How to flush a hydraulic system

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One of our readers wrote to me recently with the following question:

"Aside from replacing oil in the reservoir, what is the best way to purge contaminated oil from hydraulic system plumbing and components?"

Techniques for flushing hydraulic systems vary in cost and complexity. Before I discuss some of these methods, let's first distinguish between flushing the oil and flushing the system.

The objective of flushing the oil is to eliminate contaminants such as particles and water from the oil. This is usually accomplished using a filter cart or by diverting system flow through an external fluid-conditioning rig.

The objective of flushing the system is to eliminate sludge, varnish, debris and contaminated or degraded oil from conductor walls and other internal surfaces, and system dead spots. Reasons for performing a system flush include:

  • Oil degradation - resulting in sludge, varnish or microbial deposits.
  • Major failure - combined with filter overload disperses debris throughout the system.
  • New or overhauled equipment - to purge 'built-in' debris.

Common methods for flushing hydraulic systems include:

  • Double oil and filter change.
  • Mechanical cleaning.
  • Power flushing.

The technique or combination of techniques employed will depend on the type of system and its size, reliability objectives for the equipment and the reason for the flush.

Double oil and filter change

This technique involves an initial oil drain and filter change, which expells a large percentage of contaminants and degraded oil. The system is then filled to the minimum level required and the oil circulated until operating temperature is reached and the fluid has been turned over at least five times. The oil is drained and the filters changed a second time. An appropriate oil analysis test should be performed to determine the success of the flush. To maximize the effectiveness of this technique, the system should be drained as thoroughly as possible and the reservoir mechanically cleaned.

Mechanical cleaning

Although not technically a flushing technique, the selective use of mechanical cleaning may be incorporated in the flushing strategy. This can involve the use of a pneumatic projectile gun to clean pipes, tubes and hoses (see exhibit 1), and disassembly of the reservoir and other components for cleaning using brushes and solvents. Mechanical cleaning is labor intensive and therefore costly. It carries with it reliability risks associated with opening the hydraulic system and intervention by human agents.

Pipe cleaning projectile.

Exhibit 1. Pipe/hose cleaning projectile (Compri Technic).

Power flushing

Power flushing involves the use of a purpose-built rig to circulate a low viscosity fluid at high velocities to create turbulent flow conditions (Reynolds number > 2000). The flushing rig is typically equipped with a pump that has a flow rate several times that of system's normal flow, directional valves, accumulators, fluid heater and chiller and of course, a bank of filters. The directional valves enable the flushing direction to be changed, the accumulators enable pulsating flow conditions and the heater and chiller enable the fluid temperature to be increased or decreased, all of which can assist in the dislodgment of contaminants. Analysis of the flushing fluid is performed regularly during the flushing operation to determine the point at which the system has been satisfactorily cleaned.

What about components?

The question of how to deal with system components arises when contemplating a system flush. Plumbing should be flushed first in isolation from pumps, valves and actuators. Once the conductors have been flushed clean, valves and actuators can be gradually included in the flushing circuit. The decision to disassemble and mechanically clean components will depend on the type of equipment, your reliability objectives and the reason for the flush.

Prevent or cure?

With the exception of new or overhauled equipment, the need to flush a hydraulic system generally represents a failure of maintenance. If you follow an effective proactive maintenance program like the one I outline in Insider Secrets to Hydraulics, it's likely that you'll never need to flush.

Related articles:

How to remove water from hydraulic oil
Priority number one in hydraulics maintenance
How to make your hydraulic oil last longer

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