In the November issue we began looking at factors that have an effect on a valve-positioner combination’s ability to faithfully respond to the control signal. The effect of dead band was discussed and the responses of three typical valves were compared on a graph. The discussion continues, taking a look at the symbiotic dynamics of the actuator-positioner design.
Actuator and positioner design must be considered together
The combination of these two pieces of equipment greatly affects the static performance (dead band), as well as the dynamic response of the control valve assembly and the overall air consumption of the valve instrumentation.
Positioners are used with the majority of control valve applications specified today. Positioners allow for precise positioning accuracy and faster response to process upsets when used with a conventional digital control system. With the increasing emphasis upon economic performance of process control, positioners should be considered for every valve application where process optimisation is important.
The most important characteristic of a good positioner for process variability reduction is that it be a high gain device. Positioner gain is composed of two parts: the static gain and the dynamic gain.
Static gain is related to the sensitivity of the device to the detection of small (0,125% or less) changes of the input signal. Unless the device is sensitive to these small signal changes, it cannot respond to minor upsets in the process variable. This high static gain of the positioner is obtained through a preamplifier, similar in function to the preamplifier contained in high fidelity sound systems.
Once a change in the process variable has been detected by the high static gain positioner preamplifier, the positioner must then be capable of making the valve closure member move rapidly to provide a timely corrective action to the process variable. This requires much power to make the actuator and valve assembly move quickly to a new position. In other words, the positioner must rapidly supply a large volume of air to the actuator to make it respond promptly. The ability to do this comes from the high dynamic gain of the positioner. Although the positioner preamplifier can have high static gain, it typically has little ability to supply the power needed. Thus, the preamplifier function must be supplemented by a high dynamic gain power-amplifier that supplies the required airflow as rapidly as needed. This power amplifier function is typically provided by a relay or a spool valve.
Spool valve positioners are relatively popular because of their simplicity. Unfortunately, many spool valve positioners achieve this simplicity by omitting the high gain preamplifier from the design. The input stage of these positioners is often a low static gain transducer module that changes the input signal (electric or pneumatic) into movement of the spool valve, but this type of device generally has low sensitivity to small signal changes. The result is increased dead time and overall response time of the control valve assembly.
Some manufacturers attempt to compensate for the lower performance of these devices by using spool valves with enlarged ports and reduced overlap of the ports. This increases the dynamic power gain of the device, which helps performance to some extent if it is well matched to the actuator, but it also dramatically increases the air consumption of these high gain spool valves. Many high gain spool valve positioners have static instrument air consumption five times greater than typical high performance two-stage positioners.
Typical two-stage positioners use pneumatic relays at the power amplifier stage. Relays are preferred because they can provide high power gain that gives excellent dynamic performance with minimal steady-state air consumption. In addition, they are less subject to fluid contamination.
Positioner designs are changing dramatically, with microprocessor devices becoming increasingly popular. These microprocessor-based positioners provide dynamic performance equal to the best conventional two-stage pneumatic positioners. They also provide valve monitoring and diagnostic capabilities to help ensure that initial good performance does not degrade with use.
In summary, high-performance positioners with both high static and dynamic gain provide the best overall process variability performance for any given valve assembly.
This article is adapted from the Fisher-Rosemount Control Valve Handbook.
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