S.K. Verma - IOCL - Guwahati Refinery
Delayed Coker Unit (DCU), being a thermal cracking process, results in gradual buildup of coke on the internal surface of the heater coils. This gradual buildup of coke slowly increases the skin temperature as well as the pressure drop across the heater coil, which in turn leads to higher inlet pressure and higher skin temperature. Based on the allowable skin temperature and the allowable pressure drop across the heater coil, intermittent steam air decoking needs to be performed to clean this deposited coke inside heater coil.
A case study has been presented on failure of heater radiant coil return bends and outlet swing elbow in Delayed Coker Unit during Steam air decoking operation. Detailed root cause analysis was carried out, including steam velocity calculation during SAD operation, literature survey for studying impact of abrasive/coke particle on erosion rate on tube and tube fittings, evaluation of historical data of heater, non-destructive examination of failed piece, experience of other oil refineries’ DCU heater performances, etc.
The possible explanation of leak appears to be high velocity steam with coke fines impacting on heater coil return bends and outlet swing elbows leading to localized erosion at the point of impact.
The presentation examines further the reason for failure and suggest mitigation measures with respect to process change and hardware modification for reliability improvement of the DCU heater return bend.
S.K Verma is the Chief Manager of Inspection for Indian Oil Corporation Ltd. He has a Masters in Metallurgical and Materials Engineering, specializing in welding, corrosion, and failure analysis of static equipment in oil refineries.