What is Control Valve?- Types and How its Work

What is Control Valve?

A control valve is a device designed to regulate fluid flow by adjusting the size of the passage through which the fluid moves, based on signals received from a controller. This adjustment allows precise control over the flow rate, which in turn influences other process variables such as pressure, temperature, and liquid level.

In the language of automatic control systems, the control valve is often referred to as the “final control element.”

As the name implies, the primary function of a control valve is to manage process variables. Common applications include controlling flow, temperature, pressure, or liquid levels. The valve’s position—open or closed—is adjusted by a control signal, which may be pneumatic, electric, or hydraulic in nature.

Within various process industries—including oil and gas, refining, petrochemicals, specialty chemicals, pharmaceuticals, and power generation—control valves play a critical role. They serve as the final control elements essential for managing both core processes and related utilities, particularly in fluid handling operations.

How Control Valves Work?

Control valves play a crucial role in regulating the flow of liquids or gases by adjusting internal passages. These valves are integral components of a control loop that manages various processes. They operate based on signals received from a controller, which directs them to modify the valve openings accordingly.

To manage the flow, control valves can operate in two primary ways: either fully opening or closing (on/off control), or partially adjusting the opening (modulating control).

Modulating valves specifically adjust the flow by varying the size of the opening, allowing precise control over the media passing through. While they can be positioned anywhere between fully closed and fully open, a valve alone cannot function without an actuator. The actuator relies on signals from an external source to initiate movement, enabling the valve to open or close as needed.

This is provided by one of the following:

• Pneumatically actuated control valve: a pneumatic pilot or liquid level controller
• Electrically actuated control valve: an electric pilot, electric liquid level controller, Programmable Logic Controller (PLC), or Remote Terminal Unit (RTU)

Purpose of control valves

A control valve regulates the flow rate passing through it by adjusting the valve stem’s position. When the stem moves, it changes the size of the opening, which in turn increases, decreases, or maintains the flow.

This adjustment occurs whenever the process parameter being monitored deviates from its intended value, known as the setpoint.

How Does a Control Valve Hold Back Pressure?

Here’s how a pneumatically actuated High-Pressure Control Valve installed on a separator holds back pressure:

• Pneumatic Pilot: In this system, the control signal originates from a pneumatic pressure pilot. The pilot detects the pressure upstream and transmits a corresponding signal—usually processed supply gas—through tubing to the valve’s actuator.
• Pneumatic Actuator: Upon receiving the pilot’s signal, the actuator responds by either opening or closing the valve accordingly.
• Electric Pilot: For electrically actuated High Pressure Control Valves, such as those installed on separators to maintain backpressure, the signal source differs. Here, an electric pilot or a PLC receives an analog 4-20 mA signal from a sensor monitoring a specific process condition. This signal is then processed and relayed as a 4-20 mA control loop to the valve’s electric actuator.
• Electric Actuator: The actuator responds to this signal by adjusting the valve position—either opening or closing it as required.

Operation of Control Valve

Automatic control valves are typically operated by electrical, hydraulic, or pneumatic actuators, which handle the opening and closing processes. In the case of modulating valves—those capable of positioning anywhere between fully open and fully closed—valve positioners play a crucial role in ensuring the valve reaches the exact intended position.

Among these, air-actuated valves are widely favored due to their straightforward design, relying solely on a compressed air supply. In contrast, electrically operated valves demand additional infrastructure such as cabling and switchgear, while hydraulically actuated valves require both high-pressure supply and return lines for the hydraulic fluid.

Traditionally, pneumatic control signals operate within a pressure range of 3–15 psi (0.2–1.0 bar). More recently, electrical control signals have become common, particularly a 4–20 mA current for industrial applications and a 0–10 V signal in HVAC systems.

Modern electrical controls often incorporate “smart” communication signals layered onto the 4–20 mA current. This advancement allows for real-time monitoring of the valve’s condition and verification of its position, with common communication protocols including HART, Fieldbus Foundation, and Profibus.

An automatic control valve is composed of three primary components, each available in various designs and types: the valve actuator, which moves the modulating element such as a ball or butterfly; the valve positioner, responsible for confirming that the valve achieves the desired opening and addressing issues like friction and wear; and the valve body, which houses the modulating element—whether it be a plug, globe, ball, or butterfly.

What Are the Important Control Valve Parts?

Here is a look at some of the key internal parts of our pneumatic Stem Guided High-Pressure Control Valve. These are linear valves that have a stem that guides the ball into the seat.

They can operate in modulating control or on/off applications.

• Bonnet: The bonnet forms the upper section of the valve, enclosing both the diaphragm and the actuator spring.
• Spring: The spring maintains tension on the stem, ensuring the valve remains in its designated fail-safe position.
• Control Valve Actuator: This actuator receives pneumatic signals, either from a pressure pilot or another pneumatic controller, and adjusts the valve’s position in response. It is designed to be disassembled and inverted, allowing conversion of the valve’s fail-safe mode from fail-closed to fail-open, or vice versa.
• Stuffing Box Assembly: Contained within the stuffing box assembly is the valve packing, which creates a seal to prevent leakage while permitting the stem to move vertically as required for valve operation.
• Yoke and Travel Indicator: The yoke and travel indicator provide a clear, visual representation of the valve’s current position.
• Stem and Seat: The stem and seat serve to block fluid flow through the valve. These components are critical to valve function but are also particularly vulnerable to wear due to exposure to abrasive or corrosive substances.

Why Control Valves used?

Process plants are composed of numerous control loops—often numbering in the hundreds or even thousands—that are interconnected to manufacture a product for the market. Each control loop is specifically designed to maintain key process variables, such as pressure, flow, level, and temperature, within predefined operating limits to ensure the final product meets quality standards.

These loops experience disturbances generated internally, which negatively impact the process variables, while interactions among different loops in the network introduce additional external disturbances affecting those variables.

To mitigate the influence of such disturbances, sensors and transmitters continuously gather data on the process variables and their deviation from desired set points.

This data is then processed by a controller, which determines the necessary actions to restore the process variable to its target value following a disturbance. After the measurement, comparison, and calculation phases, a final control element executes the corrective measures as directed by the controller’s strategy.

Types of Control Valve

The most common valve types in flow control industries include:

• Gate valves.
• Ball valve.
• Globe valves.
• Pinch valves.
• Diaphragm valves.
• Needle valves.
• Butterfly valve.
• Plug valve.

Continue reading to learn more about each of these types of flow control valves and their functions.

#1. Gate Valves.

Gate valves are commonly employed as general service valves, primarily designed for on/off control without throttling capabilities. They are especially suitable for situations where maintaining a straight-line fluid flow with minimal restriction is important. Operation of these valves involves rotating the stem either clockwise to close (CTC) or clockwise to open (CTO).

The gate itself moves vertically along the threaded stem as the operator turns it. Because of this threaded mechanism, gate valves require multiple turns to transition fully between open and closed positions. This gradual movement helps to reduce the risk of water hammer effects during operation.

Engineers often choose gate valves when it is essential to minimize pressure loss and ensure an unobstructed flow path. Since the design of typical gate valves does not impede the flow, they result in very low pressure drop across the valve.

#2. Ball Valves.

Ball valves are widely employed in flow systems across various industries because of their affordability, robustness, and reliable shutoff performance. However, like butterfly valves, they are less suited for applications demanding precise flow control and fine regulation.

This limitation stems partly from the fact that operating a ball valve requires considerable torque, which restricts an operator’s ability to make subtle adjustments.

#3. Globe Valves.

A globe valve is a type of linear motion valve designed to start, stop, and regulate fluid flow. It achieves closure through a plug, which typically has a flat or convex bottom, that is lowered onto a horizontal seat located centrally within the valve body. When the valve is opened, the plug is lifted, allowing fluid to pass through.

Globe valves are well-suited for both on/off control and throttling applications. This versatility stems from the valve’s disk, which can be fully removed from the flow path to permit unrestricted flow or lowered to completely obstruct it.

Although globe valves generally create higher pressure drops compared to straight-through valves such as gate, plug, or ball valves, they remain appropriate in contexts where pressure loss is not a critical consideration.

#4. Pinch Valves.

Pinch valves offer a cost-effective solution for controlling the flow of slurries or liquids with a high content of suspended solids. Their sealing mechanism relies on one or more flexible components, typically rubber tubes, which are pinched to stop the flow.

In these valves, the rubber sleeves are the only parts that come into contact with the fluid, and their flexibility enables a secure closure even around trapped solids. Actuation is achieved by applying air or hydraulic pressure directly to the elastomer sleeve.

Because the valve body itself serves as an integrated actuator, there is no need for additional hydraulic, pneumatic, or electric operators, contributing to the overall cost efficiency of pinch valves.

#5. Diaphragm Valves.

Diaphragm valves feature a flexible disc that presses against a seat located at the top of the valve body to create a seal. This diaphragm is both flexible and responsive to pressure, enabling it to transmit force for opening, closing, or regulating the valve.

Although similar to pinch valves, diaphragm valves differ in that they use an elastomeric diaphragm instead of an elastomeric liner within the valve body. The diaphragm, which is connected to a compressor, acts as a barrier between the flow stream and the closure mechanism. These valves are particularly well-suited for applications involving corrosive, erosive, or contaminated fluids.

Several advantages make diaphragm valves a preferred choice: they maintain a highly clean environment, provide a leak-proof seal, ensure a tight shut-off, are straightforward to maintain, and minimize environmental leakage. Additionally, diaphragm valves can often be repaired without the need to halt pipeline operations.

#6. Needle Valves.

Needle valves are specialized volume control devices designed to regulate flow in small-diameter lines. As fluid passes through the valve, it changes direction by 90 degrees and moves through an orifice, which acts as the seat for a rod with a cone-shaped tip. The size of this orifice varies depending on the position of the cone relative to the seat, allowing precise control of flow.

While needle valves share certain design elements and advantages with globe valves—such as enabling operators to adjust flow rates through a threaded, rotating stem—the key distinction lies in the level of precision. Needle valves offer a much finer degree of control, making them particularly well-suited for calibration tasks where exact flow adjustments are required.

Additionally, needle valves are capable of providing a positive shutoff, which ensures that gauges and other measurement devices can be safely installed or removed. This functionality explains their widespread application across diverse sectors, including petrochemical and biofuel industries.

#7. Butterfly valve.

A butterfly valve functions by rotating a disk within the flow passage, which results in non-linear flow characteristics. Because of this design, these valves tend to be less precise compared to more commonly used flow control valves.

Consequently, butterfly valves are sometimes overlooked as a flow control option, despite their usefulness in applications where extreme accuracy is not critical. Additionally, their cost-effectiveness makes them a practical choice for suitable situations.

#8. Plug valve.

Plug valves are available in multiple configurations and function by rotating a cylindrical or cone-shaped plug inside the valve body to control flow through the plug’s hollow section.

In flow control applications, the eccentric plug valve is the most frequently used design. This type features a half plug that generates a higher seating force while maintaining low friction during opening and closing. This design offers enhanced shut-off performance, making it particularly suitable for regulating flow.

What are the factors that must be considered while installing a control valve, so that we can avoid future maintenance?

The control valve will function properly if the installation is done perfectly so we must consider certain factors while installing a control valve to operate the valve perfectly without any trouble they are

• Valve installation should avoid locations where vibrations or external stresses might interfere with proper valve operation. When installation in such areas is unavoidable, appropriate measures must be taken to mitigate these effects.
• It is important to ensure that control valves are installed in locations that provide ample space for operation and allow easy access for maintenance activities.
• To prevent the valve’s weight from placing undue stress on the process piping, support structures should be provided either directly for the valve or for the associated pipe runs.
• Valves must not be installed in areas prone to submersion by water or snow, or where excessive freezing conditions occur.
• Adequate precautions must be implemented to protect valves from corrosive environmental conditions.

How to do the quick check and maintenance of a control valve

What are the major parts that must be inspected in a control valve?

• Actuator diaphragm
• Actuator shafts
• Trim components such as plug seat cage and stem
• Packing box must be inspected which is in the bonnet
• Gasket seating surface
• NDT test for body valve
• We must check the condition of the valve positioner
• Bolts and nuts of the body
• Actuator spring
• Handwheel if present
• The surface of the flange gasket

Actuators, like other mechanical components, are susceptible to various mechanical issues that can impact the overall process. To address these problems effectively, it is essential to identify any faults in the actuator by comparing the valve position indicator with the position requested by the valve controller. This comparison helps in detecting discrepancies that may signal actuator malfunction.

Certain types of actuators include manual operators, which can be utilized to resolve issues when the actuator fails. Additionally, some actuators feature manual bypass lines located near the malfunctioning components, providing an alternative means to address problems.

It is also important to examine the actuator’s spring tension, as improper tension may contribute to operational difficulties.

When inspecting the diaphragm, attention must also be given to the condition of the springs. Furthermore, ensuring that the actuators and valve positioners are correctly sized is critical for optimal performance.

Routine maintenance

During routine maintenance, it is important to inspect the gland nuts and tighten them if any leakage is detected through the gland packing. Additionally, the nuts on both the top and bottom flanges should be examined to ensure that the gasket is not leaking.

FAQs