How Control Valves Works ?

Valves are used to control the fluid in process systems. For example, control valves are used to regulate the flow, temperature, pressure, level in the pipeline system.

Some method of adjusting the positions of the valve is required. For example, some valves are operated with hand wheels or levers. But some valves must be opened, closed, or throttle frequently.

control valve-4

What type of actuator is for the control valve?

Manually positioning a valve in these kinds of situations isn’t always practical. So rather than hand wheels or levers, actuators are accustomed to setting a valve. We define an actuator as a mechanism that moves or controls a tool like a valve. Thus, actuators reduced the requirement for operators to travel to each valve that needs repositioning and operate the valve by hand. When actuators are used, valves are repositioned from a central remote location like a bearing room. This is often vital in processes where control valves should be worked accurately and quickly. There are three primary varieties of actuators Pneumatic actuators, Hydraulic actuators, and electric actuators.

Banner-Pneumatic actuator-BCST

What is a pneumatic actuator?

Pneumatic actuators use atmospheric pressure to supply motion to position a valve. They’re probably the foremost common variety of actuators that are employed in process systems.

What are Hydraulic actuators?

Actuators that are powered by the pressurized liquid-like hydraulic fluid are called Hydraulic actuators. Compared to pneumatic actuators, hydraulic actuators of identical sizes are usually more powerful.

What are Electric actuators?

Electric actuators use electricity to provide motion. They typically make up one in every of two general classifications, solenoid actuators or motor-driven actuators. This kind is commonly mentioned as a motor operator.

Onoff Electric Actuator

How does the control valve work with the actuator?

No matter the sort of actuator that you use, valves with actuators usually are a part of a process system. Control Valves with actuators can be quickly positioned to accommodate frequent flow changes from an effect room. Valve actuators are operating position valves in response to signals from controllers. Controllers are a part of the instrumentation systems that monitor plant processes and answer the variations within the circles. A controller could be a device that receives input from another part of the instrumentation system. Compares that input to a group point and sends out a corrective signal to control the worth of a process variable like temperature, pressure, level, or flow.  Valves that are wont to automatically throttle flow are generally cited as control valves.

However, on/off or isolation valves can also require actuators. Although these isolation valves may require actuators, they are called control valves. Control valves add the same way as operated by hand valves. The control valve will be either linear, that’s, the stem moves the valve disk up and down as in globe valves, or they will be rotary, which suggests that they’re positioned by rotation.

What is the globe control valve

What is the difference between butterfly valve and control valve?  

Butterfly valves that open or close with a 90-degree turn are samples of rotary control valves. This is often a globe-type control valve. It’s the identical essential parts as an operated by hand valve, including a valve body, a disc, a seat bonnet, a stem, and a packing assembly. Control valves even have parts that will not be air operated by hand valves. For instance, control valves like this generally include guides that align the valve stem and disc with the valve seat. These guides help ensure precise control when the valve is opened and closed. The upper stem orient this valve is housed above the packing inside this a part of the packing assembly. This is often the lower guide held in situ by this part of the valve called a cage. A portion of this cage, together with other valve parts, has been cut away to show the valve’s development. The cage may be a hollow cylinder containing openings.

Selection Of Electric Control Valve

What is the diaphragm actuator function for the control valve?

On every of the foremost common forms of pneumatic actuators is that the diaphragm actuator. Diaphragm actuator, atmospheric pressure acts on a versatile diaphragm to position a valve. A single-acting diaphragm actuator is named single-acting because atmospheric pressure works on only one side of the diaphragm to place a valve. A typical single-acting diaphragm actuator consists of a casing, a diaphragm, an air supply port, an air vent, a spring, a stem, and a valve position indicator. The indicator shows the position of the valve.

This arrangement divides the casing into two chambers, an upper chamber and a lower chamber. The upper end of the actuator’s stem is connected to the metal plates within the diaphragm, and also the lower back moves the valve disk. When the actuator receives a symptom from a controller to shut the valve, atmospheric pressure is applied to the upper chamber. The diaphragm and the actuator stem are pushed downward, the spring is compressed, and the valve closes. The air within the lower section is exhausted through the air vent. When atmospheric pressure to the upper chamber is reduced, the spring moves the actuator stem upward, and also the valve opens.

ZH Serial Pneumatic Diaphragm Actuator

What is air to close and air to open for the control valve?

These actions make stem movement and thus valve position proportional to the quantity of gas pressure applied to the actuator. Controlling the applied pressure enables the actuator to position the valve anywhere within the boundaries of travel for the actuator. This sort of actuator will be called an Air-to-Close Spring-to- Open actuator because gas pressure moves the stem to shut the valve, and spring moves the branch to open the valve.

This suggests that if gas pressure to the actuator is lost, the control valve connected to the actuator will fail open since spring pressure would raise the stem and open the valve.

What is a single-acting diaphragm actuator and a double-acting diaphragm actuator? 

A single-acting diaphragm actuator that’s designed to shut a valve or fail closed when atmospheric pressure is lost appears like this. As you’ll see, it’s very kind of like the actuator we just checked out. During operation, atmospheric pressure is fed into the lower chamber. This causes the3 diaphragm to lift, which expands the spring, raises the stem, and opens the valve. When atmospheric pressure is reduced, the spring pulls the diaphragm and stem downward, closing the valve. A bearing using this type of actuator is claimed to Fail closed because if gas pressure is lost, the spring within the actuator will close the valve. A diaphragm actuator with an airline to only the lower chamber is usually an air to open spring to shut type, which causes a con troll valve to fail closed. An actuator with an airline to only the upper chamber is usually an air to shut spring to open type, which causes an effect valve to fail open. By controlling the applied pressure, these actuators are accustomed to positioning a valve anywhere within the boundaries of travel for the actuator.


Single-acting diaphragm actuators are usually represented on process piping diagrams by this symbol, no matter how they operate. A double-acting diaphragm actuator is named double-acting because gas pressure acts on each side of a versatile diaphragm to position a valve. A double-acting diaphragm actuator consists of a casing, a versatile diaphragm, an upper air supply port, a lower air supply port, a stem, and a valve position indicator that shows the position of the valve. Metal plates support the middle of the diaphragm. This arrangement divides the case into two chambers an upper chamber and a lower chamber. The upper end of the actuator stem is connected to the metal plates and also the diaphragm.

And therefore, the lower end is connected to the valve stem. When the gas pressure is applied to the upper chamber, the diaphragm and the actuator stem are pushed downward, and the valve closes. The air within the lower section is exhausted through the lower air supply port. When gas pressure is applied to the lower chamber, the diaphragm and, therefore, the stem is pushed upward, and the valve opens. The air within the upper section is exhausted through the upper air supply port.


How to control valve stem move  

These actions make stem movement and thus valve position proportional to the difference between the two gas pressures applied to the chambers of the actuator. Controlling the applied pressures enables the actuator to position the valve anywhere within the bounds of travel for the actuator. In most applications, accurate positioning is achieved by employing a device like a positioner to feature air to 1 side of the diaphragm and bleed it off the opposite side. On a piping system diagram, a double-acting diaphragm actuator could also be represented by this symbol.


What is a piston actuator? 

However, it will even be represented by the identical symbol accustomed to representing a single-acting diaphragm actuator. Some valves require relatively long stem travel or a large amount of force to be positioned. In these situations, a piston actuator is perhaps more suitable than other sorts of actuators. This is often one style of piston actuator; it’s called a single-acting piston actuator because a controller or similar device controls the gas pressure on one side of a piston. When air is fed through this air supply port, the piston moves along during a cylinder that compresses a spring and opens the valve. Air on the opposite side of the piston leaves the cylinder through an air vent; when gas pressure is reduced, the spring expands, moving the piston within the wrong way closing the valve. If atmospheric pressure drops below a predetermined value or is lost completely, the spring will force the piston right down to close the valve. In other words, the valve will fail closed since atmospheric pressure is simply fed to 1 side of the piston. This kind of actuator is termed single-acting. However, there are piston actuators that have atmospheric pressure on either side of the piston. This sort of actuator is named double-acting. The parts of a double-acting piston actuator are the same as those of the single-acting actuator.

When pressure is fed into this supply port, it pushes against one of the pistons, which moves the piston to open the valve. Air on the opposite side of the piston is bled off through the opposite air supply port. When air is fed in through the available port within the opposite end, the piston is pushed in the other direction closing the valve. Air is bled off through the opposite supply port filling the cylinder with air, and bleeding air from the cylinder may be controlled by a positioner tool. On a piping system diagram, the single-acting and double-acting piston is also represented by this symbol.

What is a piston actuator 

What is a vane actuator?

However, a double-acting piston actuator may additionally be represented by this symbol. A vane actuator uses gas pressure acting against a paddle or vane to position a valve. In general, a vane actuator is straightforward in design and comparatively small for the quantity of force it can supply to open or close a valve. Vane actuators are used primarily with rotary type valves like a ball, plug, and butterfly valves because the vane rotates the valve disk. Here’s a cutaway of a vane actuator that operates a valve. When gas pressure is supplied through this air supply port of the vane actuator, it pushes against a vane causing the vane to swing across a housing. This turns a shaft because the shaft turns it opens the valve. Air on the opposite side of the vane is bled off through another supply port. When atmospheric pressure is supplied to the alternative supply port, the vane is moved within the other way closing the valve. Again, air on the opposite side of the vane leaves through the opposite supply port. Vane actuators, like other actuators, can be spring-loaded or set up with other devices.

This symbol will represent a vane actuator. Pneumatic actuators are commonly operated by air signals that come from a controller. However, in some cases, the controller signal alone isn’t enough to beat forces on the actuator like friction within the actuator, valve stem, or fluid pressure acting against the valve disk. Therefore, some actuators must be supplied with a unique supply of air capable of applying higher forces than the air signals from the controller. this can be accomplished by employing a positioner together with the actuator.

What are a valve positioner and its function?

A positioner could be a device that uses a separate supply of air. Ensure that an actuator correctly positions a valve in response to a controller’s change within the air signal. In other words, a positioner’s job is to place the control valve within the position needed by the controller.

The positioner shown here has three gauges, one gauge indicates the availability of atmospheric pressure to the positioner, and another shows the output gas pressure to the actuator. The third gauge indicates the signal pressure from the controller. A mechanical linkage joins the actuator stem to the positioner. This mechanical linkage may be called a feedback linkage. Because the actuator stem moves up or down, it also carries the linkage. The position of the linkage indicates to the positioner when enough movement has occurred; this will correspond with the air signal from the controller. When a positioner is employed with an actuator,  the signal from the controller goes to the positioner. The positioner controls a source of air that enters here and is distributed to the actuator through this line. When the controller sends a symptom to shut the valve, the positioner receives the signal and converts it to the acceptable air pressure using this air supply.

Pneumatic Regulating globe control valve
Pneumatic Regulating globe control valve

What is troubleshooting for positioners and actuators?

This pressure is then applied to the actuator to close the valve. The feedback linkage indicates to the positioner how far the valve is from the desired position. In this case, fully closed as the valve moves toward the fully closed position, the feedback linkage causes the positioner to change the air signal to the actuator. So that as the valve reaches the fully close position, actuator movement is stopped. On a piping system diagram, a positioner may be represented by this symbol. Actuators, like other process components, are susceptible to mechanical problems.

Since an actuator problem can adversely affect the operation of a process, it’s essential to know how to identify actuator problems when they occur. The following discussions are not intended to enable you to isolate the exact cause of an actuator or control valve failure. Still, they can help you narrow down the scope of a problem to minimize its effect on a process. Often an operator can detect an actuator problem by looking at its valve position indicator and comparing it to the position called for by the controller. For example, suppose the position indicator shows that the valve is closed, but the flow indicator on the controller indicates that the flow is continuing through the valve. In that case, the valve seat and disc are probably warm, allowing leakage through the valve.

pic 2 two major forms of Pneumatic Control Valve actuators

However, if the position indicator shows that the valve is open when it should be closed, the problem could be in some other part of the control valve or the actuator. For example, a diaphragm actuator will not properly position its control valve if its diaphragm ruptures or if air pressure to the actuator is lost. Loss of air pressure could result from leaks in the actuator around the diaphragm edge.  Therefore, if a positioner is used on an actuator, its gauges should be checked to ensure they have the appropriate air pressure readings.

Sometimes problems with a positioner can also result from a loose or broken feedback linkage, so it should be checked – regardless of the reason for the failure of an actuator or control valve. However, some steps can be taken to minimize the loss’s effect on the process. For example, some actuators have manual operators on them that may be used to regain control of the process flow in the event of an actuator failure, while other actuators and control valves may have manual bypass lines around the failed components. Bypass lines can usually be placed in service by isolating the failed actuators control valve and positioning the bypass valve to restore flow. When either a manual operator on an actuator or a bypass line around an actuator is used, it will probably be necessary to maintain communications with the control room to ensure that the proper flow rates are held in the process.



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