– Single stage pneumatic spool valve positioner
– 2-stage high performance pneumatic positioner
– Digital high performance positioner
The mechanical feedback from the actual valve position allows the positioner to adjust its output to the actuator to maintain a desired position in proportion to the process controller input signal within the limitations of the system. Most spring diaphragm type actuators can withstand air pressures up to 60 psig without damage to the diaphragm. This is the limiting factor in determining thrust requirements for a given actuator size.
The most important feature of a good positioner for increased dynamic performance is that it be a high gain device. Unless the positioner is sensitive to small input signal changes, the valve assembly will not be able to respond to minor disturbances in the process variable. Therefore the positioner must be designed such that it responds quickly to these small changes in input signal.
A second feature of a good positioner is that it must supply the power (in the form of air supply) needed to move the valve quickly. This power comes in the form of rapid air flow as needed to move the valve.
The simplest positioner is the single state pneumatic spool valve positioner. In order to meet the second feature, the spool valve must be modified in order to supply the rapid air flow to the actuator. However this modification increases overall air consumption for the control valve assembly.
Two-stage positioners, although more complicated, give excellent dynamic performance with minimal steady-state air consumption.
EDS 2006/Inst-101
Positioner is the secondary controller in a cascade loop
Valve Position Controller (secondary) 4-20mA
The control valve position controller must have a speed of response (dynamic response) superior to that of the loop (primary) controller.
This is very important in fast control loops such as flow.
Control Valve Performance
(Positioner/Cascade Loop)
For any control loop, the response time of the control and measuring equipment must be less than the response time of the process.
For the control valve positioner, it’s speed of response must be superior to that of the loop controller.
For flow loops or other fast loops, only high performance positioners will help provide good valve performance.
EDS 2006/Inst-102
±1.5% Slow Simplest NO
Two Stage
±1.0% Fast Complex NO
Digital HP
±1.0% Fast Complex YES
Because of its simplicity, the single state positioner is the most common positioner provided with control valve assemblies. A single stage positioner may have the simplest design but provides the worst position control. With the increasing emphasis upon economic performance of process control, better performing
positioners should be considered. In the past, maintenance personnel often liked the single stage positioner for it’s simplicity and ease of maintenance. The performance of the single stage spool valve, internal to the positioner, in controlling the air pressure in the control valve actuator, limits the ability of the control valve to provide position control to no better than +/- 1.5%.
The two stage design provides better position control and if set up correctly can provide position control to within +/- 1 %. It is more difficult to set up than a single stage. Good quality dry and oil free air is required for good performance otherwise the air quality effects the performance of the first stage.
A digital valve controller (positioner) should provide the same level of control as a two stage positioner with the added benefits of the digital interface for set-up and diagnostics. Digital high performance positioners are invariably easy to set up, provide very good valve position control and often have the capabilities to perform control valve diagnostic tests.
Time (seconds) Filtered Flow Rate
Open Loop Step Test
The above flow loop test response shows that valve “A” responds to 0.5% step changes in control signal. Valve “B”, typical of many installed control valves, starts responding correctly to step changes of magnitude 5% and greater in control signal.
Control Valve Performance
(Positioner Testing)
Single Stage Positioner Valve B Digital Positioner Valve A
Unless users start specifying dynamic performance criteria when purchasing control valves , then they will always have to tolerate the type of control valve response shown by Valve “B”. Dead band is a major contributor to excess process variability, and the control valve assembly is often a primary source of dead band in the control loop.
In both valves, the actuator stem motion (green) changes in unison with the input signal (black) changes. However for Valve B, 2% step changes resulted in the valve faithfully moving in conjunction with changes in input signal. This can also be seen in Valve A with changes in flow rate (red). Step changes on the order of 0.5%
in input signal resulted in changes in flow rate; where as with Valve B, step changes in the order of 2% were required to obtain a corresponding change in flow rate response.
The real test of a control valve is how well it handles disturbances (i.e, load changes) and how well it tracks the controller setpoint.
The following is a rough measure of variability, deviation away from set point:
Very good variability is 0.5 to 2 % Good variability is 2 to 4 %
Poor variability is 5 to 10 %
Extremely poor variability is >10%
Therefore during normal operation with the process in automatic, a control audit could be used to measure variability. The various control loops could be evaluated based on criticality and variability. Once identified as a control loop with high (poor) variability and deemed as a critical loop, an investigation could be launched to identify and improve the dynamic response of the control loop.
EDS 2006/Inst-104