An off-line process is characterized by the fact that the system to be cleaned has to be taken out of operation in order to inject the cleaning body(ies) and to execute the cleaning procedure. An additional distinction must be made between active and passive cleaning bodies.
Passive cleaning bodies may be a matter of brushes or special constructions like scrapers or so-called "pigs", for instance, which are conveyed through the tubes by means of pressurized air, water, or other media. In most cases, cleaning is implemented through the oversize of the cleaning bodies compared to the tube inner diameter. The types range from brushes with bristles of plastic or steel to scrapers (with smaller tube diameters) and more expensive designs with spraying nozzles for pipelines. This method is applied for tube and pipe diameters from around 5 mm to several metres. Also belonging to this field is the cleaning of obstructed soil pipes of domestic sewage systems that is done by means of a rotating, flexible shaft.
The active cleaning bodies are more or less remote controlled robots that move through the tubes and fulfill their cleaning task, pulling along with them not only cables for power supply and communication but also hoses for the cleaning liquid. Also measuring devices or cameras are carried along to monitor the function. To date, such devices have still required minimum diameters of around 300 mm, however a further diminution is being worked on. The reasonable maximum diameter of this kind of devices is 2 m because above this diameter an inspection of the pipe would certainly be less expensive. For such large diameters a robot application is imaginable only if health-hazardous chemicals are in use.