Air Bleed Valves and Hydraulic Pumps

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.

Note that these are normally open valves. In other words, an internal spring will shift and hold the valve poppet in the open position, allowing flow through the valve. An orifice is located just upstream of the poppet and a pilot passage is connected upstream of this orifice. Any pressure in the pilot passage acts against the internal spring. If this pressure is sufficient to overcome the spring force, then the poppet will close. 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.

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.

Once all the air has been removed, a back pressure will build in the pilot passage. This is due to the higher viscosity (resistance to flow) of the oil. Once this back pressure overcomes the force of the internal spring, the poppet will shift into the closed position. This is illustrated in the picture at the left. The poppet will then remain closed so long as the pump is running and no air is present.

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