On-Line Steam Spalling Typical Procedure
Make provision to record process conditions, including skin temperatures, at 30-minute intervals during the procedure. (See step 16.)
1. Adjust the steam or steam condensate rate in the pass to be spalled to 10 lb/sec/ft2.
2. Lower the pass outlet temperature to 800°F. Take burners out of service as appropriate.
3. Block the flow of hydrocarbon to the pass. Immediately increase condensate flow to 25 lb/sec/ft2. All changes of condensate flow rate should be made quickly—elapsed time of 15 minutes or less. [Notice that steam mass velocities in many steps of this method are considerably higher than the maximum used in steam-air decoking, Section 961. This is because operating pressure is higher in on-line steam spalling. Maximum linear velocity is about the same.]
4. A momentary pressure rise will occur following step 3. If pressure continues to rise, assume that the pass is blocked and begin the emergency shutdown procedure.
5. Increase the outlet temperature to 1100°F or the maximum skin temperature to 1300°F, whichever occurs first. Hold for 1 hour.
6. Increase the condensate rate to 35 lb/sec/ft2. Hold for 1 hour.
7. Increase the condensate rate to 40 lb/sec/ft2 and the outlet temperature to 1150°F or the maximum skin temperature if it occurs first. Hold for 2 hours.
8. Increase the condensate rate to 45 lb/sec/ft2 and quickly reduce the outlet temperature by 100°F. Opening the burner air registers and closing the temperature controller (fuel) valve helps to make temperature change quickly.
9. Decrease the condensate rate to 40 lb/sec/ft2 and quickly raise the outlet temperature by 100°F. Hold for 2 hours.
10. Repeat steps 8 and 9, but do not hold in step 9; proceed immediately.
11. Increase the condensate rate to 50 lb/sec/ft2 and maintain temperature. Hold for 2 hours.
12. Increase the condensate rate to 60 lb/sec/ft2 and reduce the outlet temperature to 1000°F.
13. Decrease the condensate rate to 45 lb/sec/ft2 and increase firing until a maximum skin temperature of 1300°F is reached. Heatup should occur over 20 to 30 minutes. Hold for 2 hours.
14. Repeat steps 12 and 13 and then 12 again.
15. Decrease the condensate rate to 35 lb/sec/ft2, and adjust the outlet temperature to 1100°F.
16. Compare inlet pressure and skin temperatures to those recorded during step 6. Reduction of either is an indication that spalling has occurred. Comparison with the results of previous spallings will give a rough measure of the extent. If additional spalling is needed, repeat steps 12-16.
17. Increase the condensate rate to 55 lb/sec/ft2 and decrease the outlet temperature to 750°F. Allow all skin temperatures to drop below 800°F before proceeding. Decrease outlet temperature further, if necessary, to get skin temperatures down.
18. Slowly reduce the condensate rate to 10 lb/sec/ft2. Take about 10 minutes for this step. Meanwhile, reduce firing of the adjacent heater pass so that roof and convection tubes of the pass just spalled will cool.
19. Resume hydrocarbon flow to the pass just spalled. Slowly raise it to half of the design rate. Observe plug headers for leakage. If inlet pressure approaches the normal operating maximum, reduce condensate injection.
20. When pass outlet temperature has dropped 200° to 300°F, indicating that hydrocarbon flow has filled the coil, return all burners to service.
21. Slowly increase the hydrocarbon flow rate to 90% of normal and reduce the condensate rate to 5 lb/sec/ft2.
22. Increase firing and return all conditions to normal.