Cutting time varies with the hardness of the coke, the condition of the hydraulic cutting nozzles, and the available cutting water pressure. Of equal significance is the skill and experience of the person cutting the coke.
Initially, a 4-ft diameter hole is hydraulically drilled down the center of the coke drum using the pilot cutting head. Once the hole is completed and coke is cut out of the bottom cone, the stem is retracted and the cutting head installed. Now, starting 6 ft below the top of the coke level in the drum, slices of coke are cut out. The chunks of coke fall through the hole and out through the bottom of the coke drum.
How can this standard procedure be expedited? First, avoid producing very hard or soft coke. The soft coke (i.e., 14% VCM and higher) makes a lot of coke fines, which contaminate the cutting water and bog down the coke dewatering equipment. Hard coke, 10% VCM and lower, takes longer to cut. Second, a cutting water pressure—at the pump discharge—of 120 psi per ft of drum diameter is necessary. For large-diameter drums (24 ft and greater), specially designed nozzles on the cutting head are required to focus the jet of water near the walls of the coke drum. For 26-ft, 0-in. inside diameter (ID) drums the optimum conditions at the cutting head are 950 gpm at 2,900 psig.5
The cutting nozzles, which screw into the pilot and cutting heads, must be kept in first-class condition. This is simply a matter of calipering the nozzle diameters several times a week and changing out the worn nozzles. The rate of wear on these nozzles varies with the concentration of coke fines in the cutting water. A worn nozzle cuts like a dull knife. The jet of water is more diffuse and hence does not cut the coke cleanly. Rather, the coke is ground out instead of being cut. This produces additional fines to increase the wear in the nozzles. The self-defeating cycle is best controlled by adequate clarification of the cutting water.
Improper cutting techniques can also produce excessive coke fines. Interestingly, the same procedures that speed coke cutting also minimize coke fines. Cutting coke quickly makes less fines. By undercutting the coke 5-6 ft, chunks of coke are produced that will not jam the stem or plug the bottom outlet of the coke drum. Many workers cutting coke prefer to undercut the coke by 2—3 ft and avoid the possibility of encountering a chunk of coke big enough to jam the stem or plug the bottom hole. The amount of work to correct either of these difficulties is appreciable. Unfortunately, undercutting coke by 2 ft instead of 5 ft doubles cutting time and coke fines.
How long, then, should it take to cut a drum of coke? Assuming a typical size drum (20-ft diameter and 70-ft tangent), the following time intervals have been observed:
- Drill pilot hole 20 minutes
- Retract stem and change from pilot head to cutting head 10 minutes.
- Cut coke 75 minutes
Cutting a drum of coke in 13/4 hours is no mean trick. It can only be done by an experienced person determined to make every minute count. For example, he must be able to decide when a section of coke is undercut by the sound of water hitting the sides of the drum. He must have a “feel” for the stem at the maximum safe speed; he must take care not to leave sizable chunks of coke adhering to the sides of the drum. Any coke left in a drum goes through a second coke cycle and becomes superhard the next time the drum is opened for decoking.
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