A 13 3/8” casing string had been run to the setting depth. The last few joints were washed down using a third party circulating head. The final joint to be run was a 15 foot pup joint, however, the Company Man instructed the crew to lay down the pup joint and pick up a full joint. The circulating head was re-installed onto the full joint by the crew on the catwalk. The joint was then made up and the hose connected. As soon as the pumps were brought on line to 30 SPM, the circulating head blew off. The web sling that was being used to secure the circulating head came unfastened and allowed the head to fall approximately 15m to the rig floor. No one was injured.