The Shipment of Heavy-duty Roller Screen for Vietnam Customers
In mid-June, a Vietnamese customer purchased a Heavy-duty Roller Screen from our company. Now the shipment has been arranged.
This roller screen is used in the raw coal screening project of thermal power plants. The vast majority of thermal power plants use coal as fuel. The coal entering the plant is raw coal containing large pieces and various impurities, while the boilers burn coal particles or coal powder. The coal mill of the boiler pulverizing system requires that the particle size of raw coal should not exceed 50mm. However, in actual raw coal, there are often large-sized coal blocks and gangue blocks. Therefore, raw coal must be crushed by mineral sizer before entering the coal mill.
In order to enhance the crushing efficiency of mineral sizer and save energy consumption, the customer decided to purchase a roller screen to screen the raw coal before crushing. The coal that meets the required particle size will directly enter the belt conveyor through the roller screen. Larger-sized coal lumps are sent to the mineral sizer for crushing.
The customer got in touch with us in June 2023, discussing the purchase of a mineral sizer. It was officially put into operation in early 2024, and the customer was very satisfied with the crushing effect. Later, considering improving the crushing efficiency of the mineral sizer and saving energy consumption at the same time, the customer consulted us and decided to add a pre-screening procedure before crushing. After two months of technical communication, the customer placed an order for a Heavy-duty Roller Screen. The production has been completed and the goods have been dispatched smoothly.
The Roller screen is a sorting equipment composed of multiple parallel arranged roller shafts. Screen discs are alternately installed on the roller shafts and rotate through sprockets or gears. The rotation direction is consistent with the flow direction of the materials. Now let’s share some knowledge about roller screen with you all.
The composition and working principle of Roller screen
Roller screen is mainly composed of the driving mechanism, screen shaft, frame, bypass system and protection system. Among them, the screen shaft is a key part, composed of a long shaft connected in series with multiple plum blossom-shaped screen plates. When multiple screen shafts and screen plates are interlaced and the axes are arranged parallel, the screen surface is formed. During operation, each shaft rotates in the same direction under the drive of the motor. When raw coal enters through the feeding port and is spread on the screen surface, it moves forward in parallel under the push of the screen shaft.
As the screen surface is composed of multiple screen rollers, its forward trajectory is wavy and undulating up and down. The raw coal moves almost in a jumping manner up and down, and at the same time, the toothed screen plates stirs it. Therefore, under the combined effect of multiple movements such as parallel movement, up and down jumping, and continuous stirring, The coal flow moves forward in a rolling manner. During this process, small pieces of raw coal fall from the screen holes under the effect of gravity, while large pieces are discharged from the screen, achieving particle size classification and screening.
Fault analysis
The Roller screen is arranged in the coal conveying system of the thermal power plant. The screening operation is not carried out independently but starts simultaneously with the entire conveying line of the belt conveyor, conducting “online screening” during the raw coal conveying process. Raw coal is continuously poured into the feeding port. The roller screen must discharge the raw coal at the same speed and in the same amount while screening. Once problems such as jamming, blockage and stagnation occur, the roller screen will be filled immediately, which will affect the operating efficiency of the coal conveying system, cause the equipment to idle and bring economic losses.
Based on the above mechanism and application methods, the failure causes of roller screen can be classified as follows:
The Roller screen shaft is mechanically jammed
The raw coal entering the roller screen contains large pieces of gangue, wood blocks, and even iron blocks and iron bars, which often get stuck at the screen holes, causing the adjacent two screen shafts to slow down until they get stuck, resulting in local coal flow obstruction. For the roller screen driven centrally, the entire screen surface is directly stuck, leading to coal blockage faults.
The coal flow spreading effect is not good
The stirring effect of the coal flow is not good, and the screen holes are stuck and clogged. Especially during the rainy summer season, raw coal contains a large amount of moisture and mud, and has strong viscosity. The screen shaft is in long-term contact with it. The screen shaft, which is covered with raw coal and screen plates, becomes a smooth “roller”, losing the stirring force on the coal flow and reducing the driving force for the coal flow to move forward. The screen holes also become narrow, greatly reducing the screening efficiency and conveying capacity. This is precisely the key reason for the decline in roller screen’s efficiency and the coal blockage.
Handling methods and recommended measures
The transmission device is arranged one-to-one on each shaft and adopts the transmission mode of reducer – motor assembly. Each screen shaft is equipped with mechanical overload protection device and overcurrent protection. It will not be burned out within 100 seconds of locked rotor and has signal output. Moreover, if any group of screen shafts fails, the coal can be pushed by the previous screen shaft to pass over the faulty screen shaft and continue to move forward, and the roller screen can operate normally.
The rotational speed of the screen shaft is higher than that of the belt. Based on the operating speed of the coal conveying belt conveyor, the linear speed of the screen shaft rotation can be calculated. The linear velocity of the screen shaft must be higher than that of the belt to ensure that the coal flow does not get stuck.
The inclined arrangement of the screen shaft is conducive to longitudinal material distribution. The layout of the screen shaft, which is lower at the front and higher at the back, can quickly spread the coal flow by taking advantage of gravity and downward impact force, and is also conducive to the forward movement and rolling of the coal flow.
Install a screen shaft cleaner. Under all the screen shafts, cleaning devices are installed. The cleaning materials are made of No. 45 steel, with surface quenching treatment and a knife-shaped structure. They are embedded in the gaps between the screen plates and can remove the coal adhering to the screen shafts and the entangled debris when the moisture content of the materials is too high.
Design a reversing function. For the single-axis independently driven roller screen, install a speed sensor at the end of the shaft. If the screen shaft does not get stuck and stops rotating, immediately rotate it in the opposite direction for several turns. If we return to the main point, it can usually be automatically removed without causing coal blockage.
Set functions such as coal blockage alarm in the coal drop cylinder and roller screen shutdown alarm to prompt the operating personnel to eliminate faults in time.
