Dust Control Handbook for Industrial Minerals Mining and Processing

Overall PerforationsThe use of overall perforations is a critical factor to consider when evaluating the effectivenessof bag performance. Each ply of paper is individually perforated during the manufacturingprocess, creating staggered perforations throughout the bag. The perforations (pinholes) do notgo directly through all layers (2, 3, or 4 plies/layers) and are not aligned with each other. Thegoal is to allow the fluidizing air to work its way out of the bag while still containing theproduct/material. Perforations allow the bag to depressurize, minimizing the possibility of itrupturing during bag filling. Since the perforations are staggered for each of the differentplies/layers, it is uncommon for the product being bagged to escape through the perforations,however, this can sometimes occur for very fine product sizes—i.e., 325 mesh (0.045 mm) andfiner. As previously mentioned, the bag strength can be lessened by up to 20 percent when usingperforations. Standard perforation sizes are 1/8, 3/32, and 1/16 inches, and patterns range from3/4 x 3/4 inch centers up to 2 x 3 inch centers (Figure 6.3).Figure 6.3. Undervalve bag perforations (left) and overall perforations (right).Undervalve PerforationsUnlike overall perforations, undervalve perforations are created after all plies are positioned intothe finished bag stage. They are punched through all 2, 3, or 4 plies/layers at the same time andare aligned. Since the fluidizing air is most likely to escape out of the bag valve around the fillnozzle, (product blowback), having perforations as close as possible to the valve area is mostadvantageous/effective. In addition, this location also minimizes the amount of bagcontamination as the excess air and product escape. The perforation pattern can range from 1/4 x1/4 inch centers up to 1 x 1 inch centers. Perforation sizes are the same as for the overallperforations, being: 1/8, 3/32, or 1/16 inches. Using an exhaust hood with a local exhaustventilation (LEV) system is most beneficial in this application because of the likelihood ofproduct escaping from the bag during the filling process. Figure 6.4 shows an example ofundervalve perforations and how they depressurize the bag to keep it from rupturing orexploding during bag filling.Bagging 161