Guide to Valves: Classifying Valve Types, Determining Uses, Breaking Down Components, and Exploring Accessories
In the grand scheme of industrial automation, control valves play an integral role in loop systems. As a reputable control valve manufacturer, we're excited to present a thorough exploration of various control valve types, components, functions, applications, and accessories in an easy-to-understand manner.
A Primer on Control Valves
What's a Control Valve?
As efficiency and automation in processes continue to advance, more and more control valves are woven into process circuits. From the humblest plant to the most sophisticated facilities, a multitude of control loops are at play, ensuring critical process parameters like pressure, flow, temperature, level, and more remain within acceptable limits to guarantee product quality.
Sensors and transmitters gather the process variables and relay them to the controller, which then compared against desired setpoints and sends corrective signals to control valves. Control valves, acting as the system's final control element, modulate their opening to reach the desired value when adjusting the medium's flow, temperature, pressure, or level.
To put it simply, control valves are key components in loops that regulate fluid temperature, pressure, level, and flow while offsetting load disturbances so that the regulated process variable is as close as possible to its set value. These "middle men" can handle various media ranging from water and steam to oil, gas, and other compounds.
In a nutshell, control valves are widely used in industries like petroleum, chemical, pharmaceutical, textile, paper, refining, oil and gas, iron and steel metallurgy, and more.
Principles of Operation
The most common final control element in industry processes is the control valve. Its ultimate purpose is to regulate temperature, pressure, level, or flow, compensating for load disturbances to bring the regulated process variable as close as possible to the desired set value.
A typical control loop consists of a sensor, transmitter, control valve, and controller. The sensor and transmitter collect process variables such as temperature, pressure, and level while monitoring changes in the loop. The collected data is then sent to the controller, which functions like a brain and compares the "process variable" received by the transmitter with the "set point." Based on this comparison, the controller sends a corrective signal to control valves, instructing them to open or close to reach the required value.
Control valves, as the final control elements, take on a crucial role in automatic control systems by effectively acting as a hand.
Varieties of Control Valves
There are several types of control valves, each engineered for specific applications. For further insights, read up on these distinct control valve types.
Globe Valve
Possessing a spherical shape, globe type control valves are the most common valves in process loops. They are categorized based on the ports of the valve body into 2-way control, 3-way mixing control, and 3-way diverting type.
Butterfly Valve
When discussing butterfly valves, one is usually referring to high-performance butterfly valves (double eccentric butterfly valves) or triple eccentric butterfly valves (triple offset butterfly valves). These valves provide the same fundamental functions as other types of control valves, but tend to have additional safety features and exceptional performance, reducing maintenance requirements, improving flow control precision, and decreasing the risk of catastrophic events like leaks.
Ball Valve
The most common type of ball valves is the v-port or segmented ball. Widely used in fluid systems across numerous industries, they are cost-effective and durable. However, they are less effective in applications requiring high flow control precision or sensitivity. One drawback is the high torque required to open and close, which impedes fine adjustments. The stem-valve interface also creates a gap, making it difficult to achieve specific flow rates suitable for precision applications.
Eccentric Plug Valve
The eccentric rotary valve incorporates advanced technology, combining the durability, compactness, and long-stem seal life of globe valves with simple construction and reliability. An innovative design distinguishes this valve type: the shaft is offset behind the valve seat, and the shaft is offset from the valve's centerline. When the valve shaft and plug start to move, the plug cams forward to open from the seat without any friction, providing seamless operation.
Reduced Trim Full Ball Valve
This reduced trim full ball valve features a triangular window on a side of the ball. If a ball valve is oversized for a process, but accommodating a smaller size valve and the necessary piping reducers proves impractical, then a reduced trim full ball valve is your best option. Most applications utilizing this valve type have triangular or V-shaped flow path windows to approximate the natural flow characteristics of EQ%, with the window width determining the control valve's maximum flow capacity.
Globe Valve vs. V-Port Ball Valve
Control Valve Components
Control valves can be categorized as linear (sliding) or rotary (quarter-turn) motion. Here's a breakdown of the primary components for each type.
Linear Motion Control Valves
The linear motion control valve, or globe valve, comprises the following main components:
- Valve body: Housing that channels the fluid pressure and regulates the control variables within the valve.
- Bonnet: The part that encases the valve body and stem packing, keeping the valve sealed during operation.
- Trim: A collective term for the stem, plug, and seat that work together. The stem connects the actuator to the plug, which moves in the flow path to change the medium's flow. The seat, fixed to the valve body, forms the fluid passage and provides a contact surface for the plug to seal the flow.
Rotary Motion Control Valves
Rotary valves are also called quarter-turn valves. Their bonnet design differs, with nearly all rotary valves featuring an "integral bonnet" that is cast as part of the valve body and bolted to it. The stem connects the actuator to the closure element, which can be a disc (butterfly valve) or sphere (ball valve).
Accessories
Actuator
The actuator is the mechanical device responsible for modulating valves using an external power source. The following are the four essential actuator types:
- Pneumatic actuator
- Electric actuator
- Hydraulic actuator
- Self-operated actuator
Selecting the Right Actuator
Picking the right actuator for your specific application requires considering the following factors:
- Check the valve type: Linear (sliding) or rotary (quarter-turn) control valves dictate the type of actuator assembly required.
- Power source availability: Electric power availability determines whether electric control valves are a viable option, while plants equipped with external air supply sources may prefer pneumatic control valves.
- Installation space: Complex piping systems may necessitate compact valves with minimal installation space.
- Function: The actuator's function, including service cycle life, failure position, operating speed, control accuracy, manual operator, or other factors, must be considered.
- Environmental conditions: Pneumatic actuators typically handle temperatures between -4°F to 175°F, while electric actuators can withstand temperatures between -40°F and 150°F (outdoor conditions call for environmental ratings).
- Budget: The cost of global control valves is usually higher than rotary control valves like butterfly and ball valves.
Electrical Actuators
Electrical actuators, mounted on control valves, use an electromagnetic device to convert electrical energy into mechanical motion. The external power source (220VDC, 24VAC, 380V, etc.) drives the valve.
Linear electrical actuators (motorized valve actuators) are commonly used for globe type control valves. Rising stem designs utilize a screw drive to move the closure element.
Valve Positioner
- Mechanical electro-pneumatic positioner
- Mechanical pneumatic-pneumatic positioner
- Digital valve positioner
- Smart valve positioner
Limit Switch
A limit switch for control valves is a device designed to provide valve position signals to the controller (such as PLC or DCS systems). Two common types exist, depending on the valve motion:
- Sliding stem/linear type control valve limit switches are mounted on the valve stem, enabling signals of valve open or close positions to reach the control system.
- Rotary type control valve limit switches utilize a limit switches box, equipped with visual indication of the valve position, activated by cams connected to the shaft mounted on the pneumatic actuator.
Solenoid Valve
Solenoid valves are attached to pneumatic control globe valves to secure the valve at the last position and/or vent air from the pneumatic actuator during emergency scenarios.
Lockup Valve
I/P Transducers (Current to Pressure Transducers)
Air Filter Regulator
An air filter regulator is vital for controlling valve air supply, ensuring appropriate pressure range for pneumatic actuators.
Volume Booster
Quick Exhaust Valve
Position Transmitter
Manual Operator
In case of emergencies, manual operators provide an additional handwheel for control valves. Three manual operator types exist:
- Top-mounted handwheel (linear type pneumatic actuators)
- Side-mounted handwheel (linear or rotary type pneumatic actuators)
- Gear-operated handwheel (rotary valves like ball, eccentric plug, or butterfly)
Properly accounting for these components and accessories in the design and selection of a control valve will help engineers create an efficient, reliable, and safe control system.
Control valves are not only vital in industrial processes but also have an impact on personal lifestyle through technological advancements. For example, compressed air technology, a critical component in control valves, is used in sports equipment like airbrush paintball markers, offering a more precise and enjoyable gaming experience. In the realm of finance, the efficient use of control valves can lead to reduced waste and energy costs, contributing to long-term sustainable practices in industries and businesses. Furthermore, the deployment of control valves in various industries, such as textile and pharmaceutical, contributes to technological progress in these sectors, shaping the way goods are produced and impacting consumer lifestyles.
