Abstract: A pipe junction for concentrically mixing fluid flow streams is fabricated entirely of stock dimension plate and pipe materials with a faired main stream flow around the disruption of the injection flow pipe penetration and with all assembly seams, welded or bonded, externally applied. A slot is cut into the main flow pipe to receive a close tolerance fit of a prefabricated fillet assembly with the injection flow pipe. A continuous seam around the main flow pipe slot perimeter seals the main flow channel interior and structurally reinforces the concentric position of injection flow pipe.