49 C.F.R. § 192.175
(b) Each pipe-type or bottle-type holder must have minimum clearance from other holders in accordance with the following formula:
C = (3D*P*F)/1000) in inches; (C = (3D*P*F*)/6,895) in millimeters
in which:
C = Minimum clearance between pipe containers or bottles in inches (millimeters). D = Outside diameter of pipe containers or bottles in inches (millimeters). P = Maximum allowable operating pressure, psi (kPa) gauge. F = Design factor as set forth in § 192.111 of this part.
[35 FR 13257, Aug. 19, 1970, as amended by Amdt. 192-85, 63 FR 37503, July 13, 1998; Amdt. 192-123, 82 FR 7997, Jan. 23, 2017]