Working principle and application of mill discharge overflow lock air valve
October 02 03:01:33, 2025
Air leakage is a common and critical issue in cement mill systems. It not only increases the size requirements of auxiliary equipment, leading to higher investment and operational costs, but also affects the system's overall performance by reducing output and increasing energy consumption. While some air leakage is inevitable due to system design needs (such as air control devices), most of it results from poor sealing at the device interfaces or improper installation and usage. In grinding systems that involve mills, the effectiveness of the grinding tailgate directly impacts the ventilation and grinding efficiency. Therefore, selecting an efficient air lock device is crucial for addressing air leakage.
The Nanjing Cement Industrial Design and Research Institute developed an overflow air lock valve that uses advanced principles and a simple structure, making it one of the more ideal solutions for air leakage control. It has been widely applied in many cement plants with generally good results. However, some manufacturers have reported unsatisfactory performance, which we found was mainly due to users' lack of understanding of the device’s principle and structure, along with improper installation and operation.
To help readers better understand and apply this device, the following sections provide an overview of its structure, working principle, key parameters, and important considerations during use.
1. **Main Structure and Function**
The overflow air lock valve consists of several main components: the valve body, maintenance door, adjustment plate, and inflatable box. The valve body is divided into two chambers (Chamber I and Chamber II) by a fixed partition, forming a U-shaped fluidized bed to achieve effective air locking. The maintenance door allows access for adjusting the plate and checking the air lock effect. The adjustment plate controls flow rate and facilitates the removal of coarse particles like iron slag. The inflatable box includes a breathable layer and perforated plates, which distribute air evenly from a Roots blower to fluidize the material in the upper part of the bed. To ensure proper discharge of coarse materials, the inflatable box should be installed horizontally or at an angle of no less than 30 degrees.
2. **Working Principle**
The overflow air lock valve operates based on fluidization technology. During the process, the bed can exist in three states: fixed bed, fluidized bed, and continuous fluidized bed. In the fluidized bed state, the void fraction increases as the fluid passes through, but the flow velocity and resistance remain relatively constant. According to fluidization characteristics, the condition for stable material flow must satisfy the equation:
F + (Lâ‚ - Lâ‚‚)·Ï·g + (Pâ‚ - Pâ‚‚) - V²·ξ·Ï/2 = 0
Where F is the impact force of the material per unit area, Lâ‚ and Lâ‚‚ are the effective material heights in each chamber, Ï is the bulk density, Pâ‚ and Pâ‚‚ are static pressures, V is the flow velocity, ξ is the flow resistance coefficient, and g is gravitational acceleration.
The pressure drop across the fluidized bed can be calculated as:
ΔP = L·(1-ε)·Ï_p·g = L·Ï·g
This shows that the pressure drop is proportional to the weight of the bed per unit area, independent of the air velocity. The overflow air lock valve utilizes this property to create a pressure barrier that prevents air leakage while allowing material to pass through.
3. **Key Operating Parameters**
Several parameters determine the performance of the overflow air lock valve:
- **Wind Pressure**: The Roots blower supplies air to overcome three resistances: the effective fluidized bed resistance, the material layer resistance below the barriers, and the resistance of the breathable layer and intake pipe. Practical experience shows that the wind pressure required is typically between 20–30 kPa.
- **Air Volume**: The appropriate airflow is essential for maintaining the fluidized bed. The air volume per ton of material is usually in the range of 0.35–0.40 m³.
- **Discharge Capacity**: This depends on the flow velocity and cross-sectional area. Based on experimental data, the discharge capacity is approximately 400–800 t/h/m².
4. **Important Considerations During Use**
During operation, users should pay attention to the following:
- Before installation, check the breathable layer for damage or moisture exposure. If damaged, replace it immediately to prevent blockage.
- Ensure the connecting pipes between the Roots blower and the valve are properly supported to avoid vibration.
- Monitor the material flow initially and adjust the cross-section using the adjustment plate to control the flow rate.
- If the height difference between Lâ‚ and Lâ‚‚ is too small, the air may penetrate the material bed, causing ineffective sealing.
- Check the pressure at the maintenance door; if slightly under-pressurized, the air lock is functioning well.
- When adjusting the plate on the outlet side, ensure it doesn’t fully close the lower passage to prevent blockage from iron slag. However, it shouldn’t be too open to avoid short-circuiting.
By following these guidelines, users can maximize the efficiency and reliability of the overflow air lock valve, ensuring long-term performance in cement milling systems.
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