How do you choose the right pneumatic valve? A practical guide

Valves come in dozens of versions, configurations and sizes. Whether you are designing your first pneumatic system or optimising an existing installation: with the right questions, you will quickly arrive at the right choice. This article takes you through the selection process step by step.

⏱ approx. 8 minutes reading time

An incorrectly sized valve does not work or works unreliably. A valve that is too small restricts the flow and slows down your system. A valve that is too large is wasteful and can cause unstable behaviour in control applications. Moreover, the type of valve determines whether the system works at all at your pressure, medium and environment.

The good news: the selection breaks down into five concrete steps. You do not need to be a valve specialist to work through them.

Step 1. What are you going to control? Medium and working pressure

Before you look at any other criterion, the valve must be compatible with the medium flowing through it and the pressure at which the system operates. The wrong choice of material leads to leakage, corrosion, swelling of seals or worse.

Ask yourself these questions

Which medium flows through the valve? Compressed air is the most common application for pneumatic valves and places the fewest demands on materials. Water, hydraulic oil, fuel, aggressive chemicals or high-purity gases each require specific material compatibility of the valve seal, the O-rings and the valve body.

What is the inlet pressure and the minimum pressure? This determines not only which materials are needed, but also which type of valve works. A pilot-operated valve always needs a minimum pressure to open. A direct acting valve works from vacuum up to the maximum working pressure. If your system can become depressurised or works with varying pressures, direct acting is the only reliable choice.

What is the temperature of the medium and the environment? Elastomer seals such as NBR are suitable up to approx. 80 degrees. For higher temperatures or aggressive media, EPDM, FKM (Viton) or PTFE seals are required. Always check the maximum medium and ambient temperature in the valve specifications.

Material compatibility is non-negotiable

Most valve manufacturers provide a compatibility table per medium. Always check both the valve body (aluminium, 316L stainless steel, brass) and the seals (NBR, EPDM, FKM, PTFE) for the specific medium and concentration. If in doubt, choose the most inert version.

Step 2. How is the valve operated? Method of actuation

A valve is a passive component until something moves it. The method of actuation determines how the valve fits into your system and which energy or signals must be available for it.

1. Solenoid (electromagnetic)

A copper coil generates a magnetic field that moves a plunger. Suitable for automated systems in which a PLC, relay or other electrical signal operates the valve. Available as direct acting (works without system pressure) or pilot-operated (uses system pressure as working force).

2. Manual

Operated by an operator via a lever, push button or foot pedal. Suitable for applications in which human control is desired or as an emergency override alongside an automated valve. No power connection required.

3. Mechanical

The valve is activated by contact with a component in the process, such as a cam, ball or plunger on a machine. Used for position detection or sequential control without electronics.

4. Pneumatic pilot signal

An output signal from another valve or a pressure signal from the system operates this valve. Used in cascaded pneumatic logic or with large valves in which a small pilot signal valve drives a larger main valve.

Tip for automation

Solenoids are by far the most widely used actuation in automated production environments. Make sure the supply voltage of the coil matches the available power supply in your system. Common coil voltages are 12 VDC, 24 VDC and 230 VAC.

Step 3. What does the valve do when the signal is lost?

Every valve has a rest state: the position it takes without an active signal. This is one of the most important safety decisions in valve selection.

Spring return

After the signal is removed, the valve automatically returns to its starting position thanks to a built-in spring. This is also called "monostable" behaviour. You need a continuous signal to keep the valve in the activated position. As soon as the power is removed, the valve returns. This is the safest behaviour in most applications: in the event of a power failure, you know exactly what the valve does.

A normally closed (NC) valve closes on power failure. A normally open (NO) valve opens on power failure. Which is safer depends on the process: in a fuel line you want normally closed; in a cooling water line for emergency cooling you want normally open.

Detented (memory position)

A detented valve remains in its last position after the signal is removed, even without further energy supply. This is called "bistable" behaviour. The valve needs a pulse to switch, not a continuous signal. Advantage: no energy consumption at rest. Disadvantage: in the event of a power failure, the position of the valve is unknown unless you have recorded or monitored it.

Fail-safe is a safety decision, not a preference

Ask yourself: what is the safest state of this system in the event of a power failure? Open or closed? The choice between NC and NO is therefore directly a safety requirement. Document this choice in your risk analysis.

Step 4. Which type of valve do you need? 2-way, 3-way or 4-way

The configuration of a valve describes the number of active ports and the switching logic. Humphrey Products uses the terms 2-way, 3-way and 4-way valve for this. The more ways, the more switching options the valve offers.

2-way valve

The simplest configuration: one inlet and one outlet. The valve opens or closes the passage. Suitable for simple on/off control of a media flow. A normally closed 2-way valve passes nothing at rest; a normally open version always passes unless activated.

3-way valve

A 3-way valve adds a third port, typically as an exhaust or as a second inlet. This allows the pressure at the outlet to be vented when the inlet is blocked. This is essential when controlling single-acting pneumatic cylinders: on deactivation, the air behind the cylinder has to go somewhere.

4-way valve and double-acting cylinders

To control double-acting pneumatic cylinders, a 4-way function is required: on activation, the compressed air switches from one side of the cylinder to the other, while the other side is vented. Humphrey achieves this by combining two 3-way valves in a normally open or normally closed version. This provides the same functionality as a traditional 5-port directional valve, but with the advantages of the direct actuation principle.

2-way

Simple on/off control of a media flow. No exhaust. Use for simple opening and closing without pressure build-up behind the valve.

3-way

Control with exhaust. Suitable for single-acting cylinders, diverter functions and selector switches. The most flexible basic valve: one type, four configurations.

4-way

Control of double-acting cylinders and actuators. Switches compressed air from one side to the other while the other side is vented. At Humphrey, realised with two combined 3-way valves.

Step 5. How large should the valve be? The Cv value

The Cv value is the flow coefficient of a valve: a universal measure of the amount of medium that can flow through a fully open valve at a given pressure differential. The higher the Cv, the more flow the valve passes.

The definition: a Cv of 1.0 passes exactly 1 gallon of water per minute at a pressure differential of 1 psi at 60 degrees Fahrenheit. In the metric world, you will also see the Kv value: Kv = 0.857 × Cv. Both describe the same thing, but in a different system of units.

Why is the Cv value so useful?

Manufacturers state the Cv of their valves in the specifications. Once you know the Cv your application needs, you can directly compare which valves are suitable and which are too small or too large, regardless of brand or type. Use the Cv calculator to calculate the required Cv for your specific situation directly.

The calculation for gases (air, N₂, CO₂ etc.)

A different formula applies to gases than to liquids, because gases are compressible. The most commonly used simplified formula for air in subcritical flow is:

Does your system work in SI units? Then use the Kv formula or first convert your parameters to the imperial system. The easiest way is the online Cv calculator: enter your process values and the required Cv value follows immediately, including unit conversion.

Calculation example

1. System: compressed air, inlet pressure 6 bar (87 psig), outlet pressure 5.5 bar (79.8 psig), temperature 20 °C (68 °F), required flow 500 Nl/min.

2. Convert: 500 Nl/min ≈ 1060 SCFH. T = 68 + 460 = 528 R. P1 = 87 + 14.7 = 101.7 psia. P2 = 79.8 + 14.7 = 94.5 psia. ΔP = 7.2 psi.

3. Fill in: Cv = 1060 × √(528 / (519 × 7.2 × (101.7 + 94.5))) ≈ 0.17.

4. Choose a valve with Cv ≥ 0.20 to have a safety margin. A Humphrey 310 series valve typically has a Cv of 0.15 to 0.35 depending on the model.

If the outlet pressure P2 is less than half the inlet pressure P1, supercritical (choked) flow occurs. The flow then no longer depends on P2 and reaches a maximum. The formula above only applies to subcritical flow. Always check whether ΔP is less than P1 / 2.

The calculation for liquids

For liquids, the formula is simpler because liquids are hardly compressible:

Rules of thumb for valve size

Never choose a valve that sits exactly at the calculated Cv value. Maintain a margin of 20 to 30 percent above the calculated value. This compensates for measurement uncertainty, future flow variations and pressure losses elsewhere in the system. Do not oversize excessively either: a valve that is far too large opens fully at a fraction of the intended flow and causes unstable behaviour in control applications.

Valve too small

• Insufficient flow, slower filling
• Greater pressure drop than expected
• System does not reach the desired cycle time
• Higher flow velocity, more erosion

Valve too large

• Unstable control at low flow
• Waste of space and budget
• With ON/OFF: pressure overshoot
• More difficult to isolate in the event of faults

Summary: the selection checklist

Work through the five steps and answer the questions below for each valve in your system. With the answers, you can compare the valve specifications directly with manufacturers' datasheets.

Valve selection checklist

• Medium: Which gas or liquid? Which temperature? Which maximum pressure? Are there corrosion or purity requirements?
• Minimum inlet pressure: Is there always sufficient pressure for a pilot-operated valve, or is direct acting necessary?
• Actuation: Solenoid (which voltage?), manual, mechanical or pneumatic pilot signal?
• Rest state: Normally closed or normally open? Spring return or detented? What is the safest state in the event of a power failure?
• Port configuration: 2-port (on/off), 3-port (with exhaust, diverter or selector), or 5-port (double-acting cylinder)?
• Required Cv: Calculate the required flow coefficient based on medium, pressure and desired flow. Add a margin of 20 to 30 percent.
• Mounting and connection: Inline, subbase, manifold? Which port connection (G1/8, G1/4, NPT)? Which IP protection class is required?
• Environmental requirements: Explosion-proof (ATEX, Zone 2)? High ambient temperature? Humidity or corrosive atmosphere?

Help with your choice

Humphrey Products supplies an extensive range of direct acting solenoid valves with the universal 3-port multi-purpose design. With one valve type configurable as NC, NO, diverter or selector, you simplify the selection process and reduce the number of part numbers required. Available via Inacom for the Dutch and Belgian markets.