What Is an Actuator? Valve Automation

An actuator is a mechanical device that converts energy (pneumatic, electric, or hydraulic) into motion to operate a valve. Actuators replace manual handwheel operation with automated control, enabling remote operation, integration with control systems, and fail-safe shutdown capability. Every automated valve in an oil and gas facility relies on an actuator.

When to Use an Actuator

Actuators are required when a valve must be operated remotely (from a control room), must respond to process signals (automatic control), must achieve fail-safe position on power/signal loss, must operate faster than manual capability, or is physically inaccessible. Emergency shutdown (ESD) valves, control valves, and remotely operated block valves all require actuators.

Actuator Types

Type	Energy Source	Motion	Speed	Typical Application
Pneumatic (diaphragm)	Compressed air	Linear	2-10 seconds	Globe control valves
Pneumatic (piston/scotch-yoke)	Compressed air	Rotary (quarter-turn)	1-5 seconds	Ball, butterfly valves
Electric (MOV)	Electricity	Rotary or linear	15-120 seconds	Gate, globe valves, remote locations
Hydraulic	Hydraulic fluid	Linear or rotary	0.5-5 seconds	High-thrust, subsea, pipeline
Electro-hydraulic	Electric pump + hydraulic	Linear or rotary	1-10 seconds	Remote locations without air supply

Key Specifications

Feature	Details
Torque/thrust output	Must exceed valve torque/thrust requirement with safety factor (typically 1.25-1.5x)
Supply	Air (4-7 bar), electric (24 VDC, 110/220/380 VAC), hydraulic (70-210 bar)
Fail-safe	Spring return to safe position (pneumatic), battery/capacitor backup (electric)
Signal input	4-20 mA (modulating), 24 VDC discrete (on/off)
Feedback	Limit switches (open/closed), positioner (position transmitter)
Hazardous area	ATEX/IECEx certified for Zone 1/2 or Div 1/2
Standards	ISO 5211 (mounting interface), IEC 61491 (partial stroke testing)
Environmental	IP66/IP67 enclosure, -40 to 80 degC ambient

Spring Return vs Double-Acting

Parameter	Spring Return	Double-Acting
Fail-safe	Yes (spring drives to safe position on air/power loss)	No (stays in last position on power loss)
Energy to operate	Air/hydraulic for one direction; spring for return	Air/hydraulic for both directions
Torque output	Lower (spring absorbs part of available force)	Higher (full force in both directions)
Size	Larger (spring housing adds bulk)	Smaller
Cost	Higher (spring mechanism)	Lower
ESD application	Standard choice	Not suitable without additional fail-safe mechanism

Actuator Sizing

The actuator output torque (for quarter-turn valves) or thrust (for linear valves) must exceed the maximum required valve torque/thrust under worst-case conditions:

Torque/Thrust Component	Source
Seating torque	Force to seat disc/ball against pressure
Unseating torque	Force to break disc/ball from seat (often highest)
Running torque	Force during mid-travel
Dynamic torque	Flow-induced torque on disc/ball
Packing friction	Stem packing resistance

Safety factor of 1.25 minimum (1.5 for critical ESD valves) is applied to the maximum of these values.

Mounting Interface

ISO 5211 defines standardized flange mounting dimensions between actuator and valve:

ISO 5211 Flange	Valve Size Range
F05 / F07	1/2” to 2”
F10 / F12	2” to 6”
F14 / F16	6” to 12”
F25 / F30	12” to 24”
F35 / F40	24” to 48”

Pro Tip

For ESD valves, specify partial stroke testing (PST) capability per IEC 61491. PST moves the valve 10-20% from the open position to verify the actuator and valve will actually move on demand, without fully interrupting the process flow. This confirms the valve is not stuck; a common hidden failure mode.

Read the full guide to valve types