In hydraulic design, sequencing is all about control. It ensures that Event B only happens once Event A is complete.
A sequence valve is a pressure-controlled device that acts like a mechanical logic gate. It is simple, robust, and surprisingly effective at ensuring hydraulic functions occur in the correct order, without requiring any electronics.
Let’s explore what sequence valves do, how they compare to PLC-controlled sequencing, and when each makes sense for your next design.
What Is a Sequence Valve?
A sequence valve allows one action to start only after another has reached a set pressure. It acts as a priority manager, controlling flow based on pressure rather than timing or position.
Classic examples:
- Clamp then cut. The tool will not move until the clamp hits full pressure.
- Retractable doors. Doors open fully before actuators behind them move.
- Multi-stage presses. Secondary rams engage only after the main platen makes contact and builds pressure.
How it works:
- Inside is a spring-loaded spool or poppet.
- Oil flow to the secondary circuit is blocked until upstream pressure in the primary circuit rises to the valve’s set value.
- Then the valve opens, allowing flow to the secondary circuit. Due to internal design, which often includes a pressure differential or hysteresis, the valve typically stays open until pressure drops significantly below its opening set point. Proper operation also depends on how the spring chamber is drained, either internally or externally.
Where Sequence Valves Shine
Designers reach for sequence valves in situations that demand mechanical certainty without the complexity of software or sensors.
Use cases include:
- Simple jigs and fixtures. Clamp-and-pierce stations with two cylinders and no wiring.
- Harsh or explosive environments. No ATEX requirements for electronics and no sensitive components to fail under heat or vibration.
- Specific safety interlocks. Ensures a critical safety device, such as a mechanical lock, only engages after a primary action is complete and pressure has shifted appropriately.
Design tips:
- Set pressure. Adjust to be 10–15% higher than the pressure required to complete the preceding operation, but well below the system relief valve setting. This avoids premature sequencing and reduces issues like chatter.
- Drain type. Use an external drain if there is significant or fluctuating back-pressure in the return line to maintain a stable set pressure.
- Pilot source. An internal pilot is used for simplicity when sensing pressure directly at the valve. Use an external pilot for remote pressure sensing when the control point differs from the flow path.
- Pressure drop. Always account for the pressure drop across the valve when open, especially in systems requiring precise flow rates.
- Holding performance. Use poppet-type sequence valves if the downstream actuator must not creep due to leakage.
PLC-Controlled Sequencing: When Smarts and Flexibility Matter
When a process involves multiple steps, changeable logic, or feedback from various sensors, a Programmable Logic Controller (PLC) becomes essential.
Advantages of PLC sequencing:
- Fully programmable for complex sequences.
- Supports any trigger, including pressure, position, time, temperature, flow rate or operator input.
- Easily adapted and updated through software changes.
- Built-in diagnostics, alarms and trend logging.
- Seamlessly integrates with robots, conveyors and HMI panels.
Comparing Both Approaches
Trigger Mechanism:
- Sequence valve. Triggered by pressure reaching a preset mechanical threshold.
- PLC system. Triggered by any monitored condition programmed into the logic.
Installation and Commissioning:
| Task | Sequence Valve | PLC Control |
|---|---|---|
| Piping effort | More complex. Each valve requires dedicated piping. | Often simpler using integrated manifolds and directional valves. |
| Wiring | None for the valve itself. | Required for PLC, I/O modules, sensors and solenoid valves. |
| Setup | Manual adjustment via screw and locknut. | Requires laptop and software (ladder logic, structured text, etc). |
Flexibility and Adaptation:
| Feature | Sequence Valve | PLC-Based Control |
|---|---|---|
| Logic changes | Requires mechanical modification. | Software change uploaded to PLC. |
| System expansion | Limited and hardware-intensive. | Scalable with added I/O and updated logic. |
| Safety integration | Basic failsafe behaviour. | Can meet high safety standards (PL e / Cat 4 / SIL 3). |
| Remote diagnostics | Not supported. | Fully available via networked diagnostics or HMIs. |
Cost Comparison: Valves vs PLC
Upfront Hardware (Typical Ranges)
| Component | Sequence Valve System | PLC-Based System |
|---|---|---|
| Valves | £80–£250 each | £30–£80 for standard solenoids (more for IO-Link types) |
| Sensors & PLC | None | £200–£2,000+ for PLC and I/O; sensors vary widely |
| Wiring & I/O | None | £150–£1,000+ |
| HMI (optional) | Not applicable | £300–£1,200+ |
Verdict. For basic sequences involving up to three actuators and fixed logic, sequence valves offer a lower initial cost. As the number of actuators and control complexity increases, PLC systems become more economical and scalable over time.
Operating Costs and Maintenance
| Aspect | Sequence Valves | PLC Systems |
|---|---|---|
| Energy use | Higher due to pressure drop and throttling. | More efficient through pump control and energy-optimised logic. |
| Troubleshooting | Manual, using gauges and circuit familiarity. | Digital diagnostics and software tools improve response times. |
| Fault finding | Slower and more trial-and-error based. | Rapid, thanks to sensor feedback and status logging. |
| Skill required | Hydraulic technician with circuit knowledge. | Controls engineer or technician with software and electrical skills. |
Verdict. For rugged or remote applications with minimal electronics expertise, sequence valves are simpler to maintain. In automated plants with trained personnel, PLC systems reduce downtime and improve transparency.
Hybrid Systems: The Best of Both Worlds
Modern machines increasingly combine sequence valves and PLCs to benefit from both approaches.
Examples include:
- A pressure switch tied to a sequence valve feeding status to a PLC.
- IO-Link or CANopen valves that allow digital monitoring and set-point adjustment via HMI.
- Sequence valves for fixed safety interlocks, PLC logic for adaptable operations.
- Manual levers for maintenance, with PLC-based control during production.
How to Choose: A Quick Decision Checklist
| Scenario | Recommendation |
|---|---|
| Fewer than three sequential steps, fixed pressure-based logic | Sequence valve |
| Position, timing, or temperature inputs required | PLC |
| Frequent logic or configuration changes expected | PLC |
| Safety compliance (e.g., PL or SIL) needed | PLC with certified components |
| Explosive or extremely hot environment | Sequence valves or ATEX-rated PLC gear |
| Mechanically skilled maintenance team, limited software support | Sequence valves |
Sequence valves are robust, mechanically simple devices, ideal for small-scale machines with fixed, pressure-dependent sequences. When your design demands flexibility, diagnostics, integration and complex control logic, PLCs are the better choice.
The most effective designers do not rigidly pick one approach. They understand the strengths of both and apply them where appropriate: mechanical logic, where pressure is the key trigger, and software-defined logic, where adaptability and diagnostics are essential.
Design for what the machine needs today, but always consider leaving doors open for the potential needs of tomorrow.
