1. Why is there always a problem with the tail flue? — — Double test of high-frequency vibration + high-temperature flue gas
Anyone who has worked in a boiler knows that the tail flue is one of the most troublesome positions in the equipment. Especially for high-frequency boilers, the flue gas temperature is always three or four hundred degrees. When the fan turns, the whole flue vibrates with high frequency, with a small amplitude but a high frequency, and hundreds of thousands of cycles a day. In this condition, metal expansion joints (such as we often sayCorrugated expansion joint for power station industryOrHigh-temperature axial expansion joint) has become a weak link.
The most common failure mode is that the root of the bellows is cracked, the weld is corroded through, and even the guide tube is blown off. Two days ago, a friend from a power plant complained to me that the tail flue of one of their high-frequency boilers took less than a year, and two of the four expansion joints were replaced. When I asked, the fatigue life was not considered at all when selecting the model, only calculated according to static displacement. Didn't you say it was a waste of money?
High-frequency vibration brings alternating stress. If the high-temperature flue gas contains sulfur and chlorine, it is stress corrosion superimposed fatigue, which is a double blow. So,Metal expansion joint of tail flue of high frequency boilerSelection is definitely not something that can be determined by just flipping through a sample.
2. Metal expansion joint vs. non-metallic expansion joint: Who is more suitable for the tail flue?
Can you use non-metallic ones? Cheap and corrosion resistant. Let me say the conclusion directly: it is best not to use non-metal in the high-frequency area of the tail flue. Why is it?
The advantages of non-metallic expansion joints (fabric fiber expansion joints) lie in vibration reduction and corrosion resistance, but the upper limit of temperature resistance is usually only about 250℃ (the special design of silicone coating can be higher, but the life is also short). The exhaust temperature of high-frequency boilers is generally 300-450℃, which cannot be held by non-metals. Coupled with high-frequency vibration, the fabric layer is easy to delaminate and wear, and once the hole is directly injected, the risk is great.
On the metal expansion joint,Metal rectangular expansion joint、Corrugated expansion joint for power station industryAnd other products, the temperature resistance can be above 600℃, and the structural strength is good. But metal is afraid of fatigue, so it must be specially selectedHigh temperature axial expansion jointThis class of products designed for thermal displacement. If the flue section is square, use rectangular metal expansion joints; For round ones, use circular bellows. My advice is: Don't try to replace non-metals cheaply unless you can confirm that the flue gas temperature is below 200℃ and the vibration is extremely low.
3. Select three key parameters: displacement, temperature and fatigue life. One less won't do
You think choosing an expansion joint is just reporting a caliber? Too naive. None of the three parameters of the tail flue can be missed: displacement, medium temperature and fatigue life requirements.
displacement amountIncludes axial, transverse and angular displacement. Because of thermal expansion and fan vibration, the actual displacement of flue of high-frequency boiler is often larger than the theoretical calculation. It is recommended to measure on site, or leave at least 20% margin.
TemperatureDecide on the material grade. 304 stainless steel should be used below 350℃, 316L or equivalent heat-resistant steel should be used at 350-500℃, and INCOLOY alloy should be considered above 500℃.
Fatigue lifeIs the easiest to overlook. Conventional expansion joint design requires 10³-10⁴ cycles, but high-frequency boilers have tens of thousands of cycles a day. If you choose according to ordinary standards, it will crack in three months. The supplier must be required to provide a fatigue life curve with an explicit requirement of ≥10⁶ times. Such asCorrugated expansion joint for power plant industryThere are reinforcement designs dedicated to high frequencies, and the number of corrugated layers, wave height and wall thickness are different. You ask the supplier: "How long will this expansion energy saving last?" If he can't answer, you will change it.
I have seen a case where an ordinary general-purpose corrugated expansion joint was used in the tail flue of a biomass power plant, and a penetrating crack appeared at the root of the corrugated pipe half a year later. When removed, the thickness of the material is only 0.8mm, and the design cycle life is only 2000 times. Replace withHigh temperature axial expansion jointWith a wall thickness of 1.5mm, a layer of reinforcing ring is added, and it will be fine after two years of use.
4. Five Common Mistakes in Installation and Routine Maintenance
If you choose the right thing, if you pretend to be broken, it will be useless. The following pits, I can run into them almost every time I go to the site.
- The direction of the deflector is reversed.The function of the guide tube is to direct the airflow to the center and avoid direct flushing of the bellows. The direction of the arrow must coincide with the direction of the media flow. Install it backwards, and the high-speed dusty smoke directly hits the ripples, which will wear out in one month. The question and answer of this site specifically talks about "the specific function of the expansion joint guide tube", which can be turned over.
- The transport tie rod runs without disassembly.Some manufacturers use pull rods to fix the transportation state, which must be loosened or removed after installation. If it is not dismantled, the expansion joint can't move at all, which is no different from the straight pipe section. The thermal stress is all held in the pipe.
- The cold tightness value was not adjusted according to the design.The amount of pre-stretching or pre-compression is forgotten to do, resulting in insufficient actual compensation. Especially for high-temperature pipelines, cold tightness is the key to compensation.
- The insulation layer holds down the expansion joint.The tail flue is often wrapped with thermal insulation cotton, but workers directly wrap the thermal insulation cotton on the bellows to save trouble, which limits the deformation and causes local overheating.
- The inspection only looks at the surface and not the inside.From the outside, the corrugation is intact and it is not possible to see whether the internal guide tube has fallen off. It is recommended to check endoscopy every time the machine is shut down for maintenance, or open the manhole to see.
5. Don't rush to replace it if it is broken-you can save half the money by checking these places first
When there is a problem with the equipment, the leader's first reaction is to "replace it with a new one". Wait, spend half an hour checking first. In many cases, you can recover by changing the nut and adjusting the tie rod.
Step 1: Check the deflector.If there are wear marks on the inside of the bellows, there is a high probability that the guide tube falls off or is displaced. It is 80% cheaper to weld back the old deflector than to replace the entire expansion joint.Step 2: Check the tie rod nut.The role of the expansion joint tie rod is to limit and protect, and if the nut is loose, it will cause the bellows to overstretch. Just readjust to the design stroke. This site has the answer of "How to adjust the expansion joint tie rod nut".Step 3: Check the pipe support.If the bracket is displaced or the sliding mount is stuck, it can cause the expansion joint to undergo additional displacement. Adjusting the bracket tends to be immediate.Step 4: Judge fatigue fracture or corrosion.If the crack extends transversely along the root of the corrugation, typical fatigue; If reticulated or pitted, it is corrosion. For initial cracks, you can grind and repair them after welding, but pay attention to heat treatment. It is only worth replacing if there is a large area of cracking or a wall thinning of more than 30%.
The bad ones are not necessarily all the problems with the expansion joints themselves. Sometimes it is careless installation, loose support and crushed insulation cotton. You can save at least half the money by checking these out. If you really want to change, it is recommended to chooseMetal rectangular expansion jointOrCorrugated expansion joint for power station industryAnd require manufacturers to provide fatigue life curves and material reports.
Metal expansion joint of tail flue of high frequency boilerSelection depends on the temperature, displacement and fatigue life, install the guide tube and the tie rod, maintain frequently and check, and don't change it blindly. If you follow this routine, the life of the device will at least double.