How bad is the working condition of pulverized coal pipeline? — — High-speed airflow, particle scour, and temperature fluctuation are many
Friends who have worked in power plants or cement plants know that pulverized coal pipelines are not worry-free at all.
Think about it, what's going down the pipeline? High-speed airflow is wrapped in pulverized coal particles, and the flow rate is 25~35m/s. What about the temperature? The normal temperature jumps back and forth to 300℃, and the temperature difference changes more drastically when starting and stopping. Coupled with the hard components of pulverized coal particles-quartz and feldspar, these things are at the corners and diameters of pipes, just like sandpaper polishing metal, 24 hours a day.
Even more headache, the pipeline system also has to withstand the displacement caused by thermal expansion and contraction. A straight pipe section without compensation, when the temperature rises by 100℃, one meter of pipe can expand 1.2mm. Pipelines tens of meters long, with cumulative displacements of more than ten millimeters or even tens of millimeters, hard connections? No way.
Therefore, choosing an expansion joint in this scenario is by no means just buying a general-purpose corrugated expansion joint. If you dare to make do, it will dare to wear out, crack and leak powder for you within half a year-the consequences? Stopping the furnace for maintenance, losing hundreds of thousands a day.
Why can metal corrugated expansion joints hold up? — — From structural design to material selection, explain its core advantages
Then what exactly does the metal corrugated expansion joint of pulverized coal pipeline rely on to survive in such a harsh environment?
The biggest difference between it and the ordinary general-purpose corrugated expansion joint is that it has an additional guide tube. This guide tube is not an ornament. It is installed on the inner wall of the bellows, directly facing the high-speed pulverized coal gas flow, which is equivalent to wearing a layer of armor on the bellows body. The material of the guide tube is generally made of wear-resistant alloy steel or stainless steel, and its thickness is much larger than the wall thickness of the bellows, which is 3mm, 5mm or even thicker.
Another important role of the guide tube is to direct the airflow. You go to look at the direction of the arrow on the expansion joint, that is the direction of installation of the deflector, which must coincide with the direction of media flow. Got it backwards? The consequence is that the pulverized coal particles directly flush the bellows trough, and it takes less than a few months to perforate. On this point, the question and answer on our site "The specific function of the expansion joint guide tube" makes it very clear: the function of the guide tube is to protect the bellows, while reducing flow resistance and preventing vortex.
Let's talk about the materials. Corrugated pipe itself should be made of corrosion-resistant and fatigue-resistant austenitic stainless steel, such as 304 and 316L, and GH series superalloy should be used under high temperature conditions. However, whether the bellows is wear-resistant or not is not enough, and the surface of the guide tube often has to be hardened or welded with wear-resistant layer. Some manufacturers even coat the inner wall with ceramic coating, and the wear resistance is even higher.
Another key point is the choice of structural form. When the displacement of pulverized coal pipeline is relatively large, it is not enough to rely on axial expansion joints alone-you also need to consider lateral displacement and angular displacement. At this time, the double hinge transverse expansion joint or the straight tube pressure balance expansion joint comes in handy. They absorb multi-dimensional thermal displacement while avoiding pressure thrust acting on the fixed bracket. These products are introduced in detail on our station: corrugated expansion joints for power station industry, metal corrugated expansion joints for cement industry, double hinge transverse expansion joints, straight pipe pressure balance expansion joints, etc., each with its own applicable scenarios.
The easiest pit to step on in model selection: it is not just a general-purpose expansion joint that can be installed on pulverized coal pipeline
Two days ago, I met a customer, who bought several general-purpose corrugated expansion joints on a cheap basis and installed them on pulverized coal pipelines. As a result, in less than half a year, the guide tube was worn out, and the bellows were covered with trachoma, leaking a mess. So what's the problem?
First, the general-purpose expansion joint often has no guide tube design specifically for granular media, or the guide tube is not thick enough, or even directly coil welded with thin plates, which simply can't bear the high-speed pulverized coal erosion.
Second, improper selection of materials. In order to control the cost of general-purpose products, ordinary carbon steel may be used, or the stainless steel model is wrong. Pulverized coal pipeline often contains sulfur, which is corrosive at high temperature. If the material is not good, stress corrosion cracking will soon occur.
Third, fatigue life is ignored. The pulverized coal pipeline starts and stops frequently, and the temperature cycle causes the bellows to expand and contract repeatedly. The design cycle times of ordinary expansion joints may be several thousand times, while the pulverized coal pipelines in power stations or cement plants often require tens of thousands or even more. A poor design, fatigue cracking is a matter of time.
Then how do you choose? There are three cores: First, define the design temperature, pressure, medium (pulverized coal concentration, particle size distribution) and displacement (axial, transverse and angular) of the pipeline; Second, calculate the flow rate to ensure that the wear-resistant layer of the guide tube can match the working conditions; Third, according to the results of pipe stress analysis, the type of expansion joint is determined-whether it is single axial type, compound tie rod type or pressure balance type. Don't pat your head when selecting a model, and you can't save any calculations that should be done.
The doorway of installation and maintenance: how to do the direction of the deflector, the adjustment of the pull rod and the daily inspection to avoid accidents
If you choose the right product, if you go wrong with the installation, you will fall short. Let's say a few of the most common mistakes:
- Install the guide tube in reverse direction— This is the easiest low-level mistake for beginners. The direction of the arrow of the guide tube must coincide with the direction of the medium flow. Install backwards, pulverized coal is directly hit on the bellows, and the wear speed is doubled. Be sure to check the drawings before installation, and don't be superstitious about experience. Sometimes it is possible that the pipeline flow direction is marked opposite to the actual situation.
- Tie rod nut locked or not adjusted in place-The role of the tie rod is to limit the axial displacement of the expansion joint and prevent excessive stretching or compression. During installation, the tie rod nut needs to be adjusted to the predetermined installation length, and it should be checked regularly for loosening during operation. How to adjust the expansion joint tie rod nut? Generally, the locking nut is loosened first, and the adjustment nut is turned until the initial compression or stretching amount required by the design is met, and then locked. For specific values, refer to the manufacturer's installation instructions.
- Ignore fixed bracket and guide bracket— — The fixed brackets on both sides of the expansion joint must be able to withstand the blind plate force, and the guide bracket should ensure that the pipeline expands and contracts along a straight line. If the strength of the fixed bracket is not enough, the whole pipe system will deviate during thermal expansion, the expansion joint will be unevenly stressed, the bellows will be twisted and deformed, and the life will be greatly reduced.
What about daily inspections? Don't wait until you leak powder to remember to check it. It is recommended that you take a look at the appearance of the expansion joint every week: whether there is any abnormal deformation, whether there are cracks on the corrugated surface, whether there are any signs of perforation in the guide tube (you can listen to whether there is any abnormal noise from the outside), and whether the tie rod nut is loose. In addition, when the machine is shut down for maintenance, open the pipeline to clean the dust, and check the wear degree of the inner wall of the guide tube with an endoscope. When the amount of wear is found to exceed half of the design allowance, it is time to prepare spare parts.
Common failure modes and prevention: wear and perforation, powder accumulation in troughs, fatigue cracking, how to avoid it
Wear perforation— This is the number one killer. Long-term high-speed pulverized coal scour, once the guide tube is worn out, then the bellows. Preventive measures: First, improve the wear resistance grade of the guide tube, and adopt a thicker wear-resistant layer or ceramic lining; The second is to control the flow rate in the pipeline (try not to exceed 30m/s); The third is to add anti-wear plates in easy-to-wear parts such as elbows.
Trough powder accumulation— — The pulverized coal particles are fine and easy to deposit in the trough of the bellows. Too much powder accumulates, on the one hand, it affects the expansion and contraction of the expansion joint; on the other hand, it may clump after the accumulated powder is wet, and even cause spontaneous combustion. Solution: Use unreinforced U-shaped bellows as much as possible to reduce the dead angle of wave trough; Or design an expansion joint with a purge port, and regularly pass compressed air to clean up the accumulated powder.
Fatigue cracking— — Repeated expansion and contraction under thermal cycle cause material fatigue, and cracks often start from the trough. Use waveform designs with high fatigue life (such as multi-wave thick-walled structures), materials with better fatigue resistance (such as 316L or Incoloy 800), and strictly control the displacement of expansion joints to not exceed 80% of the design value, leaving a safety margin. In addition, pre-stretching or pre-compression must be in place during installation, otherwise the displacement in actual operation exceeds the design range, and the fatigue life of tens of thousands of times may be scrapped after thousands of times.
In the final analysis, the metal corrugated expansion joint for pulverized coal pipeline is not a one-time sale. Only by selecting the right type, installing it in place and keeping an eye on it can it hold up to the overhaul cycle under bad working conditions. If you want to save trouble and cheap, you will have to pay back twice as much sooner or later.