An employee was dewatering water base mud from inside the dewatering unit. They were standing at the transfer tank monitoring the discharged fluid from the centrifuge to make sure it was free of solids. The employee rotated the 6" pipe and 90-degree elbow so that the fluid would dump into the other side of the transfer tank and reached with his right hand to pull on the handle attached to the elbow. When the employee pulled on the handle to rotate the elbow, the elbow fell off and landed on his left hand which was resting on the divider wall below. The employee suffered blunt force trauma to the left hand that caused a laceration to the ring fingertip requiring stitches and an amputation to the middle fingertip.