AN EQUIPMENT CLEANER HELPER WAS STANDING ON A PIPE APPROXIMATELY FOUR (4) INCHES ABOVE GROUND LEVEL WHILE TIGHTENING UP BOLTS ON A HEATER. THE EE SLIPPED OFF OF THE PIPE AND STRUCK HIS LEFT UP PER BODY AGAINST AN ADJACENT VALVE HANDLE. THE EE SUSTAINED A FRACTURED RIB, MAKING THIS A REPORTABLE INCIDENT.