PURPOSE:To remove scale well even from the position where the tube diameter will vary in step by inverting the passing direction of fluid one or two times. CONSTITUTION:In order to scavenge between the ends (a) and (b) of water supply pipe, mixture fluid of air and sand produced from fluid generator 14 is flowed from the terminal (a') of valve box 12 through the ends (a), (b) to the terminal (b') then discharged to the fluid collecting device 16. Then the flow of said fluid is inverted through changeover operation of the valve box 12 to flow the fluid from the end (b) to (a) thus to scavenge between said ends (a), (b). Similarly, it is scavenged sequentially between the ends (a) and (c), (a) and (d), (a) and (e) and (a) and (f) to remove the scale at the step section in the water supply pipe completely thus to finish scavenging work of entire pipe.
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