Special Working Conditions of Kiln End Preheater-Why Ordinary Expansion Joints Can't Hold
The kiln tail preheater of cement plant, to put it bluntly, is the heat exchange system composed of cyclone and pipeline. What's the temperature? The inlet flue gas temperature is usually between 900°C and 1100°C, and the outlet is also around 300°C. What about the stress? Negative pressure operation, fluctuation range from-5000Pa to-20000Pa. Even more ruthless is the medium-high concentration alkaline dust, and the wind speed is always above 20m/s. Under this working condition, the ordinary carbon steel expansion joint will be wasted in less than three months.
Why Stainless Steel? 316L has insufficient creep resistance at high temperature, and stress corrosion cracking caused by chloride ions is common in cement lines. If you can really carry it, you have to use heat-resistant alloys, such as Incoloy 825 or high-temperature stainless steel, and you have to cooperate with multi-layer corrugated structure. However, the light material is useless, and the design details are deadly.
Three common failure modes: high temperature fatigue, dust wear, corrosion cracking
After doing this for a long time, you will find that there are only a few failure modes. The first, high temperature fatigue. The preheater starts and stops frequently, and the temperature cycle leads to the thermal stress concentration of the bellows, and cracks appear in the peaks and valleys. The second, dust wear. High-speed dusty airflow polishes the bellows surface like sandpaper, especially after the guide tube is not installed or worn, the dust directly washes the corrugations. Third, corrosion cracking. The flue gas in the kiln tail contains SO₂ and Cl⁻¹, which form acidic corrosion when encountering condensed water. Coupled with the action of stress, the crack propagates along the grain boundary.
I've seen a 5000t/d line that wore out the expansion joint bellows in less than half a year. After removing it, the guide tube has already fallen off, and the dust directly cuts a trench in the bellows. You think it's bad or not?
How to choose: from metal corrugated expansion joints in cement industry to high-temperature axial expansion joints
Metal Corrugated Expansion Joints in Cement IndustryAndHigh temperature axial type expansion joint. The former is specially designed for cement line, with multi-layer thin-walled structure of corrugated pipe and heat-resistant alloy, which can withstand temperatures above 900℃. The latter focuses more on axial displacement compensation and is suitable for straight sections of pipelines, usually with external tie rod limits.
How to choose? Calculate the thermal displacement first. The temperature of the preheater pipe varies greatly, and the axial expansion and contraction amount is often tens of millimeters. Then count the pressure and caliber. If the diameter exceeds 1 meter, the stability of the bellows must be considered to avoid columnar instability. In addition, the deflector must be added-it not only deflects the flow, but also protects the bellows from being directly washed by dust. What about the material? The guide tube is generally made of carbon steel or heat-resistant steel, with a thickness of 4-6mm, and the inner wall can be welded with wear-resistant layer.
If the site space is limited and multi-directional compensation is required, considerDouble three-way expansion joint in cement industry— — It can support three directions, saving space. However, note that the duplex structure requires high installation accuracy, and the tie rod adjustment must be in place.
Installation and maintenance: adjustment of pull rod, direction of guide tube, whether screw is removed or not
In this step of installation, the probability of problems is higher than that of model selection. Let's start with the pull rod. The role of the expansion joint tie rod is to limit and protect the bellows from excessive stretching or compression. The adjustment method is very simple: first loosen the locking nut, rotate the adjustment nut according to the amount of pre-tension or pre-compression, and lock it after adjustment. Note that the tie rods at both ends should be adjusted synchronously, otherwise the pole effect will cause the bellows to twist.
Never install the direction of the guide tube backwards. The arrow points to the flow direction of the medium, which must follow the direction of the airflow, otherwise it will not only fail to divert the flow, but also stick the dust and accelerate the wear. A few days ago, a customer asked me: Where is the direction arrow of the deflector? I said you imagine if the arrow points to the bellows and the airflow directly hits the trough, wouldn't it be over?
There is also the problem of whether the screw can be removed or not. The transport screw (also called the mounting screw) is temporarily fixed at the factory to prevent the transport vibration from damaging the bellows. Once installed in place, it must be removed! If it is not disassembled, the screw will lock the bellows when the pipeline is thermally expanded, and the compensation function will fail. The pipeline will be cracked at the slightest, and the preheater bracket will be pulled and deformed at the worst. Remember: All transportation screws and positioning screws shall be removed after installation and debugging, and only the working tie rods shall be retained.
A Case: Transformation of Expansion Joint of Preheater in a 5000t/d Production Line
Last year, I helped to transform a 5000t/d production line. They originally used an ordinary axial expansion joint, made of C276 material, and as a result, there were three leaks in less than a year. At the scene, all the guide tubes were worn out, and penetrating cracks appeared at the trough of the bellows. The reasons for the analysis are: the medium temperature is on the high side (measured at 950℃), the corrosion resistance of C276 decreases at high temperature, and the failure is advanced with dust washing.
Replace withHigh temperature axial expansion joint, the material of the corrugated pipe is upgraded to SUS 316L, and the design temperature is 1050℃. The guide tube is thickened to 6mm, and the inner wall is welded with wear-resistant layer. The tie rod is made of high-strength alloy steel, and the pre-stretching amount is calculated according to the actual installation temperature. During installation, the transport screw was removed strictly according to the regulations, and the pull rod was adjusted three times before it reached the design displacement. After the transformation, there has been no leakage in operation for 20 months, and Party A called it long ago.
What does this case say? The metal expansion joint of the kiln tail preheater in cement plant is not something that can be dealt with by just buying a general part. Only when the working conditions are thoroughly understood, and every step of type selection, installation and maintenance is in place, can the long-term stable operation of the production line be ensured.