Double-Acting Pneumatic Actuator — Anodized Aluminum, NAMUR, ISO 5211, DIN3337, 13 Models

The double-acting pneumatic actuator is the correct automation mechanism for butterfly and ball valves when the highest available torque in both directions — opening and closing — is required, or when the valve must operate at high frequency with independently controlled speeds in each direction. Unlike single-acting actuators where an internal spring creates resistance and reduces the net available torque in one direction, double-acting actuators use compressed air in both cylinder chambers — the torque available for opening and closing is maximized throughout the operating range without the spring's penalty. Constructed from high-strength anodized aluminum, with NAMUR certification for standard solenoid and accessory mounting, and ISO 5211 mounting compatible with any quarter-turn valve in the Cematic catalog and the market in general.

Double-acting vs. single-acting — when double-acting is the correct specification

When double-acting is mandatory — not just preferable

• Knife gate valves in slurry service: The knife must actively cut solids when closing — double-acting applies the cylinder's maximum cutting force. A single-acting spring may not have enough torque to cut a thick or compacted slurry.
• Large diameter valves with high torque: For 12" to 24" butterfly valves and 6" to 10" flanged ball valves at high pressures, the required torque may exceed the net available torque of the equivalent single-acting actuator — double-acting provides maximum torque without spring penalty.
• Very frequent cycles (>10,000 cycles/year): The single-acting spring fatigues with very frequent cycles — its return force progressively decreases. Double-acting actuators do not have this problem as they do not rely on a spring for either direction.
• Differentiated speed control: When the process requires fast opening and slow closing (or vice versa) with precise and independent adjustment of each speed — flow regulators on each air port of the double-acting actuator allow for this adjustment.
• "Last position" failsafe accepted: In services where keeping the valve in its last position upon air failure is the accepted safe state by risk analysis — the double-acting actuator is the correct and most economical specification compared to adding a pressure accumulator to a single-acting actuator.

Standards and certifications

• NAMUR (VDI/VDE 3845): The NAMUR standard defines the standard interface between the pneumatic actuator and control accessories — control solenoid, positioner, limit switch box, and flow regulators. The integrated NAMUR mounting on the actuator body allows direct mounting of any NAMUR accessory on the market without adapters or additional brackets — NAMUR solenoids from any manufacturer (Festo, SMC, Parker, IMI) are interchangeable.
• ISO 5211: Defines the mechanical interface between the actuator and the valve — the bolt pattern and the square or splined stem that connects the actuator to the valve. ISO 5211 conformity ensures that the actuator is interchangeable with any valve from any manufacturer that has the same mounting — without adapters or mechanical modifications.
• DIN3337: Supplementary German standard that defines dimensions for rotary pneumatic actuators for valves — ensures dimensional interchangeability with actuators from other manufacturers compliant with this standard.

Anodized aluminum — the right material for pneumatic actuators

• Low weight: Aluminum has a density of ~2.7 g/cm³ vs. ~7.8 g/cm³ for steel — an aluminum actuator weighs approximately one-third of its steel equivalent, reducing the load on the valve body and supporting structures.
• High mechanical strength: Structural aluminum alloys have a tensile strength of 200–400 MPa — sufficient for the operating pressures of pneumatic actuators up to 7 bar.
• Anodizing — corrosion resistance: Anodizing forms a hard, continuous aluminum oxide layer on the surface — it protects the base aluminum from corrosion in industrial environments with humidity, moderate acidic vapors, and condensation.
• Precise machining: Aluminum allows for high-precision machining — tight tolerances on the cylinder and on the ISO 5211 and NAMUR mounting surfaces ensure a correct fit without play.

Available models — by cylinder diameter

The cylinder diameter determines the available torque — a larger cylinder diameter means a larger piston area and greater torque for the same air pressure. The 13 available models cover the complete range of quarter-turn valves in the Cematic catalog:

• 32 mm — small valves, ½" to 1½" threaded ball valves, 2" to 3" butterfly valves
• 40 mm — 1" to 2" threaded ball valves, 2" to 4" butterfly valves
• 52 mm — 3" to 4" ball and butterfly valves
• 63 mm — 4" to 6" butterfly valves
• 75 mm — 6" butterfly valves
• 83 mm — 6" to 8" butterfly valves
• 92 mm — 8" butterfly valves
• 105 mm — 8" to 10" butterfly valves
• 125 mm — 10" to 12" butterfly valves
• 140 mm — 12" butterfly valves
• 160 mm — 12" to 14" butterfly valves
• 190 mm — 14" to 16" butterfly valves
• 210 mm — 16" to 20" butterfly valves

The diameter-valve correspondences are indicative — the correct model depends on the actual valve torque at maximum process differential pressure and minimum air network pressure. Cematic sizes the correct actuator at no additional cost.

Selecting the correct model — 4 steps

• Step 1 — Required valve torque: Obtain the maximum operating torque of the valve at the maximum process differential pressure from the Cematic technical data sheet.
• Step 2 — Minimum safety factor 1.25×: The actuator torque must be at least 1.25× the maximum valve torque — verify at the minimum guaranteed air network pressure.
• Step 3 — ISO 5211 mounting: Verify that the mounting of the selected model is compatible with the ISO 5211 mounting of the valve (F05, F07, F10, F12, F14, F16).
• Step 4 — Failsafe position: Confirm with the process risk analysis that "last position" is acceptable — if NC or NO failsafe is required, specify the single-acting actuator.

Compatible accessories

Control options and compatible accessories

The double-acting pneumatic actuator integrates with three levels of control according to process requirements — from simple position confirmation to modulating control with online diagnostics:

1. Limit Switch — open/closed position confirmation

The limit switch with IP67 protection mounts directly on the actuator's NAMUR body and sends two discrete signals to the PLC: confirmation of fully open position and confirmation of fully closed position. It is the most frequent accessory in ON/OFF valve automation — simple, reliable, and economical.

• Two mechanical microswitches or magnetic sensors — open and closed position signal to the PLC
• IP67 protection — suitable for installation in areas with water and dust
• Direct NAMUR mounting — no adapters
• Compatible with any PLC or control system with digital inputs
• First choice for: ON/OFF valves where only knowledge of whether the valve is open or closed is required

See limit switch →

2. Digital Positioner — 4–20 mA modulating control

The digital positioner receives the 4–20 mA signal from the PID controller and positions the actuator at any point between 0° and 90° with high repeatability. It includes 4–20 mA position feedback to the control system — the controller knows the actual valve position at all times.

• Input signal: 4–20 mA
• Position feedback: 4–20 mA
• Automatic calibration of end stops
• Local position display in percentage
• Direct NAMUR mounting on the actuator
• First choice for: proportional modulating control of flow, temperature, or pressure without advanced diagnostic requirements

See digital positioners →

3. Smart Positioner with HART — modulating control + online diagnostics

The smart rotary positioner with HART protocol is the version with the highest functionality — the same 4–20 mA input signal and position feedback as the digital positioner, plus bidirectional HART communication that allows for valve status diagnostics, remote calibration, and predictive monitoring from the DCS or SCADA system without interrupting the process.

• Input signal: 4–20 mA + bidirectional HART communication
• Position feedback: 4–20 mA
• Online diagnostics: actuator friction, seat wear, hysteresis — detects problems before they cause an in-service failure
• Partial Stroke Test (PST): partial stroke test without interrupting the process — verifies that the valve can close in an emergency without taking it out of service
• Remote calibration and configuration from the DCS or from a portable HART communicator in the field
• Direct NAMUR mounting on the double-acting actuator
• First choice for: systems with DCS, critical processes where predictive diagnostics reduce unplanned downtime, safety valves where PST is required by regulations

See smart positioner with HART →

Comparison of control options

Additional accessories

• NAMUR Solenoid 24 VDC / 110 VAC / 220 VAC: ON/OFF control from PLC — direct NAMUR mounting without adapters
• Flow regulators (speed controllers): Independent adjustment of opening and closing speeds
• Air filter regulator: Regulation and filtering of supply — recommended in all cases

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  Data Sheet

  Torque Guide