Control valves are devices that regulate the dimensions of the flow path to control and manage fluid flow, pressure, temperature, or liquid level. Control valves are extensively used in the processing systems of production wells, oil and gas industries, chemical plants and petrochemical industries, refineries, and power plants to control or manage any of the process parameters.
The pipe systems of industrial, corporate, residential, and other urban infrastructure carry the lifeblood of contemporary civilization. And the valves in such pipe systems perform the duties of permitting, halting, regulating, and controlling the flow to achieve the system’s desired objectives.
Valves are critical components of any pipeline system that transmits fluids, gases, vapors, slurries, and blends of liquid and gaseous phases of diverse flow substances. Some valves are self-actuated, while some are manually controlled or have actuators that are electric motors, pneumatic or hydraulic actuators, or a combination of the three to control the valve. Parts of the Valves are made of both metals and nonmetals.
Qualities of Control Valve
The globe valve is the most prevalent form of valve body used as a control valve. Although various body shapes are used, including angle valves, three-way valves, eccentric rotary plug valves, semispherical ball valves, ball valves, butterfly valves, and so on, the globe valve is the most effective way to regulate and control flow.
Control valves use signals from devices placed throughout the piping system to make modifications that manage the fluid inside the pipe efficiently. Though control valves can perform a variety of tasks, they are most commonly used to regulate the flow of a fluid inside a pipe or to control its pressure.
Control valves should be positioned along a pipe so that they can be easily controlled. Control valve manifolds are used to do this. Control valve manifolds allow plant workers to easily access control valves.
Specifying Control Valve
The first stage in specifying a control valve is defining its purpose in the particular application. In most cases, it will function as an on-off valve that opens and closes in response to commands from a programmed controller on, say, a batch process. In others, it will be used to remotely control the flow rate in a process, functioning as a manually controlled variable orifice in a pipe (an open-loop application).
Finally, in more advanced applications, the control valve will act as the final control element in a process control loop, reacting to perhaps infinitely small variations in a signal from a controller (typically a computer). The signal will be created to a deviation in a process fluid’s required temperature, pressure, or level as measured by a transmitter. More than 90% of all control valves are pneumatically activated, either by spring-opposed diaphragms or by pistons.
Materials for Control Valves
Carbon steel is the material chosen for non-corrosive applications (ASTM A216 Grade WCB, if cast; and A105, when forged). 316 stainless steel valve housings are constructed of type CF8M for moderately corrosive applications. For highly corrosive fluids, however, Teflon-lined housings and exotic alloys such as Hastelloy, Monel, or titanium are available.
Control Valve Selection
The selection of control valves is influenced by two primary factors:
- Service condition
- Load characteristics
Other factors that influence control valve selection include
- The ability to handle the flow rate
- The absence of turbulence or flow resistance when the valve is fully open — turbulence reduces head pressure.
- Rapid opening and closing mechanism: In an emergency or for safety reasons, a quick response is often required.
- Tight shut-off – prevents high-pressure leakage.
- Capability to allow only one direction of flow – prevents return
- Opening at a pre-set pressure – procedure control to prevent damage to equipment
- Ability to handle abrasive fluids – hardened material prevents rapid wear.
Pressure Control Valve
Pressure control valves assist in controlling system pressure within a certain limit. They are put in any system where pressure fluctuations can occur and cause damage if not properly handled. Pressure control valves are essentially closed valves with a pressure restriction. Pressure control valves work on two different mechanisms:
- Direct-Acting Valve
- Pilot Operated valve
Pressure control valves include relief valves, reducing valves, and so on.
How to install a Control Valve?
Control valves are often installed horizontally in straight pipe runs, especially away from elbows. The actuator is always kept upright. During control valve inspection and maintenance, a bypass valve is inserted to ensure continuous operation. To isolate the control valve for maintenance, upstream and downstream isolation valves and drains are connected to the piping system.
Control valve sealing classification
A metal-seated control valve’s sealing quality is very important. In general, the lower the allowable leakage rate in a closed position, the higher the pressure class of the valve.
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