PURPOSE:To conduct high pressure water discharge type on-line decoking by a method wherein in the connecting condition of a cracking gas cooler (TLE) and thermal cracking tubes, the high pressure water in the TLE is discharged after the outlet temp. of the thermal cracking temp. is reduceed to a specific temp. or less and the outlet temp. of the TLE is controlled in a specific temp. range. CONSTITUTION:Combustion-support gas is caused to flow through the cooling tubes of a cracking gas cooler (TLE) cooling indirectly cracking gas with water, containing ethylene obtained by thermally cracking hydrocarbon under the existence of steam, to remove thermally coke sticking on the cooling tubes. In this method, high pressure water is discharged under the condition that steam or the mixed gas of steam and air is supplied at a temp. of 450 deg.C or less through the thermal cracking tubes in the connecting condition of the TIE and the thermal cracking tubes and decoking is carried out under low temp. condition such as a temp. of 350-450 deg.C at the outlet temp. of the TLE. Consequently, the temp. of the cooling tubes is increased only by 100 deg.C at most in comparison with the temp. in normal operation time and large thermal stress is not generated in the TLE and decoking can be carried out safely.
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