Pneumatic solenoid valves control the opening/closing or switching of airflow paths. They consist of two main components: the valve body (pneumatic section) and the coil (electrical drive). Selection requires first determining the operating conditions and then verifying the parameters.
1. Determine the Type
1. Positions and Ports (The Basics)
Positions: Valve spool operating positions; Ports: Number of air connections on the valve body.
2-Position 2-Port (2/2): On/off control (unidirectional flow); functions like a switch; Normally Closed (NC) or Normally Open (NO).
2-Position 3-Port (2/3): For single-acting cylinders (spring-return); controls intake and exhaust for a single chamber.
2-Position 4-Port / 5-Port (2/4 or 2/5): Most common; for double-acting cylinders; controls cylinder extension and retraction; 5-port versions have independent exhaust ports for better heat dissipation.
3-Position 5-Port (3/5): Features a center position function (closed center, pressure-release center, or pressurized center); allows the cylinder to stop mid-stroke.
2-Position: Automatically resets upon power loss; 3-Position: Maintains center position upon power loss (requires dual coils).
2. Actuation Method
Single Coil: Switches when energized; spring-returns when de-energized (most common and cost-effective).
Dual Coil (Double Solenoid): Switches when energized; maintains current position when de-energized; requires separate signals for each end; must not be energized simultaneously for extended periods.
II. Key Fluid Parameters (Valve Body Section)
1. Port Thread
Ports: G1/8, G1/4, G3/8, G1/2 (British Standard Pipe Thread—pneumatic industry standard).
Orifice size determines maximum flow rate; bigger isn’t always better—it must match the required cylinder speed. Larger cylinders require solenoid valves with larger orifice sizes.
2. Flow Parameters: Cv Value (or Kv, Qn)
A critical indicator representing the valve’s flow capacity.
Qn: Air flow rate under standard conditions (L/min); commonly used in the pneumatic industry.
Cv: US standard flow coefficient.
Estimation: Cylinder movement speed is determined by the solenoid valve’s flow rate. Insufficient valve flow rate leads to slow or weak cylinder movement.
3. Operating Pressure Range
Common range: 0.15–0.7 MPa (1.5–7 bar). Note:
Below minimum pressure: The valve spool cannot be shifted; the valve fails to switch.
Above maximum pressure: Seals may be damaged, causing air leaks. Factory air compressors typically operate at 0.4–0.6 MPa, so standard valves suffice; high-pressure applications require high-pressure rated valves.
4. Medium and Temperature
Medium: Compressed air; must be dry and clean. If it contains significant oil, water, or dust, a pre-filter is required to prevent valve jamming.
Ambient temperature: Standard valves usually operate between 0–50°C; high or low-temperature environments require special seals (e.g., FKM/Viton).
Seal material: NBR (Nitrile Rubber) for standard air; FKM (Fluoroelastomer) for oil resistance, high temperatures, and special gases.
III. Coil Electrical Parameters (Electrically driven section; easy to select incorrectly)
1. Voltage: AC 220V / DC 24V (most common in industry); AC 110V and DC 12V are also available.DC 24V recommended: Generates less heat, offers longer service life, and is safer for explosion-proof applications; AC coils are prone to burnout. ⚠️ Coil voltage must exactly match the power supply; incorrect connection will cause immediate burnout.
1. Coil power: High-power and low-power options; low-power versions are suitable for low-output PLC terminals.
2. IP Protection Rating: Indoor dry environments: IP40; humid or dusty environments: IP65.
Lead-wire or connector/socket types available; connectors often include an indicator light and a flyback diode (essential for DC coils to protect the PLC).
1. Explosion-proof requirements: In flammable or explosive environments, explosion-proof solenoid valves must be used; standard valves are unsuitable.
IV. Other Key Selection Factors
1. Mounting Style
Individual valve: Standalone installation;
Valve manifold (sub-base): Centralized mounting for multiple solenoid valves sharing a common air supply; saves piping and is commonly used in automation equipment. 1. Response time: Standard valves take over ten milliseconds; high-speed valves take a few milliseconds. High-frequency operation (multiple cycles per second) requires consideration of both response time and service life.
2. Air leakage and service life: Typical pneumatic valves have a service life in the tens of millions of cycles; for frequent operation, prioritize major brands.
3. NC (Normally Closed) / NO (Normally Open): NC—closed when power/air is off, open when energized (used in the vast majority of scenarios); NO—open when power/air is off, closed when energized (used in specific safety circuits).
Summary of Quick Selection Process:
1. Actuator: Single-acting / Double-acting cylinder → Determine valve configuration (single-acting with spring return → 2-position, 3-port; double-acting → 2-position, 5-port).
2. Cylinder bore, stroke, and required speed → Calculate required air flow → Select Qn/Cv rating and port size.
3. Air supply pressure → Verify valve pressure range.
4. Control power supply: DC 24V or AC 220V.
5. Operating conditions: Temperature, dust, humidity, explosion-proof requirements, and whether the medium contains oil.
6. Installation: Individual valve or manifold.