This International Standard specifies pipe cleaning and cleaning levels of hydraulic oil pipe systems. The cleaning of pipes and components in hydraulic oil pipe systems is essential for the trouble-free operation of hydraulic systems. It indicates methods and equipment for the practical execution of the cleaning of specific parts of hydraulic systems with appurtenant components. The purpose of the cleaning process is to remove installation dirt and to check that the piping and hydraulic system have been adequately cleaned. The cleaning process of a system is considered a “washing through” process when the Reynolds number, Re, ≤ 3 000, and a flushing process when Re ≥ 3 000. The Reynolds number is an indicator of whether a fluid flow is considered laminar or turbulent. This International Standard presupposes that the pipe sections of the hydraulic system have been cleaned partly by pickling and partly by mechanical cleaning. It is furthermore assumed that both dynamic and static components from system suppliers are adequately clean when delivered (see Clause 5). The specifications given in this International Standard are supplementary to, and not a replacement for, the guidelines specified by the various manufacturers. The manufacturer's guidelines, where available, take precedence.