Steam-Air Tube Decoking Typical Procedure
Procedures vary in detail from unit to unit. The following procedure is “generic”. Alternatives are discussed as appropriate. The various safety checks involved in any firing of the heater are not detailed here; see Fired Heater Operations Guide for that information. An outside operator or scanning service operator should monitor conditions during spalling and burning. The frequency of logging temperatures, pressures, and flowrates varies from one site to another, usually every hour if done by hand, every 15 minutes if done automatically.
1. With the heater steamed free of oil and all decoking connections made, put protective steam through non-process coils, fire the heater, and raise bridgewall temperature at the rate of 200-300°F per hour. Adjust burners to minimum
excess air to minimize oxidation of tube external surfaces. (Note: the practice of “dry firing” as described here is not universal. It has the obvious possibility of tube overheating. Some operators start steam flow in process coils before firing. In that case, firing should be delayed until steaming has removed any pockets of condensate collected in the tubes. You don’t want sudden vaporization.)
2. When 300°F skin temperature is reached, start flow of dry steam through all passes. Continue increasing temperature at 100-200°F per hour until maximum tubeskin temperature of 1200°F (for carbon steel tubes) is reached. During remaining decoking steps, maintain minimum flow through all passes. Minimum flow is 5-6 lb/sec/ft2 or whatever is needed to keep measured tubeskin temperatures from exceeding 1200°F (carbon steel tubes). This corresponds to a bridgewall temperature of about 1350°F. [Note that metal temperatures well above oxidation limits for continuous operation are permitted for short-term operations such as decoking. For carbon steel, the oxidation limit is 1000°F; the short term limit is 1275° F; this is 50°F below the “lower critical temperature” (API Recommended Practice 530, Table 4), where tube toughness is affected. For 9Cr-1Mo steel, the oxidation limit (Spec. HTR-MS-1350-Q, Fig. 2) is 1250°F; the short-term limit is 1450°F; this is 65°F below the lower critical temperature.]
3. Start quench water into the effluent line. To minimize hazard to personnel, effluent water from quench knockout drum should be maintained at 150°F maximum. If this is not possible, due to limited water supply or sewer capacity, then protection of plastic sewer pipe may set the next limit at 170-180°F.
4. Increase steam rate in the pass(es) being spalled to 16-18 lb/sec/ft2. A heater may be spalled one pass or one cell (two or four passes) at a time. The number of passes is limited by steam or quench water capacity or by the amount of operator attention required. Participation by an infrared scanning service is particularly useful here.
5. Spalling is promoted by reversing steam flow, cycling temperature by 100-200°F (rate of change 100°F/hr), alternately increasing and decreasing steam flow, and intermittently adding small amounts of air. (Notice, on Figure 900-1, that reversing flow involves closing two valves and opening two others; this must be done quickly to avoid overheating the tubes during the period of no flow.) During spalling, water from the decoking knockout drum will be grey to black depending upon size and concentration of coke particles. Particles will be of various sizes, but if a preponderance of small coke particles (less than one-eighth of an inch) is produced, this suggests that velocity is too high and erosion of metal surfaces may be occurring; reduce the steam rate. (Particle size may be checked by sampling the water from the decoking knockout drum; be careful, prevent scalding.) Completion of spalling is indicated by the effluent water turning milky or clear and remaining so through repetition of the spalling promotion measures described above.
6. When spalling of all passes is complete, adjust the tubeskin temperature to 1200°F, adjust steam flow to 6 lb/sec/ft2, and carry out the coke burning step one pass at a time. Again, passes may be burned simultaneously provided that air supply is adequate and operator coverage is provided to closely monitor tube temperatures.
7. Coke burning is not done in reverse flow through convection tubes unless tube and extended surface metallurgy is known to be suitable. Suggested minimum requirements are 5Cr-½Mo tubes and 11-Cr fins.
8. In the pass(es) being decoked, begin by adding an air rate of 0.5-1.0 lb/sec/ft2 to the 6 lb/sec/ft2 of steam already flowing. Observe each pass, beginning at the inlet tube, for presence of a red spot moving along the tube, indicating progress of the burning wave through the tube. For carbon steel tubes, the spot should be dull red (1100-1200°F); 9Cr-1Mo and stainless steel tubes can be cherry red (1300-1400°F). Infrared scanning is helpful here if tubes are of carbon or chrome-moly steel. Reduce the air rate if the temperature of the burning spot is too high.
9. When the burning wave has passed through, flush the pass with steam at the spalling rate and then reverse the flow of the air-steam mixture.
10. When no further burning is apparent, double the air flow and look carefully for signs of additional burning. Wait until both CO and CO2 are below 0.04% in the effluent gas. Then repeat this step, doubling air flow again. Red iron oxide usually appears in the effluent water near the end of the burn, but this is not conclusive evidence of completion.
11. Reduce steam flow to zero while maintaining air flow. Check for any additional burning.
12. Cut fires and air flow and flush the coil with steam at full line pressure. Reverse the flow several times to remove all loose ash and scale.
13. If tubes are of carbon or chrome-moly steel, maintain low steam flow to cool tubes at 300°F per hour until tube temperature has dropped to 350-400°F. Forced draft fans may be used to continue cooling down to 200°F. Flush with water in preparation for repair or startup.
a. If tubes are of stainless steel, then measures must be taken to prevent stress corrosion cracking. Maintain low steam flow to cool tubes at 300°F per hour until the tubewall temperature falls to about 100° above the water dew point temperature. Discontinue steam injection and depressure. Purge with dry nitrogen and maintain purge flow until blinds are installed. Positive nitrogen pressure should be maintained on the system after blinding.
b. (Alternative to step 13a for stainless steel tubes.) Maintain low steam flow to cool tubes at 300°F per hour until the tubewall temperature falls to about 100° above the water dew point temperature. Discontinue steam injection and depressure. Wash with soda ash solution and blow clear with dry air before blinding. Note: this alternative may not be appropriate for vertical tube coils.