A center feed system for injecting residual byproduct into a coke drum during a delayed coking process, the center feed system comprising: an inlet sleeve configured to attach to the coke drum or to a spool coupled to the coke drum; and a retractable injection nozzle (1300, 1600, 1700) contained within the inlet sleeve, the retractable injection nozzle sliding within the inlet sleeve to extend into and retract from the coke drum or spool for introducing the residual byproduct into the coke drum or spool, the retractable injection nozzle including one or more openings (1301, 1302) through an exterior surface of the retractable injection nozzle, the one or more openings being exposed within the coke drum or spool when the retractable injection nozzle is extended, the residual byproduct exiting the retractable injection nozzle through the one or more openings; characterized in that: the or each of the one or more openings (1301, 1302) has threads; and the or each of the one or more openings (1310, 1302) includes an insert (1301a, 1302a), the or each insert having corresponding threads to allow it to be threaded into a corresponding opening of the one or more openings (1301, 1302), the retractable injector nozzle having an exterior contour and the or each insert having the same exterior contour as the exterior contour of the retractable injector nozzle so that, once the or each insert is inserted into the one or more openings, the exterior contour of the assembled retractable injection nozzle remains constant. an inlet sleeve configured to attach to the coke drum or to a spool coupled to the coke drum; and a retractable injection nozzle (1300, 1600, 1700) contained within the inlet sleeve, the retractable injection nozzle sliding within the inlet sleeve to extend into and retract from the coke drum or spool for introducing the residual byproduct into the coke drum or spool, the retractable injection nozzle including one or more openings (1301, 1302) through an exterior surface of the retractable injection nozzle, the one or more openings being exposed within the coke drum or spool when the retractable injection nozzle is extended, the residual byproduct exiting the retractable injection nozzle through the one or more openings; characterized in that: the or each of the one or more openings (1301, 1302) has threads; and the or each of the one or more openings (1310, 1302) includes an insert (1301a, 1302a), the or each insert having corresponding threads to allow it to be threaded into a corresponding opening of the one or more openings (1301, 1302), the retractable injector nozzle having an exterior contour and the or each insert having the same exterior contour as the exterior contour of the retractable injector nozzle so that, once the or each insert is inserted into the one or more openings, the exterior contour of the assembled retractable injection nozzle remains constant. the or each of the one or more openings (1301, 1302) has threads; and the or each of the one or more openings (1310, 1302) includes an insert (1301a, 1302a), the or each insert having corresponding threads to allow it to be threaded into a corresponding opening of the one or more openings (1301, 1302), the retractable injector nozzle having an exterior contour and the or each insert having the same exterior contour as the exterior contour of the retractable injector nozzle so that, once the or each insert is inserted into the one or more openings, the exterior contour of the assembled retractable injection nozzle remains constant.