Control Valve Accessories

Positioners
Pneumatically operated valves depend on a positioner to take an input signal from a process controller and convert it to valve travel. These instruments are available in three configurations:

1. Pneumatic Positioners—A pneumatic signal (usually 3-15 psig) is supplied to the positioner. The positioner translates this to a required valve position and supplies the valve actuator with the required air pressure to move the valve to the correct position.

2. Analog I/P Positioner—This positioner performs the same function as the one above, but uses electrical current (usually 4-20 mA) instead of air as the input signal.

3. Digital Controller—Although this instrument functions very much as the Analog I/P described above, it differs in that the electronic signal conversion is digital rather than analog. The digital products cover three categories.

  •  Digital Non-Communicating—A current signal (4-20 mA) is supplied to the positioner, which both powers the electronics and controls the output.
  •  HART—This is the same as the digital non-communicating but is also capable of two-way digital communication over the same wires used for the analog signal.
  •  Fieldbus—This type receives digitally based signals and positions the valve using digital electronic circuitry coupled to mechanical components. An all-digital control signal replaces the analog control signal. Additionally, two-way digital communication is possible over the same wires. Fieldbus technologies benefit the end user by enabling improved control architecture, product capability and reduced wiring.

Use of a single, integrated analog I/P positioner or digital controller (figure 1) instead of a combination of pneumatic positioner and transducer (two instruments) results in lower installed cost.

Figure 1. Modern Control Valves

Figure 1. Modern Control Valves

The ability to embed software commands into the memory of the device represents the real difference between digital and analog I/P segments. This allows automatic configuration and setup of the valve when equipped with a digital controller. Most importantly, it allows two-way communication for process, valve, and instrument diagnostics. Users purchase digital valve controllers for several reasons:

  •  Reduced cost of loop commissioning, including installation and calibration.
  •  Use of diagnostics to maintain loop performance levels.
  •  Improved process control accuracy that reduces process variability.

Two aspects of digital valve controllers make them particularly attractive:

  •  Automatic calibration and configuration. Considerable time savings are realized over traditional zero and spanning.
  •  Valve diagnostics. Through the Distributed Control System (DCS), PC software tools, or handheld communicators, users can diagnose the health of the valve while it is in the line.

FIELDVUE instruments enable new diagnostic capabilities that can be accessed remotely. This single element requires a look at the potential impact of the technology as it applies to control valves.
An in-plant person, with the aid of the FlowScanner system, can diagnose the health of a valve through a series of off-line tests. The FlowScanner system consists of a portable, ruggedized computer and travel and pressure sensors. The sensors are connected to the valve to enable diagnostic tests, which are conducted with the valve off-line. A skilled maintenance technician can determine whether to leave the valve in the line or to remove the valve for repair.

Digital instruments allow an extension of this service with added enhancements:

  •  Because sensors are part of the instrument, tests can be run easily at appropriate times.
  • It is now possible to diagnose the health of a valve remotely via HART or Foundation fieldbus.
  • On-line diagnostics enable predictive maintenance without disrupting the process.
Figure 2. Positioner Schematic for Diaphragm Actuator

Figure 2. Positioner Schematic for Diaphragm Actuator

These additional elements are extremely important. The remote capability allows monitoring valves. Those who make, supply and service valves for a living now assist the customer in the diagnosis of valve condition to a level never before possible. Predictive maintenance offers additional savings for the customer. It is now possible to see the performance of the valve as it operates. Watching performance decline over time enables the user to predict when replacement or repair is necessary.

Other Control Valve Accessories

Figure 5 illustrates a top-mounted handwheel for a direct-acting diaphragm actuator. This unit can be used as an adjustable travel stop to limit travel in the upward direction or to manually close push-down-to-close valves.

Figure 6 illustrates a top-mounted handwheel for a reverse-acting diaphragm actuator. This unit can be used as an adjustable travel stop to limit travel in the downward direction or to manually close push-down-toopen valves.

Limit Switches

Limit switches operate discrete inputs to a distributed control system, signal lights, small solenoid valves, electric relays, or alarms. The cam-operated type (figure 7) is typically used with two to four individual switches operated by movement of the valve stem. An assembly that mounts on the side of the actuator houses the switches. Each switch adjusts individually and can be supplied for either alternating current or direct current systems. Other styles of valve-mounted limit switches are also available.

Figure 3. Positioner Schematic for Piston Actuator

Figure 3. Positioner Schematic for Piston Actuator

Figure 4. Volume Booster

Figure 4. Volume Booster

Figure 5. Top-Mounted Handwheel for Direct-Acting Diaphragm Actuator

Figure 5. Top-Mounted Handwheel for Direct-Acting Diaphragm Actuator

Figure 6. Top-Mounted Handwheel

Figure 6. Top-Mounted Handwheel

Figure 7. Cam-Operated

Figure 7. Cam-Operated

Solenoid Valve Manifold

The actuator type and the desired failsafe operation determine the selection of the proper solenoid valve (figure 8). The solenoids can be used on double-acting pistons or single-acting diaphragm actuators.

Supply Pressure Regulator

Supply pressure regulators (figure 9), commonly called airsets, reduce plant air supply to valve positioners and other control equipment. Common reduced-air-supply pressures are 20, 35 and 60 psig. The regulator mounts integrally to the positioner, or nipple-mounts or bolts to the actuator.

Figure 8. Solenoid Valve

Figure 8. Solenoid Valve

Figure 9. Supply Pressure Regulator with Filter and Moisture Trap

Figure 9. Supply Pressure Regulator with Filter and Moisture Trap

Pneumatic Lock-Up Systems

Pneumatic lock-up systems (figure 10) are used with control valves to lock in existing actuator loading pressure in the event of supply pressure failure. These devices can be used with volume tanks to move the valve to the fully open or closed position on loss of pneumatic air supply. Normal operation resumes automatically with restored supply pressure. Functionally similar arrangements are available for control valves using diaphragm actuators.

Fail-Safe Systems for Piston Actuators

In these fail-safe systems (figure 11), the actuator piston moves to the top or bottom of the cylinder when supply pressure falls below a pre-determined value. The volume tank, charged with supply pressure, provides loading pressure for the actuator piston when supply pressure fails, thus moving the piston to the desired position. Automatic operation resumes, and the volume tank is recharged when supply pressure is restored to normal.

Electro-Pneumatic Transducers

Figure 12 illustrates an electropneumatic transducer. The transducer receives a direct current input signal and uses a torque motor, nozzle-flapper, and pneumatic relay to convert the electric signal to a proportional pneumatic output signal. Nozzle pressure operates the relay and is piped to the torque motor feedback bellows to provide a comparison between input signal and nozzle pressure. As shown, the transducer can be mounted directly on a control valve and operate the valve without need for additional boosters or positioners.

Figure 12. Electro-Pneumatic Transducer Mounted on a Diaphragm Actuated Control Valve

Figure 12. Electro-Pneumatic Transducer Mounted on a Diaphragm Actuated Control Valve

Electro-Pneumatic Valve Positioners

Electro-pneumatic positioners (figure 13) are used in electronic control loops to operate pneumatic diaphragm control valve actuators. The positioner receives a 4 to 20 mA DC input signal, and uses an I/P converter, nozzle-flapper, and pneumatic relay to convert the input signal to a pneumatic output signal. The output signal is applied directly to the actuator diaphragm, producing valve plug position that is proportional to the input signal. Valve plug position is mechanically fed back to the torque comparison of plug position and input signal. Split-range operation capability can provide full travel of the actuator with only a portion of the input signal range.

Diagnostics

Digital valve controllers incorporate predefined instrument and valve diagnostics within firmware to provide alerts if there are problems with instrument mounting, electronics, hardware or valve performance.

Figure 13. Electro-Pneumatic Positioner on Diaphragm Actuator

Figure 13. Electro-Pneumatic Positioner on Diaphragm Actuator

HART-based handheld field communicators when connected to the digital valve controllers enable user-configured alerts and alarms. These flags provide notification of current status and potential valve and instrument problems. Typical alerts include travel deviation, travel limit, cycle count and travel accumulation.

AMS ValveLink software allows tests that identify problems with the entire control valve assembly. Using the valve stem travel feedback, actuator pressure sensor and other sensors on the instrument, the health of the control valve can be evaluated while the valve is still in service and fully operational. This helps to pinpoint problems before the equipment fails, without disrupting the process.

 

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