How "ruthless" is the heating furnace flue? - -Three mountains of high temperature, corrosion and displacement
Anyone who has worked in a heating furnace knows that what runs in the flue is not air, but "fire". The flue gas temperature fluctuates with the working conditions, soaring to five or six hundred degrees at every turn, and even reaching a thousand degrees in some processes. This is not the end. The smoke contains sulfur, chlorine and water vapor. When it condenses, it turns into acid, which is more corrosive than concentrated sulfuric acid. Coupled with the thermal expansion and contraction when the pipeline starts and stops, the displacement of several meters long flue pipe sections can exceed 100 mm. With a three-pronged approach, ordinary pipe connectors will have to be scrapped if they can't last for three months.
What about that? Expansion joints can only be installed in key positions, which is what we often call compensators. Don't fool the general bellows. The expansion joint used in the flue of the heating furnace must withstand high temperature, be corrosion-resistant, and be able to absorb large displacements. If one of these three things is missing, it will be a matter of time before the equipment lies down.
Metal Corrugated Expansion Joints vs Non-Metal Expansion Joints: Who Can Better Hold the Smoke From the Furnace?
The first level of selection is stuck here. Let's start with the metal corrugated expansion joint-I used the one in the stationHigh temperature axial expansion joint, andCorrugated expansion joints are used in power stations. These products rely on stainless steel bellows to handle thermal displacement and have strong pressure resistance, but they have a fatal shortcoming: they are afraid of corrosion. Once the sulfide in the flue gas condenses between the bellows pitches, stress corrosion cracking is a sure thing. And guess what? Many heating furnace owners choose ordinary stainless steel for cheap, and as a result, they leak in half a year.
Look againNon-metallic expansion joints (fabric fiber expansion joints)This thing is made of glass fiber cloth, silicone cloth and polytetrafluoroethylene film. Its temperature resistance is generally within 600℃, and it can reach 800℃ with heat insulation cotton. The advantage is that the corrosion resistance throws metal several blocks, and the ability to absorb large displacements is extremely strong-axial, lateral and angular displacements can be eaten down. The disadvantage is that the pressure is weak, and the flue pressure exceeds 0.1MPa. So which is better? There is no absolute answer-the flue gas temperature exceeds 600℃ for a long time, and the pressure is higher than normal pressure. Honestly, the metal corrugation is lined with an anti-corrosion layer; In slightly lower temperature and negative pressure conditions, non-metallic expansion joints are more durable schemes. The flues of many power plants and chemical plants now tend to useRectangular non-metallic expansion jointMatching the low-temperature wet flue gas after desulfurization is to see its corrosion resistance and vibration reduction characteristics.
Don't Ignore Deflectors and Insulation: Two Little Things Determine How Long Expansion Energy Saves Live
Let's talk about a real case. Four metal expansion joints were used in the flue of the heating furnace of a refinery in northwest China. In less than a year, all the bellows cracked from the inside. When disassembled, the guide tube was eroded to only half a circle. What is the role of a deflector? Simply put, it is to block the direct scouring of high-temperature smoke, let the airflow go along the inner wall, and not directly hit the bellows. Inside the stationexpansion joint guide tubeIt is standard, but many people think that "fewer tubes are okay"-this is jumping into the pit.
Insulation is more critical. Non-metallic expansion joint if not addedThermal insulationWhen the fabric fibers are directly exposed to high temperatures, the aging rate of the silicone cloth is doubled and the life of the cloth is halved. The correct method is: the inner layer is filled with temperature-resistant ceramic fiber blanket, and the outer layer is fixed with stainless steel wire mesh or thin metal sheath. In this way, the heat of flue gas is weakened layer by layer, and the temperature has dropped by more than 200 degrees when it reaches the pressure-bearing layer. If these two things are not paid attention to, the expansion joint will be a ticking time bomb.
Expansion joints alone are not enough-how can smoke baffle doors and insulation doors be used together?
The expansion joint solves the displacement compensation, but there is another rigid need for the flue system: cutting off the flue gas during overhaul. That's got to goFlue gas baffle doorAndIsolation doorYeah. Commonly used such asDouble-sealed single-axis circular baffle doorWith two sealing structures when closed, the leakage rate is less than 1%, which is much more reliable than a single-layer flap door. Heating furnace outlet flue usually equipped with electric actuatorElectric plug-in insulation doorCan be completely closed within 15 seconds, which is convenient for cold air maintenance when the furnace is shut down.
Installation sequence of expansion joints and baffle doors. If the expansion joint is not installed upstream of the baffle door (near the furnace side), condensed water will easily accumulate in the expansion joint after closing the baffle door, which will accelerate corrosion. matchDesulfurization flue gas baffle doorWhen, the expansion joint must be added with a drain valve-this is a lesson learned in the industry.
Should the screw be removed during installation? — — A few big truths to help you avoid stepping on pits
This is a question that almost every new customer asks. Watch the Q&A on the site"Does the screw of the expansion joint need to be removed?"The standard answer is: the installation locking screw (also called the transport screw) must be removed! The reason is simple. The function of the screw is to prevent the expansion joint from being compressed or stretched and deformed during transportation and hoisting. Once the pipeline system is fixed and the screw is not disassembled, the expansion joint cannot expand and contract freely at all, which is equivalent to becoming a rigid connection-why install it?
However, some customers said, "Can you not dismantle it? Anyway, the displacement of the pipeline is not big". Let me tell you, the design compensation amount of the flue expansion joint of the heating furnace is less than 20 or 30 millimeters. If you weld the screw to death or screw it with a wrench, it won't loosen. During thermal expansion, the hard tensile stress of the bellows will be forced to the pipe flange or support, which will lead to water leakage and air leakage, and the pipe wall will be torn. Therefore, be sure to loosen the screw and take it off after the cold alignment of the pipeline is completed and the bolts are tightened. Really worry, after disassembly, you can put it back in place but loosen twice-but it's best to remove it completely. Tsk, this is a lazy job.
Suggestions for selection: Don't just look at the model, but focus on the working condition and life
Finally, let's be honest. Many purchasers like to compare prices. "You guysUniversal corrugated expansion jointHow much is it? ThatLarge diameter thick wall expansion jointHow much more? " -But the most important one is overlooked: Have you taken out your heating furnace flue gas composition list? What about the temperature graph? What about the pipe stress analysis report? Without these, the reported model is gambling.
First, clarify the flue gas temperature and pressure range. Beyond 600℃ preferredHigh temperature axial expansion joint(In stock at the station), with liningCeramic fiber insulation layerAndguide tube。 Second, look at corrosiveness. The flue gas after wet desulfurization must be selectedNon-metallic expansion jointOrPTFE-lined hoseClass of anti-corrosion structure, don't buy ordinary stainless steel to save 20,000 yuan. Third, the displacement amount and fatigue life are calculated. useMetal expansion joint weight tableAndCalculation formula of rigidity of bellowsReview whether the design meets the standard.
After all, the excerpt of flue expansion of heating furnace is right, and the whole flue system will run stably for three to five years; Wrong choice, shut down once a half year for replacement, and the maintenance cost is enough to buy three new equipment. Do you want to buy the right one-time, or do you fight with maintenance every year? This is not a difficult account.