A system and method whereby on-line cleaning of black oil heater tubes and delayed coker heater tubes may be effectuated by injecting a high pressure water charge through the tubes during normal process operations so as to prevent tube fouling and heater downtime. The high pressure water charge begins the on-line cleaning process once it enters the heater tube by undergoing intense boiling and evaporation. The cleaning occurs by two methods—a scrubbing action and a shocking action. The scrubbing action occurs because of the complete turbulence caused by the water charge's intense boiling within the heater tubes. The shocking action is caused by the expansion and contraction of the heater tubes resulting from the colder water charge flowing through the heater tubes which is then followed by the hotter process fluid flowing through the heater tubes.