Control Valve-FAQ 

A control valve controls the fluid flow, such as water, gas, or water. It regulates the flow of fluid as the position of the valve plug is changed by the actuator. It maintains the process variable close enough to the desired set point. We at Instruthinkmanufacture durable and high-quality control valves at the most affordable price.

In the guide, we will discuss the common FAQs about the control valve. Hopefully, the information will help understand the control valve in detail.

Q 1-What is a control valve?  

Control valves are a kind of valve that can regulate and control the flow, pressure, and temperature of a flowing fluid, such as gas, steam, water, or the chemical mixture, by opening or closing internal passages. They control the amount of flow in processes. The valve stem moves, altering the size of passage, which increases, decreases, or holds steady flow.

A control valve is used in many processes to control fluid flow, temperature, pressure, or other variables. The type of control valve is dependent on the size of the pipe, the overall pressure of the flowing media, and other factors. It is one of the most common control elements in the process control industries. They operate in the closed-loop.

The control valve is the most common control element that is used in the industry. You can operate the control valve pneumatically, hydraulically, or electrically. The control valve requires a device to convert the electrical signal to operate effectively. Therefore, a control valve is also called the final control element in automatic control terminology.

The control valve body can be categorized as below. The body part of the control valve stays in contact with the products. To handle the temperature, pressure, and chemistry of the medium.

1. Globe control valve

1.1 Single-seat globe control valve body

1.2 Double seat globe control valve body

1.3 Balanced plug cage globe control valve body

1.4 Cage guided globe control valve body

1.5 Three-way control valve body

2. Angle control valve

2.1 Cage style control valve body

4. Rotary control  valves

4.1 Butterfly v control  valve  body

4.2 V-notch ball control valve body

4.3 Eccentric-disk control valve body

4.4 Eccentric-plug control valve body

5. Sliding cylinder control valves
6. Air-operated valve

6.1 Air-operated valve

6.2 Relay valve

Q2-How does the control valve work?  

The control valve has three main parts: Control Valve body – in which the modulating element, a globe, plug, ball, or butterfly is contained.

Control Valve actuator -which moves the valve’s modulating elements, such as ball or butterfly

Control  Valve positioner- which ensures the valve has reached the required degree of opening. This overcomes the problem of friction and wear.

The control valve actuator is used to move the valve stem. It opens or closes the control valve according to the change required in the flow. Depending on the control valve’s body type, we can have linear or rotary actuators for the control valve. For example, a butterfly control valve requires a rotary actuator, and a globe control valve usually works with a linear actuator. The rate of flow through the control valve depends on the pressure difference between the inlet and outlet.

Three different systems to move happen -pneumatic, hydraulic, and electrical for the control valve. Pneumatic actuators of the control valve convert air pressure to a linear or rotary movement. The hydraulic actuator of the control valve does the same with liquids instead of air. And electric actuators of the control valve use motors to convert electrical energy into mechanical torque. All the systems work forward and backward, or indirect or reverse actions. In direct actions, the input will push the actuator down the control valve stem, and the spring with push it back up. The input pushes the valve stem up for reverse auctions, and the spring pushes it back down.

The control valve positioner is to deliver pressurized air to the control valve actuator. Control valve positioners are usually used when a control valve requires throttling action. A control positioner requires position feedback from the control valve stem or shaft and delivers pneumatic pressure to the actuator to open and close the control valve. The positioner should be mounted on or near the control valve assembly. There are three main types of positioners, depending on the type of control signal, the diagnostic capability, and the communication protocol: pneumatic analog or digital.

Q3 – What is the control valve used for?

The control valve is the power-operated device that regulates or manipulates fluid, gases, oil, water, or steam by opening or closing internal passages. It is used in many processes to control flow, temperature, and other variables. For example, the movement of the valve stem alters the size of the passage that determines the flow range.

The control valve opening is altered when the controlled process parameter does not equal the value meant to be. Thus, there is a balance between the cost of the valve and the cost benefits with tighter control. The setpoints of the control valve include pressure, temperature, and flow rate, and the other product parameters of the control valve are density, concentration level, and liquid level.

Q4 – What are the types of control valves?

The different types of the control valve are:

• Butterfly valves

Butterfly valves use less space and have a lower cost than other valves. They have larger diameters.

• Globe valves

Globe valves are more precise and provide a variety of flow characteristics. However, they are more expensive and not well suited for anything except clean fluids.

• Ball valves

Ball valves have two types, complete and segmented. You can use a whole ball for modulating service and a segmented ball for controlling slurry flows.

• Gate valves

You can use gate valves for shutoff and isolation. You can use them for throttling service as well, but they are not suitable for that purpose.

• Plug valves

They are similar to ball valves but are less expensive. You can use them where there is a high flow rate and low-pressure drop.

Q5 – How does the control valve work?

The control valve can be operated manually or automated. Automation is through the rotary actuators for butterfly, plug, or ball valves; the constant pressure drop determines the valve’s inherent flow characteristics. The pressure drop consists of equal percentage, linear, or quick opening.

The equal percentage is most commonly used in industrial processes. For example, butterfly and ball valves are often equivalent percentage types. It is important to install a valve with full capabilities as a poor valve actuator can eliminate the effectiveness of the process control.

Q6 – What are the characteristics of the control valve?

The relationship between control valve capacity and valve stem travel is called the flow characteristic of the control valve. The control valve capacity is dependent on the trim design of the valve. Because of the variation of the trim design, valves are not linear.

Valve trims are characterized to meet a large variety of control application needs. The flow capacity of the control valve increases with valve trim travel. The quick opening provides significant changes in flow for small changes in lift.

Q7- What is the purpose of the control valve?

The control valve controls the fluid flow in pneumatic systems and hydraulic circuits. It consists of electro-hydraulic valves with simple openings that adjust the temperature and pressure.  In hydraulic systems, the function of the control valve is to control the flow rate to motors and cylinders, hence maintaining the speed of these components.

The control valve also manages the rate of energy, based on the Physics concept of work, power, and energy.

Q8 – What are the flow characteristics of the control valve?

The flow characteristics of the control valve define the relationship between ‘valve opening’ and flow rate under constant pressure conditions. The valve opening is relative to the valve plug to the closed position against the valve seat. The orifice pass area is the narrowest area between the valve plug and seat.

For any valve, the flow rate and orifice pass area are always directly proportional to one another. Therefore, a valve of any size with the same volumetric flow rate and differential pressure has the same orifice pass area. However, different valve sizes will give valve openings for the same pass area.

Q9 – What are the challenges of hydraulic flow control valves?

The hydraulic control valve has many benefits to control and manage the flow rate. It provides a cost-effective solution to issues surrounding the flow rate. However, the obstruction can lead to a loss of pressure in a control valve, which impacts its performance.

The flow control valves can change the flow rate due to temperature, pressure, or other variables. In addition, the changes in flow rate can lead to reliability concerns. To overcome these issues, you may require installing a sophisticated control valve by selecting the suitable hydraulic flow control valve.

Q10 – What are the factors to consider for hydraulic flow control valves?

The flow control valve controls the actuators in the hydraulic circuit. The variables that influence the flow rate are temperature, pump output, and other variables. Therefore, when designing the appropriate valve for an application, you should consider various factors that can influence the performance of the control valve.

Some of the factors that can directly affect the control valve are fluid density, corrosive property of the liquid, maximum and minimum flow rate, maximum noise tolerance, connectivity to the process, etc. Fluid density and flow rate are essential to determine the proper sizing of the valve.

Q11 – What are the advantages of a control valve?

Control valve offers many advantages as flow controlling equipment. Some of the advantages of using a control valve are as follow:

• Control valves provide rapid and effective functioning.
• It is reliable and has a durable service life.
• The control valve has a compact design which facilitates less space consumption.
• It gives laminar flow, so the loss of pressure is lesser.
• Due to the simple design of the control valve, it is easy to open and close.
• You can easily repair them. It has a low maintenance cost.
• You can get reliable and safe service under high temperatures and pressure.

Q12 – What are the disadvantages of a control valve?

Although a control valve offers many benefits, there are certain drawbacks to using a control valve. The main disadvantage of the control valve is that the opening and closing are not prompt; there is a need for large space for installing, assembling, and maintaining it.

When you operate them at high temperatures, there is a risk of leakage because of periodic fluctuations. Another issue with the control ball valve is the collision of fluids can cause abrasion and leakage. The control plug valves include fast movement, which requires a large amount of force, which is expensive.

Q13 – What are the functions of the control valve?

A pressure control valve is found in every hydraulic system. It assists in different functions like maintaining the system pressure below a desired upper limit safely. The control valve manages the flow of fluid by changing its size by responding to a signal from the container. The control valve assists in many functions, such as maintaining the set pressure in part of a circuit.

The control valve effectively controls the pressure, temperature, flow, and liquid level by opening and closing it based on the signals received from the controller. Pneumatic, hydraulic, and electric actuators regulate the opening and closing of the control valve. The ones controlling the opening and closing of these actuators are the positioners.

Q14 – What are the applications of the control valve?

Control valves are used widely in many applications and for various purposes in industrial, engineering, manufacturing, and scientific communities. They are most commonly used to control process qualities, including temperature, pressure, and liquid level.

The control valve has usage in the process industry, whether it is refinery, chemicals, petrochemicals, and pharmaceuticals, to control the process-related utilities. The control valve is also used in various fluid handling methods.

Q15 – What are the components of the control valve?

A control valve regulates the fluid flow and pressure within the system. It controls the fluid flow by stopping and starting the fluid flow according to the signal given by the controller. There are three main parts of the control valve. That includes the valve body, the actuator, and the controller.

The main purpose of the control valve is to regulate the fluid flow. You can perform by two primary forms of action, the sliding stem, and the rotary action. The control valve in the sliding stem includes the globe valve, angle body valve, and angle seat piston valve. The control valve in the rotary action consists of the butterfly valve and ball valve.

Q16 – How to specify a control valve?

If you want to specify a control valve, the first step is to define its functions in the given application. For example, some applications will operate as an on-off valve that open and close the valve as per the controller’s instructions. In other applications, a control valve sets a flow rate in a process which means it acts as a manually controlled variable orifice in the pipe.

In sophisticated applications, the control valve needs to be specified differently. In these applications, the control valve will serve as a final control element in process control and respond to the small variation of signals coming from the controller.

Q17 – What is the working process of the control valve?

The control valve is a power-operated device that can be electric, pneumatic, or hydraulic. The control valve responds to the signals from the controller and responds accordingly. The valve stem’s movement alters the passage’s size, which decreases or holds steady the flow.

When the process parameter does not equal the value meant to be, the opening of the control valve changes, the control loops set the temperature, pressure, and flow for maintenance. Next, the sensors measure the internal disturbances of the control loops. The controllers then correct the disturbances, if any.

Q18 – What is the principle of operation of the control valve?

The Control valve regulates pressure and flow rate at different pressure and temperature variations. Globe valves are usually used for control. Depending on the size of the supply, the disk is moved by the hydraulic, pneumatic, electrical, or mechanical actuator. The valve control and regulates the flow through the movement of the valve plug. The valve plug is attached to the valve stem, and the valve stem is attached to the actuator.

The collector receives the signal, compares them with pressure drops, and adjusts the control valve to increase or decrease the flow.

Q19 – How to tune a control valve?

The tuning of the control valve is done based on the valve response to the changing process parameter. Tuning is essential as it sets the speed and intensity of the valve and detects the error—the valve response when the need for a correction is needed. Finally, the container produces an output to the actuator to move the stem.

You have to check the size between the setpoint and the current value of the parameter. Then you have to check the length of the time the discrepancy has been present and how much speed it has been changing.

Q20 – What are the limitations of the control valve?

Despite the advantages, control valves are subject to several problems. The gate valve requires a lot of space, so it isn’t easy to operate and manage. The large size of these control valves causes issues when fine adjustments are needed.

A control valve has excessive lag because of the sensor’s location, which is too far away. There are chances of leakage in the actuator air supply, which produces insufficient force to move the stem. Also, the control valve stem can become too sticky if the packing is tight. The presence of friction in the control valve increases the force from the actuator, and the problem arises when static and dynamic forces are combined.

Q21 – How does the level control valve work?

The purpose of the level control valve is to maintain liquid levels for various process equipment, including vessels, distillation column side liquid draw, etc. The liquid is in contact with the gas or other liquid phase. Level measurement is done using level instruments that send a signal to the controller.

The controller checks the readings and gives input to the control valve. The control valve is designed to operate its functions in the closed-loop. It takes the information back to the controller to provide automatic corrections to the process variable.

Q21 – How does the control valve affect the control loop?

Process plant consists of many control loops built together to form a product. These control loops aim to keep the essential process variables such as temperature, pressure, and fluid level within the limit. However, the loop creates internal disturbances that can affect the process variables.

Sensors collect the data from the process variables, and the control valve manages and controls the process based on this data. Hence, the process variable is sent to where it should be by the control valve.

Q22 – What are the primary sections of the control valve?

The control valve regulates fluid flow by the varying resistance that the valve introduces into the system. A control valve consists of four primary sections:

• The body has the orifice and is the main port through which the controlled fluid flows.
• The trim is the part of the control valve, excluding the body that comes in contact with the fluid. It is comprised of a valve seat, plug, disc, stem, and disc holder.
• The bonnet provides the mounting for the actuator and the guide through which the stem must pass. It is composed of the centerpiece, packing, packing guide, and packing nut.
• The actuator consists of pneumatic or electric means to provide the force to stroke the valve.

Q23 – What are the inherent control valve flow characteristics?

The inherent characteristic of the control valve is derived based on tests. These tests are performed carefully to ensure that the pressure drop is constant at all valve openings and flow rates. So, the inherent characteristic of the control valve shows the relationship between valve flow capacity and valve opening.

The inherent control valve characteristics include linear, equal percentage, quick opening, modified parabolic characteristics, hyperbolic, and square root. As shown in the below image, a small increment of valve position at small opening results in a small increase in flow capacity. However, the same increment of valve position at a large opening results in a significant increase in the flow capacity of the control valve.

Q24 – What are the installed control valve flow characteristics?

When the actual flow in the system is plotted against the valve opening, this curve is the installed flow characteristic of the control valve. When the control valve opens, the resistance due to fluid flow decreases the pressure drop; the installed flow characteristic causes this decrease.

The installed characteristic of the control valve is the relationship between the valve system and the flow in the system. You have to take into consideration the changes in the pressure differential to the control valve. The processing system includes a significant amount of pipe and several fittings; this results in a relationship between the flow in the system and the pressure drop.

Q25 – What are the factors to consider while choosing the control valve?

The control valve handles and controls a variety of functions in controlling the flow of liquids and gases. They can be utilized in a variety of applications. It is essential to look at certain factors to select the correct control valve for your application.

1. Media type

The valve material is selected based on the characteristic of liquid or gas. For monitoring corrosive materials, polytetrafluoroethylene (PTFE) is the best choice.

2. Application conditions

The selection of a suitable valve for your application is most important. If your control valve has an oversize valve, it can cause operation issues within the valve. In most cases, the metal valve can withstand higher temperatures and pressure.

3. Valve function

For some applications, two-way valves perform well in the system; for others, valves can be used in three ways.

4. Maintenance requirements

You need to select the ball valve if maintenance needs to be done regularly. Ball valves are the best control valve as they resist clogging and are more straightforward in structure.

Conclusion

The control valve is an essential product used in many industries. For example, it is an effective component in various oil and gas industries. The essential purpose of the control valve is to control the fluid flow based on input from the controller.

Our company has successfully designed, manufactured, and delivered high-quality control valves to our clients for decades. If you are looking forward to ordering a control valve, we can provide you with the best control valve in the market. If you have any inquiries or any concerns, you can email us at sales@instruthink.com.