A hard black organic substance that forms when petroleum products and most vegetable, or mineral oils are over heated. During the heating process a varnish like solid forms on the metal parts. These solids will intefer with seal sliding components and o-ring sealing surfaces.
Restricting the free movement of a mechanical seal will cause the lapped faces tp open and the seal to leak prematurely. You’ll find the black coke attached to all of the seal components, especially the carbon face where the stuffing box temperature is the hottest..
Oxygen doesn’t 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 oil below its coking temperature.
Some pump bearing cases have installed coolers that can retard the formation of coke in the bearing oil.