How fierce is the temperature in the desulfurization flue? Find out the working conditions first and then talk about the type selection
Everyone in dry desulfurization knows that the temperature resistance of the expansion joint of desulfurization flue can't be dealt with by just finding a metal bellows. The flue gas temperature at the inlet and outlet of the desulfurization tower usually fluctuates between 80°C and 180°C, but when it meets the bypass of the flue gas heat exchanger (GGH) or the rear section of the booster fan, it can instantly rush above 350°C. This is not the end. In the start-stop stage, hot and cold alternate, and there are frequent thermal shocks-today, it runs at full capacity during the day, stops at night for a cool time, and heats up again the next morning. The test of thermal fatigue on materials is much harsher than that under constant temperature conditions. What's more troublesome is that acid dew point corrosion is indispensable in the desulfurization system: as soon as the temperature is lower than the acid dew point, SO3 condenses to form sulfuric acid. Even if the pipe wall temperature is only ten degrees lower, the corrosion rate can be doubled several times. If the temperature resistance problem is not dealt with properly, the expansion joint will be scrapped in a few months. The shortest record I have ever seen is three months-ordinary stainless steel cracks directly into fishing nets.
Therefore, before selecting, you must get the real operating temperature curve and pressure fluctuation, and don't just look at the nominal value on the design drawings. In the design stage of many projects, it is written "flue gas temperature 150°C". In actual operation, the local temperature can soar above 380°C because of GGH bypass or induced draft fan rush. Anyone who stepped on this kind of pit knew it hurt.
Temperature-resistant core in material and structure
How many degrees can each carry metal bellows, non-metallic fabric and PTFE lining?
Metal bellows: like in our siteHigh temperature axial expansion jointAndCorrugated expansion joint for power station industryWith stainless steel (such as 316L, 254SMO), it can carry 400-500°C. But there is a premise: you have to calculate the fatigue life. For every 100°C increase in temperature, the fatigue life of stainless steel may decrease by an order of magnitude. If the actual temperature exceeds 400°C for a long time, ordinary stainless steel will not be enough, and nickel-based alloys, such as Inconel 625, will be required. However, it is expensive, several hundred yuan per kilogram, so most projects will add deflectors for heat insulation to block the high-temperature flue gas and lower the actual temperature of the bellows.
Non-metallic expansion joint(Fabric Fiber Expansion Joint,Rectangular non-metallic expansion joint): Temperature resistance is usually 200-350°C, which is better than shock absorption and corrosion prevention. It is especially suitable for the later stage of wet desulfurization-the flue gas has high humidity and low temperature after passing through the desulfurization tower, so metals are easily corroded, while non-metals are solid. However, it should be noted that the inner layer must be added with radiation-proof heat insulation cotton, otherwise the fabric layer will burn through quickly. Last year, there was a cement line desulfurization project. When the non-metallic expansion joint was designed, it was forgotten to add thermal insulation cotton. Less than a month after it was put into operation, the inner glass fiber became crisp, and the flue gas came out directly from the outer layer, baking the control cable next to it.
PTFE-lined hoseAndPTFE compensator: First-class corrosion resistance, can carry almost all common acids, alkalis and salts, but the upper temperature resistance limit is only 260°C. High-temperature areas are prone to deformation – PTFE begins to decompose above 300°C, releasing toxic gases. Therefore, if the flue gas temperature exceeds 250°C, the PTFE-lined scheme can basically pass.
For high temperature sectionMetal rectangular expansion jointOrCorrugated expansion joint for power station industryFor low temperature sectionNon-metallic expansion jointOrrubber compensator。 Put one in the middleFlue gas baffle doorIsolation overhaul. This set of matching, as long as the materials are selected correctly and installed in place, will be used for five or six years.
Feedback from a power plant project two days ago
The expansion joint of the original flue of desulfurization of a 1000MW unit is made of ordinary stainless steel 304, and cracks appear along the wave peak in less than half a year. The owner asked us for investigation, and the flue gas temperature was measured on the spot-it actually exceeded 400°C, and the sulfide concentration was high. Stress corrosion and high temperature made 304 unable to bear it at all. What's the right thing to do? Either a guide tube is added to separate the bellows from the high-temperature flue gas; Either go straight on a nickel-based alloy, such as Inconel 625, or simply useHigh temperature axial expansion jointMatching insulation. Later, we replaced them with the bellows of Inconel 625, and added a layer of high-temperature resistant ceramic fiber insulation felt on the inside. It has been two years now, and there has been no problem again.
For desulfurization of a certain cement line, the choice isNon-metallic expansion joint (fabric fiber expansion joint)However, the inner layer forgot to add radiation-proof heat insulation cotton. As a result, the fabric layer burned through three days after it was put into operation. When we remove it, the lined silicone cloth is directly carbonized and shattered when touched. Later, it was suggested that they replace it with a non-metallic expansion joint with three layers of thermal insulation structure (ceramic fiber blanket + stainless steel wire mesh + fluororubber coating) in the inner layer, and install a temperature monitoring point to get it done.
Desulfurization flue expansion joint is temperature resistant, and it is not done simply by looking at the material label. The design of the guide tube, the thickness of the insulation layer, and whether the adjustment of the tie rod nut leaves enough compensation during installation, every detail determines the life span.
Details of installation and maintenance determine temperature resistance
When it comes to installation, there is one of the most common low-level mistakes: the expansion joint tie rod nut is pre-tightened cold before leaving the factory to prevent damage to the bellows during transportation and hoisting. However, before it is put into operation, it must be loosened or adjusted to the design displacement value-many workers try to save trouble. As a result, as soon as the system heats up, thermal expansion pulls the bellows hard, which directly exceeds the limit and fails. The most exaggerated case I have ever seen: a 4MPa steam pipe expansion joint, the tie rod nut is not loose, and the bellows is forcibly cracked after heating up, and the steam spewing out almost hurts people. This is inHow to adjust the tie rod nut of expansion jointIt was said in the question and answer, so you must remember.
In addition, the direction arrow of the guide tube cannot be installed backwards. The function of the guide tube is to let the flue gas pass through from the inside, so as to avoid directly washing the working face of the bellows. Once installed backwards, the flue gas directly hits the bellows, and the local temperature is tens of degrees higher than the design value, which will accelerate corrosion and fatigue. How to judge the direction? Look at the arrow pointing in the direction of the medium flow, just consistent with the direction of the pipe flow.
And cooperationManual plug-in insulation doorOrDouble-sealed single-axis circular baffle doorWhen using, pay attention to the sequence of opening and closing-first open the baffle door to form a channel, and then slowly raise the temperature. If the temperature is raised first and then the baffle door is opened, the internal pressure of the expansion joint may bulge or even burst. The operating system must be written into the SOP and affixed to the field control cabinet.
Suggestions for model selection and implementation
To summarize the selection principles, you can use them directly:
- Temperature below 200°C: PriorityNon-metallic expansion jointOrrubber compensator, taking into account both corrosion prevention and cost. It is commonly used in the later stage of wet desulfurization.
- 200-400°C: AdoptHigh temperature axial expansion jointOr stainless steel bellows, pay attention to adding a guide tube. If sulfide-containing corrosion is severe, it is recommended to upgrade the material to 254SMO or SUS304.
- Above 400°C: Use composite structures such as Inconel 625, SUS 316L) or lined with thermal insulation. Highly recommended withFlue gas baffle doorLinkage design to avoid long-term high-temperature direct burning-such as adding one before the expansion jointSingle-axis double-flapper doorCut off the smoke during maintenance and let the expansion joint cool.
If the customer has the need for technical transformation of desulfurization system, be sure to ask the operator about the actual operating temperature curve and pressure fluctuation. The design parameters are often "planned values", and the data actually run out on the spot is reliable. By the way, GB/T 12777 and JB/T 12235 are national standards for expansion joint design-check the former for metals and the latter for non-metals. Have a bottom in your heart, and you can have more confidence when communicating with the owner. After all, the problem of temperature resistance of desulfurization flue expansion joint depends on experience not to suffer, and depends on luck to overturn sooner or later.