The role of control valves in the industrial process loop is becoming increasingly important. As a reliable control valve manufacturer, we would like to introduce the different types of control valves, components, functions, application scenarios, and valve accessories, in a systematic and professional way in this article.
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In this section we are going to introduce a piece of brief information about control valves, in case help you to know what control valve is and which role has it played.
As industrial automation processes are upgraded, more and more control valves are being used in process circuits. Almost every plant consists of hundreds of control loops. All loops are in turn networked together and used to keep a number of important process variables within defined limits to ensure the quality of the final product. These parameters include pressure, flow, temperature, level, etc. The sensors and transmitters collect the process variables to the controller, after comparing with the desired setpoints, the controller sends a corrective signal to control valves, in case modulating the valve opening to reach the required value.
In brief, a control valve is a key part of a control loop, a critical control element in industry processes, which consists of a pneumatic actuator, electric actuator, or hydraulic actuator assembly with a valve for regulating the flow capacity or temperature of fluids, or upstream pressure and downstream pressure, or level. It is widely used in petroleum, chemical, pharmaceutical, textile, paper industry, refining, oil and gas, iron and steel metallurgy, and other industries.
By now we all know that the most common final control element in the process control industry is the control valve. The ultimate purpose of a control valve is to regulate the temperature, pressure, level or flow of a medium while compensating for load disturbances so that the process variable being regulated is as close to the set value as possible. The medium could be water, steam, oil, gas, or other compounds.
Typically, a control loop consists of a sensor that detects the process parameters, a transmitter, a control valve, and a controller.
The sensor and transmitter collect process variables in the loop and act as an eye to monitor changes in individual parameters such as the temperature, pressure, or level of the medium. The collected value is then sent to the controller. Through the controller, which functions like a brain, the “process variable” received by the transmitter is compared with the “set point” and a corrective signal is sent to the control valve. So the control valve, as the final control element, plays the role of the hand in the control loop, making it the most important part of the automatic control system.
There are many types of control valves, each designed for specific types of applications, read this post for more details on different types of control valves.
Globe type control valve is the most common final control element during the process loop, and it gets its name as its spherical shape. According to the ports of the globe valve body, it includes 2-way control, 3-way mixing control, and 3-way diverting type.
When we talk about butterfly control valves, we generally refer to high-performance butterfly valves (double eccentric butterfly valves) and triple eccentric butterfly valves (triple offset butterfly valves).
In most applications, butterfly valves can replace other types of control valves because they provide the same basic functions, but often have more safety features and better performance. Using butterfly valves in your application can reduce maintenance requirements, improve the accuracy of flow control, and reduce the risk of disasters such as leakage. Butterfly control valves are also used to control water treatment plants because they provide the best seal and can withstand water pressure. Especially in large-diameter pipelines, the use of butterfly control valves is cost-effective.
The v-port or segmented ball is the most common type for ball control valves. Ball valves are widely used in fluid systems in many industries due to their low cost, durability, and excellent closing performance. Similar to butterfly valves, they are not as effective in flow control applications that require high precision and control sensitivity. One of the reasons is that the ball valve requires high torque to open and close, which prevents the operator from making fine adjustments. There is also a certain gap between the stem and the ball, which makes it difficult to find a specific flow rate suitable for fluid control applications.
The eccentric rotary valve is an innovative technology that combines many of the ruggedness, compactness, and long-stem seal life of globe valves with simplicity and reliability.
The eccentric plug rotary control valves have two offset for construction design. One is the shaft is offset behind the valve seat and the other is the shaft is offset from the centerline of the valve.
When the valve shaft and plug of the eccentric plug control valve start to move, the plug cams away forward open from the seat once the valve just started. There is no friction between the plug and the seat.
The reduced trim full ball valve is the design that has a triangular window on side of the ball.
This reduced trim ball valve is not commonly used in initial selection and sizing, most applications are when you find after the plant starts up that a ball control valve is oversized, the capacity is way large than the process required, so if you are not able to alter the piping to accommodate the smaller size valve and the necessary piping reducers, then reduced trim full ball valve is your best choice.
Most of the flow path windows are triangular or V-shaped to get as close as possible to the inherent flow characteristics of EQ%. The wide width of the window determines the maximum flow capacity of the control valve.
There are two major types of control valves, linear and rotary motion. The rotary type control valve is a quarter-turn motion, and the linear type control valve is a sliding motion. So we will separate those two types and list each component for control valves.
Above is a cross-sectional view of a globe type control valve, which clearly shows the main components. globe type control valves can also be called sliding globe type control valves.
Rotary control valves are also known as quarter-turn control valves, and the definition of each part is similar to that of linear type control valves. Only the name of the bonnet is different. The bonnet of almost all rotary valves is cast as a part of the valve body as an entirety and is usually called an “integral bonnet”. Instead, the bonnet is bolted to the valve body as in the case of globe valves.
The part that connects the actuator to the closure element is called a stem by some manufacturers, and a shaft by others.
The closure element of a butterfly valve is called a disc, while a ball valve is called a sphere or ball.
The actuator for the control valve is a mechanical device that uses an external power to modulate valves.
There are four different types of actuators for the control valve, pneumatic actuator, electric actuator, hydraulic actuator, and self-operated actuator. An actuator is an important component for control valves, which drive to open or close valve. For the industrial process control loop, pneumatic and electric actuated are the most common types.
The pneumatic actuator includes a multi-spring diaphragm pneumatic actuator, single-spring diaphragm pneumatic actuator, sing-acting and double-acting piston/cylinder pneumatic actuator, and rotary type pneumatic actuator(rack&pinion, and scotch yoke).
The multi-spring membrane pneumatic actuators of spring return type are applied for control operation of control valves, and other positioning elements in industrial automatic systems. There are two following design options for the actuator:
2. Single-Spring Diaphragm Pneumatic Actuator
4. Rotary Type Pneumatic Actuator
We know that an automated control valve needs an actuator to operate the valve, pneumatic actuator and electric actuator are different, so it may cause confusion when you select an actuator for your application. The following step-by-step method can help you figure out which actuator is exactly perfect for your condition.
Step 1: Check the valve type
Linear type control valve
Linear type control valve includes globe type, gate type, and pinch valve. The actuator assembly for these control valves must provide linear motion to operation.
Rotary type control valve
The rotary type control valve applies the actuator produces the rotational motion to operate a valve. Eccentric plug valve, butterfly valve, segment ball Valve, and full port ball valve are all categories by rotary type control valve.
Electric actuators have high levels of precision, control, and energy efficiency. They are usually used for on/off control with fail in the last valve position. And its cost is more expensive than pneumatic rotary actuators.
Pneumatic rotary actuators are known for fewer and simple components, so it makes easier and cheap for maintenance work. Pneumatic actuators with spring return are very commonly used for the failsafe condition, such as FC(fail to close valve) or FO(fail to open valve).
Step 2: Check the available power source on your site
This is a key issue if the site can only provide electric power, not equipped with external air source equipment, in order to save costs, generally use an electric control valve. On the contrary, if equipped with sufficient air supply, then pneumatic control valves are a better option for the process loop.
Pneumatic diaphragm actuators usually required an airpower range of 1.4 bar to 4.5 bar, and piston-type pneumatic actuators required 3 bar to 10 bar power. It’s difficult for plants to guarantee above 6bar at all times for the process equipment, it’s one of the reasons why diaphragm actuators are so commonly assembled for control valves. If low airpower which under 5 bar will require a large size of pistons to generate torque to operate valves.
Piston pneumatic actuators are available in two main types: rack&pinion, and scotch yoke.
Both of these rotary pneumatic actuator types provide a compact and economical solution for quarter-turn(90-degree) full port ball valves, eccentric plug control valves, high-performance butterfly valves, and segment ball valves.
In many plants, electric actuators are commonly available power voltages with 12 and 24 VDC, and 24, 120, and 220 VAC, 1-phase. For power plants, most use 380V, and 400V 3-phase power.
Step 3: Installation Space
Usually, in complex piping systems, we need to consider the installation space of the control valve, for the narrow installation space, the engineer needs to first consider the compact rotary type control valve, such as eccentric plug control valve, segment ball valve, and butterfly control valve.
Step 4: Function
Most engineering has requirements for actuator function, such as on/off or modulating, service cycle life, failure position, operating speed, control accuracy, assemble with manual operator/handwheel, local control, or others.
We also need to consider the media characteristics if it’s flammable, then pneumatic actuators are safer than electric actuators. So pneumatic actuators are very commonly used in hazardous or explosive working areas, however, any electrical accessories such as limit switch, valve positioner, and solenoid valve assemble for pneumatic actuator still need to meet NEMA standards.
The electric actuator also can be used in hazardous areas, just more expensive than the pneumatic actuator. Just need electrical components and enclosure to meet the NEMA standards.
Step 5: Environmental conditions or ambient temperature
General pneumatic actuators can handle temperature ranges between -4°F to 175°F, due to the piston seal ring, bearing, and mounting design can’t bear high temperatures.
Since low-temperature environments can cause air sources to condense, the condensate may freeze and block the air supply tube, causing the pneumatic actuator to fail to operate.
Electric actuators can be used in the temperature range of -40 to 150°F, but are also affected by ambient temperature and humidity. When used outdoors, electric actuators should be rated for that environment to prevent moisture buildup from damaging electrical components. Also in outdoor low-temperature environments, the heat from the motor can cause condensation to form on the electric actuator housing, so the actuator should be fitted with thermostat accessories or a heater to maintain a constant temperature and avoid the occurrence of condensation issues.
Step 6: Budget
In general, the cost for globe type control valve is higher than rotary type control valve, such as butterfly type and ball type.
The electrical actuator is mounted on the control valve which uses an electromagnetic device converts electrical energy into mechanical movement, it uses an external power source like 220VDC, 24VAC, 380V, or other power voltage to drive valves. Most electrical actuators use limit switches to turn the motor off when the actuator reaches the final valve position, due to it is activated by cams that are mounted on the stem drive shaft.
Electric actuators can be classified as linear and rotary electrical actuators.
The linear electrical actuator also called motorized valve actuator, which commonly used for globe type control valves. For rising stem control valves assemble electric actuators to operate with a screw drive. The closure element of the valve is moved up and down by the rotation of a threaded rod that is connected to the valve stem.
A limit switch for control valves is a device designed to provide a valve position signal to the controller, such as PLC or DCS control system. This accessory can be integral to an electrical actuator and be separately assembled into a pneumatic actuator or valve yoke.
Generally, there are two common types of limit switches used for control valves regarding valve movement.
For sliding stem/linear type control valve, these limit switches are assembled at the valve stem and enable the signal of valve opening or close position to the control system. Some conditions only require one signal of valve closing or opening to the controller, but some require both positions, valve close, and valve open position.
Two available options:
For rotary type control valves, like ball valve, butterfly valve, and eccentric plug rotary control valve, they use a limit switches box, which has a visual indication of the valve position. The limit switches box is typically activated by cams connected to the shaft and equipped on the pneumatic actuator to the control valve.
Limit switches send electrical feedback from two micro switches to indicate valve position. Typically for on/off type control valve, which confirms either open or close position in site.
Solenoid valve is equipped with pneumatic control globe valves that main function is to lock the valve at the last valve position, and also it can vent out the air from the pneumatic actuator, in case drive valve to the safe position.
I/P transducer means making a current signal 4-20mA converts proportional pressure output of 3-15psi. This accessory is used for an application that does not require a high level of positioner accuracy of a positioner, only needs an electro-pneumatic sensor to transfer the signal.
An air filter regulator is a necessary accessory for the control valve, usually assembly before the actuator, in case the air power reduces the right pressure range for the pneumatic actuator. For example, if the diaphragm pneumatic actuator requires 2.4bar air power, then we need to adjust the air filter regulator to reduce the pressure to meet 2.4bar, otherwise, the air power will damage the diaphragm. Also, it can filter the Impurities in the air.
Volume booster is used for large volume needed pneumatic actuator, the airpower is not enough to provide large air volume in a short time, so in case of speed up the response of control valve, volume booster can provide extra pneumatic thrust output capacity to the valve.
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Typically single-acting pneumatic actuators only request one volume booster, and double-acting pneumatic actuators require at least two sets.
Compressed air flows from the control valve and through the quick exhaust valve to the cylinder, speeding up the opening time of the valve.
Sometimes control valve required a positioner transmitter that can send a 4-20mA output signal to control system, in case to feedback a actual valve position.
A manual operator is an additional handwheel for the control valve, in the case of an emergency situation we can operate by a manual operator.
There are three types of manual operators for control valves.
Notes
There are many different types of flow control valves that can be used in a variety of industries. Some valves are complex enough to adjust automatically to pressure and temperature variations, which is why they are called flow control valves. Regardless of their construction, flow control valves are designed to modulate the flow capacity or pressure of fluids, and they respond to signals from flow meters, senses, and transmitters.
The control valve flow coefficient Cv is the flow rate in Gallons Per Minute of 60°F pure water when there is a pressure differential of 1 PSI through the valve at specified travel.
The control valve flow coefficient Cv is the flow rate in Gallons Per Minute of 60°F pure water when there is a pressure differential of 1 PSI through the valve at specified travel.
Here are some numbers that show you relatively how much flow capacity will go flowing through the control valve. For example, a control valve with a Cv rate of 100 will go flowing through 250GPM. Under the same process conditions, a Cv of 200 will go flowing through 500GPM, and a Cv of 300 will go flowing through 750GPM.
Cv 100 = 250GPM
Cv 200 = 500GPM
Cv 300 = 750GPM
So we know the bigger of Cv rate, we got the larger the flow capacity. Typically, doubling the size of the valve will increase the Cv about three to four times.
During the selection process of a control valve, we all need to confirm the flow characteristics required for the valve. As a valve manufacturer, we do not know what kind of system this control valve will be installed in (except for the complete solution), so generally, we only provide the inherent flow characteristics which are the relationship between valve opening and flow rate.
In fact, a control valve has two characteristics, one is the inherent flow characteristics, and the other is installed flow characteristics. The end-user is often concerned with the installed flow characteristic which is the relationship between the control valve opening and the flow rate in the particular system being installed.
Cavitation is a very serious problem that could break the control valve performance. The pressure recovery factor of control valve FL represents its tendency to cause cavitation problems. Generally, the higher capacity of rotary valves has a higher recovery factor that has a greater tendency to cavitate. But it doesn’t mean butterfly valves, eccentric plug rotary control valves, segment ball control valves, and other rotary type control valves are not good for modulating function. However, we need to pay more attention that these valves are selected and sized right.
There are many specifications that will tendencies valve cavitation, different valve type has different tendencies, despite they have a low FL but still have a greater potential for cavitation.
In conclusion, the selection and calculation of control valves need to consider many factors, the above content can not display all the knowledge, but we will continue to update the content as much as possible, at the same time, if you are interested in the calculation formula and sizing information, you can go to our download page for free download.
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In the simplest terms, a valve is a device used to control the flow of a medium — liquid, gas or solid — through a piping system. Most commonly, valves are used to stop and start the flow of media. Some valves are also able to control the rate of flow, these are commonly referred to as control valves.
Because there are so many different uses for valves, they are available in a wide range of mechanical variations. Using the appropriate valve in a given scenario means your application will run smoothly and safely, accomplishing the intended goal most efficiently.
Though there are many different valve classifications, most valves fall within three categories: rotary, linear and self-actuated.
Rotary valves utilize a rotating closure component to block flow within a piping system. Most often, this rotation is limited to 90 degrees, which is why rotary valves are also commonly referred to as quarter-turn valves. These types of valves are closed at 0 degrees and open at 90 degrees.
Some rotary valves can operate with a larger degree of rotation and include more than two positions. Common examples of rotary valve types are butterfly, plug and ball valves.
Linear valves use a flow obstructer — such as a disc, slat or diaphragm — that moves in a straight line to start, stop or adjust flow through a piping system. These valves can be separated into two distinct types: rising stem (multi-turn) and axial.
Multi-turn linear valves work particularly well in control applications and include globe valves, gate valves and needle valves. Axial valves, such as coaxial and angle seat valves, are fast-acting and used primarily in on/off process applications.
Compared to other categories of valves, linear motion valves tend to have longer cycle times than rotary valves.
Unlike linear and rotary valves, self-actuated valves do not require direct input from the operator. Instead, they use the pressure within the process line to open or close a pressure control valve. This valve type is commonly used as a pressure relief valve and only opens once maximum allowable pressure is reached within the system. As a result of this mechanism, self-actuated valves are common safety requirements in certain applications.
Common self-actuated valves include relief valves, safety valves, check valves and steam traps.
Since quarter-turn valves allow for a wide range of motion with the turn of a handle, these valve types are commonly used in applications that prioritize quick and easy opening/closing over precision.
Ball valves can be used in most applications where a fluid flow needs to be shut off. In fact, they are the most widely used process control valve. These rotary valves use ported spheres that swivel in the pipe stream, working to start or stop flow.
Since the valve’s port can be fitted precisely to match the pipe’s diameter, ball valves offer low head loss. Ball valves may be more expensive than other rotary valve options, but they offer better sealing than types such as butterfly valves.
Butterfly valves work using a center-mounted, disc-shaped obstructer that sways in and out of the piping system’s flow. These quarter-turn valves are frequently used with larger pipe sizes and in wastewater plants, power plants and other process plants where shut-off, regulation, and isolation are called for.
Advantages of using butterfly valves include lower cost and smaller size. However, these common valve types are more vulnerable to leaks and head losses than ball valves and may not work as well against high-pressure flows.
Plug valves are very similar to ball valves in their construction and function. However, instead of using a ball-shaped obstructer, plug valves obstruct or allow flow via a tapered cylinder that swings into or out of the flow stream. There are two main types of plug valves: lubricated and unlubricated.
This type of rotary valve is frequently used for shut-off and as a control valve for chemical processing industries, processing plants and wastewater treatment facilities. Plug valves may be favored for their lack of voids and cavities, and their ability to be adjusted for leaks that develop over the long term. As a result, these valves are often used in extreme service scenarios — think corrosive environments like oil refineries and chemical plants.
Commonly used in flow control applications, multi-turn valves work by cranking a handle to move an obstructer element into the flow path. This restricts flow within the piping system. Some types of multi-turn valves enable the flow to be released or blocked at variable speeds. These types of valves are commonly referred to as control valves.
Globe valves work by using a globe-shaped disc to block flow when closed against a restriction orifice. These multi-turn valves are commonly used in on/off and throttling applications. Globe valves can seal both against or with fluid flow.
Two-way globe valves are recommended for precision, while 3-way globe valves are suitable for combining media from two inlet ports and sending the resulting mixture through an outlet port. Globe valves are frequently used in wastewater plants and food processing services. Although this valve type is available in many variations, the most prevalent is the Z-style valve.
Gate valves are multi-turn valves commonly used to block flow streams and occasionally (and sometimes ineffectively) for throttling. This multi-turn valve type utilizes a plate-like barrier to block a flow stream.
Gate valves offer lower head losses when open compared to other valve types. Although their operation is comparable to globe valves, gate valves offer less flow restriction and regulation capabilities.
Needle valves closely resemble globe valves aside from several key factors. First, they are smaller than globe valves and allow for more precise flow control within smaller systems. Additionally, they consist of a cone-shaped needle — as opposed to a disc-shaped plug — that moves into and out of an orifice to start and stop flow.
Needle valves offer a precise way of modifying fluid flow through a system. They can be used for on/on applications but may not be the best option in such instances, as they require many turns in order to close.
Much as the name suggests, on/off valves are fast-acting devices that either enable unrestricted flow or block flow entirely. Though these types of valves do not allow for precise flow adjustment, they are useful in various applications where speed is essential, for example when used for situations that require emergency shut-off.
Solenoid valves use a linear sliding obstructer to open or shut the valve or redirect the flow from one outlet to another. These valve types are available in many different sizes, from as small as one millimeter to as large as 100 millimeters. Furthermore, they can be composed of various materials such as cast iron, stainless steel, aluminum, plastic and brass.
Solenoid valves are commonly used in emergency shut-off service applications and high-pressure systems. They can also handle a wide range of temperatures, with some models able to handle temperatures as extreme as -418° F to ° F, making solenoid valves a popular choice for cryogenic processes.
A coaxial valve is a 2-position valve that can be either pneumatically or electrically actuated. Electric coaxial valves use an electromagnetic coil vs. a spring. Pneumatic coaxial valves use either air pressure vs. a spring or double acting air pressure (air pressure that comes from both directions). These valves use a shuttle obstructer to open or close and to divert flow to the correct outlet.
Coaxial valves are a good substitute for ball valves in many on/off applications. In fact, they are a more affordable, smaller, lighter, safer and faster alternative. Disadvantages of coaxial valves include lower flow capacity and limited fluid compatibility.
Closing out the list of different types of valves and their uses is the angle seat valve, another type of valve that can serve as a substitute for a ball valve in on/off applications. Seat angle valves use a built-in pneumatic actuator that can be double acting or spring return.
This 2-position valve includes an obstructer plug that goes into the valve at an angle in order to fit into the angled seat situated within the flow path. When the valve is open, the plug is retracted almost entirely out of the flow path, making this type of valve the highest flow fast-acting valve with the lowest pressure drop.
Compared to ball valves, angle seat valves are faster, less expensive and just as durable. The primary disadvantage an angle seat valve has when compared to a ball valve is that angle seat valves do not perform as well with media that carries particulates. Due to their linear nature, an angle seat valve is more prone to trapping objects against the seal when closing, which can lead to failure to create a tight seal and can damage soft seal components. Ball valves, on the other hand, will tend to wipe away any particulates as they close which keeps objects from getting caught between the ball and soft seals. Angle seat valves also tend to be better suited for applications with lower operating pressure.
Why is it important to choose a certain type of valve?
Different valve styles will perform differently in any given application. This may mean differences in both functionality (cycle speed, position control, etc.) and the lifespan of the valve. Some types of valves may be entirely inappropriate for certain applications.
What’s the difference between linear valves and rotary valves?
Rotary valves utilize a rotating closure component to block flow within a piping system, while linear valves function by raising or lowering a flow obstructor, such as a gate, globe or diaphragm. Rotary valves typically require only 90 degrees of rotation to cycle on or off, whereas linear valves require multiple rotations of the shaft to move the closure component.
Why should I consider an on/off valve?
On/off valves should be used in applications where the intended function of the valve is to stop or start the flow of media. On/off valves should not be used to modulate flow rate.
Why should I consider a control valve?
Control valves should be utilized in applications where more precise control of the media flow rate is required, though not all control valves are recommended for throttling.
Can all linear valves be used as control valves?
No; gate valves, for example, are not recommended for control applications and are intended to either be fully open or fully closed.
Why are some valves more expensive than others?
Cost is influenced by a wide variety of factors including materials of construction, manufacturing processes and the complexity of the design. There are many different features and options that may influence cost as well. For example, actuated valves will generally be much more expensive than manual valves.
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