Steam-Air Tube Decoking Special Equipment Required
Much of the decoking equipment is not used between decokings; it should be carefully checked before each use.
• A temporary manifold may be used for measurement, control, and mixing of air and steam to individual heater passes. Alternatively, if decoking flow will be only in the normal flow direction, the regular inlet manifold may be used together with control of air to individual passes. A temporary manifold attached to the heater outlet tube flanges conducts effluent to the knockout drum; the heater’s regular outlet manifold is not used. Decoking manifolds are constructed of carbon steel. The effluent manifold will see temperatures approaching 1200°F; however, it is used only for a few days once every several years and it does not hold high pressure.
• The steam mass velocity for a pass being spalled is 16-18 lb/sec/ft2 of internal tube cross section. For a pass being burned or being protected from overheating, the figure is 5-6 lb/sec/ft2. The air mass velocity for the burn step may vary from 0.5 lb/sec/ft2 at the beginning of the burn up to 5 lb/sec/ft2 at the end. A multiple pass heater may be decoked one, two, or even four passes at a time. The decoking manifold must simultaneously supply the needs of all coils in the heater.
• Most Chevron heaters are decoked with flow only in the normal flow direction through the passes; however, it is recommended that the manifolds provide for reversing the flow if necessary. See Figure 900-1. When spalling, higher steam velocity near the end of a pass (lower pressure) is more effective in coke removal; reversing the flow produces more complete spalling. (A heater with different tube sizes in each pass should be spalled only in the direction of smaller tubes to larger tubes; the concern is that a coke fragment from a large diameter tube could plug a smaller tube completely.) In the burn step, it may be difficult to get the first couple tubes of a pass hot enough; reversing the flow permits complete decoking of tubes near the normal inlet. If the heater has a process convection coil connected to a radiant coil and if tubes are to be burned in the reverse flow direction, then the engineer should check the metallurgy of the convection tubes and any extended surfaces. Tube metal temperatures reach 1200°F during the burn step. It might be necessary to disconnect the convection coils at the crossover.
• Effluent is quenched by injection of utility water through the nozzle shown in Figure 900-2, Detail A. The quench water design rate is based on the highest rate of steam to be used during spalling. In most cases, all injection of water occurs through this nozzle. The liquid effluent is quenched at least to 180° F. Personnel protection, sewer material, or the water disposal system may dictate a lower temperature, say 150°F. (At El Segundo, it’s 120°F.) As a first approximation, 2.6 gallons of water at 80°F will quench 1 lb of steam at 1200°F to water at 150°F. [A modification to this quench procedure, proposed and partially implemented at the Richmond Refinery, involves condensing only a portion (one-third to one-half) of the steam at this point. Uncondensed steam is separated in the knockout drum; the condensate (only) is further cooled to 170°F in the bottom of the drum.]
• The decoking knockout drum is shown in Figure 900-2. The drum diameteris chosen for a steam mass velocity (at maximum flow condition and if uncondensed) of about 1000 lb/hr/ft2. As an example, the Richmond Refinery No. 4 Crude Unit, of nominally 190,000 barrel/day capacity, has a drum 8 ft in diameter by 12 ft tangent-to-tangent. The drum operates at 2-4 psig, whatever pressure is needed to move noncondensible gas and uncondensed steam to the heater stack. A loop seal on the liquid outlet line holds the water level in the drum and prevents discharge of vapor through that line. [In the modification proposed at Richmond, additional quench water would be injected through a manifold located below the liquid surface in the drum.]
• Liquid effluent from the knockout drum may be further cooled by injection of utility water or the use of fire hoses. In some locations, sumps are provided for separation of coke fines.
• The initiation and completion of the burn step can be detected by measuring carbon monoxide and carbon dioxide in the flue gas. It is important to be able to monitor both. Dräeger or MSA tubes are used, along with the sampling device shown in Figure 900-3. Tubes for the concentration range 0.01-0.03% will be needed.
• Port Arthur Refinery makes a strong recommendation that an infrared scanning service (such as Infrared Surveys of Houston, Texas, 800-336-3711, 713-471-8631) be used to assist in both the spalling and the burn steps. Scanning has also been used to advantage at the El Segundo coker. Constant monitoring of individual tube temperatures permits expediting the procedure and obtaining complete decoking while assuring that tube overheating does not occur. The scanner operator also provides general surveillance of the firebox, a second pair of eyes for the operator. (But note that while infrared scanning is very helpful with carbon and chrome-moly steel tubes, it may be misleading in the case of stainless steels; surface properties confuse the IR detector.)