An employee was preparing a well for chemical preservation. The employee attached a 2x6-inch metal nipple and a 2-inch metal elbow to the casing valve. The employee then opened the casing valve to bleed down the casing pressure and stepped back to allow the pressure to bleed off. The employee started to reach down to partially close the valve, but before they could make contact with the valve, the nipple and elbow disconnected from the casing valve and struck the employee on the right leg, resulting in a fractured tibia and fibula.