The Application and Fault Handling of Roller Screen in Thermal Power Plants
The Roller Screen is a screening equipment used in the coal conveying system of thermal power plants. After screening, the coal smaller than the sieve holes falls onto the equipment below the screen machine, while the coal larger than the sieve holes moves forward on the conveying surface and is sent into the coal crusher. The Roller Screen can prevent the phenomenon of clogged screens and is suitable for operation in the power plant under conditions where the original coal contains a large amount of moisture.
Working Principle
After the coal material enters the screen box through the Roller screen inlet, due to the 15-degree angle between the first 4 screen shafts and the horizontal plane, the coal material begins to move downward at a relatively fast speed under the combined action of its own weight and the rotation of the screen plates, simultaneously undergoing screening. This is the initial coarse screening stage. Most of the materials are screened out and spread evenly across the entire screen surface after this stage. When the materials enter the middle 4 screen shafts and the rear 4 screen shafts, since the arrangement of the screen shafts is nearly horizontal, with the screen shafts and the horizontal plane forming 10 degrees and 5 degrees respectively, the forward speed of the materials slows down. This is the fine screening stage of the screening process. After this stage, materials with a particle size less than 30mm have been screened out, and materials with a particle size greater than 30mm are sent out through the outlet and sent to the crusher for the next stage of crushing.
Roller screen structure and features
The screen plates of the roller screen are in the plum-shaped and are arranged alternately. This design not only facilitates the transportation of coal materials but also reduces the dead angles formed by adjacent screen shafts, preventing jamming.
Each screen shaft is driven independently by a motor reducer. That is, each group of screen shafts has a set of drive device; the reducer is connected to the coupling of the roller screen with an elastic coupling; if any one of the screen shafts fails, the coal can be pushed by the previous screen shaft to continue moving forward, and the equipment can still operate normally. All screen shafts rotate at the same speed, and there is overload capacity. All components operate flexibly without any jamming phenomenon.
Each screen shaft of the roller screen is equipped with both electrical and mechanical dual protection. When the equipment is overloaded, the nylon safety pin of the coupling will be cut, providing mechanical protection for the reducer. In addition, when the screen shaft jams, the increase in motor current beyond the critical value will trigger an alarm signal, providing electrical protection.
The screen shafts are distributed at multiple angles reasonably. At the inlet section, the screen shafts are at a 15-degree angle to the horizontal; at the middle section, the screen shafts are at a 10-degree angle to the horizontal; at the outlet section, the screen shafts are at a 5-degree angle to the horizontal. This enables the coal to enter the roller screen for preliminary coarse screening first and then undergo fine screening, effectively improving the screening efficiency and preventing coal blockage.
Roller screen failure and handling
Roller screen jammed at the shaft
The coal used for combustion is of poor quality. According to statistics, when the equipment in the coal yard takes the bottom layer of the coal pile, the probability of the roller screen’s shafts getting jammed with coal significantly increases. The reason is that the bottom layer of the coal pile in the coal yard contains a large amount of large coal blocks or coal gangue. The coal blocks are prone to stay between the two sieve shafts, resulting in an increase in the probability of coal jamming. After on-site actual observation by the maintenance personnel, the phenomenon before the occurrence of the sieve shaft jamming fault was that a large amount of coal blocks kept rolling back and forth between the two sieve shafts and stopped, showing a stationary state. At this moment, a relatively hard coal block would suddenly get stuck between the two sieve shafts, causing the sieve shaft motor thermocouple to act and the equipment to shut down.
Handling of the shaft jamming problem
Timely cleaning
The maintenance personnel promptly removed the large coal blocks from the bottom layer of the coal pile. Advantages: The solution is simple and convenient. Disadvantages: The content of large coal blocks in the bottom layer of the coal pile is large, and relying solely on manual cleaning of the large coal blocks results in extremely low efficiency.
Installing small sieve plates on the sieve shafts
Installing small sieve plates on the sieve shafts at the end of the roller shaft. After on-site actual observation, the installed small sieve plates can promptly remove the coal blocks that have stayed between the two sieve shafts, preventing the coal blocks from staying at the sieve shafts, thereby fundamentally eliminating the cause of the sieve shaft coal jamming.
Inclined linear design
When designing the Roller screen, an inclined linear design method can be adopted. That is, the centers of the cross-sections of each sieve shaft are connected to form a straight line, which forms a 10-degree angle with the horizontal line. This can effectively prevent the occurrence of the coal jamming phenomenon.
The particle size of the products screened by the roller screen is too large
Screen shaft gap
The gap between the Screen shaft might be set too large, resulting in some larger-sized coal blocks failing to be effectively intercepted and directly falling into the screen under.
Screen shaft speed
If the speed of the Screen shaft is low, the coal blocks will stay on the Screen shaft for an excessively long time, and some coal blocks will fall into the screen under before being fully screened.
Screen plate wear
If the screen plates are severely worn, their screening capacity will decrease, leading to a larger particle size in the screened products.
Targeted solutions
Screen shaft gap
The gap between the Screen shaft can be reduced by adjusting the installation position of the Screen shaft to intercept larger-sized coal blocks and prevent them from falling directly into the screen. Regularly check and adjust the gap between the sieve axes.
Screen shaft speed
The motor reducer can be debugged to appropriately increase the speed of the Screen shaft, so that the coal blocks stay on the Screen shaft for a more reasonable time and ensure that the coal blocks can be fully screened before falling into the screen under.
Screen plate wear
Regularly check the screen plates. When the screen plates are severely worn, replace them with new ones in time. When choosing screen plates, pay attention to the quality and material of the screen plates to ensure they have good wear resistance and screening performance.
Conclusion
In the practical application of roller screens in thermal power plants, there may still be some imperfections. By gaining a deep understanding of its working principle and structural characteristics, as well as implementing effective solutions to common problems such as jammed shafts and excessive particle size of the products screened out, the operational efficiency and stability of the roller screen can be significantly improved.
