The control valve actuator is like the "muscle" that moves the valve. Its job is simple: translate a control signal into mechanical action. And while the valve body provides the flow path, it's the actuator that governs how quickly, accurately, and reliably the valve responds.
In this article, we’re going to look at the primary types of control valve actuators, valve body action, actuator action, fail-safe modes, and how to choose the right one.
What is a Control Valve Actuator and How Does it Work?
In the context of industrial processes, a valve actuator provides the force necessary to move the valve stem and position the valve plug or disc. A control valve actuator serves as the "muscle" of the control valve, where its primary role is to provide the necessary force and power to move the valve's closure element (e.g., plug, ball, or disc) to regulate the flow of fluids. This means that the actuator is a critical part of the control loop. The simplest control loop would consist of the following:
Signal from controller (PLC/DCS) → Actuator → Valve movement → Process change
Note that control valve actuators support two main methods of flow control: modulating (i.e., continuous positioning) and on/off (two-position) control.
The Three Primary Types of Control Valve Actuators
There are three basic types of control valve actuators: pneumatic, electric, and hydraulic. Their key features are summarized below.
| Actuator Type | Energy Source | Key Characteristics & Advantages | Typical Applications |
|---|---|---|---|
| Pneumatic | Compressed Air | Simple and reliable, typically faster for quick-response and emergency-shutdown duty than electric types. A key safety feature is their fail-safe mode, where a spring can automatically move the valve to a safe position (open or closed) if air pressure is lost. | Widely used in hazardous or explosive environments. Often used for general process control and emergency shutdown and isolation service. |
| Electric | Electrical Energy | Utilizes an electric motor and gearbox to provide highly precise control and positional feedback. They are the easiest to integrate directly with automation systems like a PLC or DCS. | Best for applications requiring high precision and seamless digital communication with control systems. |
| Hydraulic | Pressurized Fluid | Capable of generating extremely large forces while maintaining precise control. They typically require a dedicated power source, such as a pump or accumulator. | Primarily used for large valves or applications involving high pressure differentials. |
Pneumatic Actuators
There’s a good reason why pneumatic actuators are considered the industry standard for valve actuators. First, their design is straightforward, and they’re reliable. Pneumatic actuators use compressed air, which is readily available in most industrial settings. These actuators are also well-suited to hazardous environments. The reason is simple: they reduce electrical ignition concerns (but an assembly may still include an electric positioner, solenoid, or switches, so keep that in mind).
A typical pneumatic actuator often provides faster full-stroke response than its electric counterparts. Pneumatic actuators are also able to quickly return to a safe position if they experience a loss of power or control signal. In addition, they can serve as a cost-effective alternative to other potential solutions. Oh, and don’t forget that they generally need less maintenance.
Now let’s look at the two main types of pneumatic actuators: diaphragm and piston.
Diaphragm Actuators
A diaphragm actuator is a popular option with a very simple design: an air chamber, a flexible diaphragm, and a return spring. Air pressure is applied to the diaphragm. That diaphragm passes the resultant force to a spring. When the pressure is removed, the spring returns to its original position. This is illustrated below.
As you can see, the actuator casing is divided into two parts. Note that the air-side and spring-side arrangement described is for a direct-acting actuator; in a reverse-acting actuator, the air is fed to the opposite side of the diaphragm. So here it is:
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The air chamber forms a sealed pressure vessel and is the wide part where air is pressed against the diaphragm. A control system or positioner regulates the amount of air supplied.
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The spring casing is where, as the name suggests, the spring is located. This side isn’t sealed, and usually has a small vent hole to let ambient air escape when pressure is applied.
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A heavy-duty, coiled spring pushes back against the diaphragm. As the air pressure increases, it overcomes the tension in the spring to move the valve stem. When the air pressure drops, the spring pushes the stem back.
One of the aspects of the design of a diaphragm actuator is the simple spring return mechanism, which actually enhances the reliability of this control valve actuator. Instead of using air to both open and close the valve, air only pushes in one direction, and the spring handles the return journey. Spring-return pneumatic actuators provide a built-in fail-safe action (double-acting actuators do not inherently fail open or closed).
This is configured in one of two ways based on the valve's requirements:
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Spring-to-Extend (Air-to-Retract): Air pressure is piped into the chamber to pull the valve stem inward. When the air pressure is reduced or vented, the spring pushes the stem outward, extending it.
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Spring-to-Retract (Air-to-Extend): Air pressure is used to push the stem outward against the spring's resistance. When the air is vented, the spring pulls the stem back inward, retracting it.
You’ll typically see pneumatic control valve actuators used in process controls for oil & gas, chemical plants, and power generation.
Piston Actuators
A piston actuator combines a solid piston within a cylinder to control valves. Basically, it takes hydraulic or pneumatic energy and converts it into high-force, straight-line physical movement. Because it uses a solid piston within a cylinder, a piston actuator can actually handle a higher force output and longer strokes than diaphragm actuators. And this design can work for both linear and rotary movement. So, this approach to valve control includes a higher force output, a small size relative to other valve actuators, and a fast stroking speed.
Now let’s look at single-acting vs double-acting configurations. First, there’s Single-Acting (Spring-Return). In this configuration, fluid pressure pushes the piston in one direction, where it compresses a heavy internal spring. When the fluid pressure is released, the spring automatically pushes the piston back to its starting position. Such a crucial safety feature is ideal for "fail-safe" designs (e.g., automatically closing a critical valve if power or air pressure is lost).
There is also the Double-Acting configuration. Fluid is pumped into one side to push the piston out, and then pumped into the opposite side to pull it back in. This requires a bit more plumbing but offers full, high-force control in both directions.
There are several benefits to using a piston actuator. For example, because a piston can travel down a long cylinder barrel, these piston actuators can consistently provide a significant length of travel compared to compact diaphragm alternatives. In addition, although their physical profile is fairly narrow, they can transmit a substantial amount of force.
There are, however, downsides to using a piston actuator. They’re subject to stiction and wear because the design requires piston seals. In addition, if the inner seals start to wear out, the actuator will begin to lose power or start to drift because fluid will leak across the piston chamber.
Electric Actuators
Electric actuators combine an electric motor, a gear train, and control electronics. These actuators offer highly precise positioning without the need for compressed air. They are also easy to integrate with existing digital control systems. Electric actuators also offer benefits that include high accuracy, excellent repeatability, built-in position feedback, and remote-control capability. However, they typically have a higher initial cost than the pneumatic options just discussed.
Electric actuators typically have a higher initial cost than pneumatic options. On the plus side, they eliminate instrument-air infrastructure and may require less routine maintenance in some applications. In hazardous areas, electric actuators require special protection methods, like explosion-proof or intrinsically safe rated enclosures, to be used safely.
Electric actuators are often used in water treatment, HVAC systems, and applications requiring precise flow control.
Hydraulic Actuators
Hydraulic actuators are simple in design: a hydraulic pump system provides pressurized fluid that interacts with a piston to control the flow of fluid. The primary advantage of this approach is the extremely high force output typically required for large valves or high-pressure applications, with its high power-to-size ratio and its ability to handle extreme operating conditions. However, hydraulic actuators are complex, have a relatively high initial cost, and require a hydraulic power unit to operate. They are also susceptible to hydraulic fluid leaks.
You’ll often see hydraulic actuators used in subsea valves, large pipeline valves, and heavy-duty industrial processes.
Valve Body Action and Actuator Action
Now that we’ve discussed the different types of control valve actuators, there are two types of valve body action:
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Direct-Acting Valve Body (PDTC): The stem moves down to close the flow path (e.g., standard Globe valve). This is known as Push-Down-to-Close.
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Reverse-Acting Valve Body (PDTO): The stem moves down to lift the plug off the seat, opening the flow path. This is known as Push-Down-to-Open.
Actuator action relates to which direction the stem moves as the control signal increases.
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Direct Acting (Spring to Retract) means that increasing the control signal extends the stem.
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Reverse Acting (Spring to Extend) retracts the stem as the control signal increases.
The actuator action must be matched with the valve action (direct or reverse) to achieve the desired fail-safe position. The final "fail-safe" position of an automated valve depends on two components: the actuator action (how it moves in response to the signal) and the valve body action (how the stem movement affects the flow).
Fail-Safe Modes: Protecting Your Process in an Emergency
Fail-safe modes define what happens when power or air is lost. Here are the possible fail-safe modes:
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Fail-Closed (FC): Valve closes when power or air is lost. Used for steam, hot oil, or hazardous chemical applications where stopping flow is the safe action.
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Fail-Open (FO): Valve opens. Used for cooling water or emergency venting applications where maintaining flow is the safe action.
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Fail-in-Place (FIP): Valve remains in its last position, which typically requires a double-acting actuator plus a lock-up valve, trip system, or other pressure-trapping arrangement. Used when neither fully open nor fully closed is the safest option.
You can use the following decision guide to determine the correct fail-safe mode for your application.
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Identify the hazard (e.g., what happens if this valve loses power at the worst moment?)
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Determine the safe state (does stopping flow, maintaining flow, or holding position minimize the hazard?)
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Map it: safe-when-closed = FC, safe-when-open = FO, safe-when-held = FIP
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Confirm the selection against your process hazard analysis (HAZOP) and any SIL/LOPA study.
And here’s how the fail-safe modes combine with the actuator action:
| Actuator Spring Action (No Signal) | Valve Body Action | Fail-Safe Condition | Common Term |
|---|---|---|---|
| Retract (Direct Actuator) | Push-Down-to-Close | FAIL OPEN (FO) | Air-to-Close |
| Retract (Direct Actuator) | Push-Down-to-Open | FAIL CLOSED (FC) | Air-to-Open |
| Extend (Reverse Actuator) | Push-Down-to-Close | FAIL CLOSED (FC) | Air-to-Open |
| Extend (Reverse Actuator) | Push-Down-to-Open | FAIL OPEN (FO) | Air-to-Close |
How to Select the Right Control Valve Actuator for Your Application
Here’s a practical guide to help you select the right actuator for your design:
Step 1: Determine the Required Thrust or Torque
To find the required force, calculate the total load by combining the valve size, process differential pressure, and internal packing friction. Now, while manual calculations are helpful, valve manufacturers typically provide standardized thrust and torque tables to make sizing straightforward. Here’s an excerpt for the Bonomi Double Acting Pneumatic Actuator:
Step 2: Choose the Actuator Type
For general industrial use, most engineers would agree that pneumatic is typically the best choice. Choose electric when precision and digital integration matter most, and opt for hydraulic when extreme force is required.
Step 3: Specify the Fail-Safe Mode
Select a fail-safe position based on your process safety requirements. This ensures the actuator defaults to the safest state if instrument air, power, or hydraulic pressure is suddenly lost.
Step 4: Consider Environmental Factors
Account for real-world operating conditions, such as extreme ambient temperatures and hazardous area classifications (like explosion-proof ratings). Also consider the availability of local utilities like compressed air or electricity.
Step 5: Evaluate Control Requirements
Determine whether your application requires simple on/off isolation or precise modulating control. If you need modulating control to fine-tune process parameters, a valve positioner will likely be required.
Conclusion: The Right Actuator Makes All the Difference
Choosing the right actuator is just as important as choosing the right valve. The wrong actuator can quickly lead to poor control, premature valve failure, and some serious safety risks. The experts at ValveMan, with 60+ years of valve experience, are here to help. Contact us today, and one of our valve experts can work with you to make the right choice of valve and actuator.