October 23, 2024
Valve positioner (Valve Controller) is a device used to precisely control and regulate the position of a control valve. By receiving input signals from the controller, it adjusts the opening of the valve to the desired setpoint, thus ensuring that process parameters (e.g., pressure, temperature, flow rate, etc.) remain within predetermined ranges. Positioners play a key role in industrial automation and process control, and are widely used in oil and gas, chemical, pharmaceutical, water treatment and other industries.
The stem position of a pneumatic control valve is linearly related to the air pressure applied to the actuator because mechanical springs tend to follow Hooke's Law, which states that the amount of spring motion (x) is directly proportional to the applied force (F=kx). The force applied by a pneumatic actuator is a function of air pressure and piston/diaphragm area (F=PA), and the spring, in turn, compresses or stretches, producing an equal and opposite reaction force. The end result is that the actuator pressure is linearly translated into stem movement (x=PA/k).
1. Control Valve Positioner
This linear and repeatable relationship between pneumatic signal pressure and stem position only holds true if, and only if, the actuating diaphragm/piston and spring are the only forces acting on the stem. If any other force acts on the mechanism, the relationship between signal pressure and stem position will no longer be ideal.
Unfortunately, there are many other forces acting on the stem in addition to the actuator forces and spring reaction forces. The friction of the stem packing is one of these forces, and the reaction force on the spool caused by the differential pressure in the spool area is another. These forces combine to reposition the stem so that stem travel is not precisely related to actuating fluid pressure.
A common solution to this dilemma is to add a valve positioner to the control valve assembly. A valve positioner is a motion control device designed to actively compare the stem position to a control signal and adjust the actuator diaphragm or piston pressure until the correct stem position is achieved:
The valve positioner itself is basically a control system: the valve's stem position is the process variable (PV), the command signal to the positioner is the set point (SP), and the positioner's signal to the valve actuator is the manipulated variable (MV) or output. Thus, when the process controller sends a command signal to the positioner-equipped valve, the positioner receives that command signal and applies as much or as little air pressure to the actuator as necessary to achieve the desired stem position. Thus, the positioner will “fight” any other force acting on the valve stem to achieve clear, accurate stem positioning in accordance with the command signal. A properly functioning positioner ensures that the control valve “behaves” to the command signal.
2. Example of pneumatic valve positioner
The following picture shows a Fisher Model 3582 pneumatic positioner mounted on a control valve. The positioner is a gray box with three pressure gauges on the right side:
Part of the feedback mechanism can be seen on the left side of this positioner: a metal bracket bolted to the stem connector that attaches to an arm that extends from the side of the positioner. Every control valve positioner must be equipped with some means of sensing the position of the stem, otherwise the positioner would not be able to compare the position of the stem with the command signal.
A more modern positioner, the Fisher DVC6200 (again in a gray box with a pressure gauge on the right side), appears in the next photo:
Like the earlier Model 3582 positioner, this DVC6000 uses a feedback linkage on the left side to sense the position of the valve stem. The newer DVC6200 uses a magnetic Hall effect sensor to sense the position of a magnet bolted to the stem. This non-mechanical position feedback design eliminates backlash, wear, interference and other potential problems associated with mechanical links. Better feedback is critical to better valve positioning.
Control valve positioners are typically constructed to generate and discharge high air flows, so the positioner also functions as a volume booster850 . As a result, the positioner not only ensures more accurate stem positioning, but also faster stem speeds (shorter time delays) than valve actuators that are “powered” directly by the I/P sensor.
3. Valve in position
Another advantage of adding a valve positioner to a pneumatic control valve is that the valve seals (closes tightly) better. This advantage is not obvious at first glance and therefore requires some explanation.
First, it must be understood that in a control valve, contact between the spool and seat alone is not sufficient to ensure tight closing. Instead, the spool must be pressed hard against the seat to completely shut off all flow through the valve. Anyone who has ever tightened the handle of a leaky faucet (garden spout) understands this principle intuitively: a certain amount of contact force is required between the plug and the seat in order to deform the two parts slightly, resulting in a perfect fluid-tight fit. The technical term for this mechanical requirement is seat loading.
Imagine a diaphragm-actuated, straight-through pneumatic opening control valve with a bench setting of 3 to 15 Psi. At an actuator pressure of 3 Psi, the diaphragm generates just enough force to overcome the preload of the actuator spring, but not enough to move the spool off the seat.
In other words, at 3 Psi diaphragm pressure, the spool will contact the seat, but there will be little force to provide a tight shutoff seal. If this control valve is powered directly from an I/P sensor calibrated from 3 to 15 Psi, this means that the valve will barely close at 0% of the signal value (3 Psi), rather than close tightly. In order for the spool to be fully seated for a tight seal, all air pressure must be removed from the diaphragm to ensure that there is no diaphragm force against the spring. This is not possible for an I/P with a calibration range of 3-15 Psi.
Now imagine that the same valve is equipped with a positioner that receives a 3 to 15 Psi signal from the I/P and uses it as a command (set point) for the stem position, applying as much or as little pressure to the diaphragm as needed to achieve the desired stem position. The correct way to calibrate the positioner is for the stem to start lifting only when the signal has risen to slightly above 0%, which means that at 0% (4mA) the positioner will be trying to force the valve to a slightly negative stem position. In attempting to achieve this impossible requirement, the output of the positioner will reach low saturation, exerting no pressure on the actuating diaphragm, resulting in the valve stem exerting its full spring force on the valve seat. A comparison of the two scenarios is shown in the chart below:
While positioners are helpful for spring-equipped valve actuators, they are absolutely essential for certain other types of actuators. Consider the following double-acting pneumatic piston actuator without springs:
Without a spring to provide a restraining force to return the valve to the “fail-safe” position, there is no Hooke's Law relationship between applied air pressure and stem position. The positioner must alternately apply air pressure to both surfaces of the piston to raise and lower the stem.
Motorized control valve actuators are another actuator design that absolutely requires some form of positioner system because the motorized unit cannot “sense” the position of its own shaft to move the control valve accurately. Therefore, a positioner circuit using a potentiometer or LVDT/RVDT transducer to detect the position of the valve stem and a set of transistor outputs to drive the motor is required to enable the electric actuator to respond to analog control signals.
4. Force Balance Pneumatic Positioner
A simple force balanced pneumatic valve positioner design is shown below:
The control signal for this valve is a 3 to 15 Psi pneumatic signal from either an I/P sensor or a pneumatic controller (neither shown in the diagram). This control signal pressure exerts an upward force on the force beam, causing the baffle to attempt to approach the nozzle. The increase in back pressure in the nozzle causes the pneumatic amplification relay to output more air pressure to the valve actuator, which in turn lifts the valve stem (opens the valve). As the valve stem lifts, the spring connecting the actuator to the valve stem stretches further, applying additional force to the right side of the actuator. When this additional force is balanced with the force of the bellows, the system stabilizes at a new equilibrium point.
As with all force-balanced systems, the movement of the pushrod is limited by the balancing force, so its movement is negligible in practice. Ultimately, equilibrium is achieved by one force balancing another, like two teams of people pulling on a rope: as long as the forces of the two teams are equal in magnitude and opposite in direction, the rope will not deviate from its original position.
The diagram below shows the PMV 1500 Force Balance Positioner for positioning a rotary valve actuator with the cover on (top) and under (bottom):
A 3 to 15 Psi pneumatic control signal enters the bellows and pushes down on the horizontal force beam (black). The pneumatic pilot valve assembly on the left side of the force beam detects any movement and increases air pressure to the valve actuating diaphragm if any downward movement is detected, and releases air pressure to the actuator if any upward movement is detected:
When compressed air enters the valve actuator through the pilot valve assembly, the rotary valve will begin to rotate in the open direction. The rotary motion of the shaft is converted to linear motion within the positioner by means of a cam: the cam is a disk with an irregular radius designed to produce a linear displacement from an angular displacement:
A roller follower located at the end of the gold colored beam moves along the circumference of the cam. The cam motion is converted into a straight stroke force by compression of the coil spring directly against the force of the pneumatic bellows on the force beam. When the cam movement is sufficient to compress the spring enough to counterbalance the additional force generated by the pneumatic bellows, the force beam returns to the equilibrium position (very close to the start position) and the valve stops moving.
If you look closely at the last photo, you'll see the positioner's zeroing screw: a threaded rod that extends below the gold-colored beam. This screw adjusts the compression of the bias spring so that the positioner unit “thinks” the cam is in a different position. For example, turning this threaded rod clockwise (as viewed from the slotted end of the screwdriver engagement) compresses the spring further, pushing the darker rod upward with more force, achieving the same effect as a slight counterclockwise rotation of the cam. This causes the positioner to take action and rotate the cam clockwise to compensate, bringing it closer to the 0% stem position.
Although the cam and follower in this positioner mechanism actually move in response to stem movement, it can still be thought of as a force balancing mechanism since the crossmember attached to the pilot valve does not move appreciably. By balancing the forces on the beam, the pilot valve is always in the balanced position.
5. Dynamic balancing pneumatic positioner
Motion-balancing pneumatic valve positioner designs also exist, where the motion of the valve stem counteracts the motion (not the force) from another element. The following cutaway diagram shows how a simple motion balanced positioner works:
In this mechanism, an increase in signal pressure causes the beam to advance toward the nozzle, resulting in a higher nozzle back pressure, which in turn causes the pneumatic amplification relay to deliver more air pressure to the valve actuator. As the valve stem lifts, the upward movement of the right end of the beam offsets the previous advance of the beam toward the nozzle. When equilibrium is reached, the beam will be in a tilted position where the bellows movement is balanced by the stem movement.
The following photograph shows a close up view of the FISHER Model 3582 Pneumatic Balance Positioner mechanism:
At the heart of the mechanism is a D-shaped metal ring that translates bellows movement and stem movement into baffle movement. As the pneumatic signal pressure increases, the bellows (located below the upper right corner of the D-ring) expands, causing the beam to rock along its vertical axis. When the positioner is set for direct-acting operation, this rocking motion pushes the baffle closer to the nozzle, which increases back pressure and delivers more compressed air to the valve actuator:
As the stem moves, the feedback lever rotates the cam at the very bottom of the D-ring. The roller “follower” on this cam translates the movement of the valve stem into another rocking motion on the beam, this time along the horizontal axis. Depending on how the cam is fixed to the feedback shaft, this motion may cause the valve flap to rock further away from the nozzle or closer to it. The cam direction must be chosen to match the action of the actuator: direct (air extends the valve stem) or reverse (air retracts the valve stem).
The D-ring mechanism is quite ingenious in that it allows for easy span adjustment by adjusting the angle of the baffle (stopper) assembly at various points along the circumference of the ring. If the baffle assembly is set close to horizontal, it will be most sensitive to bellows movement and least sensitive to stem movement, forcing the valve to move farther to equalize the small movement of the bellows (long stroke length). Conversely, if the valve assembly is set close to vertical, it will be maximally sensitive to stem movement and minimally sensitive to bellows movement, resulting in a very small valve stroke (i.e., the bellows will need to expand significantly to balance the small amount of stem movement).
6. Digital Valve Positioner
Recall that the purpose of a valve positioner is to ensure that the position of a mechanical valve always matches the commanded signal. Thus, the valve positioner itself is actually a closed-loop control system: applying as much or as little pressure as possible to the actuator in order to always reach the commanded stem position. Mechanical valve positioners use levers, cams, and other physical components to achieve this closed-loop control.
Digital valve positioners (such as the Fisher DVC6000 model) use electronic sensors to detect stem position, a microprocessor to compare the sensed stem position with a control signal through mathematical subtraction (error = position - signal), and then a pneumatic signal converter and relay to send air pressure to the valve actuator. Below is a simplified schematic of a common digital valve positioner:
As you can see from the diagram, the internal structure of a digital valve positioner is very complex. We have not only one control algorithm, but two control algorithms working in tandem to maintain the correct valve position: one monitors and controls the pressure applied to the actuator (compensating for variations in supply pressure that may affect the valve position), and the other monitors and controls the valve stem position itself, sending cascading control signals to the pressure control assembly.
A command signal (from a process loop controller, PLC, or other control system) tells the positioner the position of the valve stem. The first controller (PI) within the positioner calculates how much air pressure the actuator needs to reach the required stem position. The next controller (PID) drives the I/P (current-to-pressure) converter as needed to achieve that pressure. If for any reason the stem is not in the commanded position, the two controllers within the positioner will work together to force the valve to the correct position.
Not only does a digital valve positioner provide superior position control compared to a mechanical valve positioner, but its array of sensors and digital communication capabilities provide a higher level of diagnostic data for maintenance personnel and supervisory control systems (if programmed to monitor and act on that data).
The diagnostic data provided by the digital valve positioner includes:
--Air supply pressure
--Actuator air pressure
--Ambient temperature
--Position and pressure error
-Total stem travel (similar to an automobile odometer)
In addition, the microprocessor embedded in the digital valve positioner is capable of performing self-tests, self-calibration, and other routine procedures traditionally performed by instrument technicians on mechanical valve positioners. The digital valve positioner also captures measurements such as total stem travel to predict when the packing will wear out and automatically sends out maintenance alerts to notify the operator and/or instrument technician when the stem packing needs to be replaced!
7. Valve position sensor malfunction
Some “smart” valve positioners monitor the air pressure of the actuator in addition to the stem position, and thus have a useful feature of maintaining some degree of valve control in the event of a stem position sensor failure. If the microprocessor detects a failure of the position feedback signal (out of range), it can be programmed to continue operating the valve based on pressure only:
That is, the air pressure to the valve actuator is adjusted based on the pressure/position function recorded in the past. Since it cannot sense the stem position, it no longer functions strictly as a positioner, but can still function as a booster (compared to the flow rate of a typical I/P) and provide sensible control of the valve, whereas any other (non-intelligent) valve positioner will actually make the situation worse when it loses stem position feedback.
With any purely mechanical positioner, if the stem position feedback linkage is dislodged, the control valve will usually “saturate” and either open completely or close completely. This is not the case with the best “smart” positioners!
8. Actuator Pressure and Stem Position
Probably the most important diagnostic data provided by a digital valve positioner is the comparison of actuator pressure to stem position, usually represented graphically. Actuator pressure is a direct reflection of the force applied to the stem by the actuator, since the relationship between piston or diaphragm force and pressure is simply F=PA, where area (A) is a constant. Thus, the comparison of actuator air pressure to stem position is actually an expression of the force and position of the valve. This so-called valve characterization is very useful in identifying and correcting problems such as excessive packing friction, interference with valve internals, and spool/seat fit problems.
Shown here is a screen shot showing “valve characterization” (taken from Emerson's software product ValveLink, part of its AMS suite) of the behavior of an air-opened Fisher E-body straight-through control valve:
This graph shows two plots of actuator pressure versus stem position, one in red and one in blue.
The red graph shows the response of the valve in the open direction, when the valve is open (up), additional pressure is required to overcome packing friction.
The blue graph shows the valve closed, with less pressure now applied to the diaphragm to allow spring compression to overcome packing friction as the valve closes (down) to rest.
The sharp turns at the ends of this diagram show the position where the valve stem reaches its end position and cannot move further despite further changes in actuator pressure.
According to Hooke's Law, which describes the behavior of valve springs, each graph is roughly linear, with the force exerted on the spring being proportional to the displacement (compression) of that spring: F=kx. Any deviation from the individual linear graphs indicates that there are forces other than spring compression and air pressure acting on the stem. This is why we see a vertical shift in the two plots: packing friction is another force acting on the stem in addition to spring compression and the force exerted by air pressure on the actuator diaphragm. The magnitude of this offset is relatively small, and its consistency indicates that packing friction is “healthy” in this valve. The greater the packing friction experienced by the valve, the greater the vertical offset of the two charts.
The sharp drop at the left end of the graph where the valve plug contacts the valve seat is called the seat profile. The seat profile is located at the end of the chart where the valve is closed and contains much useful information about the physical condition of the valve plug and seat. As these valve internals wear in a control valve, the shape of the seat profile changes. An irregular seat profile may diagnose seat corrosion, wear, or many other conditions.
Seat contours can be examined in detail by zooming in on the lower left end of the valve feature drawing. The following figure shows the seat profile of a Fisher E-body straight-through control valve in its intact condition:
If a facility's maintenance staff is diligent enough to record the valve characteristics of its control valves after they are assembled or rebuilt, the “original” characteristics of any given control valve can be compared to the characteristics of the same control valve at any later date, allowing wear to be determined without the need to disassemble the valve for inspection.
Interestingly, this relationship between actuator pressure (force) and stem position also applies to the digital valve positioners used in some modern motorized valves. With motorized actuators, the force applied to the valve stem is directly related to the motor current, which can be easily measured and interpreted by the digital valve positioner.
As a result, the same type of diagnostic data can be presented graphically, even when different actuator technologies are used, to make it easier to diagnose valve problems. These diagnostics apply even to open/close motorized valves that are not used in throttling service and are particularly applicable to gate, plug and straight-through control valves, where seat engagement is important for tight shutoff.
