Q: Control valve positioners offer gain and speed adjustment capability, which are highly valued by instrumentation engineers. Could you explain the significance of these features and how they contribute to improved process control?
A. While the main reason for using a positioner is that it amplifies the rather low system controller signal enough (typically to between 30 and 80 psi) to enable the actuator to overcome high stem forces.
One other important reason is, because positioners offer additional ways to affect overall process dynamics. That allow the instrument engineer another way to avoid valve actuators from being unstable. It is bad enough to fight system loop instability, but it would be worse, if the control valve actuator too becomes unstable. The gain of a positioner and actuator is the percent change in actuator operating pressure for a given percent change in controller output pressure to the positioner. Remember such gain is equivalent to proportional band setting in the process controller.
To illustrate this concept, let’s consider an example:
Assuming the controller signal is 3-15 psi. Here a 5% change in positioner output signal would be 0.05 x (15-3) psi =0.6 psi. Now assuming the actuator signal is 5-20 psi. Thus, assuming
the 5% change in controller signal causes the actuator pressure to vary 10 psi, or, by 10 / (20 – 5) = 66 %. The resultant gain now is 66 % /10% or 6.6, this is the gain of the control valve with positioner (the final control element). The typical gain adjustment range is 10:1; Speed adjustment can vary over a range of 5:1.
By regulating the travel speed of the actuator, we can affect the time constant of the final control element (the actuator and positioner). This time constant is defined as the time in seconds for the actuator to travel 63% of the total rated travel. Here are typical time constants of spring-diaphragm actuators in seconds.
The above data matches an average speed setting of 0.22inch/second: *In inch per second to 63% of rated travel (for air in). Divide the rated travel by the speed of travel. This is the time constant of the valve. Example. a 54in2 actuator has a time constant of 0.75 / 0.1 in/sec = 7.5 seconds.
Here are some general guidelines for adjusting gain and speed:
- For fast systems (pressure control application) set the speed high, but the gain low.
- For slow systems such as temperature control do the opposite, set gain high, but speed low.
Note: Never set gain and speed high at the same time. You may have a dancing actuator on your hand!
Here is a table of common speed and gain settings for control valve positioners:
Example: A 50 inch2 pneumatic actuator has a rated travel of 1.5 inch. If a speed of 0.75 inch/ sec is selected, then it would take 2 seconds to travel 100%. Assuming further that, following a system upset, the controller output signal would increase by 10% or by 3 psi. With a gain setting of 5, the resultant actuator signal would then rise by 3 psi x 5 = 15 psi.
Note: Both actuator travel speed and the selected gain setting are subject to the size and volume of the selected actuator and the available air output capacity of the positioner. If electronic positioners are used, then both speed and gain settings are done electronically. In some pneumatic positioners, the gain and speed adjustments are done mechanically inside the unit’s housing.
What is the best ratio between the time constant of the final control element and that of the system?
About the author
Dr. Hans D. Baumann is an internationally renowned consultant with extensive experience in the valve industry. Throughout his career, he held managerial positions in Germany and France, and his innovative spirit led to the creation of 10 novel valve types, including the well-known Camflex valve. Dr. Baumann has authored 8 books, including the acclaimed “Valve Primer,” and has been granted 115 US patents. He also founded his own valve company, which he later sold to Emerson, and served as Vice President at Masoneilan and Fisher Controls Companies.
About this Technical Story
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