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Air To Open And Air To Close Control Valve Pdf

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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.

Valve Positioners

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. A common solution to this dilemma is to add a positioner to the control valve assembly. A positioner is a motion-control device designed to actively compare stem position against the control signal, adjusting pressure to the actuator diaphragm or piston until the correct stem position is reached:.

Thus, when a process controller sends a command signal to a valve equipped with a positioner, the positioner receives that command signal and applies as much or as little air pressure to the actuator as needed in order to achieve that desired stem position. The following photograph shows a Fisher model pneumatic positioner mounted to a control valve. The positioner is the grey-colored box with three pressure gauges on its right-hand side:.

A more modern positioner appears in the next photograph, the Fisher DVC again, the grey-colored box with pressure gauges on its right-hand side :. 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 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. Consider the following double-acting pneumatic piston actuator which has no spring:.

A positioner must alternately apply air pressure to both surfaces of the piston to raise and lower the valve stem. A simple force-balance pneumatic valve positioner design appears in the following cutaway illustration:. This control signal pressure applies an upward force on the force beam, such that the baffle tries to approach the nozzle. Increasing backpressure in the nozzle causes the pneumatic amplifying relay to output a greater air pressure to the valve actuator, which in turn lifts the valve stem up opening up the valve.

As the valve stem lifts up, the spring connecting the force beam to the valve stem becomes further stretched, applying additional force to the right-hand side of the force beam. Like all force-balance systems, the force beam motion is constrained by the balancing forces, such that its motion is negligible for all practical purposes. The following photograph shows a PMV model force-balance positioner used to position a rotary valve actuator, with the cover on left and removed right :.

The PSI pneumatic control signal enters into the bellows, pushing downward on the horizontal force beam colored black. A pneumatic pilot valve assembly at the left-hand side of the force beam detects any motion, increasing air pressure to the valve actuating diaphragm if any downward motion is detected and releasing air pressure from the actuator if any upward motion is detected:.

As compressed air is admitted to the valve actuator by this pilot valve assembly, the rotary valve will begin to rotate in the open direction. Cam motion is translated into linear force by the compression of a coil spring directly against the force of the pneumatic bellows on the force beam. When the cam moves far enough to compress the spring enough to balance the additional force generated by the bellows, the force beam return to its equilibrium position very nearly where it began and the valve will stop moving.

For example, turning this threaded rod clockwise as viewed from the slotted end where a screwdriver would engage further compresses the spring, pushing up with greater force on the dark-colored bar, achieving the same effect as if the cam had rotated counter-clockwise slightly.

Even though the cam and follower in this positioner mechanism actually do move with valve stem motion, it is still considered a force-balance mechanism because the beam connected to the pilot valve does not move appreciably. The pilot valve always comes to rest at its equilibrium position through a balancing of forces on the beam. Motion-balance pneumatic valve positioner designs also exist, whereby the motion of the valve stem counteracts motion not force from another element.

The following cutaway illustration shows how a simple motion-balance positioner would work:. In this mechanism, an increasing signal pressure causes the beam to advance toward the nozzle, generating increased nozzle backpressure which then causes the pneumatic amplifying relay to send more air pressure to the valve actuator. At the heart of this mechanism is a D-shaped metal ring translating bellows motion and valve stem motion into flapper baffle motion.

As the bellows located underneath the upper-right corner of the D-ring expands with increasing pneumatic signal pressure, it rocks the beam along its vertical axis. With the positioner set for direct-acting operation, this rocking motion drives the flapper closer to the nozzle, increasing backpressure and sending more compressed air to the valve actuator:. As the valve stem moves, a feedback lever rotates a cam underneath the bottom-most portion of the D-ring. Depending on how the cam has been fixed to the feedback shaft, this motion may rock the flapper farther away from the nozzle or closer toward the nozzle.

This selection of cam orientation must match the action of the actuator: either direct air to extend the stem or reverse air to retract the stem. If the flapper assembly is set close to horizontal, it will be maximally sensitive to bellows motion and minimally sensitive to valve stem motion, forcing the valve to move farther to balance small motions of the bellows long stroke length. Conversely, if the flapper assembly is set close to vertical, it will be maximally sensitive to valve stem motion and minimally sensitive to bellows motion, resulting in little valve stroke i.

Thus, a valve positioner is actually a closed-loop control system in its own right: applying as much or as little pressure to the actuator in order to achieve the commanded valve stem position at all times.

Mechanical valve positioners use levers, cams, and other physical components to achieve this closed-loop control. A simplified diagram of a generic electronic valve positioner is shown here:. As you can see from this diagram, there is a lot going on inside an electronic positioner. The command signal sent from the process loop controller, PLC, or other control system tells the positioner where the valve stem should be positioned.

The first controller inside the positioner PI calculates how much air pressure at the actuator should be needed to achieve the requested stem position. If anything causes the valve stem to not be at the commanded position, the two controllers inside the positioner work together to force the valve to its proper position. Not only do electronic valve positioners achieve superior position control when compared to mechanical valve positioners, but their array of sensors and digital communication ability provides a new level of diagnostic data both to maintenance personnel and the supervising control system if programmed to monitor and act on this data.

Examples of diagnostic data provided by electronic positioners include:. Additionally, the microprocessor embedded within an electronic valve positioner is capable of performing self-tests, self-calibrations, and other routine procedures traditionally performed by instrument technicians on mechanical valve positioners.

Perhaps the most significant diagnostic data provided by an electronic positioner is the comparison of actuator pressure versus stem position, usually expressed in the form of a graph. Thus, a comparison of actuator air pressure versus stem position is really an expression of force-versus-position for the valve.

Two plots of actuator pressure versus stem position are shown in this graph, one red and one blue. The sharp turns at each end of this graph show where the valve stem reaches its end positions and cannot move farther despite further changes in actuator pressure. Any departure from a single linear plot indicates some other force s besides spring compression and pneumatic force acting on the valve stem.

This is why we see two plots vertically offset from each other: packing friction is another force acting on the valve stem in addition to spring compression and the force exerted by air pressure on the actuator diaphragm. The sharp down-turn at the left-hand end of the graph where the valve plug contacts the seat is called the seating profile. Located at the end of the plot where the valve closes off, the seating profile holds much useful information about the physical condition of the plug and seat.

As these trim parts wear in a control valve, the shape of the seating profile changes accordingly. Irregular seating profiles may diagnose seat erosion, galling, or a number of other maladies. By zooming in on the lower-left end of the valve signature graph, the seating profile may be examined in fine detail. A seating profile taken of a Fisher E-body globe valve in pristine condition appears here:. Interestingly, this relationship of actuator pressure force to stem position is also available in the electronic positioners used with some modern electrically-actuated valves.

In the case of an electric actuator, force applied to the valve stem directly relates to motor current , which is easily measured and interpreted by the electronic positioner. Thus, even with a different actuator technology, the same kind of diagnostic data may be presented in graphical form for the purpose of more easily diagnosing valve problems. Don't have an Control account? Create one now. Forgot your password? Click here. Latest Projects Education.

I - Fundamentals Vol. II - Instrumentation Vol. III - Measurement Vol. IV - Control Vol. V - Reference Worksheets. Textbook Valve Positioners. 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.

The following photograph shows a PMV model force-balance positioner used to position a rotary valve actuator, with the cover on left and removed right : The PSI pneumatic control signal enters into the bellows, pushing downward on the horizontal force beam colored black. A pneumatic pilot valve assembly at the left-hand side of the force beam detects any motion, increasing air pressure to the valve actuating diaphragm if any downward motion is detected and releasing air pressure from the actuator if any upward motion is detected: As compressed air is admitted to the valve actuator by this pilot valve assembly, the rotary valve will begin to rotate in the open direction.

Motion-balance pneumatic positioners Motion-balance pneumatic valve positioner designs also exist, whereby the motion of the valve stem counteracts motion not force from another element. The following cutaway illustration shows how a simple motion-balance positioner would work: In this mechanism, an increasing signal pressure causes the beam to advance toward the nozzle, generating increased nozzle backpressure which then causes the pneumatic amplifying relay to send more air pressure to the valve actuator.

With the positioner set for direct-acting operation, this rocking motion drives the flapper closer to the nozzle, increasing backpressure and sending more compressed air to the valve actuator: As the valve stem moves, a feedback lever rotates a cam underneath the bottom-most portion of the D-ring. A simplified diagram of a generic electronic valve positioner is shown here: As you can see from this diagram, there is a lot going on inside an electronic positioner.

Examples of diagnostic data provided by electronic positioners include: Supply air pressure Actuator air pressure Ambient temperature Position and pressure errors Total valve stem travel like an odometer in an automobile Additionally, the microprocessor embedded within an electronic valve positioner is capable of performing self-tests, self-calibrations, and other routine procedures traditionally performed by instrument technicians on mechanical valve positioners.

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Air to open and Air to close Control Valve

A control valve is a valve used to control fluid flow by varying the size of the flow passage as directed by a signal from a controller. In automatic control terminology, a control valve is termed a "final control element". The opening or closing of automatic control valves is usually done by electrical , hydraulic or pneumatic actuators. Normally with a modulating valve, which can be set to any position between fully open and fully closed, valve positioners are used to ensure the valve attains the desired degree of opening. Air-actuated valves are commonly used because of their simplicity, as they only require a compressed air supply, whereas electrically-operated valves require additional cabling and switch gear, and hydraulically-actuated valves required high pressure supply and return lines for the hydraulic fluid.

By Terry Bartelt. Learners examine how air-to-open valves and air-to-close valves are used in different situations. A brief quiz completes the activity. Click here to login. Learners study the process of mathematically converting temperatures between the Fahrenheit and Celsius scales.

Air to open/close valves and direction of control action

This section describes a couple of important points which are necessary to understand prior to implementing a practical control systems. Control valves come in two sorts: air to open; and air to close. Air to open valves are normally held closed by the spring and require air pressure a control signal to open them - they open progressively as the air pressure increases.

Control valves come in two sorts: air to open, and air to close. Air to open valves are normally held closed by the spring and require air pressure a control signal to open them — they open progressively as the air pressure increases. Air to close valves are valves which are held open by the valve spring and require air pressure to move them towards the closed position. The reason for the two types of valves is to allow failsafe operation.

By Terry Bartelt. Learners examine how air-to-open valves and air-to-close valves are used in different situations. A brief quiz completes the activity. Click here to login.

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Valve Positioners

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Air to open and Air to close Control Valve

 - Я думал, это вирус. Джабба глубоко вздохнул и понизил голос. - Вирусы, - сказал он, вытирая рукой пот со лба, - имеют привычку размножаться.

Я сказала, что нашла его в парке. Я думала, что она мне заплатит, но ничего не вышло. Ну, мне было все равно. Я просто хотела от него избавиться. - Когда вы отдали ей кольцо.

Air to open/close valves and direction of control action

Колокола Гиральды по-прежнему звонили, заставляя содрогаться каменные своды. Беккер перевел взгляд на позолоченную стену под потолком.

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Air to open valves are normally held closed by the spring and require air pressure (a control signal) to open them – they open progressively as.

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