A hard black organic substance that forms when petroleum products and most vegetable or mineral oils are over heated.
Coke interferes with the mechanical seal movement and will eventually open the lapped faces causing the seal to leak. You will see the black coke attached to the seal components, especially the carbon face where the stuffing box temperature is the hottest..
Although widely believed to be true, oxygen does not have to be present for coke to form. The formation of coke is a function of temperature and time, with the coking rate doubling with each increase in temperature of 10°C or 18°F.
The only way to prevent the formation of coke in the pump stuffing box area and around the sliding seal components, is to cool the fluid down below its coking temperature.
Some pump bearing cases have installed coolers that can retard the formation of coke in the bearing oil.