Control Valve Simulator – Types, Characteristics and Sizing

A control valve is one of the most important final control elements in an industrial process. It regulates the flow of liquids, gases and steam based on the signal received from a controller.

In a typical control loop, the transmitter measures the process variable and sends the signal to the controller. The controller compares the measured value with the setpoint and sends an output signal to the control valve. The valve then adjusts its opening to control the process.

Understanding control valve operation is essential for instrumentation engineers working in power plants, oil and gas facilities, refineries and process industries.

This interactive Control Valve Simulator helps you understand valve operation, actuator fail action, flow characteristics, installed characteristics, valve body selection and basic control valve sizing.

What Is a Control Valve?

A control valve is an automatic valve used to regulate process flow by changing the flow passage opening. It receives a control signal from a positioner or actuator system and adjusts the valve position accordingly.

For example, in a boiler feedwater system, the control valve adjusts the feedwater flow to maintain the required drum level. In a steam temperature control system, it regulates the spray water flow to maintain the desired steam temperature.

The main components of a typical pneumatic control valve include:

• Valve body: The pressure-containing part through which the process fluid flows.
• Plug: Controls the flow passage by moving towards or away from the seat.
• Seat: Provides the sealing surface when the valve closes.
• Stem: Connects the actuator to the valve plug.
• Actuator: Converts pneumatic pressure into mechanical movement.
• Positioner: Controls valve position according to the input signal.
• Spring: Provides the required fail position when instrument air is lost.

1. Control Valve Actuator and Fail Action

Control valves commonly use pneumatic actuators. The controller output is converted into a pneumatic signal, which operates the actuator and changes the valve opening.

Two common actuator arrangements are:

Air to Open (ATO): The valve opens when air pressure increases. If the instrument air supply fails, the spring drives the valve to the closed position. This is called Fail Closed (FC).

Air to Close (ATC): The valve closes when air pressure increases. If the instrument air supply fails, the spring drives the valve to the open position. This is called Fail Open (FO).

The correct fail action depends on the process safety requirements.

For example, a fuel gas valve supplying a furnace may need to fail closed to stop fuel flow during an air failure. A cooling water valve may need to fail open to maintain cooling.

The safest position must always be determined through process hazard analysis and the approved control and safety philosophy.

In the simulator, change the controller output, select the actuator type and press the air supply failure button. Observe how the spring moves the valve to its fail position.

2. Control Valve Flow Characteristics

The relationship between valve travel and flow is known as the valve flow characteristic. Three commonly used characteristics are quick opening, linear and equal percentage.

Quick Opening Characteristic

A quick opening valve provides a large increase in flow with a small amount of initial valve travel. As the valve continues to open, the additional flow increase becomes smaller.

It is commonly associated with on/off applications and certain relief or fast-response duties. It is generally not the first choice for precise continuous throttling.

Linear Characteristic

In a linear control valve, the flow rate changes approximately in direct proportion to valve travel when the pressure drop across the valve remains constant.

For example, 50% valve travel produces approximately 50% of the rated flow under the specified constant pressure-drop condition.

Linear characteristics are useful in applications where a relatively constant valve gain is desirable.

Equal Percentage Characteristic

In an equal percentage valve, equal increments of valve travel produce approximately equal percentage changes in flow.

At lower openings, the flow increase is relatively small. At higher openings, the same travel increment produces a larger flow increase.

Equal percentage characteristics are widely used in process control applications, particularly where the pressure drop across the valve changes significantly with flow.

Use the simulator to select each characteristic and move the stem travel slider. Compare how the flow changes at different valve positions.

3. Inherent and Installed Valve Characteristics

One important concept in control valve selection is the difference between inherent and installed characteristics.

Inherent characteristic describes the relationship between valve travel and flow when the pressure drop across the valve is maintained constant.

Installed characteristic describes the actual relationship between valve travel and flow after the valve is connected to the process piping system.

In a real plant, the pressure drop across the control valve changes as flow increases. Pressure losses in pipelines, fittings, heat exchangers and other equipment influence the valve’s actual performance.

This means a valve that has an equal percentage inherent characteristic may behave more like a linear valve when installed in a system with suitable valve authority.

What Is Valve Authority?

Valve authority is the ratio of the pressure drop across the fully open control valve to the total pressure drop across the system at the corresponding design condition.

A simplified expression is:

Valve Authority = Valve Pressure Drop / Total System Pressure Drop

For example, if the valve pressure drop is 3 bar and the total system pressure drop is 10 bar:

Valve Authority = 3 / 10 = 0.30 (30%)

Low valve authority can distort the installed characteristic and make process control more difficult. A value around 0.3 or higher is often used as a preliminary guideline, but the appropriate value depends on the application and system design.

In the simulator, change the application, valve characteristic and authority. Observe how the installed flow curve changes and which characteristic provides the steadiest simulated loop gain.

4. Control Valve Types

Different valve body designs are used depending on process conditions, flow capacity, pressure drop, fluid properties and control requirements.

Globe Control Valve

Globe valves are widely used for accurate flow control. They are suitable for many throttling applications and can handle relatively high pressure drops when properly selected.

Ball Control Valve

V-notch ball valves provide good flow control with relatively high capacity. They are useful in applications where higher flow capacity and tight shutoff are important.

Butterfly Control Valve

Butterfly valves are compact, lightweight and commonly used in large-diameter pipelines. They are often selected for water and other utility services where space and cost are important considerations.

Diaphragm Control Valve

Diaphragm valves are used in applications involving corrosive, contaminated or hygienic fluids, depending on the lining and material selection. Their pressure and temperature limitations must be considered.

The correct valve body should be selected based on the process design requirements, not just the pipe size.

5. Control Valve Sizing – Understanding Cv

Control valve sizing is an important part of instrumentation engineering. An incorrectly sized valve can create control problems even when the actuator and controller are working correctly.

The valve capacity coefficient, known as Cv, indicates the flow capacity of a valve.

For water under the standard US Cv definition:

Cv = Q × √(SG / ΔP)

Where:

• Cv: Valve flow coefficient
• Q: Flow rate in US gallons per minute (GPM)
• SG: Specific gravity of the liquid relative to water
• ΔP: Pressure drop across the valve in psi

For water, SG is approximately 1.

The simulator uses a simplified water-sizing calculation with flow in m³/h and pressure drop in bar. It calculates the required Cv and compares it with the selected valve size.

Why Is Correct Valve Sizing Important?

An oversized control valve may operate close to the seat during normal operation. This can result in poor controllability, excessive sensitivity and unstable process behaviour.

An undersized valve may not provide the required maximum flow even when fully open.

A properly selected valve should provide sufficient capacity while operating within a suitable travel range under normal and maximum process conditions.

The simulator allows you to change maximum flow, pressure drop and valve Cv. Observe the calculated valve opening at different flow conditions.

Actual control valve sizing must also consider fluid properties, vapour pressure, choking, cavitation, flashing, noise, piping geometry and applicable sizing standards.

6. Test Your Control Valve Knowledge

The simulator includes a quiz covering important control valve topics:

• Air-to-open and air-to-close actuator operation.
• Fail-open and fail-closed action.
• Quick opening, linear and equal percentage characteristics.
• Installed valve characteristics and valve authority.
• Control valve body selection.
• Oversized and undersized valves.

Try answering the questions after completing the learning steps. You can also go back and repeat any section.

Who Should Use This Control Valve Simulator?

This simulator is useful for:

• Instrumentation and control engineers.
• Instrumentation technicians.
• Control valve maintenance engineers.
• Process control engineers.
• Engineering students.
• Engineers preparing for instrumentation interviews.
• Engineers working in power plants, oil and gas, refineries and chemical industries.

It provides a practical way to understand control valve concepts before working with actual equipment.

Final Note

Control valve selection involves more than choosing a valve based on pipeline size. Actuator fail action, flow characteristics, installed performance, valve authority, body construction and sizing all influence the performance of a control loop.

Use this interactive simulator to understand these concepts and practise basic control valve selection and sizing.

Note: This simulator is intended for learning and basic concept demonstration. Its sizing calculations and installed-characteristic models are simplified. For actual plant applications, use manufacturer sizing software, applicable standards, process data and approved engineering calculations.