EE WAS WORKING TO CHANGE OUT A DEFECTIVE 6"" BLOCK VALVE FOR #74 PRECIPITATOR. THE OLD VALVE WAS REMOVED AND PLACED ON TOP OF A SCAFFOLD. THE EE ROLLED THE VALVE OFF OF THE SCAFFOLD. THE VALVE STRUCK A DAVIT. THE DAVIT FLEW UP AND STRUCK HISLEFT ELBOW, RESULTING IN A LACERATION REQUIRING FOUR STITCHES. NO LOST TIME OR WORK RESTRICTIONS