Troubleshooting Pneumatic Conveying Systems: Common Issues and Solutions
Pneumatic conveying systems are a flexible and effective way to transfer bulk materials. They offer many benefits, including space efficiency, ease of handling directional changes, and the ability to transport materials without requiring much physical infrastructure. However, despite their apparent simplicity, pneumatic conveying systems can present operational challenges, especially since their performance is often not visible due to the enclosed pipeline design.
While many issues in pneumatic conveying systems are relatively easy to fix, others, if not addressed at the design stage, can become complex and difficult to resolve once the system is in operation. This article focuses on four major operational problems that commonly arise with pneumatic conveying systems.
Pipeline Blockage
Pipeline blockages are one of the most significant problems in pneumatic conveying, causing reduced efficiency and downtime. One of the key factors affecting pneumatic conveying is the pick-up velocity at the material feed point. There is a minimum transport velocity that must be exceeded to prevent material from falling out of suspension and building up in the pipeline, leading to blockages.
In dilute-phase conveying, the minimum velocity is the point at which material no longer remains suspended in the air. In dense-phase conveying, insufficient velocity results in the material de-aerating and accumulating inside the pipe, causing blockages. For coarse granular solids, a lack of drag force can lead to a stoppage due to wall friction.
To ensure successful and reliable conveying, the feed point velocity must exceed the minimum transport velocity, ideally with a 20% margin. This helps account for temporary surges in feed rates that could otherwise lower the velocity and cause blockages.
Common causes of pipeline blockage include:
- Inadequate material feed rate control leading to pressure surges
- Insufficient airflow control reducing air velocity
- Failure to purge the pipeline after a blockage or at the end of a conveying cycle
- Excessive air leakage, especially when using rotary valve feeders
- Changes in the material being conveyed without adjusting for altered flow characteristics
- Increasing pipeline length or adding bends without adjusting feed rates
Systems Not Achieving Rate
Systems failing to meet the required rate are often a result of intermittent blockages or flow issues. The performance of a pneumatic system is defined by the relationship between the air mass flow rate, material mass flow rate, and conveying line pressure drop.
For instance, in low-pressure systems using a rotary valve and blower combination, a surge in material mass flow rate can increase pressure demand and reduce airflow, which may push the system toward the minimum transport velocity, leading to instability and reduced conveying rates.
To prevent this, proper control of both the air and material mass flow rates is essential. If the operating point exceeds the limits of the system, it will not be able to sustain the required throughput.
Ways to improve system performance:
- Reducing pipeline resistance: By shortening pipeline lengths or reducing the number of bends, you can reduce pressure drops and increase the conveying rate.
- Modifying pipeline geometry: Using long-radius bends and larger diameter pipelines can help reduce air pressure losses.
- Upgrading air movers: Larger air movers can help increase the conveying capacity by raising the available pressure drop.
Pipeline Wear and Erosion
Pipeline wear is a significant concern, especially when conveying abrasive materials like sand or alumina. The rate of erosion is directly linked to the impact velocity of the material against the pipe walls. A higher impact velocity leads to greater erosion, following a power-law relationship where doubling the velocity increases erosion by a factor of four.
Reducing the conveying velocity is the most effective way to mitigate erosion, but it must be balanced against the need to maintain the minimum transport velocity. To minimize erosion, consider the following:
- Pipeline geometry: Use long-radius bends and select materials like basalt or ceramics to reduce wear in high-erosion areas.
- Material selection: Choose the right bend materials based on the type of material being conveyed.
Material Attrition and Product Quality
Preserving the integrity of materials during pneumatic conveying is essential, especially in industries like pharmaceuticals where particle size distribution is critical. The impact of material at bends can lead to attrition, causing changes in the size and quality of the product.
The level of particle attrition is largely determined by the velocity of impact. Reducing the impact velocity will generally reduce material breakage. However, there is a threshold where below a certain velocity, particles will not break, but above this velocity, breakage increases proportionally.
To prevent attrition, consider:
- Reducing velocity: Lowering the conveying velocity helps preserve particle integrity while maintaining the minimum transport velocity.
- Using soft curve bends: Long-radius bends with reduced impact angles help minimize particle breakage.
Conclusion
This article has covered some of the most common issues encountered in pneumatic conveying systems, such as pipeline blockages, system performance problems, wear and erosion, and material attrition. While many of these issues can be addressed during the design stage, others require attention to detail during system operation. By understanding these common problems and implementing appropriate solutions, you can significantly improve the reliability and efficiency of your pneumatic conveying system.