The Importance of Conveyor Skirtboards in Ports & Terminals

The importance of skirtboards on conveyor transfer points at ports and terminals should be self-evident.  They prevent dust emissions and product loss from the spillage of commodities that often do not belong to the port or terminal and should remain in the cargo stream. 

Most bulk handling operations have a conveyor belt with vertical loading or a transfer chute, often containing little more than a rock box to slow material on its descent.  When tons of bulk material hit a moving belt, three things happen: fines scatter in a random direction, cargo shifts as it settles to the center of the belt and dust becomes airborne.

Inside a settling zone enclosure, the impact can create turbulent air that seeks the easiest escape from any gap it can find, carrying dust and fines with it.  These gaps generally appear on the sides of the enclosure between the chute wall and the belt.  Skirting systems address the challenge of containing fugitive dust and spillage.

Anatomy of a Sealed Chute

The components of a well-designed and sealed loading zone will likely consist of a combination of components including:

• Enclosed loading chute.
• Dual seal or self-adjusting skirting system.
• Heavy-duty belt support system.
• Closely spaced idlers to avoid belt sag.
• Adjustable skirting to contain fine particles.
• Wear liners to protect the chute wall.
• Dust curtains to control air flow.
• A sealed tail box to protect the tail pulley.
• Exit curtains to prevent release of fugitive dust.

Proper skirt design for adequate sealing is important.  Usually made from natural or SBR rubber (or specialty formulations for specific applications such as underground mining, food, etc.), the skirting extends down the entire length of the transition enclosure and is generally tapered at the bottom to match the trough angle of the belt.  It is intended to maintain a seal on the enclosure and help trap any fine particles and dust that is not contained by the wear liners and chute wall.

Skirting Issues

Without a wear liner to protect the skirt and chute wall from serious damage by bouncing and shifting cargo, both will degrade and fail quickly.  However, the skirtboard is a wear part.  As the skirt edge loses its seal, gaps are created, which allow material to become entrapped in a “pinch point.”  This causes abrasion that gouges or chafes the surface of the belt down its entire length.  Often blamed on pressure from the skirt, extensive study has shown pinch points occur most often when there is inadequate belt support or loading is done before the belt is fully troughed.^[1]  Some of the major issues that result from entrapment are:

• Scalloping occurs when trapped material forms a high-pressure area between the roller and the skirt, causing excessive wear on the sealing system.[Fig.1]
• Grooves are created by trapped debris under the skirt causing friction damage. [Fig.2]
• Regardless of the belt tension provided by the takeup pulley, small amounts of belt sag will occur between the skirt and belt, creating a space for fugitive dust to escape or material to become entrapped. [Fig.3]  This is avoided by using impact cradles or edge support rather than rollers in the loading zone for a more consistent belt plane through the settling zone.  By creating a consistent plane, the straight edge of the skirt is able to retain a tight seal.

Proper Enclosure Configuration

The internal wear liner and external skirt configuration has been the industry standard for decades.  Wear liners are typically welded to the inside of the chute wall with the skirtboard attached on the outside using an adjustable clamp system.  However, the internal design [Fig.4] can allow some material buildup in the gap between the wear liner and the skirtboard, depending on the size of the aggregate.

External wear liner and skirting configurations place both pieces of equipment outside the chute wall. [Fig.5]  Placing the skirt seal and wear liner outside of the chute makes it easier for workers to perform adjustments safely.  The result is a better seal on the belt and less material entrapment, while eliminating the need for confined space entry to replace the wear liner.  Without confined space entry, in most cases, adjustment or replacement can be performed by a single worker.

Self-adjusting skirting maintains a seal automatically as belt path fluctuates due to its design, without the need for adjustment and only periodic inspection.  The use of self-adjusting skirting is also excellent for tight spaces where maintenance may be difficult.  A low-profile skirting assembly should need only 6 in. (152 mm) of clearance for installation and maintenance in small spaces.  For safer and faster maintenance, the clamps securing the skirt may be held in place with linchpins, making it an easy no-tool operation.

Skirting Best Practices

The skirt should be considered a safety device first and foremost, protecting the work environment from fugitive dust and spillage and therefore reducing the time exposed to hazards while cleaning, with the added benefit of increased efficiency.  With this in mind, any manager contemplating a capital investment in a skirting and wear liner system should consider:

• Hiring an outside engineer to design the best system for the application.
• Installing equipment that features external maintenance.
• Utilizing a skirting strip that extends the entire length of the chute to avoid seams.
• Choosing skirting material that is free of fabrics, with a lower abrasion resistance than the belt.
• Installing a self-adjusting system.
• Choosing the option with the least worker exposure to equipment hazards.

Since skirts are wear parts, it is important that they are easily installed, adjusted, maintained and replaced to avoid dust and spillage, mitigate downtime, improve workplace safety and reduce the cost of operation.

Case Study – Grain Terminal In Brazil Reduces Dust

The Port of Santos in São Paulo is the busiest in Latin America, spanning approximately 8 square kilometers (3 square miles).  Operating at Terminal 26, T-Grão manages the import and export of over 4 million st/y (3.6 million mt/y) of malt, wheat, soy and maize.  Unloading vessels using two industrial vacuum units with an offloading capacity of 330 st/h (300 mt/h) each and a loading capacity of 1433 st/h (1,300 mt/h), the company manages 42 concrete silos and 8 massive metal silos, totaling 126,000 st (114,000 mt) of storage capacity.

Transfer points at T-Grão range between 32-50 feet (10-15 m) in height.  The fugitive emissions affected air quality and visibility in the immediate work areas, forcing personnel to wear protective masks when working around any part of the conveyor system.  The dust often traveled beyond the site line.  Before the chute upgrade, a cleaning crew of 45 workers spent ~24 hours per month on dust and spillage cleanup.

After constructing a longer sealed enclosure with the ability to control airflow and give dust extra space to settle, technicians added several other critical components including external dual seal skirtboards.  A heavy-duty impact cradle was installed to protect against impact and friction wear on the belt, which reduces maintenance and prolongs operational life.  Extending from the impact cradle down the length of the settling zone are a series of slider cradles with smooth UHMW polymer “box bars” that prevent belt sag gaps associated with idlers.

When the conveyor was activated, operators immediately observed significant results.  As material moved through the system, particulates remained within the enclosure and either collected in the dust bags or settled back into the cargo flow.  With less carryback on the return side of the belt, dust was drastically reduced in the immediate area around the conveyor system.  After a lengthy observation period, operators report that there has been less downtime for cleanup and maintenance, as well as improved workplace safety.

Conclusion

By installing modern skirtboards that prevent fugitive material from leaving the loading chute, operators can reduce worker exposure to hazards, minimize labor for maintenance and cleanup, improve equipment and belt life and achieve better compliance.  The savings on labor and operating costs alone provide a sound return on investment, but the improvement to workplace safety should be the only motivation needed.