time. For example, we useMetal rectangular expansion jointOrNon-metallic expansion joint (fabric fiber expansion joint)Corrugated structures do this job.
The second action is to reduce stress. If the stress does not drop, the pipes and equipment will fail by fatigue. The expansion joint turns the rigid pipe system into an elastic system through its own flexible deformation. Think about it, an iron bar collides head-to-head, and the stress is all concentrated in the weld joint; Add a section of bellows in the middle, and the stress is dispersed and absorbed. This is the most critical function of the flue inlet expansion joint.
The third action is to maintain the seal. High-temperature smoke runs in the flue, which not only wastes energy, but also pollutes the environment and burns people. The expansion joint itself is a seal, and the design of gasket and guide tube at the connection of flange directly affects the sealing effect. Three functions, one less won't do.
Don't mess with the selection: how to build metal rectangle, non-metallic fabric and baffle door is reasonable?
When it comes to model selection, only those who have turned over the car have a say. Many people ask "which expansion joint is good" when they come up, and the standard answer is always "depending on the working conditions".
High temperature and containing corrosive flue gas (e.g. desulfurization inlet), preferredNon-metallic expansion joint (fabric fiber expansion joint)Which is temperature resistant, corrosion resistant and has a large compensation amount. But the pressure is not too high, and it is easy to use within 0.1MPa. If the pressure is too high or if more axial force is required, useMetal rectangular expansion jointStainless steel corrugated pipe plus multi-layer structure, can carry. Another situation-the flue inlet needs to be switched frequently, so it can't rely on the expansion joint alone, but it has to be equippedFlue gas baffle doorOrDesulfurization flue gas baffle door。
How do you match flapper doors and expansion joints? For example: on the boiler outlet flue, an expansion joint is usually installed first to absorb heat displacement, and then a baffle door is installed for isolation. If the baffle door is installed in front of the expansion joint, the thrust generated when the baffle acts will easily crush the bellows. So the reasonable order is: the expansion joint is close to the equipment outlet, and the baffle door is installed behind the expansion joint. In addition, some projects require zero leakage, so you have toRound Flap Door (Double Seal)OrDouble-sealed single-axis circular baffle door, the sealing effect is good, but the price has also gone up.
And there's anotherSingle-axis double-flapper doorWhich is used for working conditions requiring strict isolation. Don't forget to cooperate with the expansion joint when selecting the model: leave enough spacing between the baffle door and the expansion joint to facilitate maintenance.
Should the tie rod be removed during installation? Where does the deflector arrow go? Only those who have turned over these details will understand
If the installation details are wrong, no matter how good the product is, it will be useless. Let's start with the pull rod. Many expansion joints leave the factory with limit rods or transport rods. Before installation, should the tie rod be removed or not? Case by case:
- If the tie rod istransport tie rod(Only used to prevent transportation deformation), which must be removed after installation, otherwise the expansion joint cannot be freely expanded and contracted.
- If the tie rod isadjusting tie rod(Used to control the amount of pre-tension or limit the displacement direction), then it can't be removed, but after installation, the nut should be adjusted to leave working displacement space for the expansion joint.
You see,How to adjust the tie rod nut of expansion jointThis question is often confused. The standard practice is: first loosen the nut, let the expansion joint be in a free state, and then tighten the nut according to the pre-tension amount given by the manufacturer according to the difference between the installation temperature and the working temperature. Don't screw to death when you come up, otherwise the bellows will be relieved after heating up, and the life will be greatly reduced.
Let's talk about the deflector. What we always saySpecific Function of Expansion Joint Guide TubeFirst, guide the flow direction of the medium to prevent the vortex from washing the bellows; The second is to reduce resistance. The most critical point during installation: The direction of the arrow on the guide tube must coincide with the direction of media flow. Where does the arrow go? In the direction of flue gas flow. Install it in reverse, the guide tube becomes a baffle, and the bellows is directly washed and perforated in a few months.
When the flange is connected, the gasket should be centered, the bolt should be tightened diagonally three times, and the torque value should be according to the standard. Don't underestimate these "mother-in-law" things. Nine times out of ten, leakage accidents are caused by inadequate installation details.
Owners often step on pits: they thought that the expansion joint was just a 'section of corrugated pipe', but they were miserable at low prices
Alas, I get angry when I talk about this. Some purchases come up with lower prices: "Isn't it just a section of corrugated pipe? Others are half cheaper!" The result? The material used in low-priced products is thin, and the bellows will crack in fatigue within a few days; The flange surface is rough and the gasket cannot be pressed; The deflector is directly omitted, and the flue gas flushes the bellows-alas, it begins to leak after three months of installation.
In fact, the function of flue inlet expansion joint is far more than "connection". It is the safety valve of the whole flue system. The little purchase cost you save may be paid out in one downtime overhaul. Really reliable manufacturers will give you detailed working condition analysis: temperature, pressure, displacement, media composition, installation space... and then match the most suitable model, such asCorrugated expansion joint for power plant industryOrMetal Corrugated Expansion Joints in Cement IndustryEven if higher pressure is needed, it may be recommendedExternal pressure single axial expansion jointOrStraight pipe pressure balanced expansion joint。 These products all have strict manufacturing standards and inspection processes, and the price can't be ridiculously low.
Choose expansion joints, don't just look at the price. Figure out which of the full lifecycle costs — equipment, installation, repairs, downtime losses — is more cost-effective, and you have an idea in mind.