Sequence Valve: Structure, Applications & Types

The sequence valve is a component for controlling a hydraulic flow. It is also called a “cut-in valve”. Its purpose is to open another hydraulic circuit when a defined pressure is reached. It effectively enables an additional function once a certain pressure is attained.

Structure of a Sequence Valve

A sequence valve essentially consists of a spring-loaded shut-off valve and a bypass line. As long as the pressure in the overall system is insufficient, the hydraulic flow bypasses the closed sequence valve.

When the defined pressure is reached, the holding force of the closing spring is overcome, and the second channel opens. Optionally, the additionally controlled component is located directly behind the spring-loaded shut-off or is enabled by a hydraulic relay.

Application of a Sequence Valve

The sequence valve serves to expand the functionality of a hydraulic system without increasing the complexity of its control. Therefore, sequence valves are usually coupled to actuators that should only be enabled after a certain state is reached. Actions that occur in a defined sequence after a preceding action and must also be reset before the previous action is reversed, represent the application field for the sequence valve.

Examples of Sequence Valve Applications

Sequence valves are often used in complex hydraulic circuits where actions must be switched on and off in a specific order. The harvester is a good example here. A harvester is an attachment for forestry equipment. It has three tasks:

• Gripping the log
• Moving the log
• Cutting the log to length

Gripping the log is done by a hydraulic grapple. No further action may take place until the grapple’s holding force is fully established. There are various technical approaches for pushing the log through. In any case, cutting to length is only triggered when the clamping force of the holding grapple is fully ensured. The sequence valve thus significantly increases operational safety and efficiency.

Types of Sequence Valves

Generally, sequence valves are available with two or three ports. The simplest sequence valves consist only of the closing spring and an uncontrolled bypass line. In the next stage of development, the bypass line is equipped with a check valve. Subsequently, there are no limits to the complexity of sequence valves.

Advantages of a Sequence Valve

A sequence valve does not necessarily have to trigger another hydraulic function. If the return spring is equipped with a spool, other arbitrary applications can be added. Electrical circuits, controlled by direct switches or proximity sensors, can thus be easily switched on depending on pressure. The connection of pneumatic applications is equally common. This flexibility and simple creation of dependent switching sequences make the sequence valve a very popular component.

Technical Characteristics of Sequence Valves

A sequence valve opens when a certain pressure is reached. If this pressure drops below a specific threshold, the sequence valve closes again. The actual movement of this component is therefore rather small. The frequency and duration of mechanical movements in a sequence valve cannot be compared to those of a pump, a hydraulic cylinder, or a radial hydraulic motor. This gives the component a certain robustness and long service life. The sequence valve is relatively insensitive to pressure fluctuations, changes in viscosity, and contamination in the oil. Nevertheless, for the use of hydraulic systems, powerful filtration with replaceable filter cartridges is generally recommended. This guarantees reliable and long-lasting operability of the entire hydraulic system.