Skirtboard length refers to the additional length of steel wall beyond the impact zone. The impact zone is the area of the loading chute if it was extended down to the belt.
Skirtboard should extend in the belt’s direction of travel past the point where the material load has fully settled into the profile that it should maintain for the remainder of its journey on the conveyor.
Sometimes the load never becomes completely stable, and, consequently, skirtboard is required for the entire length of the conveyor. This is most common with very fine materials that are easily made airborne, materials that tend to roll or conveyors with multiple load points. Belt feeders, typically short in length and loaded to nearly the full width of the belt, are commonly skirted for their full length.
The minimum length for the skirtboard should be based on the total air movement and the speed of the belt, using the following guidelines:
A. Metric measurements
If airflow is under 0.5 cubic meters per second, the length of skirtboard is 0.6 meter for every 0.5 meter per second of belt speed. If airflow is greater than 0.5 cubic meters per second, the length of skirtboard is 0.9 meter for every 0.5 meter per second of belt speed.
B. Imperial measurements
If airflow is less than 1000 cubic feet per minute, the length of skirtboard is 2 feet of skirtboard for every 100 feet per minute of belt speed. If airflow is over 1000 cubic feet per minute, the length of skirtboard is 3 feet for every 100 feet per minute of belt speed.
To prevent spillage or damage to the belt, skirtboard should end above an idler rather than between idlers. This in itself may increase the overall length of the skirtboard.
What may provide a more telling answer for skirtboard length is the need for enclosing the dust-suppression and/or dust collection systems. The walls of a dust-control enclosure can effectively serve as skirtboard, with the length necessary for effective dust-control systems generally providing more than what is required for load stabilization.
Penalties for increasing the length of skirtboard are the additional maintenance cost for longer liners and seals, a minimal increase in the cost of the steel for the walls, and a slight increase in the conveyor’s power requirements. The extra power consumption results from the added friction created by the longer steel wall and the additional length of sealing strip installed. This is usually a modest increase that provides long-term benefits that greatly outweigh the minimal up-front cost.
There are also times when conditions such as the incline of the belt, the shape of the conveyed product, or the depth of the material bed require the length of the skirted area to be increased product, or the depth of the material bed require the length of the skirted area to be increased substantially to prevent material spillage.
When in doubt, it is always better to have the skirtboard a little longer than is minimally required by the above equation. An extra 25 percent in additional settling area length is a recommendation that will improve dust control with only a minimal increase in power requirements and expense for steel. When the conveyor incorporates multiple load zones, the calculation should use the total airflow from all loading points to establish the minimum dimensions of the settling zone after the last loading point.
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