The sequence valve is a pressure control valve and it is widely used in hydraulic systems. Sequence valve is a non-passing valve. The sequence valves are quite similar to the pressure relief valves.
The sequence valve is used in applications where a set of operations must be controlled in a pressure-related sequence. Sequence valve majorly performs the function of diverting the flow in a predetermined sequence.
A sequence valve is a pressure-actuated valve. The construction of a sequence valve is almost similar to that of a relief valve and is normally a closed valve.
Operating Principle of Sequence Valves
Sequence valves operate when the main system pressure overcomes the spring setting. Once the pressure overcomes, the valve spool would be moved. Thus the outlet of the sequence valve will remain closed till the upstream pressure builds up to a predetermined value. After this, the valve will open and allow the transfer of air from the inlet to the outlet.
They are generally used to force two actuators to operate in sequence. These valves have a separate drain connection from the spring chamber. This is because high pressure could result in the output port during normal operating conditions.
If the pressure increases beyond the set limit, the pressure sequence valve would allow the flow to occur in another part of the system. Pressure sequence valves are installed in pneumatic controls where the specific pressure is needed for its switching operation.
Construction of Sequence Valve
The sequence valve has a construction similar to a direct relief valve. It has two major ports. One of these ports will connect to the main pressure line. The second port is connected to the secondary circuit. The second port is generally closed by the spool.
The pressure on the spool works against the spring force. Thus if the pressure exceeds the preset value of the spring, the spool will lift up and the fluid will flow from the primary port to the secondary port. The spring chamber in the sequence valve should be drained in order to prevent build-up of the pressure at the primary port. The drain will be connected by an external line to the reservoir.
Operation of Sequence Valve
A sequence valve is used to control a sequence of operations. So when a preset pressure is reached the sequence valve will allow the flow to branch circuits rather than sending it back to the tank.
They use pressure signals to shift the spool. Due to this, the low path is opened or closed at a preset pressure. Hence, the valve can divert the flow in a predetermined sequence and it can also operate the cycle of a machine automatically.
Why are sequence valves needed?
In some processes, two or more actuators in a parallel circuit should move in sequence. This can be done only by using separate directional control valves and limit switches. But, this is not a safe practice. Hence, during such conditions sequence valves are used.
When sequence valves are used, the automatic sequencing of two or more actuators in the hydraulic circuits can be possible. In such a case, the primary actuator will move when the fluid flow is directed to the actuator section of the circuit.
The sequence valve will block the flow to the secondary actuator till a predetermined pressure is reached. After the predetermined pressure is reached, it will allow the fluid to flow to the actuator. The sequence valve has a check valve fitted to it. This allows the free flow of fluid around the valve when the actuator direction is reversed. A sequence valve is used when two circuits are supplied by one pump and one circuit has priority over the other.
Types of Sequence Valves
There are five types of sequence valves. They are:
1. Direct Acting Sequence Valve
These are spool-type valves with a spring force that holds the spool closed. Thus, if the inlet pressure will exceed the setting of the spring, then the spool will shift. So, it will direct the flow to the second function. These types of sequence valves are well-suited for low-pressure and constant flow.
2. Pilot Operated Sequence Valve
These valves have an internal pilot and external drain. The inlet pressure acts on the pilot seat area which causes the ball to move back against the spring. This will create a flow to the tank through the drain port. So when the flow passes through the orifice in the main pool, there will be a pressure imbalance. This imbalance in pressure moves the main spool back against the spring.
These valves can change from fully closed to fully open if there is a small rise in pressure. These types of sequence valves are generally used for varying flows and higher pressure.
3. Unloading valve
This type of sequence valve opens only if there is proper pressure at the inlet port. After proper pressure is received in the inlet, there is a flow to the secondary side. Otherwise, it is blocked. Hence, when the pressure on the primary side reaches a preset value, the poppet will open and allow the output to be delivered.
These types of sequence valves are usually used to unload the pump outlet to the tank at a minimum pressure drop. Because of this, we get high system efficiency.
4. Kick Down Sequence Valve
These valves block the flow till a sufficient flow is received at the inlet. When a predetermined pressure is reached at the inlet, it forces the pilot poppet off its seat. This will create a pressure difference across the valve spool. It would then cause the spool to shift and open flow from the primary to the secondary.
The spool remains open until the flow through the valve is shut off. These types of sequence valves are generally used as safety devices in circuits where overloading or overheating can cause damage if the pressure is held for an extended period of time.
5. Sequence Valve With Reverse Flow Check
These valves are used in a double-acting system when the flow has to pass through the sequence valve in the reverse direction. So to do this, a free-flow check valve is mounted around the cartridge of the sequence valve.
Advantages of Sequence Valve
Sequence valves have the following advantages:
- Sequence valves provide a sequenced series of operations, like in a clamp and lift circuit.
- Sequence valves can be used as relief valves when the return line has high or varying backpressure.
- A sequence valve can direct the fluid flow in a sequential manner.
- Some sequence valves have a check valve to do the return flow.
- A sequence valve can be used as a compensator in a pressure-compensated flow regulator circuit.
Disadvantages of Sequence Valve
Sequence valves have the following disadvantages:
- The sequence valve generates a lot of heat. This is because the first actuator takes higher pressure to move than the subsequent actuators. Thus, there will be a high-pressure drop in the sequence valve. This is a waste of energy.
Applications of Sequence Valve
Sequential valves see major applications in the following:
- Sequence valves are used in controlling the sequential action of multiple components.
- Sequence valves are used in pressure-keeping circuits.
- Sequence valves can be used as balance valves in order to prevent the free fall of cylinder pistons caused due to self-weight.
- Controlled sequence valves can be used externally as unloading valves to unload the pump.
- Controlled sequence valves can be used internally as back pressure valves.
