Manufacturing tolerances  in today’s high-pressure hydraulic systems demand tight control of system contamination. Contamination that is built into systems during manufacture and assembly must be removed before start-up to insure proper and predictable system performance throughout its service life.

A new or rebuilt hydraulic system should be flushed before it becomes operational. The concept of flushing is to loosen and remove contamination particles inside the system by forcing flushing fluid through it at high velocity. In theory, this leaves the inside walls of the fluid conductors at the same cleanliness level as the new fluid that will be installed. Then, during normal operation, the system will experience only externally and internally generated contamination that can be controlled with filtration.

Instructions for flushing usually specify a level of system cleanliness that must be achieved, and sometimes a fluid velocity that must be maintained during the flushing procedure. Typical instructions state that flushing must be accomplished at normal system-fluid velocities for a certain period of time with a certain level of filtration. More-stringent specifications may call for a particular fluid-contamination level and require documentation by fluid-contamination analysis.

One shortcoming of all these flushing methods is that they are based on procedures to clean the fluid, but ignore the interior cleanliness of the system. Even if the tubing and conductors have been installed with the greatest of visual care, the human eye can only see particles that are larger than 40 µm — well below the needs of even the crudest and most-elementary hydraulic system.

How high a velocity?

The critical variable in flushing to achieve acceptable fluid and conductor cleanliness is fluid velocity. Traditional flushing methods usually establish this velocity in one of two ways:
• the velocity must be high enough to achieve a Reynolds number (NR) of 3,000 or more, or
• the velocity must meet or exceed the system fluid’s normal operating velocity as designed.

Experience has shown that neither of these flushing velocities is sufficient to assure the cleanliness of the ID of the system’s conductors. A short review of basic fluid dynamics explains why.

Reynolds numbers are dimensionless and used (along with other factors) to classify fluid flow as either laminar, turbulent, or somewhere in between, Figure 1. Reynolds number depends on the fluid’s viscosity and velocity and the ID of the pipe. The flow condition is termed laminar when the Reynolds number is less than 2,000, signifying orderly flow with parallel streamlines. When Reynolds number exceeds 3,000, the flow is considered turbulent, defined as the condition when fluid stream lines are no longer orderly. When Reynolds numbers fall between 2,000 and 3,000, flow exists in transition, sometimes called the critical zone.

The hydraulic-fluid velocity required to achieve turbulent flow is well within the recommended fluid-velocity guidelines for hydraulic-fluid conductors. This equation reinforces that statement:

NR = VD/v

where V is fluid velocity in fps,
D is ID of the fluid conductor in ft, and
v is fluid kinematic viscosity in ft2/sec.

Two examples

Suppose the Reynolds number is 3,000, the conductor is a 1-in. tube with a wall thickness of 0.049 in., and the fluid’s kinematic viscosity is 1.288 × 10-4 ft2/sec. Calculated fluid velocity, then, is 5.14 fps, which corresponds to a flow rate of 10.24 gpm in this instance.

The viscosity (and, therefore, the Reynolds number) of a typical hydraulic fluid is influenced by temperature and pressure. That is, the hotter the oil, the higher the NR for the same fluid velocity and pressure. The higher the pressure, the lower the NR for the same fluid velocity and temperature. Thus, specifying that Reynolds number should be 3,000 is not a stringent requirement but is well within the normal operating fluid velocities of a system. By definition, turbulent flow has been created because the fluid streamlines are no longer parallel, but sufficient fluid motion to clean the inside walls of the conductors has not been generated.

Even at the recommended maximum fluid velocities and NRs for hydraulic-system working conductors, fluid flow still is not turbulent enough to greatly affect contamination on conductor walls. Boundary layer fluid at the interior surfaces of the fluid conductor remains undisturbed.

The Reynolds number for flow at normal system velocities next can be calculated using the same conductor size and kinematic viscosity as in the first example, but with the velocity increased to 20 fps. This higher velocity results in a Reynolds Number of 11,671, which corresponds to a flow rate of 39.8 gpm.

As Reynolds number increases, flow conditions go from laminar, through the critical zone, to turbulent. Once the Reynolds number exceeds 3,000, resistance to fluid flow is a combination of the effects of turbulence and of viscous drag at the conductor wall. (This region of viscous drag at the conductor wall is known as the viscous sub-layer.) There is a transition zone within the turbulent flow range where flow resistance goes from being governed by turbulence effects to being governed by the roughness of the inside wall of the conductor.

This is shown clearly when inspecting the Moody diagram, Figure 2, which graphically demonstrates the relationship between Reynolds number; friction factor, f; and the roughness of the conductor’s inside surface, e. Resistance to flow through a fluid conductor, f, is only affected by the surface roughness of the fluid conductor when the Reynolds number exceeds 4,000. Thus, the majority of the resistance to flow is created by turbulence effects. Only when the Reynolds number is high enough to cause surface projections of the conductor walls to extend beyond the viscous sub-layer does the surface come in contact with the turbulent flow, thereby affecting the pressure drop in the conductor.

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