Method for cleaning heavy hydrocarbon scale adhered to heat exchanger and piping structure for cleaning

A method for clearing an oil foulant of a heavy hydrocarbon scale adhering to a heat exchanger that is incorporated in a system which processes petroleum hydrocarbons as a raw material by using an organic solvent fluid wherein

• said heavy hydrocarbon scale contains an oil component and sludge, and
• said heat exchanger is disposed between a residual oil source and a crude oil source in an atmospheric distillation unit or a vacuum distillation unit of a petroleum refining plant,
• the method comprising:
• a dissolving/dispersing step in which an organic cleaning solvent of a mixture comprised of a petroleum derived hydrocarbon A and an oxygen containing polar compound B as active components, and the petroleum derived hydrocarbon A has a boiling point of 200° C. or higher and is a liquid at ordinary temperature is supplied to and circulated inside the heat exchanger, or the heat exchanger is soaked in the organic cleaning solvent, to dissolve and disperse the oil foulant;
• a removal by peeling step in which the remaining sludge from the heavy hydrocarbon scale is removed by peeling either concurrent with or after the dissolving/dispersing step;
• and
• a mixing and processing step in which the organic cleaning solvent is mixed into the raw material after cleaning, and processed.