Abstract: A two-piece replaceable union nut for repairs on shipboard high pressure air systems. The replacement union nut is employed when an existing union nut has failed (i.e. cracked or broken) or has been identified as sub-standard due to material type or some other shortcoming, but the piping, tailpiece and thread piece are still in satisfactory condition. The two pieces fit around the pipe and mate using machined notches to form a complete union nut that is constrained in all directions and dimensionally equivalent to a standard union nut. The two-piece union nut facilitates replacement of a union nut in service without the need for cutting piping and reattaching the accompanying tailpiece, and testing the system as conventionally required.