5.1 Control Valve Types & Actuators

Final control elements are the 'muscles' of process control systems. They take the controller's output signal and manipulate the process variable, typically by adjusting the flow of a fluid. The most common final control element is the control valve. Control valves consist of two main parts: the valve body (which contains the fluid) and the actuator (which moves the valve mechanism).

Linear Motion Valves

Linear motion valves, also known as sliding-stem valves, use a linear motion to push a closure member into a seat to stop or throttle flow.

Globe Valves

Globe valves are the workhorses of the process industries for throttling control. They feature a plug that moves linearly into a seat ring.

• Single Port Globe Valves: These have a single plug and seat. They provide tight shutoff but require a large actuator force to overcome the fluid pressure acting against the plug.
• Double Port Globe Valves: These have two plugs and two seats. The fluid pressure is balanced between the two plugs, significantly reducing the required actuator force. However, it is very difficult to get both ports to close tightly at the same time, leading to higher leakage rates compared to single port designs.
• Cage Guided Globe Valves: In this design, a cylindrical cage surrounds the plug. The cage provides guidance for the plug, reducing vibration and wear, and the ports in the cage can be shaped to provide specific flow characteristics.

Diaphragm and Pinch Valves

• Diaphragm Valves: These use a flexible diaphragm to isolate the fluid from the valve's moving parts and to control flow. They are excellent for corrosive, viscous, or slurry fluids because there are no crevices for fluid to get trapped.
• Pinch Valves: These feature a flexible tube that is 'pinched' closed by a mechanism. Like diaphragm valves, they offer a straight-through flow path and complete isolation of the fluid from the valve mechanism, making them ideal for heavy slurries.

Rotary Motion Valves

Rotary motion valves turn a closure member (typically a quarter-turn, or 90 degrees) to control flow. They generally offer higher flow capacity and lower weight than globe valves.

Ball Valves

• Full Port Ball Valves: These have a spherical closure member with a hole through it. When fully open, they provide virtually no resistance to flow. They are typically used for on/off service rather than precise throttling.
• Segmented V-Notch Ball Valves: A modification of the standard ball valve, this design uses a V-shaped notch in the ball segment. This provides a shearing action and an equal percentage flow characteristic, making it an excellent choice for throttling control, particularly for paper stock and slurries.

Butterfly Valves

Butterfly valves use a disk mounted on a rotating shaft.

• Conventional Butterfly Valves: Provide high capacity and low cost but often lack precise throttling capabilities and tight shutoff.
• High-Performance Butterfly Valves: Use eccentric shaft mounting (double or triple offset) and specialized seat designs (like PTFE or metal) to provide excellent throttling, reduced wear, and tight shutoff capabilities, bridging the gap between globe and standard rotary valves.

Plug Valves

Plug valves use a cylindrical or tapered plug with a hole through it. They are robust and often used for on/off applications, although characterized plugs can be used for throttling.

Valve Flow Characteristics

The flow characteristic of a valve describes the relationship between the valve's travel (stroke) and the flow capacity (Cv).

• Linear Characteristic: The flow capacity increases linearly with valve travel. A valve open 50% provides 50% of its maximum flow capacity. This is typically used for liquid level control or systems with constant pressure drops.
• Equal Percentage Characteristic: Equal increments of valve travel produce equal percentage changes in flow. For example, if moving from 20% to 30% open increases flow by 50%, moving from 30% to 40% will also increase flow by 50% of the new value. This characteristic is widely used for temperature and pressure control where process pressure drops vary.
• Quick Opening Characteristic: Provides a large change in flow for a very small initial change in valve travel. It is primarily used for on/off applications in which high capacity is needed early in travel; it is not a substitute for a code-rated pressure-relief device.

Flow Coefficient (Cv)

The Flow Coefficient, Cv, is a standardized measure of a valve's capacity. By definition, Cv is the number of US gallons per minute of water (at 60°F) that will flow through a valve with a pressure drop of 1 psi. The fundamental formula for liquid flow is:

Q = Cv * sqrt(DeltaP / G)

Where Q is flow rate (gpm), DeltaP is pressure drop (psi), and G is specific gravity of the fluid.

Actuators

The actuator provides the force to move the valve closure member.

Pneumatic Diaphragm Actuators

These are the most common actuators in the process industry. They use instrument air pressure applied to a flexible diaphragm to move the valve stem.

• Air-to-Open (Direct Acting): Increasing air pressure moves the valve stem to open the valve. If air is lost, a spring forces the valve closed (Fail Closed).
• Air-to-Close (Reverse Acting): Increasing air pressure moves the valve stem to close the valve. If air is lost, a spring forces the valve open (Fail Open).

Pneumatic Piston Actuators

Piston actuators use air pressure acting on a piston inside a cylinder. They can provide much higher thrust forces than diaphragm actuators.

• Double Acting: Uses air pressure on both sides of the piston to move it in either direction. It does not have a native fail-safe position without additional accessories like a trip valve and volume tank.
• Spring Return: Uses a spring to provide the fail-safe position, similar to a diaphragm actuator, but with higher thrust capabilities.

Electric and Electro-hydraulic Actuators

• Electric Actuators: Use an electric motor and gear train to move the valve. They are common in remote locations where compressed air is unavailable. They offer precise control but typically fail in their last position upon power loss unless equipped with battery backups or mechanical spring returns.
• Electro-hydraulic Actuators: Combine an electric motor, a hydraulic pump, and a hydraulic cylinder. They provide the massive force and fast response of hydraulics with the convenience of electrical power, often used in critical high-pressure applications.