A port interface for a pneumatic distribution system includes a first flowpassage configured to receive fluidized particulate material from an outletport of aheader and to output the fluidized particulate material to an outlet line. Theportinterface also includes a second flow passage configured to receive abalancingairflow and to output the balancing airflow to an external environment. Inaddition,the port interface includes a knife valve having a blade with an opening. Theblade isconfigured to move between a first position in which the opening issubstantiallyaligned with the first flow passage and a solid portion of the blade issubstantiallyaligned with the second flow passage, and a second position in which theopening issubstantially aligned with the second flow passage and the solid portion ofthe blade issubstantially aligned with the first flow passage.
