Introduction to Control Valves

A control valve, also known as a Final Control Element, is a valve that controls fluid flow by changing the size of the flow passage in response to a controller signal. This allows for direct control of flow rate and, as a result, process variables like pressure, temperature, and liquid level.

The question here is why these control valves are needed? Process plants are made up of thousands of control loops that are all interconnected just to create a product that can be sold. Each of these control loops is aimed at keeping process variables like pressure, flow, etc within a specified operating range in order to ensure the end product's quality. Each of these loops creates disturbances that have a negative impact on the process variable, and connections with other loops in the network generate disturbances that have an effect on the process variable.

Sensors and transmitters gather information about the process variable and its connection to some desired set point to reduce the effects of these load interruptions. After that, a controller analyses the data and determines what needs to be done to return the process variable to its original state after a load disturbance. After all of the measuring, and calculating is completed, the controller's method must be implemented by some form of final control element.

Components of Control Valve

Control valve consists of some interactive components:

i) Body: The controlled liquid is allowed to flow through the body of the valve. It supports in the analysis of flow regulation behaviour.

ii) Trim: Trim is a part of the valve which comes in contact with the fluid like plug or stem.

iii) Actuator: Consists of Electric, Hydraulic and Pneumatic.

iv) Bonnet: It functions as a mounting surface for guide and actuator, as well as path for the stem.

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Control Valve Actuator

The component that provides power for actuating valve mechanisms is known as the actuator of a control valve. Actuators are divided into three categories:

i) Pneumatic Actuators: Pneumatic Actuators are the most basic and commonly used control valve actuators, which create a modulating control action by using an air or gas signal from an external source. The pneumatic signal is sent from the top port to the actuator, which applies pressure to the diaphragm plate to move the valve stem. Pneumatic actuators give a fail-safe reaction when the driver loses power.

ii) Electric Actuator: They're driven by motors. They apply an electrical signal to support in the spin of the motor shaft. This movement is transformed to a linear motion, which serves in the operating of the valve stem for flow modulation.

iii) Hydraulic Actuator: Hydraulic actuators are similar in operation to pneumatic actuators except for the fact that they use a fluid. Hydraulic oil is used to control the action of the valve. When the force required to move the valve stem is high, they are preferred instead of pneumatic or electric actuated valves.

Types of Control Valves

There are many control valves available in different types:

Gate Valve: Gate valves are often designed to function in one of two states: fully open or fully closed. They are not the ideal option of control valves, but they can be utilized in applications where precise control is not necessary. As a result, they're mainly seen in flow-stopping applications. The advantage of a gate valve is that it has a lower pressure drop, making it perfect for low-pressure applications.

Ball Valve: Just like gate valves, ball valves are also suitable for services where precise control is not required. Ball valve has the advantage of lower pressure drop. They are also very light weight and compact. When flow reversal and direction change are necessary, ball valves are an excellent choice.

Globe Valve: Globe valves are a popular choice. When it comes to control, paired with an actuator , a globe valve may be used for a wide range of applications. Globe valves are used as control valves in most industrial applications in the process sector. These valves can be utilized for multiphase services as well as flashing applications with suitable trim arrangements, where other control valves are not advised.

Butterfly valve: Butterfly valves are utilised when the service is clean and there are no particulates or crystallisation to be expected. Butterfly valves are usually used in low-pressure applications. In some circumstances, the butterfly valve is the only choice since no other control valve can handle big size. Examples of butterfly valves are cooling water distribution, gas distribution, etc.

Diaphragm Valve: Diaphragm valves are well suited to abrasive services as they do not come in direct contact with the liquid. Diaphragm valves are well controlled and with good closure these valves can be used for pull service without major problems. applications where Diaphragm valves can be used for slurry service, cleaning services and microbiological fluids. In terms of limitations, diaphragm valves are expensive and closed and the diaphragm is exposed to mud and solid fluids and regular discharge may be required due to the short lifespan.

Applications:

These are some of the most common requests for control valves in various industries: