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Coke and cooled residuum have heat insulating properties that are very important to understand. A layer of coke and cooled residuum surrounding a hot pool of heater charge inside of a coke drum will have the same inch for inch insulating R factor as the same thickness of mineral wool insulation. Thus, a layer of coke or solidified residuum 3 4s thick on top of a coke bed will have the same insulating affect as a layer of mineral wool the same thickness. A partially filled coke drum that has not been properly steamed and water cooled can have a very hot center, 650+F, which can be encased by insulating layers of cooled residuum and coke. This hot isolated center will be filled with partially coked residuum and a full range of thermally cracked products at very high temperatures. This isolated hot center will not be detectable by normal coke drum temperature and pressure indicators due to the thick insulating coke and tar surrounding it. The coke and tar insulation essentially slows down or stops the flow of hot vapors coming off the coke bed, resulting in low pressure and temperature readings. A hot center, which is above auto-ignition temperature of 450F, represents a serious danger to personnel since it cannot be detected by normal instrumentation. A drum which is un-headed in this condition will result in a very serious fire and possible explosion. For the protection of all personnel, Process Technicians should seek the assistance of Management and Technical support when dealing with this type of coke drum.
It is very important to note that even a normally cooled coke drum can and will have hot spots throughout the coke bed that are detectable during hydraulic decoking. As the drilling water comes in contact with the hot spot, a large eruption of steam and vapors are emitted from out of the top and bottom heads of the drum. In some cases coke erupts from out of the top of the coke drum and onto the cutting deck. Further, some hot spots encountered during the drum cut result in coke falling out of the coke drum into the coke pad glowing and on-fire but in most cases this does not represent a hazard since the cutting water rapidly cools the extremely hot coke. Hot spots in a normally cooled coke drum are caused by the water channeling through the coke bed during coke drum cooling. Cooling water takes the path of least resistance, which in the case of a coke bed are the paths formed by the channels produced in the bed during charging and steaming of the drum. If the water is put into the coke drum too rapidly during the first hours of cooling, then the water will have a tendency to follow the path of the channels and end up on top of the coke bed rather than contacting and soaking into the bed. Thus, it is very important to follow the prescribed rates of water over specific time periods listed in the normal coke drum operating procedures during drum cooling in order to get proper contacting and soaking of the bed to avoid channeling. Delayed Coking Units that produce shot coke often have serious problems with hot spots and coke eruptions since the shot coke is more impervious to water than sponge coke. Additionally, there is also more of a tendency for water to channel through a shot coke bed, and for the shot coke drum inlet to plug during cooling and draining. The DCU Process Technicians need timely communication about any unusual problems or coke properties that are encountered during the drum cut as required, so that the next drum cycle may be adjusted to avoid similar circumstances.
in addition to what my dear friend lucky indicates there are other points to get you an idea of ​​how hot the drum is; like watching the rate of disappearance (water vaporization) of water from the level in the level detectors once you leave the Drum soaking, the temperature indicators of the fo skirt (skin) can give you a vision of channeling in the bottom of the drum
There are also units that once they begin to drain the drum they stop draining water and inject wated again for half an hour and see if the drum pressure increases, which is indicative of hot spot
and finally, and because not always will be totally cool drum, the operator must cut the pilot hole to a relatively low speed and see if suddenly out of the drum vapors , in this point the operator should immediately cut up the tool a few feet above the hot spot and leave it there a few minutes letting water cutting gradually help to cool this area. A good cutting speed is between 2 to 3 feet per minute (cut pilot hole should be about 30 minutes (depends on the height of the bed coke) No 15 or 10 minutes, also you can test several rate of cooling water in your cooling ramp, Again in my experience lower rate is preferable in order to avoid channeling and also decrease the mechanical stress at the bottom of the drum

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