The best practice indicate that the steam must be injected between 10 to 20 minutes before you switching the drum, of way to avoid losing the flow inside of the line & coke drum. In other words is not recommendable to take off a fluid without having already other fluid entering into the line or drum (the same it applies for the change the steam to cooling water). This injection can be done through the purge line located down stream of the SP-6 (3″) until that steam going by the steam control valve (utility header), then this purge is closed and the steam flow going to the normal way (is generally around 10000 lb/hr ;going to the fractionator), if you do not have this purge line you can do it through the steam control valve (utility header)
Im sorry for my english, best Regards
