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Kushal Choudhury, DGM (Prod)(Expert)   -   07-Mar,2019 17:48

Radiant coils have a limited life and failure is caused by a variety of factors, many being related to furnace operation. However, each pyrolysis plant experiences specific operational conditions and operational philosophies. There are several methods for estimating and damage detection of furnace tubes. Some of them are • Basic damaging mechanism is the combined action of carburization and creep ductility exhaustion. This results in bulging, bending and ovalization of the tubes. • The second dominant failure mechanism is brittle fracture during furnace trips (e.g ID fan failure, naphtha pump trip, naphtha feed valve malfunction etc) which result in sudden thermal shock resulting in large, longitudinal cracks on many tubes. • Mechanisms which causing the majority of failures in radiant tubes are: coke formation, creep ductility exhaustion, thermal fatigue, brittle fracture, erosion, overheating, human factor, creeping, carburization, oxidation. • Tube sheets and tube supports of LMP section should be examined to determine their physical condition and fitness for further service as these are also prone to failure & crack in tube sheets(End tube sheet or Intermediate Tube sheet) leads to failure of tube. Supports should be examined carefully for cracks, oxidation, corrosion, distortion and sagging. Several actions has been taken over the years to increase the reliability of furnace operation & prevention of the furnace trips. Process fluid temperatures, pressure drop across the coils, flue gas temperatures and drafts at various sections are monitored online. The oxygen level in the flue gas are monitored to avoid excessive oxidation of the tubes or after burning in the convection and air preheater sections. Monitoring of tube skin temperatures(TMT), metal skin temperatures of the radiant coil are checked and the possibility of any excessive coking evaluated. Specified skin temperature limit of the tubes are not exceeded. The hearth & wall burners are checked to ensure that flame does not touch the arch or the tubes. The radiant tubes are inspected for hot spots, flame impingement, excessive oxidation bowing and sagging, and the exact locations recorded. Cleaning of bad flame burners are done on regular basis. Inspection of firing conditions flame pattern, tubes and tube supports of radiant section are carried out daily. Reliability of ID fan ,BFW pump, COT thermo wells to be ensured with periodic preventive maintenance. To summarize, reliability of the furnace can be ensured by Burners healthiness, Sufficient excess O2 in the firebox , Severity of cracking & EOR TMT , Contaminants in feed ,reliability of the instruments of the furnace viz. control valves , on-off valves, proper & timely decoking of the furnace are very important for ensuring the reliability. Based on the guidelines available from Licensor typical life of a 35/45(Ni/Cr alloy) outlet coil is about 6- 6.5 years and 52 decoke cycles, Inlet tubes typically last somewhat longer (one or two years) than the outlet tubes. However frequent sudden thermal shocks to furnace tubes due to frequent trips can lower the life of the radiant coils

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