What is Solenoid Valve: A Technical Guide to Principles, Types, and Airtac Selection

 

A solenoid valve is an electromechanically operated valve designed to control the flow of liquids or gases. In automated industrial systems, it serves as the primary interface between electrical control systems (such as PLCs) and pneumatic or hydraulic actuation components.

Understanding the precise mechanical principles, functional categories, and selection metrics of solenoid valves is critical for ensuring operational reliability and preventing system downtime. This technical guide provides an objective analysis of solenoid valve technology, with a specific focus on industrial pneumatic applications and Airtac product architecture.

1. Core Working Principles of Solenoid Valves

At its most fundamental level, a solenoid valve consists of two main functional assemblies: an electrical solenoid coil and a mechanical valve body containing a moveable spool or plunger.

The Electromechanical Conversion

When an electrical current passes through the copper windings of the solenoid coil, it generates a magnetic field according to Ampere's Law. This magnetic force acts upon a ferromagnetic plunger (also referred to as an armature) enclosed within the core tube.

De-energized State: The valve relies on a mechanical return spring to hold the plunger or spool in its baseline position, either blocking or permitting fluid passage depending on the valve's default configuration.

Energized State: The magnetic force overcomes the mechanical spring resistance, pulling the plunger upward or shifting the internal spool along its axis. This physical movement opens, closes, or redirects the fluid pathways through the valve ports.

2. Classification by Actuation Type

Solenoid valves are categorized into three primary functional designs based on how the mechanical movement is achieved within the valve body.

A. Direct Acting Solenoid Valves

In a direct acting valve, the solenoid plunger is mechanically linked to the sealing orifice. When the coil is energized, the magnetic force directly lifts the seal to open the path.

Pressure Dependence: Direct acting valves do not require a minimum operational pressure differential (ΔP = 0 bar) to function. They can operate from complete vacuum up to their maximum rated pressure.

Limitations: Because the magnetic coil must overcome the fluid pressure acting directly against the orifice area, these valves are typically restricted to smaller port sizes and lower flow rates.

B. Pilot-Operated (Indirect Acting) Solenoid Valves

Pilot-operated valves utilize the pressure of the fluid medium itself to open and close the main valve seal. The solenoid coil controls a much smaller "pilot" passage.

Mechanism: When the coil is energized, the pilot port opens, venting pressure from a chamber situated above a flexible diaphragm or piston. The higher pressure from the inlet side then forces the diaphragm upward, opening the main flow path.

Pressure Dependence: Pilot-operated valves require a minimum pressure differential (typically between 0.05MPa to 0.1 MPa) to function properly. If the inlet pressure drops below this threshold, the valve will fail to open or close completely.

C. Internally Piloted Spool Valves (Standard Pneumatic Automation)

Commonly used in directional control valves for air cylinders, these valves utilize a spool layout. A small direct-acting pilot valve redirects a fraction of the incoming compressed air to one end of a larger aluminum spool, shifting the spool to change the port alignment.

3. Understanding Port and Position Configurations (Way/Position)

Solenoid valves are technically classified by the number of fluid connections (ports or "ways") and the number of switching states (positions) they possess. This is standardly noted as [Number of Ports]/[Number of Positions] (e.g., 2/2-way, 3/2-way, 5/2-way, 5/3-way).

Configuration Type	Port Designations	Typical Application
2/2-Way	1 Inlet, 1 Outlet	On/Off fluid isolation (liquids, steam, air)
3/2-Way	1 Inlet, 1 Outlet, 1 Exhaust	Operating single-acting cylinders or spring-return actuators
5/2-Way	1 Inlet, 2 Working Ports, 2 Exhausts	Operating double-acting pneumatic cylinders
5/3-Way	1 Inlet, 2 Working Ports, 2 Exhausts	Mid-position cylinder positioning (Closed, Exhaust, or Pressure Center)

Default Operational States

Normally Closed (NC): The valve remains closed when electrical power is removed; fluid flow is blocked.

Normally Open (NO): The valve remains open when electrical power is removed; fluid flow is permitted.

4. Technical Specifications and Code Standards within Airtac Product Architecture

Airtac designs and manufactures several distinct series of directional control solenoid valves, primarily categorized under their pneumatic line. Selecting the correct model requires matching system requirements to specific product series characteristics.

A. The 4V Series (Directional Control Spool Valves)

The 4V Series (such as 4V210, 4V310) represents the standard industrial benchmark for 5/2-way and 5/3-way pneumatic directional control.

Structure: Internally piloted spool structure with an aluminum body.

Working Medium: Compressed air filtered to a minimum of 40μm.

Pressure Range: Standard operating pressure range is 0.15 to 0.8 MPa (21 to  114 psi). It cannot be used in zero-pressure or vacuum environments due to the internal pilot design.

Lubrication: Not strictly required under standard operating conditions due to specialized internal dynamic seals. If a lubricator is introduced into the system, it must be maintained continuously using ISO VG32 or compatible non-synthetic mineral oils.

B. Fluid Control Series (2S, 2W, and 2P Series)

For applications handling fluids other than compressed air (such as water, light oils, or steam), Airtac provides direct-acting or specialized diaphragm series.

2S Series (Stainless Steel): Designed for corrosion resistance and steam or liquid media.

2W Series (Brass Body): Direct-acting 2/2-way configuration capable of operating at zero pressure differential (Δ P = 0MPa).

Fluid Constraints: Fluid viscosity must remain below 20 cSt (Centistokes) for standard models to prevent dynamic lag or spool stickiness.

C. Electrical Characteristics and Coil Options

The solenoid coil converts electrical input to magnetic flux. Airtac components specify specific electrical tolerances that must be matched to the control network:

Voltage Formats: Standard industrial choices include Alternating Current (AC220V, AC110V at 50Hz/60Hz) and Direct Current (DC24V, DC12V).

Power Consumption: Standard DC24V coils typically pull between 3.0W to 5.0W depending on the valve frame size.

Insulation Class: Standard coils comply with Class B or Class F insulation standards, allowing for a maximum continuous operating temperature threshold of 130℃ or 155℃ respectively.

Electrical Entry: Available in Grommet (direct wire leads) or Terminal/DIN Connector types (incorporating an ISO 4400 / EN 175301-803 standard plug connection).

5. Engineering Selection Framework for Industrial Sizing

To correctly specify a solenoid valve part number for an industrial system, engineers must calculate and cross-verify four distinct criteria:

1. Flow Capacity Calibration (Cv Factor / Flow Rate)

The physical port size (e.g., 1/8", 1/4", 3/8") does not fully define a valve's flow capacity. Selection must be evaluated using the Flow Coefficient (Cv) or nominal flow rate (L/min).

If the Cv value is too low, it acts as a system restriction, preventing downstream cylinders from achieving their designated cycle speeds.

If the Cv value is excessively high, it increases compressed air consumption during the pilot actuation phase and increases physical space requirements unnecessarily.

2. Environmental and Media Compatibility

Ambient Temperature: Standard Airtac valves operate safely within a temperature window of 20 ℃ to 70 ℃ (assuming the compressed air is dry to prevent internal freezing).

Seal Selection: Standard units utilize NBR (Nitrile Butadiene Rubber) seals, which are structurally compatible with standard air, water, and inert gases. For applications with elevated thermal requirements or chemical interactions, alternative seal compounds such as Viton (FKM) must be specified.

3. Response Time and Cycle Frequency

Pneumatic spool valves feature a response lag (the time between electrical activation and full mechanical spool shifting) ranging between 20ms to  40ms.

For continuous high-frequency sorting operations, the maximum cycling limit of the valve series (typically 5 cycles per second for standard 4V series) must not be exceeded to avoid thermal overload of the coil and accelerated dynamic seal wear.

6. Sizing and Sourcing Compliance Checklist

Prior to final processing of a technical procurement list for Airtac solenoid valves, verify the following mechanical criteria:

Verify Minimum Operating Pressure: If the application pressure is below 0.15 MPa, do not use an internally piloted valve (like the 4V series). Select a direct-acting model or an externally piloted version instead.

Confirm Thread Suffix Standards: Cross-check if the system manifold requires NPT (American standard), G / PF (Parallel pipe threads), or PT / Rc (Tapered pipe threads).

Check Coil Voltage Tolerance: Ensure the electrical source provides voltage stability within ±10 of the nominal coil rating to prevent coil burnout (from over-voltage) or failure to actuate (from under-voltage).

Assess Exhaust Filtration Requirements: For cleanroom or quiet factory floor environments, ensure appropriate pneumatic silencers/mufflers are specified for ports 3 and 5 on 5/2-way valve manifolds.