Imagine a power outage emergency where your critical cooling system suddenly shuts down, leaving equipment at risk of overheating with potentially devastating consequences. This is precisely the scenario normally open (NO) solenoid valves are designed to prevent.
Unlike their more common normally closed (NC) counterparts, NO solenoid valves remain open when de-energized, ensuring continuous system operation during power failures. Understanding these specialized valves is crucial for mission-critical applications where uninterrupted flow is non-negotiable.
What Is a Normally Open Solenoid Valve?
Solenoid valves are electromechanical devices that control liquid or gas flow using electric current. The "normally open" designation refers to the valve's default state when de-energized:
- Normally Closed (NC) Valves: The most common type. Remains closed when de-energized, only opening when powered.
- Normally Open (NO) Valves: Functions inversely - remains open when de-energized (allowing flow) and closes only when energized.
Key Applications for Normally Open Valves
NO solenoid valves serve critical roles in systems where flow continuity during power loss is essential:
- Emergency cooling systems: Maintains coolant flow during power failures to prevent equipment overheating
- Ventilation systems: Ensures continuous air circulation in fume hoods or critical air supply lines during outages
- Fire suppression lines: Keeps extinguishing agents available even with main power loss
- Process purging/drainage: Allows gravity-driven drainage to continue during system idle periods
- Bypass or drain lines: Maintains default flow paths, only closing when alternative routing is needed
5 Critical Selection Factors for Normally Open Solenoid Valves
1. Application Requirements & Fluid Compatibility
The valve must be compatible with both the controlled fluid and operating conditions:
- Fluid type: Water, air, corrosive chemicals, steam, or viscous oils require different body and seal materials
- Temperature/pressure ranges: Valve ratings must match system extremes to prevent leaks or failures
- Operating environment: Indoor/outdoor installation, exposure to dust, moisture, or hazardous substances may require special enclosures
2. Electrical Specifications
Proper coil-to-power matching is essential for reliable operation:
- Voltage (AC/DC): Coils are designed for specific voltages (e.g., 120V AC, 24V DC)
- Power consumption: Important for battery-powered systems or installations with multiple valves
- Duty cycle: Continuous vs intermittent operation capabilities
3. Flow Capacity & Orifice Size
Physical design determines fluid/gas throughput:
- Required flow rate: Expressed in GPM or LPM, matched to the valve's Cv (flow coefficient) value
- Orifice size: Larger orifices generally permit higher flow rates
- Connection types/sizes: Must match existing piping (NPT, BSP, flanged, etc.)
4. Material Construction
Materials directly impact durability, chemical resistance, and service life:
- Body materials: Brass, stainless steel, or plastic for different applications
- Seal materials: NBR (Buna-N) for petroleum-based fluids, EPDM for hot water/steam, Viton (FKM) for harsh chemicals
5. Operation Type & Response Time
Internal mechanisms affect performance characteristics:
- Direct-acting: Best for small orifices, zero pressure differential applications
- Semi-direct (servo-assisted): Combines features of direct and pilot-operated valves
- Pilot-operated: Handles larger flows/pressures but requires minimum pressure differential
- Response time: Critical for applications needing millisecond-level actuation
- Manual override: Useful for maintenance or power failure situations
Why Proper NO Valve Selection Matters
Choosing the correct normally open solenoid valve impacts:
- System reliability: Ensures failsafe flow during power interruptions
- Operational efficiency: Optimizes energy use while meeting flow requirements
- Safety: Prevents unplanned shutdowns, leaks, or pressure-related failures
- Lifecycle costs: Reduces unplanned maintenance and downtime expenses
- Coil longevity: Minimizes energized time when default open position is preferred
Frequently Asked Questions
Q1: What happens if I use DC power on an AC normally open valve?
AC coils cannot properly operate with DC power due to different impedance characteristics, leading to excessive current draw, overheating, and rapid coil burnout.
Q2: Can a normally open valve be converted to normally closed?
No. NO and NC valves have fundamentally different internal designs optimized for their specific default states and cannot be converted between types.
Q3: When should I choose NO over NC valves?
Select NO valves when continuous flow during power failure represents the safer or required default state, such as in cooling systems where uninterrupted flow prevents overheating.
Q4: Do NO valves consume power when open?
No. NO valves only consume power when energized to close, making them energy-efficient for applications requiring mostly open flow.
Q5: How do I determine the correct Cv value?
Cv calculations require knowing your required flow rate, pressure drop across the valve, and fluid specific gravity. Online calculators and charts can assist with proper sizing.