Having the valve position transmitter function integrated within the valve controller means that the expense, bracketry, and maintenance requirements of an add-on, accessory transmitter are eliminated. The discrete switch is a solid state circuit 1-amp maximum that opens and closes based on a user configurable trip point. The trip point can be based on valve travel anywhere within the calibrated travel range or on a device alert. The position transmitter is independent from the digital valve controller with the power coming from the control system analog input channel in the same manner as a standard 2-wire transmitter. It may also be field configured to the earlier HART 5 standard to match any control system requirement. With over one million units installed worldwide, FIELDVUE digital valve controllers prove highly suited to a wide variety of industries and applications where they provide unmatched valve operation.
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If any other force acts upon this mechanism, the relationship between signal pressure and valve stem position will no longer be ideal. These forces conspire to re-position the valve stem so stem travel does not precisely correlate to actuating fluid pressure.
Note 1 : One way to minimize dynamic forces on a globe valve plug is to use a double-ported plug design, or to use a balanced plug on a cage-guided globe valve. A disadvantage to both these valve plug designs, though, is greater difficulty achieving tight shut-off. A common solution to this dilemma is to add a positioner to the control valve assembly.
In the case of a valve positioner, the positioner receives a valve stem position setpoint from the main process controller. We could say that the main process controller in this case is the primary or master controller, while the valve positioner is the secondary or slave controller.
The following photograph shows a Fisher model pneumatic positioner mounted to a control valve. The even newer model DVC uses a magnetic Hall Effect sensor to sense the position of a magnet bolted to the valve stem. This non-mechanical position feedback design eliminates backlash, wear, interference, and other potential problems associated with mechanical links. Better feedback is essential to better valve positioning. Control valve positioners are typically constructed in such a way to source and vent high air flow rates, such that the positioner also fulfills the functionality of a volume booster Note 3.
Another advantage of adding a positioner to a pneumatically actuated control valve is superior valve seating tight shutoff. This benefit is not obvious at first inspection, and so some explanation is in order.
First, one must understand that mere contact between the plug and seat within a sliding-stem valve is not enough to ensure tight shut-off. Rather, the plug must be forcefully pressed down onto the seat in order to fully shut off all flow through the valve. Anyone who has ever tightened the handle on a leaking hose bib garden spigot intuitively understands this principle: a certain amount of contact force between the plug and the seat is necessary in order to slightly deform and thereby mold those two components to a perfect fluid-tight fit.
The technical term for this mechanical requirement is seat load. Imagine if you will a diaphragm-actuated , sliding-stem, air-to-open control valve with a bench set range of 3 to 15 PSI.
In other words, at 3 PSI diaphragm pressure, the plug is touching the seat but with little or no force to provide a tight shut-off seal.
In order to fully force the valve plug against the valve seat to achieve a tight seal, all air pressure would have to be vented from the diaphragm to ensure no diaphragm force opposing the spring. A comparison of the two scenarios is shown here: While positioners are beneficial on spring-equipped valve actuators , they are absolutely essential for some other styles of actuators. A positioner must alternately apply air pressure to both surfaces of the piston to raise and lower the valve stem.
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