— A Practical Guide for Engineers, EPCs, and Project Teams

The solenoid valve is one of the most ubiquitous yet least understood components in industrial automation. It is the small device that sits between the control system and the actuator, converting an electrical signal into a pneumatic or hydraulic switching action. Without it, the control valve cannot open, the cylinder cannot stroke, and the emergency shutdown system cannot isolate the process.

Despite its apparent simplicity, the solenoid valve is also one of the most common sources of failure in pneumatic control systems. A wrong voltage, an undersized orifice, an incompatible seal material, or an incorrectly wired coil can cause a control valve to fail in the wrong direction—or not to respond at all. The solenoid valve is small, but its consequences are not.

This guide explains the types, functions, and selection criteria for solenoid valves in industrial automation, with practical guidance for engineers, EPCs, and project teams.

1. What Is a Solenoid Valve?

A solenoid valve is an electromechanically operated valve. It uses a solenoid—a coil of wire that generates a magnetic field when energised—to move a plunger or armature, which opens or closes one or more flow paths.

Core components:

Basic operation: When the coil is energised, the magnetic force pulls the plunger against the spring, opening (or closing) the valve. When de-energised, the spring returns the plunger, reversing the action.

2. Why Solenoid Valves Matter

In industrial automation, solenoid valves serve three critical functions:

Key insight: In a pneumatic control valve assembly, the solenoid valve is often the single point of failure for the valve's fail-safe response. If the solenoid fails to de-energise, the valve will not fail to its safe position—regardless of the actuator's spring design.

3. Types of Solenoid Valves

Solenoid valves are classified by several criteria: the number of ports, the operating principle, the de-energised state, and the application.

3.1 Classification by Number of Ports

2-way solenoid valves are the most common for shut-off and isolation service. 3-way valves are common for piloting single-acting actuators. 4-way and 5-way valves are common for double-acting actuators.

3.2 Direct-Acting vs Pilot-Operated

Selection principle: Use direct-acting for low-pressure or zero-differential applications. Use pilot-operated for higher flow rates where a minimum pressure differential is always available.

3.3 Normally Closed (NC), Normally Open (NO), and Universal

Critical: The NC/NO designation refers to the de-energised state. For fail-safe design, the solenoid configuration must match the valve's required fail-safe position (see Section 6).

3.4 Classification by Application

4. Key Functions of Solenoid Valves

4.1 Pilot Control of Pneumatic Actuators

The most common function of a solenoid valve in industrial automation is to pilot a pneumatic actuator. The solenoid valve supplies or exhausts air to the actuator in response to a control signal.

For a spring-return actuator:

• Energise to open: Solenoid supplies air; spring closes on de-energisation

• Energise to close: Solenoid exhausts air; spring opens on de-energisation

For a double-acting actuator:

• 4-way or 5-way solenoid directs air to one side of the piston while exhausting the other

4.2 Fail-Safe Action

The solenoid valve determines the valve's fail-safe response on loss of power or signal.

Critical: The solenoid valve must be configured so that de-energisation results in the safe position. This is the fundamental principle of fail-safe design.

4.3 On/Off Control of Process Fluid

In some applications, the solenoid valve directly controls the process fluid—not just pilot air.

4.4 Emergency Shutdown (ESD)

In safety instrumented systems, solenoid valves are used to trip final elements to their safe position.

5. Selection Criteria

5.1 Valve Function

5.2 Flow Capacity (Cv/Kv)

The solenoid valve must have sufficient flow capacity to operate the actuator within the required time.

Sizing principle: Calculate the required Cv based on actuator volume, stroke time, and supply pressure. Select a solenoid with Cv equal to or greater than the calculated value.

5.3 Pressure and Temperature

5.4 Voltage and Power

Selection principle: Match the voltage to the available power supply. For ESD applications, 24V DC is standard. For low-power applications, specify low-wattage coils.

5.5 Body and Seal Materials

Selection principle: Match the body and seal materials to the process fluid. For corrosive media, use stainless steel or plastic bodies with PTFE or FKM seals.

5.6 Response Time

5.7 Explosion Protection

Selection principle: Match the Ex certification to the area classification. For Zone 0/1, use Ex ia. For Zone 1/2, Ex d or Ex e.

5.8 SIL Rating

For safety instrumented functions, the solenoid valve must be SIL-rated.

Selection principle: Verify the SIL certificate covers the specific model and application. Consider redundant solenoids for SIL 3 applications.

5.9 Manual Override

Best practice: Specify manual override for critical valves that may need to be operated during commissioning or maintenance.

6. Solenoid Valve and Fail-Safe Position

The solenoid valve is a key component in determining the fail-safe position of a control valve. The table below summarises the combinations.

Key principle: The solenoid valve must be selected so that de-energisation results in the safe position. In most industrial applications, the safe position is fail-closed, which requires an NC solenoid on an air-to-open actuator.

Common mistake: Using an NC solenoid on an air-to-close actuator. On de-energisation, the solenoid exhausts air, and the spring opens the valve—resulting in fail-open, not fail-closed. This is a potentially catastrophic error in safety-critical applications.

7. Common Applications

8. Common Mistakes to Avoid

9. Applicable Standards

10. Why Choose Anhui Tiankang for Solenoid Valve Solutions?

Anhui Tiankang (Group) Co., Ltd. has nearly five decades of experience in industrial instrumentation and valve automation. We provide complete solenoid valve solutions for control valves, actuators, and safety systems.

Solenoid valve product portfolio:

Accessories:

• Manual override

• Surge suppressor

• DIN connectors

• Cable glands

• Mounting brackets

Core advantages:

• Complete certifications: CCC Ex, ATEX, IECEx, SIL

• CNAS-accredited laboratory: full performance testing

• Engineering support: solenoid selection, Cv calculation, fail-safe verification

• Long-term supplier to CNPC, Sinopec, CNOOC, and international EPC projects

• One-stop supply: valves + actuators + positioners + solenoids + accessories

11. Conclusion

Solenoid valves are small components with large responsibilities. They determine whether a control valve opens or closes, whether an ESD system trips correctly, and whether a process can be shut down safely when something goes wrong.

Key takeaways:

The most important rule: The solenoid valve must be selected so that de-energisation results in the safe position. In most industrial applications, this means an NC solenoid on an air-to-open actuator for fail-closed service. Verify this configuration at FAT, SAT, and commissioning.

Remember: A solenoid valve that fails in the wrong direction is worse than no solenoid valve at all. The solenoid is the last link in the control chain—and often the first to be blamed when a valve fails. Select it correctly, verify it thoroughly, and keep a spare on the shelf.

Contact Us

For solenoid valve selection, fail-safe verification, or project quotations, please contact:

Yin Shuangjie
International Sales Manager
📧 Email: [email protected]
📱 WhatsApp / Zalo: +86 17856068126
🌐 Website: http://www.tiankang-global.com/

Anhui Tiankang – Your partner for reliable solenoid valve and instrumentation solutions.