Failure of Equipment /the Hammer Union failed under pressure due to worn out threads, Inadequate Procedure/ the checklist for Lowering and Raising Mast did not address the requirement of inspection and maintenance of the standpipe hammer unions. Inadequate policy on pressure testing / the pre-spud test of the standpipe did not cover working pressure of the standpipe. Lack of due care by rig floor supervisors / the union was cracked loose during rig move to facilitate lowering of mast and kept loose during the move.