Introductionto Flow Control Valves
Flow control consists of controlling the volume of oil flow in a circuit or out of a circuit. Controlling flow in a hydraulic circuit can be accomplished in several ways.
The most common way is by installing an orifice. When an orifice is installed, the orifice presents a higher than normal restriction to the pump flow. The higher restriction increases the oil pressure. The increase in oil pressure causes some of the oil to take another path. The path may be through another circuit or the path may be over a relief valve.
Also discussed are non-compensated and compensated flow control valves.
Orifice
An orifice is a small opening in the oil flow path. Flow through an orifice is affected by several factors.
Three of the most common factors are listed below:
- The temperature of the oil.
- The size of the orifice.
- The pressure differential across the orifice.
Temperature
The oil viscosity changes with changes in temperature. Viscosity is a measurement of the oil's resistance to flow at a specific temperature. Hydraulic oil becomes thinner and flows more readily as the temperature increases.
Orifice Size
The size of the orifice controls the flow rate through the orifice. A common example is a hole in a garden hose. A small pin hole will leak in the form of a drip. A larger hole will leak in the form of a stream. The hole, whether small or large, meters a flow of water to the outside of the hose. The amount of water that is metered depends on the size of the orifice.
The orifice size may be fixed or variable.
Check Valve With A Fixed Orifice
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| Illustration 1 | g01064413 |
Illustration 1 shows an example of a check valve with a fixed orifice that is commonly used in construction equipment. The fixed orifice is a hole through the center of the check valve. When oil flow is in the normal direction the valve opens. The oil flow will allow oil to flow around the valve and through the orifice. When oil attempts to flow in the reverse direction, the valve closes. All reverse flowing oil must flow through the orifice that controls the flow rate.
Variable Orifice
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| Illustration 2 | g01064414 |
Illustration 2 shows a variable orifice in the form of a needle valve. In the needle valve, the size of the orifice is changed by the positioning of the valve tip in relation to the valve seat.
The oil flow through the needle valve must make a 90° turn and pass between the valve tip and the valve seat. The needle valve is one of the most frequently used variable orifices.
When the valve stem is turned counterclockwise, the orifice becomes larger and the flow increases through the valve.
When the valve stem is turned clockwise, the orifice becomes smaller and the flow decreases through the valve.
The schematic in Illustration 3 and Illustration 4 consist of a positive displacement pump, a relief valve, and a variable orifice. The relief valve is set at 3445 kPa (500 psi) and limits maximum pressure in the system. The orifice may be adjusted to any flow between zero and 18.9 L/min (5 US gpm).
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| Illustration 3 | g01064415 |
In Illustration 3, the variable orifice allows a flow of 15.1 L/min (4 US gpm) through the orifice at a pressure of 3445 kPa (500 psi). Any increase in flow through the orifice would require a pressure greater than 3445 kPa (500 psi). If the pressure exceeds 3445 kPa (500 psi), the relief valve opens and the excess oil (1 gpm) flows through the relief valve. Therefore, the flow through the orfice is limited to 3.8 L/min (1 US gpm) by the relief valve setting.
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| Illustration 4 | g01064417 |
In Illustration 4, the variable orifice allows a flow of 3.8 L/min (1 US gpm) through the orifice at a pressure of 3445 kPa (500 psi). Any increase in flow through the orifice requires a pressure greater than 3445 kPa (500 psi). If the pressure exceeds 3445 kPa (500 psi), the relief valve opens and the excess oil 15.1 L/min (4 US gpm) flows through the relief valve. Therefore, the flow through the orfice is limited to 3.8 L/min (1 US gpm) by the relief valve setting.
Pressure Differential
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| Illustration 5 | g01064419 |
Flow through an orifice is affected by the pressure differential across the orifice. The greater the pressure differential across the orifice, the greater the oil flow through the orifice.
In Illustration 5 pressure differential is illustrated using the two tubes of tooth paste. When the tube of toothpaste is gently squeezed as in (A), the pressure difference between the inside of the tube and the outside of the tube is small. Only a small amount of tooth paste is forced out of the tube.
When the tube is squeezed with greater force as in (B), the pressure difference between the inside of the tube and the outside of the tube increases and a larger amount of toothpaste is forced out.