# How to Calculate Ideal Flushing Flow Rate

The removal of contamination ‘built-in’ during the assembly of a new hydraulic system, or the removal of internally-generated contamination following a major, catastrophic failure in an existing hydraulic system, is commonly known as flushing.

This type of flushing is usually carried out with a dedicated rig designed specifically for this purpose. And while the hydraulic oil to be used in the system can be used as the flushing fluid, it is often the case that an alternative, lower viscosity fluid with no expensive additives is used instead.

For maximum effectiveness, the flushing process must be carried out in both directions with the flushing fluid heated to a temperature of ~60°C (140°F) and with turbulent flow. In an operational hydraulic system, turbulent flows are to be avoided like the plague. But when flushing, the presence of turbulent flow conditions assist contaminant dislodgement and removal.

The flow-rate required to achieve turbulent flow conditions depends on the inside diameter of the conductor (pipe, tube or hose) being flushed and the viscosity of the flushing fluid. It can be calculated using the following formula:

Q > 0.19 x v x d

Where:

Q = flow-rate required for turbulent flow conditions in liters/minute.
v = kinematic viscosity of the flushing fluid, at flushing temperature, in centistokes.
d = inside diameter of the conductor in millimeters.

This requirement for turbulent flow means that the flushing rig must be capable of generating flow-rates greater than the hydraulic system being flushed–and its installed pumps, have been designed for.

In practice, flushing duration is determined by the cleanliness level of the flushing fluid, based on particle counts taken at regular intervals. This is important because failing to flush the system thoroughly enough can be a very costly mistake in the long run. And to discover six other costly mistakes you want to be sure to avoid with your hydraulic equipment, get “Six Costly Mistakes Most Hydraulics Users Make… And How You Can Avoid Them!” available for FREE download here.