Control valve accessories

Control valve accessories. The International Society for Automation defines a control valve as a mechanically operated device that controls the flow of fluid in a process control system.

When equipped with the right accessories, a control valve can be more useful for process control and ensure smooth and efficient operation. Pneumatic control valve accessories help ensure optimum performance and are an essential component of your process control system.

Control or control valve accessories include actuators, positioners, limit switches, position transmitters, boosters, solenoid valves, quick release valves, and block valves.

Purpose

There are five main reasons why accessories or accessories are added to a control valve:

• Improved process control
• Improving process or personnel safety
• Increased valve performance or response rate
• Monitor or check valve response
• Diagnosing Potential Valve Problems

Seat valve actuators

Pneumatic control valve actuators are the most popular type in use, but electric, hydraulic and manual actuators are also widely used. The spring-diaphragm pneumatic actuator is most often used because of its reliability and simplicity of design. Pneumatic piston actuators provide high stem output for difficult operating conditions. Spring-diaphragm and pneumatic piston actuators can be adapted for direct mounting on rotary control valves.

Pneumatic actuators come in two configurations: direct acting and reverse acting.

Direct acting means that supplying air to the actuator causes the actuator stem to extend out of the actuator.

Reverse action means that the air supply to the actuator causes the stem to retract into the actuator.

 

Control valve accessories. Rotary valve actuators

The simplest and least expensive of rotary actuators is the double acting piston actuator.

There are four common types of mechanisms for converting linear to rotary motion, rack and pinion, pinned crank, yoke, and articulated crank.

Double Acting Piston Actuator Torque Curves

 

Positioners

Pneumatic positioners use a pneumatic pressure alarm as the control setpoint for control valves. The pressure is typically modulated between 20.7 and 103 kPa to move the valve from 0 to 100%.

Analog I/P positioner. Most modern processors use a 4-20 mA DC signal to modulate control valves. This introduces electronics into the design of the positioner and requires the positioner to convert the electronic current signal into a pneumatic pressure signal.

While pneumatic positioners and analog I/P positioners provide basic valve position control, digital valve controllers add another dimension to the positioner's capabilities. This type of positioner is a microprocessor tool. The microprocessor provides diagnostics and two-way communication for easy setup and troubleshooting. The most common digital protocols are HART, Foundation Fieldbus and Profibus.

I/P converters

In some applications, the high level of positioning accuracy that the positioner provides is not required. In these cases, an electro-pneumatic (I/P) converter can be used. The I/P converter uses a converter module that converts a 4-20 mA input to a proportional pressure output signal. The pneumatic booster's internal relay provides the power needed to supply output pressure to the control valve actuator. Features: no valve position feedback.

Boosters

The positioners and I/P converters are designed to provide enough pneumatic capacity to drive a typical throttle control valve. Some applications require higher response rates. When the drive volume is large, positioning response speed may become a problem. Boosters are used to provide additional pneumatic capacity to the valve assembly.

Single acting actuators usually use one booster. Double-acting actuators require a minimum of two boosters, one to feed each side of the actuator piston. In some applications, such as anti-surge compressor (anti-surge valves) or turbine bypass, additional boosters may be required to provide the necessary air volume for fast valve response.

 

Double Acting Piston Actuator Torque Curves

 

Positioners

Pneumatic positioners use a pneumatic pressure alarm as the control setpoint for control valves. The pressure is typically modulated between 20.7 and 103 kPa to move the valve from 0 to 100%.

Analog I/P positioner. Most modern processors use a 4-20 mA DC signal to modulate control valves. This introduces electronics into the design of the positioner and requires the positioner to convert the electronic current signal into a pneumatic pressure signal.

While pneumatic positioners and analog I/P positioners provide basic valve position control, digital valve controllers add another dimension to the positioner's capabilities. This type of positioner is a microprocessor tool. The microprocessor provides diagnostics and two-way communication for easy setup and troubleshooting. The most common digital protocols are HART, Foundation Fieldbus and Profibus.

I/P converters

In some applications, the high level of positioning accuracy that the positioner provides is not required. In these cases, an electro-pneumatic (I/P) converter can be used. The I/P converter uses a converter module that converts a 4-20 mA input to a proportional pressure output signal. The pneumatic booster's internal relay provides the power needed to supply output pressure to the control valve actuator. Features: no valve position feedback.

Boosters

The positioners and I/P converters are designed to provide enough pneumatic capacity to drive a typical throttle control valve. Some applications require higher response rates. When the drive volume is large, positioning response speed may become a problem. Boosters are used to provide additional pneumatic capacity to the valve assembly.

Single acting actuators usually use one booster. Double-acting actuators require a minimum of two boosters, one to feed each side of the actuator piston. In some applications, such as anti-surge compressor (anti-surge valves) or turbine bypass, additional boosters may be required to provide the necessary air volume for fast valve response.

The position sensor is mounted directly on the valve yoke and measures the position of the valve stem or shaft.

 

Solenoid valves

A solenoid or solenoid valve is installed in the path of the pneumatic tube to the actuator. In some cases, the solenoid valve will bleed air from the actuator to allow the valve to go into a closed position. or open position. In other cases, the solenoid valve will hold air in the actuator to lock the valve in its current position. Three-way solenoids are typically used to control spring return actuators, while four-way or five-way solenoids are typically used for double-acting actuators. Solenoid valves are controlled by a discrete signal from the control system.

Limit switches

The purpose of the limit switch is to provide a discrete open or close signal to the control system when the valve reaches a certain position within its travel range. Limit switches are also used for process monitoring, troubleshooting, or startup/shutdown verification. The limit switch receives position feedback from the valve stem or shaft and sends a wired or wireless signal to the control system.

Shut-off systems - shut-off valve

Emergency shutdown systems such as block valves are used in control applications where a specific actuator action is required in the event of a loss of air supply pressure. They are used with double acting actuators or with single or double acting actuators to provide pneumatic interlocking.

The blocking valve is designed to ensure that the position of the pneumatic actuators is maintained in the event of a loss of supply air pressure.

A blocking valve is installed between the ports of the “master”, which can be a solenoid or positioner, measures the pressure of the supplied air. When the air supply pressure drops below a set point, the valve will block the actuator ports, blocking the pressure that was in the actuator at the time of shutdown, effectively holding the actuator (and process valve) in place.

When the instrument supply air pressure falls below the set point, the control valve causes an actuator failure, control valve lock in last position, or failure.

For double-acting applications, the volumetric air reservoir provides a reserve volume of air to operate the valve in the event of an air supply failure and until supply pressure is restored. When, after restoring air supply pressure above the set point, the control valve automatically resets, becomes normal and allows the system to return to normal operation.

Flywheel

 Handwheels for control valve diaphragm actuators are often used as adjustable travel stops. They also provide a manual means of positioning the check valve in an emergency.

They are connected to the control valve in two ways.

1. Side handwheel
2. Top mounted handwheel
3. The side handwheel on the control valve can be used to open or close the valve in either direction at any point in the actuator travel.