Flue gas furnace tube integrity in service run dry

[valero]

I have a FCCU Flue gas furnace that posses a superheater with SA-335 Gr P22 tubes. The tubes are 2.875” O.D. with a .218” thickness. This superheater had a tube failure during operation. The decision was made to by-pass the superheater rather than performing an entry to asses the damage. These tubes have been in operation for two years without steam flow. The internal temperature of the furnace at this location is approximately 1350º F. With the tubes seeing this type of environment for two years without internal flow what is the possibility repairing them? Should I expect weldability issues? Should I plan on replacement rather than repair? During the past brief unit outage we removed a section of tube for analysis; however, was lost. The tubes did not visually appear distorted.

 

[metengr]

Valero;
Prior to making any decision, I would recommend you remove several (two to four) representative tube samples from across the circuit and have them evaluated by a metallurgical lab. Specifically, you want to obtain dimensions of the tube samples to check for swell and wall thickness variation related to exposure to elevated temperature service. Each sample should be cross-sectioned and prepared for metallographic examination to evaluate the tube metal microstructure (check for grain growth and carbide structure), extent of oxidation on the ID and OD surfaces, and bulk hardness.

If the laboratory results are considered acceptable, you should be ok to use the T22 material. I would be concerned if the SH tubing was subjected to flue gas temperatures at 1500-1600 deg F or if the tubing had contained pressure at 1350 deg F. At 1350 deg F you are at or slightly below the lower critical transformation temperature for this material, which should not really harm the tube metal provided the tubing has not been subjected to repeated quenching upon cooling and most importantly having steam pressure.

 

[davefitz]

Just because the average furnace gas temp is 1350 F doesn't mean that there can be isolated areas that are recieving radiation heat transfer from hotter portions of the furnace, so the metal temp of those tube sections could be above 1350 F. My guess is the typical difference between avg gas temp and max uncooled tube temp at the furnace outlet is about 100F, so the "worst" tube may be at 1450 F.

The tube section only needs to be overheated above the critical temp for a few moments to cause austenizing of the crystal structure. I would not expect this superheater assembly to be reliable after extended service at or above this temperature, and would replace it. If the new superheater was to also be expected to operate uncooled at this temperature, it should be fabbed of an austenitic material , 316 H or 347 H. Due to the much larger expansion coeficient of stainless steel, the supports and design of the superheater would need to also be modified.

 

[SMF1964]

In addition to the items mentioned above:

At 1350°F, you are 275°F above the oxidation limit for T22; You may have lost a significant portion of the tube thickness due to oxidation.

At 1350°F, the material has no measureable yield strength, and the deadweight alone could induce creep damage to the material, especially at clips and attachments where the stresses will be concentrated.

I would advise against using this material without a thorough metallurgical investigation prior.