Hydraulic System Contamination (Solids, Water and Air)

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Solids Contamination

In this article I want to talk about how to deal with hydraulic systems that have been contaminated. Remember, there are generally only two conditions that necessitate changing the oil in a hydraulic system:

1. Base oil degradation
2. Additive depletion

Contaminants of the hydraulic fluid such as hard and soft particles and water can be removed from the oil and therefore don't mandate an oil change.

Techniques for flushing hydraulic systems vary in cost and complexity. Before I discuss some of these methods, let's first distinguish between flushing the fluid and flushing the system. The objective of flushing the OIL is to eliminate contaminants such as particles and water. 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 fluid from conductor walls and other internal surfaces, and system dead spots. Reasons for performing a SYSTEM flush include:

1. Fluid degradation - resulting in sludge, varnish or microbial deposits.
2. Major failure - combined with filter overload disperses debris throughout the system.
3. 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, your 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 expels a large percentage of contaminants and degraded fluid. The system is then filled to the minimum level required and the fluid 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, 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'.

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 hydraulic 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 programlike the one I outline in 'Insider Secrets to Hydraulics', it's likely you'll never need to flush!

Water Contamination

Water in hydraulic fluid:

• Depletes some additives and reacts with others to form corrosive by-products which attack some metals.
• Reduces lubricant film-strength, which leaves critical surfaces vulnerable to wear and corrosion.
• Reduces filterability and clogs filters.
• Reduces the oils ability to release air.
• Increases the likelihood of cavitation occurring.

How much water is too much?

A number of factors need to be considered when selecting water contamination targets, including the type of hydraulic system and your reliability objectives for the equipment. It's always wise to control water contamination at the lowest levels that can reasonably be achieved, but certainly below the oil's saturation point at operating temperature.

Water removal methods

Methods for removing free (unstable suspension) and emulsified (stable suspension) water include:

1. polymeric filters;
2. vacuum distillation; and
3. headspace dehumidification.

Polymeric filters - These look like conventional particulate filters, however the media is impregnated with a super-absorbent polymer. Water causes the polymer to swell, which traps the water within the media. Polymeric filters are best suited for removing small volumes of water and/or maintaining water contamination within pre-determined limits.

Vacuum distillation - This technique employs a combination of heat and vacuum. At 25 inches of mercury, water boils at 133?F (56?C). This enables water to be removed at a temperature that does not damage the oil or its additives.

Headspace dehumidification - This method involves circulating and drying the air from the reservoir headspace. Water in the oil migrates to the dry air in the headspace and is eventually removed by the dehumidifier. Vacuum distillation and headspace dehumidification also remove dissolved water.

Prevention is better than cure

Like all other forms of contamination, preventing water ingress is ten times cheaper than removing it from the oil.

AIR in Hydraulics

Air can be present in four forms:

• Free air - such as a pocket of air trapped in part of a system.
• Dissolved air - hydraulic fluid contains between 6 & 12 percent by volume of dissolved air.
• Entrained air - air bubbles typically less than 1 mm in diameter dispersed in the fluid.
• Foam - air bubbles typically greater than 1 mm in diameter which congregate on the surface of the fluid.

Of these four forms, entrained air is the most problematic.

Pre-filling components and proper bleeding of the hydraulic system during start-up will largely eliminate free air. Small amounts of foam are cosmetic and do not pose a problem. However, if large volumes of foam are present, sufficient to cause the reservoir to overflow for example, this can be a symptom of a more serious air contamination and/or fluid degradation problem. Negative effects of entrained air include:

• Reduced bulk modulus, resulting in spongy operation and poor control system response.
• Increased heat-load.
• Reduced thermal conductivity.
• Fluid deterioration.
• Reduced fluid viscosity, which leaves critical surfaces vulnerable to wear.
• Cavitation erosion.
• Increased noise levels.
• Decreased efficiency.

As pointed out above, hydraulic fluid can contain up to 12 percent dissolved air by volume. Certain conditions can cause this dissolved air to come out of solution, resulting in entrained air. When fluid temperature increases or static pressure decreases, air solubility is reduced and bubbles can form within the fluid. This release of dissolved air is known as gaseous cavitation. Decrease in static pressure and subsequent release of dissolved air can occur at the pump inlet, as a result of:

• Clogged inlet filters or suction strainers.
• Turbulence caused by intake-line isolation valves.
• Poorly designed inlet.
• Collapsed or otherwise restricted intake line.
• Excessive lift.
• Clogged or undersized reservoir breather.

Air entrainment can also occur through external ingestion. Like gaseous cavitation, this commonly occurs at the pump - as a result of:

• Loose intake-line clamps or fittings.
• Porous intake lines.
• Low reservoir fluid level.
• Faulty pump shaft seal.

Like other hydraulic problems, proper equipment maintenance will prevent the occurrence of most air contamination problems.

About the Author: Brendan Casey has more than 17 years experience in the maintenance, repair and overhaul of mobile and industrial hydraulic equipment. For more information on reducing the operating cost and increasing the uptime of your hydraulic equipment, visit his web site: www.InsiderSecretsToHydraulics.com [1]