Common Faults of Screw Ship Unloader

Advantages and Disadvantages of Screw Ship Unloaders

Screw ship unloaders are increasingly adopted in modern logistics, particularly in coal unloading at ports, due to their environmental benefits and operational efficiency. However, they come with distinct advantages and disadvantages, especially when compared to grab-type ship unloaders in port. This article explores their strengths and limitations in detail, providing insights into their practical applications and maintenance challenges.

Advantages of Screw Ship Unloaders

1. Effective Control of Coal Spillage

One of the standout benefits of screw ship unloaders is their ability to control coal spillage effectively. This is especially beneficial when unloading lignite, as it results in a cleaner dock surface and reduces the need for extensive post-operation cleaning.

2. Stable and Flexible Operation

Screw unloaders provide stable coal unloading with easy control over initial operations. Their flexibility allows operators to adjust settings according to specific unloading needs, ensuring smooth and consistent performance throughout the process.

Disadvantages of Screw Ship Unloaders

1. Mechanical Complexity and Higher Fault Rates

The intricate mechanical design of screw ship unloaders leads to a higher fault rate compared to grab-type unloaders. Maintenance issues often arise from the horizontal and vertical screw lubrication systems, increasing downtime and repair costs.

2. Lower Adaptability to Coal Types

Screw ship unloaders struggle with coal that has poor flow properties or high external moisture. Sticky or wet coal can lead to blockages in the screws, reducing efficiency and causing operational delays. However, they perform comparably to grab-type unloaders when handling high-density coal like bituminous coal.

3. Prolonged Cleaning Time

Cleaning the holds of ships using screw unloaders is a time-intensive process. For example, cleaning a single hold can take 6–7 hours, and a 70,000-ton coal ship requires 24–28 hours. This extended cleaning time is a significant drawback for high-capacity operations.

screw ship unloader

Common Issues with Screw Ship Unloaders and Solutions

1. Severe Wear on Horizontal and Vertical Screws

Causes:

The operating principle of screw ship unloaders involves rotating impellers to convey coal, which leads to wear and abrasion on both the impellers and cylinder walls.

Wear Period: Horizontal and vertical screws generally require replacement every two years or after handling approximately 4 million tons of coal. Delays in replacement result in efficiency losses of 50 tons/hour after one year and up to 200 tons/hour after two years.
Maintenance: Indicators of wear include blackened screws, sand holes, or coal leakage. Timely replacement and maintenance (which takes 3–4 days) resolve these issues and restore efficiency.

2. Coal Blockage in Horizontal and Vertical Screws

Causes:

Blockages occur due to sticky coal or foreign objects. When coal absorbs moisture (e.g., from rain or seepage), its flowability decreases, causing accumulation in the fixed gap between the impeller and the cylinder wall.

Solutions:

Mild Cases: Reversing the screws at a low speed often clears the blockage.
Severe Cases: For extreme blockages, cleaning with water or manually removing foreign objects through inspection ports may be necessary, though this process is time-consuming.

3. Excessive Vibration of Horizontal Screws

Causes:

Handling coal with poor flowability can lead to uneven adhesion of coal on the impeller or cylinder wall, causing imbalances and vibrations during operation.

Solutions:

Reduce the screw speed (e.g., shifting to a lower gear).
Limit the depth of the feeder head in the coal pile to minimize stress.
Stop the operation and clean the adhered coal before restarting if severe vibrations persist.

4. Low Efficiency During Cleaning Stages

Screw unloaders have inherent limitations during the final cleaning stages of a hold:

Swing Arm Limitations: Vertical screws typically have a swing arm angle of no more than 10°, making it difficult to reach corners.
Slow Adjustments: Vertical screws have slow upward and downward movement speeds, adding up to 1–1.5 hours per operation.
Thin Coal Layers: The feeder head struggles to capture thin coal layers at the bottom, extending cleaning times further.

Balancing Advantages and Disadvantages

While the disadvantages of screw ship unloaders, such as wear, blockages, and cleaning inefficiencies, are notable, their environmental benefits and operational stability make them a valuable asset in many logistics scenarios. These systems are particularly effective in early unloading stages and when handling high-density coal types.

To mitigate their shortcomings, many large coal-fired power plants adopt a hybrid approach, using both screw and grab-type unloaders. This combination ensures flexibility, enhances efficiency, and balances environmental and operational needs.

Screw ship unloaders have carved out a niche in coal logistics due to their ability to control spillage and provide stable unloading operations. However, their mechanical complexity, limited adaptability to certain coal types, and extended cleaning times remain challenges. By addressing these drawbacks through regular maintenance and complementary systems like grab-type unloaders, industries can optimize operations and meet environmental standards.