Considered the brain of pneumatic systems, directional control valves
are
used to start, stop, and direct the flow of compressed air. Proper valve selection (3/2,
5/2, 5/3) directly affects the precision of cylinder movements and the energy efficiency
of
the system. In this guide, we examine valve types, operating principles, and ISO
symbols.
Golden Rule in Valve Selection
A valve with a Flow Rate (Kv/Cv) suitable for the cylinder diameter
and
speed requirement must be selected. A small valve slows down the cylinder, while an
unnecessarily large valve increases costs and air consumption.
1. Types of Directional Control Valves
Pneumatic valves are classified according to the number of ports and positions. The most
common types are:
1.1. 3/2 Way Valves
Used to control single-acting cylinders or to send pilot signals. They
have
3 ports (Input, Output, Exhaust) and 2 positions.
- Normally Closed
(NC): No air passage initially, opens when a signal is received.
- Normally Open
(NO):
Air passes initially, closes when a signal is received.
1.2. 5/2 Way Valves
The standard choice for providing forward-backward movement of double-acting
cylinders. They have 5 ports (1 Pressure, 2-4 Work, 3-5 Exhaust) and 2
positions.
- Monostable (Spring Return): The valve returns to
the
starting position when the signal is cut off. Preferred in applications requiring
safety.
- Bistable (Double
Coil):
Retains memory in the last signal position. The cylinder maintains its position even
if
energy is cut off.
1.3. 5/3 Way Valves
Used to stop the cylinder in intermediate positions or to take it to a
safe position in an emergency. The 3rd position is usually the center position.
| Center Type |
Function |
Application Area |
| Closed Center |
All ports are closed. The cylinder locks in
place. |
Intermediate stops and precise positioning.
|
| Exhaust Center |
Work ports are open to exhaust. The piston
can
be moved freely (Floating). |
Situations requiring manual intervention.
|
| Pressure Center |
Pressure is applied to both sides. |
To prevent sudden movement of the piston.
|
2. Actuation Methods
The triggering mechanism that allows the valve to change position.
Solenoid (Coil)
Control by electrical signal. The
most
common type of automation.
Pneumatic Pilot
Control by air signal. Ideal for
explosive (Ex-Proof) environments.
Mechanical/Manual
Control by button, lever, or roller.
Used in operator panels.
3. Reading ISO Symbols
Pneumatic circuit diagrams are drawn with standardized ISO 1219 symbols. When reading
symbols:
-
Squares
indicate the number of positions (2 squares = 2 positions).
-
Arrows
indicate the direction of flow.
- The
letter "T" indicates a closed path.
-
1 (P):
Pressure Input, 2/4 (A/B): Work Outputs, 3/5
(R/S):
Exhaust.
"The most common mistake in system design is selecting a valve based only on port
size
(1/4", 1/2"). The determining factor is the flow coefficient (Kv) allowed by the
valve's
internal structure."
— BRS PROSES Engineering Team
6. ISO 6358 Compressible Flow Modeling: Sonic Conductance & Critical Ratio
Selecting directional pneumatic valves based on generic manufacturer flow ratings ($Q_N [ ext{Nl/min}]$) frequently introduces throttling bottlenecks or oversized valve manifolds. The international fluid power engineering standard ISO 6358 defines pneumatic valve mass flow through two fundamental thermodynamic parameters:
- Sonic Conductance ($C$): The maximum choked volumetric flow capacity per unit upstream pressure $[ ext{dm}^3 / ( ext{s} cdot ext{bar})]$.
- Critical Pressure Ratio ($b$): The pressure threshold ($P_2 / P_1 approx 0.2 ext{ to }0.5$) below which sonic choked flow occurs at the valve orifice.
When downstream pressure exceeds this critical ratio ($P_2 / P_1 > b$), the flow enters the subsonic subsonic regime governed by elliptic expansion:
Q_m = C \cdot P_1 \cdot \rho_0 \cdot \sqrt{1 - \left( \frac{\frac{P_2}{P_1} - b}{1 - b} \right)^2} \quad [kg/s]
7. Solenoid Electrical Dynamics & Decentralized Fieldbus Integration
Modern Industry 4.0 production cells require seamless integration between fluidic hardware and industrial fieldbuses. BRS PROSES engineers specify decentralized valve islands integrating IO-Link, Profinet, or EtherCAT nodes directly within the pneumatic manifold.
| Electrical Parameter |
Standard 24V DC Solenoid |
Low-Power Micro-Valve (PLC Direct) |
High-Speed Voice Coil Pilot |
| Inrush Power / Holding Power |
4.5 W / 2.8 W |
0.9 W / 0.35 W |
12 W / 4.0 W (PWM Regulated) |
| Switching Time (ON / OFF) |
18 ms / 25 ms |
12 ms / 15 ms |
2 ms / 3 ms |
| Cycle Life Expectancy |
30 Million Cycles |
50 Million Cycles |
> 100 Million Cycles |
| Diagnostic Telemetry |
None (Wire break only) |
Cycle counter via PLC software |
Full IO-Link coil temperature & response time tracking |
