Hydraulic System Flushing Procedures

Date:31 Dec 2019

Tolerances in today's high-pressure hydraulic systems r […]

Tolerances in today's high-pressure hydraulic systems require strict control of system contamination. Prior to startup, contaminants built into the system must be removed during manufacturing and assembly to ensure that the system has proper and predictable performance throughout its useful life. New or rebuilt hydraulic systems should be flushed before operation.

Typical instructions state that flushing must be performed at a normal system fluid speed for a certain period of time and a certain degree of filtration. More stringent specifications may require specific levels of fluid contamination and need to be documented through fluid contamination analysis. Flushing instructions typically specify the level of system cleanliness that must be achieved, and sometimes the fluid speed that must be maintained during the flushing process. The concept of flushing is to loosen and remove contaminant particles inside the system by forcing the flushing fluid through at high speed. Theoretically, this keeps the inner wall of the fluid conductor as clean as the new fluid to be installed.

One disadvantage of all these flushing methods is that they are based on a cleaning fluid procedure, but ignore the internal cleanliness of the system. Even with the greatest degree of visual care during pipe and conduit installation, the human eye can only see particles larger than 40 µm. Even the roughest and most basic hydraulic systems require far less than this level .

Experience has shown that dimensionless Reynolds numbers (and other factors) are used to classify fluid flows as laminar, turbulent, or transitional flows (between the two). The Reynolds number depends on the viscosity and speed of the fluid and the ID of the pipe. These flushing speeds are not sufficient to ensure the cleanliness of the inner diameter (ID) of the system conductor. A brief review of basic fluid dynamics can explain why.

The speed must be high enough to reach a Reynolds number (NR) of 3,000 or more. Reynolds numbers flow during the transition between 2,000 and 3,000. The speed must reach or exceed the normal design speed of the system fluid. When the Reynolds number is greater than 3,000, the flow becomes disordered, which is defined as the situation where the fluid flow line is no longer orderly.
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