Compensated Flow Control Circuits
In a compensated flow control circuit, the pressure differential across the orifice is not affected by a change in the upstream pressure. The constant pressure differential across the orifice will produce a constant flow through the orifice.
Bypass Pressure Compensated Flow Control Valve
 | |
| Illustration 1 | g01064426 |
Illustration 1 shows a bypass type pressure compensated flow control valve. The bypass type pressure compensated flow control valve automatically adjusts to flow and load changes.
Flow Change
The amount of flow through the valve depends on the size of the orifice. Any change in oil flow through the orifice creates a change in pressure on the upstream side of the orifice. The same pressure change acts against the dump valve and the spring.
When the pump flow is within the design flow of the orifice, the force of the upstream oil pressure that is acting on the dump valve is less than the combined force of the downstream oil pressure and the spring. The dump valve remains closed and all of the pump oil flows through the orifice.
When the pump flow is more than the design flow of the orifice, the force of the upstream oil pressure that is acting on the dump valve is greater than the combined force of the downstream oil pressure and the spring. The dump valve opens and the excess oil flows through the dump valve to the tank.
No Load Pressure
 | |
| Illustration 2 | g01064427 |
Illustration 2 shows the bypass pressure compensated flow control valve in a simple lifting circuit.
When the control valve is moved to the RAISE position, pump oil is directed to the flow control valve. The flow control valve requires a pressure differential of 1378 kPa (300 psi) to send 11.4 L/min (3 US gpm) through the orifice. To send more than 11.4 L/min (3 US gpm) through the orifice, an increase in the pressure differential is required. Illustration 2 shows an increase in the pressure differential of more than 1378 kPa (300 psi) opens the dump valve. The excessive oil flows through the dump valve to the tank. This limits the flow through the orifice to 11.4 L/min (3 US gpm).
Load Pressure Increases
 | |
| Illustration 3 | g01064429 |
When the load pressure increases, the pressure increases at the orifice and in the dump valve spring chamber.
The increase in pressure at the orifice lowers the pressure differential across the orifice and attempts to reduce the oil flow through the orifice. At the same instance, the pressure is increased in the dump valve spring chamber. The additional pressure closes the dump valve. The pressure blocks oil flow to the tank. The blocked oil causes the pressure on the pump side of the orifice to increase. The pressure increases until the pressure differential across the orifice reaches 1378 kPa (300 psi). A pressure differential of 1378 kPa (300 psi) sends 3 gpm through the orifice and 2 gpm across the dump valve.
This allows the flow control valve to respond instantly to any increases or decreases in the load pressure.
Combine Orifice and Dump Valve
 | |
| Illustration 4 | g01064431 |
The more common type of flow control valve is shown in Illustration 4. This valve combines the action of the orifice and dump valve in one moving part. The pressure compensating operation is the same as the bypass pressure compensated flow control valve.
The graphic on the left shows flow through the valve. The flow is at the flow rating or less than the flow rating of the valve.
The graphic on the right illustrates the flow beginning to exceed the flow rating of the valve. The pressure differential resulting from the flow across the orifice, has become great enough to begin compressing the spring. As the spring compresses, the valve starts to dump the excess oil, as shown.
If the flow to the valve increases, the action of the orifice will cause the spring to compress and more flow will be dumped. The metered flow will remain constant as flow to the valve increases or as the flow to the valve decreases.
Restrictor Type Pressure Compensated Flow Control Valve
 | |
| Illustration 5 | g01064436 |
A restrictor type pressure compensated flow control valve is shown in Illustration 5. The controlled oil flow is set by adjusting the needle valve.
The compensator spool and bias spring work like a pressure reducing valve. The supply oil pressure is reduced to the pressure that sends the correct oil flow past the needle valve.
When the system is off, the spring moves the compensator spool to the left.
At start-up, the compensator spool is open to full oil flow and pressure. When the oil flow becomes greater than the setting of the needle valve, the needle valve restricts oil flow. This causes the oil pressure to increase on gauge 2, as shown. The increase in oil pressure is also sensed on the left side of the compensator spool. When the force of the pressure on the left side of the compensator spool overcomes the force of the spring, the compensator spool moves to the right.
Although the supply pressure may continue to increase on gauge 1, orifice 1 reduces the oil pressure at the needle valve to the force of the spring. The controlled oil flow pressure is 0 kPa (0 psi). The pressure differential across the needle valve is 1378 kPa (200 psi) which equals the force of the spring.
The needle valve is adjusted in order to allow 7.6 L/min (2 US gpm) through orifice 2 when the pressure differential across the needle valve is 1378 kPa (200 psi).
In Illustration 5, there is a relief valve (not shown), similar to Illustration 2, that is set at 4134 kPa (600 psi). Excess supply flow would return to the tank through this relief valve.
Controlled Oil Pressure
 | |
| Illustration 6 | g01064437 |
In Illustration 6 the controlled oil pressure is 1378 kPa (200 psi) as shown on gauge 3. The oil pressure in the valve spring chamber is also 1378 kPa (200 psi). The force of the spring chamber oil pressure is added to the force of the spring. The combined forces move the compensator spool to the left. Orifice 1 opens when the compensator spool moves to the left. Orifice 1 allows the oil pressure to increase at the upstream side of the needle valve.
The increase in oil pressure is also sensed on the left side of the compensator spool. The increase in pressure moves the compensator spool to the right against the combined force of the spring and controlled oil pressure. The compensator spool moves to a new position that allows 2756 kPa (400 psi) through orifice 1. The increase in gauge 2 pressure to 2756 kPa (400 psi) on the upstream side of the needle valve, maintains a pressure differential of 1378 kPa (200 psi) across the needle valve. (gauge 2 minus gauge 3)
The pressure differential of 1378 kPa (200 psi) across the needle valve sends 7.6 L/min (2 US gpm) through orifice 2.
The pressure compensated flow control valves offer more precise cylinder speed control than non-pressure compensated valves. The pressure compensated valves automatically adjust in order to keep the flow rate constant as load conditions vary.
In Illustration 6, an upstream relief valve (not shown) returns excess supply oil to the tank.
Quick-Drop Valve, Quick-Drop Mode
 | |
| Illustration 7 | g01064439 |
Illustration 7 shows a quick-drop valve. The quick-drop valve is commonly installed on the dozer lift cylinder. When the quick-drop valve is activated, the dozer blade will rapidly drop to the ground.
When the dozer blade is raised and the operator moves the lift/lower control to the QUICK-DROP position, the lift/lower control valve allows the lift cylinder rod end oil to flow to the tank. The gravitational forces that act on the dozer blade pull the rod out of the cylinder. This causes the piston to move independent of the force of the pump oil pressure. This action greatly increases the oil flow through the quick-drop valve orifice. Then, this action creates a vacuum in the head end of the lift cylinder. The increase in oil flow through the orifice causes the pressure up stream of the orifice to increase. The increased oil pressure opens the poppet valve. The open poppet valve connects the passage from the cylinder rod end to the passage to the cylinder head end. The oil from the rod end of the cylinder flows through the open poppet valve. The oil from the rod end joins the oil from the control valve and flows to the head end of the cylinder.
A small amount of oil flows through the orifice to the lift/lower control valve and to the tank.
When the blade strikes the ground, the movement of the rod out of the cylinder ceases. This causes a rapid decrease in the oil flow through the orifice. The pressure that is caused by flow through the orifice decreases and the spring closes the poppet valve. All oil flow to the head end, now comes from the lift/lower control valve.
Quick-Drop Valve Blade RAISE Mode
 | |
| Illustration 8 | g01064440 |
Illustration 8 shows the quick-drop valve when the blade is being raised.
When the operator moves the lift/lower control to the blade RAISE position, pump oil flows from the lift/lower control valve, through the orifice and to the rod end of the lift cylinder. The orifice creates a restriction to the oil flow which increases the upstream pressure. The higher pressure oil flows through the passage to the spring chamber behind the poppet valve. The pressurized oil assists the spring. The pressurized oil holds the valve closed when pressure is applied to the rod end of the cylinder.
Oil from the head end flows through the quick-drop valve and the control valve to the tank.
ISO Symbols
 | |
| Illustration 9 | g01064441 |
The ISO symbols for the basic flow control components are shown in Illustration 9.
The non-pressure compensated flow control ISO symbols are the fixed orifice and the variable orifice.
The pressure compensated flow control devices are the pressure compensated flow control valve and the pressure compensated flow control valve with a bypass.
The ISO symbol does not give any information on the actual physical structure of the component.