Normally Open vs Normally Closed vs Universal Solenoid Valves: Key Differences and Selection Guide

One of the most common specification errors in industrial automation is choosing the wrong valve function:

• Normally Closed (NC)
• Normally Open (NO)
• Universal (UNI)

In real-world applications such as oil and gas, water treatment, chemical processing, food manufacturing, OEM skid systems, and factory automation, incorrect valve function selection can lead to unsafe fail conditions, unexpected shutdowns, pressure loss during power failure, compressor unloading failures, process instability, and unnecessary energy consumption.

This guide explains the engineering differences between Normally Closed, Normally Open, and Universal solenoid valves, including their working principles, fail-safe behavior, typical applications, and selection criteria for industrial automation systems.

Understanding the "Normal" Position of a Solenoid Valve

In solenoid valve terminology, the word "normal" refers to the valve position when the solenoid coil is de-energized, meaning no electrical power is supplied.

This de-energized condition is one of the most important factors in valve selection because it defines how the system behaves during power loss.

Normally Closed Solenoid Valves

A Normally Closed solenoid valve remains closed when the coil is de-energized. When power is applied, the coil generates magnetic force, lifts the plunger, and allows media to flow. When power is removed, the spring returns the valve to the closed position and flow stops automatically.

This makes NC valves one of the most widely used fail-safe configurations in industrial automation.

Key Characteristics of Normally Closed Valves

Typical Applications of NC Valves

Normally Closed solenoid valves are commonly used where flow must stop during power failure or emergency shutdown. Typical applications include:

• Emergency shutdown systems
• Fuel gas isolation
• Burner management systems
• Chemical dosing isolation
• Process isolation valves
• Backflow prevention
• Safety interlock systems
• Pneumatic actuator control
• CIP isolation in food and beverage plants

Advantages of Normally Closed Valves

The main advantage of an NC valve is its safer fail-state. During power loss, the valve automatically closes, reducing the risk of uncontrolled media flow, leakage, or hazardous release.

For safety-critical systems, NC valves are often preferred because they naturally support a de-energize-to-trip shutdown philosophy.

Normally Open Solenoid Valves

A Normally Open solenoid valve remains open when the coil is de-energized. In its normal state, media flows through the valve. When the coil is energized, the valve closes and stops flow. When power is removed, the valve automatically reopens.

NO valves are selected when the safer failure condition is continued flow rather than shutoff.

Key Characteristics of Normally Open Valves

Typical Applications of NO Valves

Normally Open valves are used where flow must continue during power loss or where venting/unloading is required for equipment protection. Typical applications include:

• Cooling water circulation
• Fire Fighting
• Compressor unloading
• Pressure venting systems
• Air release circuits
• Drain and vent valves
• Steam safety systems
• Pneumatic exhaust applications
• Continuous circulation systems

Advantages of Normally Open Valves

NO valves provide fail-open safety logic. This is important in systems where stopping flow could cause overheating, pressure buildup, compressor damage, or unsafe process conditions.

They are also efficient in systems that remain open most of the time because the coil is energized only when the valve needs to close.

Universal Solenoid Valves

A Universal solenoid valve is designed to operate in multiple flow configurations. Depending on how the ports are connected, the same valve can function as Normally Closed, Normally Open, a selector valve, a diverting valve, or a mixing valve.

Universal valves are widely used in pneumatic automation, instrumentation control, actuator systems, and OEM machine designs where flexible porting logic is required.

Key Characteristics of Universal Valves

Typical Applications of Universal Valves

Universal solenoid valves are commonly used in:

• Pneumatic actuator control
• Air diversion systems
• Pilot air switching
• Signal control
• OEM automation panels
• Rotary actuator systems
• NAMUR-mounted pneumatic systems
• Diverting or mixing fluid paths

NC vs NO vs Universal Solenoid Valve Comparison

How Solenoid Valve Selection Affects Actuated Valve Behavior

The choice between NC, NO, and Universal solenoid valves directly affects the fail-safe behavior of actuated valves. The impact depends on whether the actuator is single-acting or double-acting.

Single-Acting Actuators

Single-acting actuators use spring return to move the valve to a predefined safe position when air pressure is removed.

• With an NC solenoid valve, air supply is blocked when the solenoid is de-energized. The actuator loses air pressure, and the spring drives the valve to its fail-safe position.
• With an NO solenoid valve, air supply may remain available when the solenoid is de-energized. This can keep the actuator pressurized during power loss, depending on the pneumatic circuit.

For single-acting actuators, solenoid selection strongly influences whether the valve moves to its spring-defined safe state during power failure.

Double-Acting Actuators

Double-acting actuators require air pressure for both opening and closing. They do not have a spring-return mechanism.

For double-acting actuators, 5/2 or 5/3 solenoid valves are typically used rather than standard 3/2 NC, NO, or Universal valves. On power loss, behavior depends on spool design. The actuator may hold last position, vent, block, or drift depending on the pneumatic circuit.

Control Valves

Control valves usually use spring-return actuators with positioners. In trip applications, an NC solenoid valve is commonly used to cut off air supply when de-energized, allowing the valve to move to its fail-open or fail-closed position.

This is common in emergency shutdown systems where de-energize-to-trip philosophy is required.

In special logic schemes, an NO solenoid may be used for energize-to-trip operation, where the trip action occurs only when the solenoid is energized.

3/2, 5/2, and 5/3 Solenoid Valve Selection by Application

Quick Selection Guide

Direct Acting vs Pilot Operated Solenoid Valves

NC, NO, and Universal solenoid valves are available in direct-acting, internal pilot-operated, and external pilot-operated designs.

Direct-acting valves are suitable for low-pressure, vacuum, or zero-pressure differential applications. Pilot-operated valves are preferred for larger flow rates but require sufficient pressure differential to operate reliably.

External pilot-operated valves are useful when media pressure is low, fluctuating, or unsuitable for internal piloting.

Energy Consumption Considerations

Valve function has a direct impact on power consumption.

A Normally Closed valve consumes power while it is open. A Normally Open valve consumes power while it is closed.

• If the valve remains open most of the time, an NO valve may reduce energy consumption.
• If the valve remains closed most of the time, an NC valve is usually more efficient.

Low-power coil designs can further reduce heat generation, cabinet temperature rise, and overall energy usage in industrial automation panels.

Hazardous Area and Industrial Availability

NC, NO, and Universal solenoid valves are available in industrial configurations suitable for demanding environments, including:

• Explosion-proof designs
• Intrinsically safe designs
• Weatherproof IP65 and IP67 enclosures
• SIL-compatible configurations
• NAMUR-mounted options
• Direct-acting and pilot-operated versions

These configurations are commonly used in oil and gas, petrochemical plants, chemical processing, refinery automation, water treatment, and process industries.

Common Solenoid Valve Selection Mistakes

Selecting NO Instead of NC in Shutdown Systems

Using a Normally Open valve where flow should stop during power failure can create an unsafe fail-open condition.

Ignoring Fail-State Logic

The valve position during power failure must always be evaluated before selection.

Using Pilot-Operated Valves Without Minimum Pressure

Pilot-operated valves may fail to operate correctly if minimum differential pressure is not available.

Incorrect Universal Valve Porting

Universal valves are flexible, but incorrect porting can reverse the intended logic.

Oversizing the Valve Orifice

Oversizing may increase coil size, power consumption, heat generation, and system cost.

Final Selection Logic

Selecting between Normally Closed, Normally Open, and Universal solenoid valves is fundamentally a fail-safe engineering decision.

Conclusion

The difference between Normally Closed, Normally Open, and Universal solenoid valves is not only about flow direction. It is about how the system behaves during power loss, shutdown, startup, and emergency conditions.

A Normally Closed valve stops flow when de-energized. A Normally Open valve allows flow when de-energized. A Universal valve provides flexible porting and can be configured for multiple functions depending on the application.

By selecting the correct solenoid valve function, engineers can improve system safety, reliability, energy efficiency, process stability, and lifecycle performance while reducing operational and maintenance risks.