1. How "evil" is the working condition of the dilution fan? — — High temperature, corrosion, vibration, none of them can be missed
Brothers who have engaged in desulfurization and denitrification projects all know that it is not easy to live with dilution fans. It pumps hot air after heat exchange, and the temperature fluctuates with the working conditions, at least two or three hundred degrees. When the waste heat recovery system changes the pipeline, four or five hundred degrees is not uncommon. There are still residual acidic gases in the air duct-sulfuric acid mist, sulfurous acid and hydrofluoric acid, which are no worse than the corrosion of the smoke side. What is even more headache is the vibration of the fan body, the impeller speed is high, the dynamic and static balance is slightly worse, and the pipeline will shake. When these three demons joined together, the compensation element connected to the terminal was the Hell level assessment.
So,Metal expansion joint of dilution fanIf you just buy a general-purpose bellows and screw it on, you will basically wait for it to be replaced once every six months. Two days ago, a customer complained to me that their factory changed expansion joints three times a year, and it stopped for half a day once it was disassembled and assembled. The labor loss alone was enough to buy several good ones. You say it was wrong or not?
2. Universal type? High temperature type External pressure type — — Pick up the type of expansion joint suitable for dilution fan
There are all kinds of expansion joints on the market, whatUniversal corrugated expansion joint、High temperature axial expansion joint、External pressure single axial expansion joint…Which one could carry it? Let's talk, talk.
- Universal corrugated expansion joint: The design temperature generally does not exceed 200℃, and the material is mostly 304 or 316L. It is used in the dilution fan outlet? As soon as the temperature exceeds, the fatigue life of the bellows drops like a cliff, and it will crack in half a year. It's not that it can't be used, but it depends on whether God gives face or not.
- High temperature axial expansion joint: Temperature resistance can reach 400-600℃. The core is the addition of temperature-resistant lining and thermal insulation layer, and the design of the guide tube is also critical-to prevent the high-temperature airflow from directly washing the corrugations. The disadvantage is that it is expensive, and the axial compensation is limited, so it can only chew the thermal expansion and contraction of the straight line segment.
- External pressure single axial expansion joint: Structurally, the bellows bears external pressure instead of internal pressure, and has good stability, which is suitable for occasions with large compensation amount and low pressure. The wind pressure of the dilution fan is generally several kPa, and the anti-instability ability of the external pressure type has become an advantage. Moreover, the external pressure type expansion joint is usually equipped with a tie rod and a guide sleeve, which can absorb a little vibration by the way.
If the outlet pipe of the dilution fan is simple in direction and has sufficient space, it is preferredExternal pressure single axial expansion jointWith high-temperature-resistant alloy bellows; If the pipe turns too much or needs to absorb lateral displacement at the same time, then you have toCompound hinge transverse expansion jointOrCurved tube pressure balance expansion joint— — But the latter two are a bit extravagant to use on dilution fans, unless Party A is not short of money.
By the way, there's a niche but useful thing- -Non-metallic expansion joint(fabric fiber expansion joint). This thing is corrosion-resistant, water-absorbing and maintenance-free, but its shortcomings are obvious: it is not resistant to ultra-high temperature (generally capped at 350℃), and its pressure-bearing capacity is weak. If the outlet temperature of the dilution fan is stable below 300℃, it will last longer instead of the metal expansion joint. You think about it.
3. Don't pat your head when selecting: how to calculate temperature, pressure, compensation amount and fatigue life
Selection is not just a DN. List four hard indicators first:Design temperature(Take the highest sustained temperature +20% margin),Design pressure(Fan total pressure ×1.5 safety factor),Axial compensation amount(Calculated according to the thermal expansion of the pipeline, formula: Δ L = α × L × Δ T, α is taken as 0.012 mm/m·℃),Fatigue life(Usually, ≥1000 times are required, but ≥5000 times are recommended when the dilution fan starts and stops frequently).
A section of 3-meter-long dilution fan outlet pipeline with a temperature rise of 250℃ has a calculated thermal elongation Δ L =0.012×3×250=9mm. If the pipe has only one expansion joint between the two fixed brackets, it must absorb 9mm axial displacement. The single-wave compensation amount of the general-purpose expansion joint is only 4-5mm, so two or three waves have to be used, but the wave number is too large and the column is afraid of instability-at this time, the advantages of the external pressure type come out, and its own design is to resist instability with long stroke.
Let's talk about fatigue life. Don't believe the "100,000 times" that the manufacturer blows-it is measured at room temperature, no pressure and no corrosion. In this working condition of dilution fan, the actual fatigue life is often only 1/10 of nominal. How to protect? Choose bellows with high fatigue level (such as multi-layer bellows, where interlayer friction can damp vibration, but the cost increases by 30%), or increase wall thickness. However, the wall thickness increases, the stiffness comes up again, and the compensation force becomes larger, which is another challenge to the design of pipeline support. Tsk, it's all about balance here.
4. Bad installation is equivalent to buying for nothing-installation details and common failure pits in those years
A few years ago, in a project in our factory, the expansion joint was selected correctly, but as a result, the direction of the arrow was not paid attention to when installing-the guide tube was installed backwards. One month after startup, the guide tube was washed away by the high-temperature airflow, and the debris was stuck in the trough of the bellows, which directly wore the crest through the air leakage. Are you saying this is about the expansion joint? It was a man-made disaster.
installMetal expansion joint of dilution fanRemember three dead rules:
- The flow direction arrow must point in that direction of the media flow。 The function of the guide tube is to divert the flow and protect the bellows, which is equivalent to streaking when installed backwards.
- Cold tightening (pre-tension/pre-compression) must be accurate。 Compensate according to the ambient temperature at the time of installation, pre-stretch amount = (thermal elongation/2) ± cold tightness adjustment value. Many construction teams try to save trouble and directly install them to tighten the bolts. As a result, the expansion joint is hard-pulled as soon as the pipeline heats up, and the bellows deforms and fails.
- Tie rod nut must be loosened！ In many scenes, the locking nut of the expansion joint tie rod is tightened, thinking that it can be reinforced-this is a standard error operation. The tie rod nut is for adjustment. After installation, it should be loosened to the design distance to allow the bellows to expand and contract freely. Refer to "How to Adjust the Tie Rod Nut of Expansion Joint" (detailed instructions in the FAQ of this site). If the nut is not loosened, the expansion joint becomes a rigid section.
The outer surface of the bellows is damp and corroded by insulation cotton (stainless steel is afraid of chloride ions, and chloride in insulation cotton forms pitting corrosion under moisture); Failure to replace the guide tube in time after it is worn, resulting in the erosion and thinning of the bellows; When the fan starts, the instantaneous impact pressure exceeds the design value (the fan surge coefficient is not considered in the selection)... These problems are most likely due to the failure to communicate thoroughly with the design institute and the construction team in the early stage.
5. How to do routine maintenance? Three Indigenous Ways to Prolong Life
Equipment is not all right. The dilution fan turns all year round, and the expansion joint has to be ground even if it is made of iron. The following three methods are effective in personal testing:
- Measure wall thickness periodically。 Use an ultrasonic thickness gauge to measure the thinnest part of the bellows crest every time the machine is stopped. Record the data and establish the attenuation curve. When the wall thickness reduction exceeds 70% of the design wall thickness, prepare spare parts in advance, and don't wait for air leakage before emergency repair. It will take less than half an hour, but it will prevent unplanned downtime.
- Flexible bypass with metal hoses。 If there are flanges at both ends of the expansion joint, connect one section in seriesMetal hose(Such as this site'sMetal hoseProduct) as redundancy compensation. The metal hose itself can absorb small displacements and buffer vibrations at the same time, which is equivalent to putting double safety on the expansion joint. The cost is not high, but whoever uses it knows the effect.
- The outer layer is painted with temperature-resistant and corrosion-resistant coating。 For corrosive environments, brush a layer of silicone temperature-resistant paint on the outer surface of the expansion joint (including bellows and end tubes) to insulate moisture and acidic condensate. Be careful not to brush to the moving part of the bellows, otherwise the coating will crack and water will accumulate. This method has low cost and can prolong the life by more than 50%.
Of course, local methods treat the symptoms rather than the root cause. The real once-and-for-all approach is to putMetal expansion joint of dilution fanWhen the core components are treated-the working condition is confirmed in place, the selection calculation is solid, the installation guidance is in place, and the maintenance record is closed. You Pinpin, is that the truth?