Accessories

Description

A common control valve accessory is the valve position controller, also called a positioner. The fundamental function of a positioner is to deliver pressurized air to the valve actuator, such that the position of the valve stem or shaft corresponds to the set point from the control system. Positioners are typically used when a valve requires throttling action. A positioner requires position feedback from the valve stem or shaft and delivers pneumatic pressure to the actuator to open and close the valve.

The positioner must be mounted on or near the control valve assembly. There are three main categories of positioners, depending on the type of control signal, the diagnostic capability, and the communication protocol.

Pneumatic

The first category of positioners are pneumatic positioners. Processing units may use pneumatic pressure signaling as the control set point to the control valves. Pressure is typically modulated between 20.7 to 103 kPa (3 to 15 psig) to move the valve from 0 to 100% position. In a common pneumatic positioner design, the position of the valve stem or shaft is compared with the position of a bellows that receives the pneumatic control signal. When the input signal increases, the bellows expands and moves a beam. The beam pivots about an input axis, which moves a flapper closer to the nozzle. The nozzle pressure increases, which increases the output pressure to the actuator through a pneumatic amplifier relay. The increased output pressure to the actuator causes the valve stem to move.

Stem movement is fed back to the beam by means of a cam. As the cam rotates, the beam pivots about the feedback axis to move the flapper slightly away from the nozzle. The nozzle pressure decreases and reduces the output pressure to the actuator. Stem movement continues, backing the flapper away from the nozzle until equilibrium is reached.

Analog I/P

The second type of positioner is an analog I/P positioner. Most modern processing units use a 4 to 20 mA DC signal to modulate the control valves. This introduces electronics into the positioner design and requires that the positioner convert the electronic current signal into a pneumatic pressure signal (current-to-pneumatic or I/P).

Digital

While pneumatic positioners and analog I/P positioners provide basic valve position control, digital valve controllers add another dimension to positioner capabilities. This type of positioner is a microprocessor-based instrument. The microprocessor enables diagnostics and two-way communication to simplify setup and troubleshooting. In a typical digital valve controller, the control signal is read by the microprocessor, processed by a digital algorithm, and converted into a drive current signal to the I/P converter. The microprocessor performs the position control algorithm rather than a mechanical beam, cam, and flapper assembly. As the control signal increases, the drive signal to the I/P converter increases, increasing the output pressure from the I/P converter. This pressure is routed to a pneumatic amplifier relay and provides two output pressures to the actuator. With increasing control signal, one output pressure always increases and the other output pressure decreases.

Description:

In some applications, the high level of positioning accuracy that a positioner provides is not required. In these applications, an electro-pneumatic (I/P) transducer can be used. An I/P transducer uses a converter module that converts a 4 to 20 mA current input to a proportional pressure output. An internal pneumatic amplifier relay provides the capacity necessary to deliver output pressure to the control valve actuator.

There is no valve position feedback and responsiveness is very quick.

Description

Positioners and I/P transducers are designed to provide enough pneumatic output capacity to drive a typical throttling control valve. However, some applications require faster stroking speeds. When the actuator volume is large, the positioning speed of response can become more of a concern.

Volume boosters are used to provide additional pneumatic output capacity to a valve assembly. A large, sudden change in input signal (output pressure from the positioner) causes a pressure differential to exist between the input signal and the output of the booster. When this occurs, the diaphragms move to open the supply port or the exhaust port, whichever action is required to reduce the differential. The port remains open until the difference between the booster input and output pressure is within the deadband limit of the booster.

With the bypass restriction adjusted for stable operation, signals having small magnitude and rate changes pass through the bypass restriction and into the actuator without initiating booster operation. Both the supply and exhaust ports remain closed, preventing unnecessary air consumption and possible saturation of positioner relays.

Single-acting actuators typically use one volume booster. Double-acting actuators require at least two volume boosters, one to feed each side of the actuator piston. Some applications, such as compressor antisurge or turbine bypass, may require additional volume boosters to provide the needed pneumatic volume for fast valve response.

Description

The purpose of a limit switch is to provide a discrete open or close signal to the control system when the valve reaches a specific position within its range of travel. 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 will send either a wired or a wireless signal to the control system. There are many different switch technologies available, such as proximity, solid state, magnetic, and contact closure.

Description

A solenoid valve is installed in the pneumatic tubing path to the actuator. In some applications, the solenoid valve will vent the air from the actuator to allow the valve to move to its no air, fail state. In other applications, the solenoid valve will trap air in the actuator to lock the valve in its current position. Three-way solenoids are generally used to operate spring-return actuators and four-way solenoids are generally used for double-acting actuators. The solenoid valve is activated by making or breaking a discrete electrical signal from the control system.