PROBLEMS CAUSED BY GASES IN HYDRAULIC FLUIDS

 PROBLEMS CAUSED BY GASES IN HYDRAULIC FLUIDS 

Gases can be present in a hydraulic fluid (or any other liquid) in three ways: free air, entrained gas, and dissolved air. 

Free Air 

Air can exist in a free pocket located at some high point of a hydraulic system (such as the highest elevation of a given pipeline). This free air either existed in the system when it was initially filled or was formed due to air bubbles in the hydraulic fluid rising into the free pocket. Free air can cause the hydraulic fluid to possess a much lower stiffness (bulk modulus), resulting in spongy and unstable operation of hydraulic actuators. 

Entrained Gas 

Entrained gas (gas bubbles within the hydraulic fluid) is created in two ways. Air bubbles can be created when the flowing hydraulic fluid sweeps air out of a free pocket and carries it along the fluid stream. Entrained gas can also occur when the pressure drops below the vapor pressure of the hydraulic fluid. When this hap pens, bubbles of hydraulic fluid vapor are created within the fluid stream. En trained gases (either in the form of air bubbles or fluid vapor bubbles) can cause cavitation problems in pumps and valves. Entrained gases can also greatly reduce the hydraulic fluid's effective bulk modulus, resulting in spongy and unstable operation of hydraulic actuators. 

Vapor pressure is defined as the pressure at which a liquid starts to boil (vaporize) and thus begin changing into a vapor (gas). The vapor pressure of a hydraulic fluid (or any other liquid) increases with an increase in temperature. 

Petroleum-based hydraulic fluids and phosphate ester fire-resistant fluids have very low vapor pressures even at the maximum operating temperatures of typical hydraulic systems (150°F, or 65°C). However, this statement is not true for water based fire-resistant fluids such as water-glycol solutions and water-in-oil emul sions. Because water-glycol solutions and water-in-oil emulsions contain a high percentage of water, they possess vapor pressures of several inches of Hg abs at operating temperatures of 150°F, or 65°C. On the other hand, petroleum-based fluids and phosphate ester possess vapor pressures of less than 0.1 in. of Hg abs. As a result, water-glycol solutions and water-in-oil emulsions have a much greater tendency to vaporize in the suction line of a pump and cause pump cavitation. 

Figure  gives the vapor pressure-VS.-temperature curves for pure water, water-glycol solutions, and water-in-oil emulsions. Note that this relationship for pure water and water-in-oil emulsions is essentially the same and thus is represented by a single curve. We know from experience that water starts to boil at 212°F (100°C) when the pressure is atmospheric (30 in. Hg abs, 14.7 psia, 1.01 bars abs). However, as shown in Fig. , water will also start to boil at 150°F (65°C) when the pressure is reduced to about 7.7 in. Hg abs (3.77 psia, 0.26 bars abs). Similarly, at 150°F (65°C), water-glycol boils at about 5.5 in. Hg abs (2.70 psia, 0.18 bars abs) and water-in-oil boils at about 7.7 in. Hg abs (3.77 psia, 0.16 bars abs). Thus, for water-in-oil emulsions at 150°F (65°C), if the suction pressure  at the inlet to a pump is reduced to 3.77 psia, boiling will occur and vapor bubbles will form at the inlet port of the pump. This boiling causes cavitation, which is the formation and collapse of vapor bubbles 

Cavitation occurs because the vapor bubbles collapse as they are exposed to the high pressure at the outlet port of the pump, creating extremely high local fuid velocities. This high-velocity fluid impacts on internal metal surfaces of the pump. The resulting high-impact forces cause flaking or pitting of the surfaces of internal components, such as gear teeth, vanes, and pistons. This damage results in premature pump failure. In addition the tiny flakes or particles of metal move downstream of the pump and enter other parts of the hydraulic system, causing damage to other components. Cavitation can also interfere with lubrication of mating moving surfaces and thus produce increased wear. 

One indication of pump cavitation is a loud noise emanating from the pump. The rapid collapsing of gas bubbles produces vibrations of pump components, which are transmitted into pump noise. Cavitation also causes a decrease in pump flow rate because the pumping chambers do not completely fill with the hydraulic fluid. As a result, system pressure becomes erratic. 

Frequently entrained air is present due to a leak in the suction line or a leaking pump shaft seal. In addition, any entrained air that did not escape while the fluid was in the reservoir will enter the pump suction line and cause cavitation. 

Dissolved Air 

Dissolved air is in solution and thus cannot be seen and does not add to the volume of the hydraulic fluid. Hydraulic fluids can hold an amazingly large amount of air in solution. A hydraulic fluid, as received at atmospheric pressure, typically contains about 6% of dissolved air by volume. After the hydraulic fluid is pumped, the amount of dissolved air increases to about 10% by volume. 

Dissolved air creates no problem in hydraulic systems as long as the air remains dissolved. However, if the dissolved air comes out of solution, it forms bubbles in the hydraulic fluid and thus becomes entrained air. The amount of air that can be dissolved in the hydraulic fuid increases with pressure and decreases with temperature. Thus dissolved air will come out of solution as the pressure decreases or the temperature increases. 

To avoid pump cavitation, pump manufacturers specify a minimum allowable vacuum pressure at the pump inlet port based on the type of fluid being pumped, the maximum operating temperature, and the rated pump speed. The following rules will control or eliminate pump cavitation by keeping the suction pressure above the vapor pressure of the fluid: 

1. Keep suction velocities below 5 ft/s (1.5 m/s). 

2. Keep pump inlet lines as short as possible. 

3. Minimize the number of fittings in the pump inlet line. 

4. Mount the pump as close as possible to the reservoir. 

5. Use low-pressure drop-pump inlet filters or strainers, 

6. Use a properly designed reservoir that will remove the entrained air from the fluid before it enters the pump inlet line. 

7. Use a proper oil, as recommended by the pump manufacturer. 8. Keep the oil temperature from exceeding the recommended maximum temperature level (usually 150°F, or 65°C).