Relief Valves
Hydraulic systems are designed to operate within a certain pressure range. Exceeding this range can damage the system components. Exceeding this range will cause the system to become dangerous to personnel. The relief valve maintains the pressure within the designed limit by opening and allowing excessive oil to flow either to another circuit or back to the tank.
Simple Pressure Relief Valve, Cracking Pressure
Illustration 1 | g01062797 |
Illustration 1 shows a simple relief valve in the cracking pressure position.
The simple relief valve (also called direct acting relief valve) is kept closed by spring force. The spring tension is set to the relief pressure setting. The relief pressure setting is not the pressure when the valve first begins to open.
When a condition develops that will cause a resistance to the normal oil flow in the circuit, excessive oil flow will cause the oil pressure to increase. The increasing oil pressure is sensed at the relief valve. When the force of the increasing oil pressure overcomes the force of the relief valve spring, the valve moves against the spring and starts to open. The pressure that is required to begin the opening of the valve is called the cracking pressure. The valve will open enough to allow the excess oil to flow through the valve.
Simple Pressure Relief Valve, Relief Pressure Setting
Illustration 2 | g01062798 |
An increase in the resistance to oil flow will increase the volume of excess oil. An increase in the resistance to oil flow will increase the circuit pressure. The increase in circuit pressure overcomes the new spring tension allowing for the relief valve to open.
The process that is shown in Illustration 2 is repeated until the maximum volume of oil (full pump flow) is flowing through the relief valve. The relief pressure setting is shown in illustration 2.
The simple relief valve is commonly used where the volume of excess oil flow is low or where there is a need for a quick response. This makes the simple relief valve ideal for relieving shock pressures or as a safety valve.
Pilot Operated Relief Valve, CLOSE Position
Illustration 3 | g01062799 |
The pilot operated relief valve (illustration 3) is often used in systems that require a large volume of oil and a small differential between the cracking pressure and the full flow pressure.
In the pilot operated relief valve, a pilot valve (simple relief valve) is used to control the unloading valve (main valve).
The pilot valve is much smaller. The pilot valve does not handle large volume oil flow. Therefore, the spring in the pilot valve is smaller allowing for a more precise pressure control. The difference between the pilot valve cracking pressure and maximum pressure is held to a minimum.
The unloading valve is large enough to handle the full pump flow at the designed maximum relief pressure. The unloading valve uses the system oil pressure and the spring to keep the valve closed. Therefore, the unloading valve spring does not need to be strong and heavy. This allows the unloading valve to have a more precise opening pressure.
The system oil flows into the relief valve housing, through the unloading valve orifice, filling the unloading valve spring chamber. The oil in the unloading valve spring chamber comes in contact with a small area of the pilot valve. This design allows the pilot valve to use a small spring to control a high pressure.
When the oil pressure increases in the system, the same pressure is in the unloading valve spring chamber. Therefore, the oil pressure is the same on both sides of the unloading valve. The combined force of the system oil pressure in the unloading valve spring chamber and the spring force on the top of the unloading valve is greater than the force of the system oil pressure against the bottom of the valve. The combined force in the spring chamber keeps the unloading valve closed.
Pilot Operated Relief Valve, OPEN Position
Illustration 4 | g01062800 |
When the system oil pressure exceeds the pilot valve spring setting (illustration 4), the pilot valve opens. The open pilot valve allows the oil in the unloading valve spring chamber to flow to the tank. The pilot valve opening (orifice) is larger than the unloading valve orifice. Therefore, the oil will flow past the pilot valve much faster than through the unloading valve orifice.
This differential inflow allows the pressure to decrease in the unloading valve spring chamber. The force of the higher system oil pressure moves the unloading valve up against the spring. The excessive pump oil flows through the throttling holes in the unloading valve to the tank. The throttling holes allow the unloading valve to dump the volume of oil necessary to maintain the desired relief pressure.
Relief Valve ISO Symbol CLOSED
Illustration 5 | g01062802 |
The relief valve ISO graphic symbol that is shown in illustration 5 can represent either a simple relief valve or a pilot operated relief valve. Both types of relief valves can use the same ISO graphic symbol.
The above relief valve ISO symbol shows a single valve envelope in the CLOSED position. The system pressure is sensed through the pilot line at the top of the envelope. The system pressure works in order to move the valve (arrow) against the spring. During normal operations, the pump flow is blocked at the closed valve.
Relief Valve ISO Symbol OPEN
Illustration 6 | g01062805 |
The relief valve ISO symbol that is shown in illustration 6 shows a single valve envelope in the OPEN position.
When the force of the system oil pressure overcomes the spring force, the arrow moves down causing the valve to open. This will cause the oil line from the pump to connect with the oil line to the tank. The pump oil flows through the valve to the tank.
Variable Relief Valve ISO Symbol
Illustration 7 | g01062806 |
Illustration 7 shows the ISO symbol for a variable relief valve.
The variable relief valve is a single envelope valve with an arrow through the spring. The arrow shows that the spring tension can be adjusted.
Sequence Valve, CLOSE Position
Illustration 8 | g01062807 |
The sequence valve (illustration 8) is simply a pilot operated relief valve between the pump and the second circuit. The sequence valve is used when two circuits are supplied by one pump and one circuit has priority over the other.
The sequence valve blocks pump oil flow to circuit 2 until circuit 1 is satisfied. When pump oil fills circuit 1, the oil pressure begins to increase. The increase is sensed throughout the circuit as well as at the bottom of the unloading valve and in the unloading valve spring chamber of the sequence valve.
Sequence Valve, OPEN Position
Illustration 9 | g01062808 |
When the pressure in the unloading valve spring chamber exceeds the setting of the pilot valve spring, the pilot valve opens. The open pilot valve allows the oil in the unloading valve spring chamber to flow to the tank. This allows the pressure to decrease in the unloading valve spring chamber. The force of the higher system oil pressure will move the unloading valve against the unloading valve spring force. This will cause the passage to circuit 2 to open. Pump oil flows through the sequence valve to circuit 2. The sequence valve remains open until the pressure in circuit 1 decreases to less than the pressure setting of the sequence valve.
Note: The sequence valve is used on self loading scrapers to open the floor door prior to starting the ejector forward.
Sequence Valve ISO Symbol
Illustration 10 | g01062809 |
The operation of the sequence valve is the same as the operation of the relief valve.
In the relief valve, the spring chamber is drained internally to the outlet passage. In the sequence valve, the outlet passage connects to the second circuit. Because the second circuit is under pressure when the sequence valve opens, the pilot valve spring chamber must be externally drained to the tank.