Skirting disaster with skirtboards

Belt conveyors without an enclosure at the transfer point loading zone may still exist in some cement operations but are becoming a thing of the past due to dust violations and excessive spillage. Whether the transfer chute is a dead drop, rock box design, or sloped design, dust and spillage from dry bulk material will still exist and must be controlled. Spillage can limit access to a system for maintenance, foul rolling components, add to labour costs for clean-up, and reduce workplace safety.

A skirtboard on either side of the conveyor belt that is sealed with a cover certainly helps but operators have found that the air turbulence from loading still causes fugitive dust to escape if a wear liner and skirting is not applied. Moreover, there are nuanced details that conveyor engineers should consider when designing a conveyor transfer point.

While controlling belt wear and the release of fugitive materials, all components of a skirtboard system must work together to contain the load as it forms a stable profile in the centre of the belt. Several skirtboard system design approaches can be used based on industry historical practice and the application. This article covers some of the common approaches bulk handlers use to mitigate dust and spillage and ensure a safe and compliant workplace with a lower cost of operation.

Configuring the skirtboard

By far the most common configuration is the vertical skirtboard. It is the easiest to fabricate and is a common detail for most engineering design firms (Figure 2). The height of the skirtboard is based on the sealing system components and is commonly at least 300 mm high. The double wall skirtboard is sometimes used with dust extraction for very fine free flowing materials.

The perpendicular and angled configurations are used in some industries. Angled skirtboards are designed to allow the load to centre. Perpendicular skirtboards relieve side pressure on the skirtboard seal. In theory, the perpendicular arrangement should allow for light seal contact but in reality, the angle of attack of the seal is not nearly as important as having a running flat belt surface for the seal and liner system to function best.

Skirt sealing configurations

A vertical seal with a rubber or elastomeric material is the most common sealing system (Figure 3). The seal is held in place with a series of clamps which can be loosened to adjust the seal against the belt. The main drawback to the vertical seal is that an undulating or vibrating belt can break the sealing contact unless the belt is supported. The lay-in and lay-out seals are self-adjusting depending on the elastic nature of the sealing material. The double skirting configuration is the most effective in retaining a belt seal. Even if the belt profile fluctuates, the secondary seal rides softly on the belt, retaining the seal. Any material that gets in between the double seal strips is non-abrasive, being carried by the belt, and rolls back to the centre once the skirtboard ends.

It is a common belief that the seal material must be softer than the belt but the real property of concern is that the abrasion resistance of the seal should be less than the belt top cover. The seal should be considered sacrificial and designed for easy adjustment and replacement without the need for excessive sealing pressure. Over adjustment can cause excessive friction heat of the seal, leading to heat damage on the belt, as well as premature wear of skirting. In extreme cases, the heat generated can cause the seal to stick to the belt during shut down, which can prevent startup.

The sealing pressure should be light with the skirtboard or the liners designed to reduce pressure on the seal. There is not much published on seal pressure values. For the self-adjusting seals, use 15 kPa contact pressure. CEMA proposes added belt tension of about 4 kN/m per side without considering the seal thickness.

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