The inorganic solid contaminant group includes dust, paint particles, dirt, and silicates. These and other materials are often drawn into hydraulic systems from external sources. The wet piston shaft of a hydraulic actuator may draw some of these foreign materials into the cylinder past the wiper and dynamic seals. The contaminant materials are then dispersed in the hydraulic fluid. Also, contaminants may enter the hydraulic fluid during maintenance when tubing, hoses, fittings, and components are disconnected or replaced. To avoid these problems, all exposed fluid ports should be sealed with approved protective closures.

Glass particles from glass bead peening and blasting are another contaminant. Glass particles are particularly undesirable because glass abrades synthetic rubber seals and the very fine surfaces of critical moving parts.

FLUID CONTAMINATION

Hydraulic fluid can be contaminated by air, water, solvents, and foreign fluids. These contaminants and their effects are discussed in the following text.

Air Contamination

Hydraulic fluids are adversely affected by dissolved, entrained, or free air. Air may be introduced through improper maintenance or as a result of system design. Air is sometimes introduced when changing filters. You can minimize this kind of contamination by putting hydraulic fluid into the filter holder before reassembling the filter. By doing this, you have introduced less air into the hydraulic system. The presence of air in a hydraulic system causes spongy response during system operation. Air causes cavitation and erodes hydraulic components. Air also contributes to the corrosion of hydraulic components.

Water Contamination

Water is a serious contaminant of hydraulic systems. Corrective maintenance actions must be taken to remove ail free or emulsified water from hydraulic systems. Hydraulic fluids and hydraulic system components are adversely affected by dissolved, emulsified, or free water. Water may be induced through the failure of a component, seal, line or fitting, poor or improper maintenance practices, and servicing. Water may also be condensed from air entering vented systems.

The presence of water in hydraulic systems can result in the formation of undesired oxidation products, and corrosion of metallic surfaces will occur. These oxidation products will also cause hydraulic seals to deteriorate and fail, resulting in leaks. If the water in the system results in the formation of ice, it will reduce fluid flow and impede the operation of valves, actuators, or other moving parts within the system. This is particularly true of water located in static circuits or system extremities and subject to high-altitude, low-temperature conditions. Microorganisms will grow and spread in hydraulic fluid contaminated with water. These microorganisms will clog filters and reduce system performance.

Solvent Contamination

Solvent contamination is a special form of foreign-fluid contamination. The original contaminating substance is a chlorinated solvent introduced by improper maintenance practices. It is extremely difficult to stop this kind of contamination once it occurs. This type of contamination can be prevented by using the right cleaning agents when performing hydraulic system maintenance. Chlorinated solvents, when allowed to combine with minute amounts of water, hydrolyze to form hydrochloric acids. These acids attack internal metallic surfaces in the system, particularly those that are ferrous, and produce a severe rustlike corrosion that is virtually impossible to arrest. Extensive component overhaul and system decontamination are generally required to restore the system to an operational status.

Foreign Fluids Contamination

Contamination of hydraulic fluid occurs when the wrong fluids get into the system, such as oil, engine fuel, or incorrect hydraulic fluids. Hydraulic oil ceders, which are used in some aircraft, leak and cause contamination of hydraulic fluids. If you think that contamination has occurred, the system must be checked by chemically analyzing fluid samples. This analysis is conducted by the cognizant engineering activity, which verifies and identifies the contaminant and directs decontamination procedures. The effects of foreign fluid contamination depend upon the nature of the contaminant. The com-patibility of the construction materials and the system hydraulic fluid with the foreign fluid must be considered when dealing with contamination. Other effects of this type of contamination are hydraulic fluid reaction with water and changes in flammability and viscosity characteristics. The effects of con-tamination may be mild or severe, depending upon the contaminant, how much is in the system, and how long it has been in the system.