control valve

What is a sequence valve?

A sequence valve is widely used in a hydraulic system these valves are pressure control valves. The sequence valves are similar to a pressure relief valve and these valves are used where a set of operations are to be controlled in a pressure-related sequence. The major function of a sequence valve is to divert the flow in a predetermined sequence. This valve is a pressure-actuated valve and its construction is very much similar to a relief valve and normally a closed valve. The operating principle of the sequence valve is that when the main system pressure overcomes the spring setting then the valve spool will be moved. So the outlet of the sequence valve would remain closed till the upstream pressure builds up to a predetermined value so then the valve would open to allow the transfer of air from the inlet to the outlet. The sequence valves are mostly used to force two actuators to operate in sequence. The sequence valve has a good feature and it is that this valve has a separate drain connection from the spring chamber and it is because a high pressure could occur in the output port during the normal operation conditions. When the pressure rises beyond its limit the pressure sequence valve would allow the flow to occur in another part of the system. The pressure sequence valves are installed in pneumatic controls in which a specific pressure is needed for its switching operation.

What is the need for a sequence valve and how can it control the pressure?

A sequence valve is called a non-passing pressure control valve which allows one operation to occur before another based on pressure. In order to do certain processes, two or more actuators working in a parallel circuit must move in sequence and it can only be done by using separate directional control valves and limit switches but it won’t be a safe process so for this purpose we can use a sequence valve.

By using a sequence valve the automatic sequencing of two or more actuators in the hydraulic circuits can be done. The primary actuator would move when the fluid flow is directed to the actuator section of the circuit. The sequence valve would block the flow to the secondary actuator until a predetermined pressure is reached after that it would allow the fluid to flow to that actuator. A sequence valve is fitted with a check valve and this would allow the free flow of the fluid around the valve when the actuator direction is reversed. The sequence valve is used when two circuits are supplied by one pump and one circuit has priority over the other. The sequence valves are used in the hydraulic circuits.

How does a sequence valve operates and what are the parts of sequence valve?

As the name implies the sequence valve can control a sequence of operations, If a preset pressure is reached the sequence valve would allow the flow to branch circuits, instead of sending flow back to the tank. The sequence valve would use a pressure signal to shift the spool and thereby opening or closing a flow path at a preset pressure. So this valve could divert the flow in a predetermined sequence it can operate the cycle of a machine automatically. The sequence valve could be a direct pilot operated or a remote pilot operated type. The construction of this type of valve is similar to the direct relief valve. The sequence valve has two major ports, one of these ports connects the main pressure line and the other one is connected to the secondary circuit. Mostly the secondary port is closed by the spool and the pressure on the spool works against the spring force. So if the pressure exceeds the preset value of the spring the spool lifts and the fluid will flow from the primary port to the secondary port.

The spring chamber of the sequence valve must be drained in order to prevent the build-up of pressure caused by the leakage past the spool from the pressure at the primary port this drain is connected by an external line to the reservoir.

What is the difference between a sequence valve and a relief valve?

The only difference between a sequence valve and a relief valve is that the former has a spring chamber drain to a separate port. This would reduce the stream pressure increasing the setting of the valve. The major advantage of using a sequence valve over a pilot is to close check, is that when centering the directional valve the pressure can be set higher than any residual pressure.

What are the types of sequence valve?

Direct acting sequence valve

This type of sequence valve is a spool-type valve with a spring force that holds the spool closed. So if the inlet pressure exceeds the setting of the spring, then the spool would shift and thus it would direct the flow to the second function. This type of sequence valves is well suited for low pressure and constant flow.

Pilot operated sequence valve

This type of sequence valve has an internal pilot and external drain. The inlet pressure that acts on the pilot seat area causes the ball to move back against the spring and this would create a flow to the tank through the drain port. When the flow passes through the orifice in the main pool, there would be a pressure imbalance that moves the main spool back against the spring. The valve can change from fully closed to fully open if there is a small rise in pressure. This type of sequence valves can be used for varying flows and higher pressure.

Unloading valve

The valve would only open if there is proper pressure at 1 and only after that there will be flow to 2 otherwise it will be blocked so if there is sufficient pressure at 1 the poppet would open and allow flow from 2 to 3. If the pressure at 1 drops below the pilot ratio value multiplied by the predetermined spring setting the valve would close and thus blocks the flow at 2. These valves are mostly used to unload a pump outlet to the tank at a minimum pressure drop and thus we could get high system efficiency.

Kick down sequence valve

This is another type of pilot-operated sequence valve, this valve blocks the flow at 1 until sufficient pressure exists at 1 to force the pilot poppet off its seat. This would create a pressure differential across the spool and this would cause the spool to shift and open flow from 1 to 2. The spool will remain open till the flow through the valve is shut off. Mostly these valves are used as a safety device in circuits where overloading or overheating could cause damage if the pressure is held for an extended period of time.

Sequence valve with reverse flow check

This type of valve is used in a double-acting system for the flow to have to pass past the sequence valve in the reverse direction. In order to do this, a free flow check valve will be mounted around the cartridge.

What are the advantages of sequence valve?

  • It can provide a sequenced series of operations such as in a clamp and lift circuit
  • It can be used as a relief valve where the return line has high or varying backpressure.
  • It can direct the fluid flow in a sequential manner
  • In certain cases it has check valve to do the return flow
  • This valve can be used as a compensator in a pressure compensated flow regulator circuit

What are the disadvantages of a sequence valve?

A sequence valve could generate a great amount of heat because the first actuator to move takes higher pressure than the subsequent actuators. So because of this, there will be a high-pressure drop in the sequence valve and it is wasted energy.

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