Air bleed valves can be used anywhere in a hydraulic system when it is necessary to remove trapped air. However, they are most often used to remove air from the outlet side of a hydraulic pump. When used in this manner, they are sometimes referred to as “startup” valves. The use of a properly installed air bleed valve will aid in pump priming and improve overall system performance. In addition, component life will be improved.
When a hydraulic pump is turned off for an extended period of time, oil can drain back to the reservoir through the internal pump clearances if the pump is located above the reservoir fluid level. This can allow air pockets to form. An air bleed valve will automatically remove the air from the oil when the pump is restarted and comes in basically two designs. Figure 1 shows an inline valve and figure 2 is an example of a cartridge-type valve.
Although the two designs look different, the operation is essentially the same. The operating principle of an air-bleed valve is based on the difference in behavior of gases (air) and liquids (oil) under pressure due to their relative difference in viscosity. Simply put, viscosity is a measure of a fluid’s resistance to flow. A low viscosity means that the fluid is less resistant to flow. Conversely, a higher viscosity means more resistance to flow. Shown here are two of the most common schematic symbols used for these valves.
The illustration at right shows what will happen when air is present. Since air has a very low viscosity, it will easily flow through the orifice. The pressure in the pilot passage will be very low. Therefore, the spring will hold the poppet in the open position which ports the air through the valve.
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