Dual Purpose Cutting Head
This coke-cutting procedure involves the use of a combination coke-cutting/drilling tool made by Pacific Pumps (Dresser Industries), as shown in Figure 2-12. This cutting head is currently in use in a large minority of U.S. cokers. The Worthington dual-purpose cutting head was found to be impractical and is no longer available.
The head permits switching from the pilot bit mode to the cutting mode, using the same head. Switching is a five-minute manual operation.
The tool design has jet nozzles that create a tighter stream of water that maintains its energy at the wall of large-diameter coke drums. The nozzle concentrates the power from the jet pump and provides a sharper hydraulic jet of water to more effectively cut the coke.
One operator reported that the combination head reduced the cutting time (time from completion of pilot drilling to the time the jet pump is stopped) from 2.5 hours using another bit design, to about 1.6 hours with the combination bit.
When cutting soft, fuel-grade coke or shot coke from a large-diameter drum, and utilizing a high-discharge-pressure jet pump, the main impediment to rapid coke cutting is coke cave-in which can bind the drill stem.
The following drilling and cutting procedure is designed to be used with the combination cutting head to help reduce the cave-in problem and increase the speed with which the coke can be removed from the drum:
- Drill the pilot hole while gradually lowering the stem at a rate of approximately 10 ft/min. A maximum drill stem rotational speed of 15-20 rpm is recommended to create a pilot hole diameter of about 2.5 ft at the top of the coke bed. Do not cut coke out of the bottom cone.
- After the pilot hole is completed, stop the drilling unit and oil the pneumatic motor.
- Withdraw the drill stem with the flow of cutting water stopped. Remember that the top 5-10 ft of the coke bed is like sand and can easily collapse, jamming the drill stem. This can be prevented by careful removal of the stem and by avoiding undercutting the soft coke layer. Undercutting can be prevented by carefully enlarging the pilot hole by running the cutting head down again.
- Switch the combination cutting tool from the pilot drilling mode to the cutting mode by applying air pressure to a fitting on the combination tool and observing the position of a 1 -in. steel pin (refer to manufacturer’s instructions).
- Open a 3- or 4-in. water connection located under the drum-relief valve to allow quench water to suppress the evolution of steam from the top head of the drum. This will improve the operator’s visibility when the tool mode is switched.
- Adjust the drill stem motor to rotate at a rate of 8-10 rpm and begin coke cutting.
- As cutting progresses, observe the size of the coke lumps leaving the drum. Control the vertical speed of the drill stem so that the lumps leaving the drum are about fist size. Increase the vertical speed if the lump size increases, and decrease the speed if the lumps get smaller. Controlling the lump size is important to avoid cave-ins that result in bottom-head plugging.
- The first pass of the cutting head should enlarge the pilot hole to 4-5 ft in diameter at the top of the coke bed (with water flowing on both the downstroke and the upstroke).
- Make two more passes with the drill stem turning at 8-10 rpm. After the third pass, the level of coke in the drum will be reduced by about 20 ft. This means that, to save time, the cutting head should not be raised to the original starting point after each pass. Gauging the residual level of coke in the drum can be done by observing the evolution of steam from the top head. If no coke is being cut and no coke is falling through the drum, steam evolution will increase. Experienced operators can also note the sound of water striking the metal sides of the drum, indicating coke has been removed from that level in the drum.
- The fourth and subsequent passes should be made with the drill stem readjusted to rotate at 6-8 rpm. Reducing the head rotation speed keeps the tangential velocity of the water jet at the coke interface at a relatively constant value as the coke wall approaches the drum wall.
- In a typical operation, four to six more passes will be required to fully remove the remaining coke. Each of these remaining passes will take about 10-12 minutes.
- After the last cutting pass, a tough ring of coke, several feet high and several feet above the bottom of the drum, will adhere to the drum wall. This can be removed using the high-energy jets of the combination tool. This generally takes only a few minutes. With other head types, this layer has to be removed by grinding.
The procedure depends on a good source of clarified water supply to the cutting head and frequent inspection of the cutting nozzles for wear.
The procedure will reduce cutting time for large coke drums making primarily fuel-grade coke. Field experience indicates that the procedure can decoke drums containing up to 1,000 tons of green coke in about two hours.
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