A circumferential back-to-back seal assembly (1,21) with bifurcated flow comprising: (a) an annular seal housing (2, 22) disposed between a pair of compartments (5, 6 or 58, 59); (b) a first annular seal ring (3, 23); (c) a second annular seal ring (4, 24), said first annular seal ring (3, 23) and said second annular seal ring (4, 24) separately disposed within said annular seal housing (2, 22), a gas communicable between said first annular seal ring (3, 23) and said second annular seal ring (4, 24); (d) a rotatable runner (15, 35), said first annular seal ring (3, 23) and said second annular seal ring (4, 24) disposed around said rotatable runner (15, 35); and (e) a plurality of groove structures (17, 37,41) disposed along an outer annular surface (16, 36) of said rotatable runner (15, 35), each said groove structure (17, 37, 41) extends radially inward into said rotatable runner (15, 35) from said outer annular surface (16, 36), said gas communicable onto said groove structures (17, 37, 41), each said groove structure (17, 37, 41) separates said gas so that a first portion of said gas is directed onto said first annular seal ring (3, 23) to form a first thin-film layer between said rotatable runner (15, 35) and said first annular seal ring (3, 23) and a second portion of said gas is directed onto said second annular seal ring (4, 24) to form a second thin-film layer between said rotatable runner (15, 35) and said second annular seal ring (4, 24), each said groove structure (17, 37, 41) has at least two grooves (19, 38, 43, 49); characterised by each said groove (19, 38, 43, 49) includes at least two adjoining steps (62a-62e) wherein each said adjoining step (62a-62e) bounded by said outer annular surface (16, 36) and a base (63a-63e) disposed at a depth (h) with respect to a line, said line intersects said outer annular surface (16, 36) at an inlet end and an outlet end of said groove (19, 38,43, 49), each said base (63a-63e) is non-planar whereby said depth (h) of each said base (63a-63e) represents an average, said depth (h) decreases from at least one said adjoining step (62a-62e) to another said adjoining step (62a-62e) in the direction opposite to rotation, a shoulder disposed between two said adjoining steps (62a-62e), said shoulder being an abrupt change or discontinuity of said depth (h) between two said adjoining steps (62a-62e), said shoulder locally redirects said gas outward toward said first annular seal ring (3, 23) or said second annular seal ring (4, 24) so that flow of said gas is turbulent, each said groove (19, 38, 43, 49) has a length and disposed along said rotatable runner (15. 35) with a runner radius (r r), each said groove (19, 38, 43, 49) outward opening and curved at said outer annular surface (16, 36), each said adjoining step (62a-62e) defined by a distance ratio less than 1 determined by the equation R = r − h r r where r = r r 2 − L 2 2. (a) an annular seal housing (2, 22) disposed between a pair of compartments (5, 6 or 58, 59); (b) a first annular seal ring (3, 23); (c) a second annular seal ring (4, 24), said first annular seal ring (3, 23) and said second annular seal ring (4, 24) separately disposed within said annular seal housing (2, 22), a gas communicable between said first annular seal ring (3, 23) and said second annular seal ring (4, 24); (d) a rotatable runner (15, 35), said first annular seal ring (3, 23) and said second annular seal ring (4, 24) disposed around said rotatable runner (15, 35); and (e) a plurality of groove structures (17, 37,41) disposed along an outer annular surface (16, 36) of said rotatable runner (15, 35), each said groove structure (17, 37, 41) extends radially inward into said rotatable runner (15, 35) from said outer annular surface (16, 36), said gas communicable onto said groove structures (17, 37, 41), each said groove structure (17, 37, 41) separates said gas so that a first portion of said gas is directed onto said first annular seal ring (3, 23) to form a first thin-film layer between said rotatable runner (15, 35) and said first annular seal ring (3, 23) and a second portion of said gas is directed onto said second annular seal ring (4, 24) to form a second thin-film layer between said rotatable runner (15, 35) and said second annular seal ring (4, 24), each said groove structure (17, 37, 41) has at least two grooves (19, 38, 43, 49);