Anyone who has worked in the flue knows that when hundreds of degrees of hot smoke rush into the pipe, the temperature of the pipe wall instantly soars. The thermal expansion and contraction of steel-the temperature of the pipeline per meter rises by 100℃ and elongates by about 1.2mm-the displacement of the flue tens of meters long can easily go tens of millimeters. The problem is that the pipeline is dead, and the flange, bracket and equipment interface are all rigid connections. If the top is hard, either the weld joint will be torn or the top of the fan shell will be deformed. That's when flue expansion joints come in handy. To put it bluntly, its working principle is to rely on the folds of the bellows to remove the thermal expansion and contraction force by elastic deformation.
1. Core principle: How to remove the thermal expansion and contraction of bellows?
The essence of the bellows is a thin-walled metal container. After it is made into a corrugated shape, the axial stiffness drops sharply-what can't be pressed in the straight pipe section can be retracted by the bellows with a gentle press. This is the same as spring: the smaller the stiffness, the greater the deformation under the same force. The working principle of flue expansion joint is to put it bluntly: when the pipe is heated and elongated, the bellows is compressed; When cooling shrinks, the bellows is stretched. By this amount of elastic deformation, the thermal displacement of the pipe is eaten.
A "pre-stretch" must be mentioned here. Experienced masters will pre-stretch the expansion joint by a certain amount during cold installation. For example, if the predetermined compensation amount is 40mm, then pull open 20mm first. Why? Because the working temperature of most flues is much higher than the installation temperature, the thermal elongation of the pipe will compress the expansion joint. If it is pulled to the middle position in the cold state, it can be taken care of when it is compressed in the hot state and when it is cooled, so as to avoid exceeding the limit in one direction. This detail is overlooked by many people, but it is precisely the key to guarantee longevity.
2. How to deal with different displacement directions?
The thermal displacement of the pipe is not only axial expansion and contraction. There is a lateral displacement at the turn and an angular displacement at the tee. Different working conditions have to choose different types of expansion joints. Let's take three typical products to explain clearly.
High temperature axial expansion joint— — Specially dealing with the axial expansion and contraction of straight pipe segments. The structure is simple, that is, a set of bellows with flanges. For example, the main steam pipeline of the power station has a temperature of five or six hundred degrees and an axial displacement of tens of millimeters. Use it directly. If you check the "high-temperature axial expansion joint" in the product information on our site, the temperature resistance grade and wave number calculation are clearly marked.
Compound hinge transverse expansion joint-To deal with the lateral displacement of L-shaped pipes. It consists of two bellows with a hinge structure in the middle, which restricts axial movement and allows only angular swing. The combination of the two angles becomes a lateral displacement. The dust removal pipeline of the old steelmaking plant is often used in this way.
Straight pipe pressure balanced expansion joint— That's interesting. The ordinary axial type will generate huge blind plate force (pressure × cross-sectional area) when absorbing displacement, which can push the bracket askew. The pressure balance type digests the thrust generated by the pressure itself through a balance bellows in the middle, and does not transmit it to the pipeline. It is used in high-pressure, large-diameter situations, such as the main line of the heating network.
When selecting, don't just look at the displacement, but also the direction of the pipeline, pressure level and medium temperature, otherwise it will be a time bomb.
3. Three details that are most prone to rollover at the installation site
No matter how well you said earlier, installing a rollover is all for nothing. According to the case of our after-sales feedback, these three things are most likely to be mistaken.
Install the guide tube in reverse direction.The function of the guide tube is to guide the medium to flow smoothly through the bellows and avoid the high-speed airflow directly washing the corrugations (refer to the question and answer "Specific function of the guide tube of the expansion joint"). Install so that the bell mouth of the deflector is directed in the direction of media inflow. Once installed backwards, the airflow hits the ripples directly, creating vortices and vibrations that wear out in less than a week. And guess what? There is a power plant flue expansion joint guide tube installed backwards, and smoke began to leak after three days of operation. When it was disassembled, the bellows was flushed out of a trench.
Should the tie rod nut be removed?For transportation protection when the expansion joint leaves the factory, the tie rod nut is usually locked, limiting the expansion and contraction of the bellows. After installation in place, the tie rod nut must be loosened to the designed position to allow the bellows to move freely. Many people forget to loosen it, and as a result, the thermal expansion of the pipe directly destroys the bracket. Some tie rods are also used for adjustment (see Q&A "How to adjust the tie rod nut of expansion joint"). You have to adjust the nut position according to the cold tightening amount, and then lock it. Something will happen if you get mixed up.
The mystery pointed at by the arrow.There is usually an arrow on the expansion joint cylinder, which indicates not the direction of the medium flow, but the direction of expansion and contraction of the bellows-that is, the direction of displacement compensation (refer to the question and answer "What is the direction of the arrow of the expansion joint"). Some manufacturers target the flow direction of media, but the standards are not uniform. The safest thing to do: read the manufacturer's installation manual, don't take it for granted.
4. Selection is not spelling parameters: metal vs non-metal, what should I do if high temperature contains sulfur?
The working conditions of the flue vary widely, so when you choose the expansion joint, you can't go up because the parameters look good. Let's start with the broad categories:Metal expansion joint(For example, stainless steel bellows) has high temperature resistance and high pressure, but is afraid of corrosion. There are often sulfur dioxide and sulfur trioxide in the flue, especially after wet desulfurization, the flue gas temperature drops below the acid dew point, forming dilute sulfuric acid. At this time, 316L stainless steel may not be able to hold it, so you have to consider high nickel alloy or lining anticorrosion layer.
Non-metallic expansion joint (fabric fiber expansion joint)It's the other way. It uses glass fiber cloth, silicone cloth, fluororubber and other composite materials as loop belt, and its temperature resistance is generally about 400℃, but it has good elasticity, large compensation and low cost. The key is that it is not afraid of corrosion. Desulfurization flue is its most typical application scenario. The "rectangular non-metallic expansion joint" series on our station is specially designed for rectangular flue.
Andrubber compensatorAndPTFE compensator。 The rubber compensator is suitable for occasions with low pressure, normal temperature and large vibration, such as fan inlet and outlet. However, the temperature resistance does not exceed 100℃, and it is easy to age as soon as there is water vapor in the flue gas. Polytetrafluoroethylene compensator has excellent corrosion resistance, and the temperature resistance can reach about 200℃, which is suitable for chemical flue gas. However, its stiffness is too large, and its compensation ability is limited.
The baffle door is used to isolate smoke and switch maintenance channels. The expansion joint should be installed in front and behind the baffle door to absorb the displacement caused by heat. If the flue continues to cool and contract after the baffle door is closed, the displacement margin of the expansion joint must be sufficient. The product description of our "Desulfurization Flue Gas Baffle Door" specifically marks the recommended expansion joint model, so just follow it.
5. When should I change? Failure Signs and Life Judgment
Expansion joints are consumables, and there is no once and for all. According to the "Service Life of Expansion Joints" question and answer, the design life of metal expansion joints is generally 10~15 years, but this depends on the actual working conditions. So when should I change it? Give engineers an actionable manual:
- Bellows crack: Tortoise cracks visible to the naked eye, or sharp sounds of metal fatigue can be heard. This is the most dangerous, and it may leak or even burst at any time.
- Corrosion perforation: Pay special attention to the heat-affected zone of the weld and the bottom end of the trough. Check regularly with a thickness gauge, if the thickness is thinned by more than 30%, it must be replaced.
- stiffness decay: If it is found that the expansion joint cannot be retracted in the cold state, or the hand pressure feels particularly loose, it means that the bellows has been plastically deformed and lost its elastic compensation ability. This can be judged by measuring the free length against the factory data.
- Aging of non-metallic band: The surface of the fabric fiber expansion joint is hardened, brittle, delaminated, or damaged and leaky. Generally, it has to be replaced in 3~5 years.
The most afraid thing in the industrial site is "use it first if you can use it". Wait until it leaks before replacing it, and the downtime loss and repair cost are much greater than the new one. It is recommended that the expansion joint be included in the annual maintenance list, focusing on checking the high-frequency vibration parts and the contact sections of corrosive media. Is that the case?