Control Valve

A control valve is a valve used to control fluid flow by varying the size of the flow passage as directed by a signal from a controller.

Control valves are used in many processes to control flow, pressure, temperature or other variables. The type of valve used will depend on the size of the pipe, the overall pressure that the system operates, the flowing media, process conditions, and other factors.

Process plants consist of hundreds, or even thousands, of control loops all networked together to produce a product to be offered for sale. Each of these control loops is designed to keep some important process variable such as pressure, flow, level, temperature, etc. within a required operating range to ensure the quality of the end product. Each of these loops receives and internally creates disturbances that detrimentally affect the process variable, and interaction from other loops in the network provides disturbances that influence the process variable.

To reduce the effect of these load disturbances, sensors and transmitters collect information about the process variable and its relationship to some desired set point. A controller then processes this information and decides what must be done to get the process variable back to where it should be after a load disturbance occurs. When all the measuring, comparing, and calculating are done, some type of final control element must implement the strategy selected by the controller.

Principles of Operation

The most common final control element in the process control industries is the control valve. The control valve manipulates a flowing fluid, such as gas, steam, water, or chemical compounds, to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired set point.

Control valves may be the most important, but sometimes the most neglected, part of a control loop. The reason is usually the instrument engineer's unfamiliarity with the many facets, terminologies, and areas of engineering disciplines such as fluid mechanics, metallurgy, noise control, and piping and vessel design that can be involved depending on the severity of service conditions.

Any control loop usually consists of a sensor of the process condition, a transmitter and a controller that compares the "process variable" received from the transmitter with the "set point," i.e., the desired process condition. The controller, in turn, sends a corrective signal to the "final control element," the last part of the loop and the "muscle" of the process control system. While the sensors of the process variables are the eyes, the controller the brain, then the final control element is the hands of the control loop. This makes it the most important, alas sometimes the least understood, part of an automatic control system. This comes about, in part, due to our strong attachment to electronic systems and computers causing some neglect in the proper understanding and proper use of the all important hardware.

What is a Control Valve..

Control valves automatically regulate pressure and/or flow rate, and are available for any pressure. If different plant systems operate up to, and at pressure/temperature combinations that require Class 300 valves, sometimes (where the design permits), all control valves chosen will be Class 300 for interchange-ability. However, if none of the systems exceeds the ratings for Class 150 valves, this is not necessary.

Globe valves are normally used for control, and their ends are usually flanged for ease of maintenance. Depending on their type of supply, the disk is moved by a hydraulic, pneumatic, electrical or mechanical actuator. The valve modulates flow through movement of a valve plug in relation to the port(s) located within the valve body. The valve plug is attached to a valve stem, which, in turn, is connected to the actuator.

Control Valve Arrangement

The image below shows how a control valve can be used to control rate of flow in a line. The "controller" receives the pressure signals, compares them with pressure drop for the desired flow and if the actual flow is different, adjusts the control valve to increase or decrease the flow.

Comparable arrangements can be devised to control any of numerous process variables. Temperature, pressure, level and flow rate are the most common controlled variables.

Valve Types and typical Applications

Legend..

• DC = Directional Change
• IoS = Isolation or Stop
• PR = Pressure Relief
• TH = Throttling

What are valve standards?

Many kinds of valves are used in the industrial process, and to ensure proper functionality, compatibility, and safety for end-users, EPC, or engineering, every country has particular organizations to make standards.

The major standard organizations include the American National Standards Institute(ANSI), American Petroleum Institute(API), The American Society of Mechanical Engineers(ASME), British(BS), China valve standards(GB, JB, HG), and so on. THINKTANK provides standard resources for all interesting valves friends study.

ANSI Valve Standards

American National Standards Institute

API Valve Standards

The American Petroleum Institute

ASME Valve Standards

The American Society of Mechanical Engineers

BS – British Standard Valves

British – BS – valve standards

China and International Valve Standards

Fluid Control Institute (FCI)

The Fluid Control Institute (FCI) provides standards and other materials to assist purchasers and users in understanding and using fluid control and conditioning equipment such as control valves, solenoid valves, and regulators.

Quality Control

A valve can be described as a mechanism that controls the passage of liquids through a pipe. Research states that valves being manufactured in China are amongst the top valve competitors in the valve industry.

Nondestructive Test

Radiography Inspection guarantees no failures inside of the castings or forgings. Ultrasonic inspection acts an alternative of inspection. Dye penetrant testing and magnetic particle testing can examine surface failures on castings and forgings.

Cryogenic Treatment

-196 ℃ Cryogenic Treatment System is a basic treatment for LNG usage valves or similiar applications.

Leakage Test

Leak testing is a procedure that inspectors use to determine whether an object or system is functioning within a specific leak limit.

Chemical Analysis

All our materials are strictly made under supervision of ladle analysis instrument in the foundries and inspected by high precision spectroanalysis instrument when they arrive in our plant.

Mechanical Properties Test

All mechanical properties test will act before we use the materials to secure the performance and safety.

Heat Treatment

Up to 1200 ℃ electric furnance provide an availability of various heat treatment cycles.