Hydraulic Fluid Power. Andrea Vacca
equals 874.9 italic k g slash m cubed"/>
Therefore, from the expression
we have
It is therefore possible to observe how, in this (gaseous) cavitation condition, the reduction in outlet flow is about 4.3%, much more pronounced with respect to the case (a), where there was no cavitation.
A final remark can be made on the evaluation of the equivalent (or effective) bulk modulus. For the typical operating pressure of hydraulic control systems, the elasticity of the material is also not negligible. Consider again the case of Figure 2.12 while also including the elasticity of the walls according to the bulk modulus of the material (similar to the Young modulus definition):
(2.33)
The equivalent bulk modulus for the system becomes
(2.34)
2.8 Contamination in Hydraulic Fluids
A hydraulic oil is subject to various forms of contamination:
Solid contamination. The solid contamination is the most common and intuitive form of contamination; it affects the operation and life of all hydraulic circuits. The solid contamination is due to the presence of undesired solid particles (metallic, plastic, fibers of different types, etc.) within the hydraulic fluid. These particles can accelerate the wear of the hydraulic components or produce the blockage of small flow connections (such as those of small hydraulic orifices). Solid contamination can cause malfunctioning of the hydraulic system and can also lead to the catastrophic failure of some components, like pumps and motors.
Liquid contamination. In general terms, liquid contamination refers to the presence of other liquids in the working fluid that can either be chemically aggressive to the hydraulic components or can deteriorate the properties of the hydraulic fluid. In the majority of the cases, the liquid contaminant is water. For example, water causes rusting in hydraulic components (which can be visible, for example, in the reservoir of the system), reduces the lubrication characteristics of the oil, and can cause unwanted reactions leading to the formation of alcohols, acids, or sludges. In addition, water has a higher vapor pressure values when compared with typical oils. Therefore, the presence of water can lead to instances of vapor cavitation that can cause instabilities and damages of the mechanical parts.
Gas contamination. Gas contamination usually refers the presence of undissolved or entrained air in the hydraulic oil. As mentioned before, this can lead to the premature wear of certain hydraulic components, caused by the implosion of the air bubbles, or can affect the controllability of parts of the hydraulic system. The physics behind the gas contamination phenomenon was described in detail in Section 2.7 of this chapter.
For every type of contaminant, three different sources can be distinguished:
1 Native contamination. Contaminant particles can be present in the system as residual from the manufacturing of the components or from the assembly or repair of the system. Residual from machining or welding processes, or excessive sealants, can also be present in a brand‐new hydraulic system before the hydraulic fluid is introduced for the first time. Humidity from the environmental air can also cause undesired amount of water to condense in the components.It is also important to remark that a new hydraulic fluid may not be clean enough for a modern hydraulic system. Manufacturing, handling, and storage processes for the hydraulic fluids need to be strict enough to guarantee that the level of contaminants complies with the requirements of the hydraulic system.
2 Ingressed contamination. All the three types of contaminants (solid, liquid, gas) can enter the hydraulic system from the surrounding environment. The airflow into the reservoir, to compensate the changes in the fluid level during the operation of the system, might bring dirt particles and excessive humidity. The breather caps typically used in hydraulic reservoirs include filters to limit the amount of contaminants entering the system. Another typical source of ingressed contamination can be through hydraulic cylinders. Particularly when the cylinder is worn, the contaminants (dust and dirt particles) can enter from wiper seal during the retraction of the piston rod. This is a typical problem of many off‐road hydraulic applications such as construction equipment.
3 Internally generated contamination. Many hydraulic components are subjected to wear. The most critical components subjected to wear are usually hydraulic pumps and motors. These components have internal parts in relative motion, and consequently, particles can be removed from the interior parts. These particles can further promote wear or cause damage to other surfaces not subjected to wear.
Numerous studies have identified the fluid contamination as the most frequent cause of failure of a hydraulic system. Therefore, it is important to always take all possible precautions against possible cavitation damages when designing or maintaining a hydraulic system.
Necessary precautions against gas cavitation were already described in Section 2.7: the designer has to make every possible effort to eliminate or limit chances of entrained air or fluid pressure falling below the saturation conditions.
As pertains to liquid contamination, usually related to an undesired water content in the oil, it is important to maintain the water level below the saturation conditions. As a matter of fact, like air, water can also dissolve into the hydraulic fluid without interfering with the main fluid properties. For many hydraulic machines, the humidity of the surrounding air does not cause the water level within the working fluid to rise above saturation conditions. However, instances of water ingression from the reservoir or from worn cylinders can cause the water level to rise to levels that hamper the safe operation of the system.
Several techniques are nowadays available for removing water from hydraulic fluids using devices that are generally called “separators.” Separating tanks are a common method based on the gravity and take advantage of the higher density of water with respect to most of the hydraulic fluids. If there is a sufficient resident time for the fluid in the hydraulic reservoir, water tends to settle at the bottom, which makes its removal possible. Other techniques include centrifugal separation, coalescing separation, and absorbent polymer separation [27]. These methods are sometimes used in special filters.
Solid contamination is always unavoidable in a typical hydraulic system due to all the three sources of contamination listed above. For this reason, one or more filter elements that ensure proper cleanliness level of the working fluid need to be present in a well‐designed hydraulic system.
2.8.1 Considerations on Hydraulic Filters
The hydraulic components most affected by solid contaminants are those where mechanical parts are in relative motion with minimum clearances.