Charring or glazing of the inner circumference of the packing rings in a centrifugal pump, is caused by __________.

Correct Answer: Option D — insufficient lubrication of the packing

Packing requires a small, controlled amount of leakage to act as a lubricant and coolant. When this leakage is restricted or absent, friction between the rotating shaft sleeve and the packing generates excessive heat, leading to the charring or glazing of the packing material and potential shaft sleeve scoring.