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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 .

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 .

Actuator size :

Rated Travel

Air in Air out I Speed *

32 in 2 0.5 in 1.1 2.5 I 0.30 32 in 2 0.75 1.5 2.5 I 0.33 54 in 2 0.75 5 7 I 0.10 105 in 2 2.25 10 12 I 0.15

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 54in 2 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 :

1 . For fast systems ( pressure control application ) set the speed high , but the gain low .

2 . 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 :

0.50 inch / sec 6 0.75inch / sec 5 1.00 inch / sec 3

Example : A 50 inch 2 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 ?

Answer : Since the time constant of the system can be considered as fixed , due to the many variables , it is up the valve ’ s time constant to provide loop stability . Here is another rule of thumb : In order to have a stable process , make sure the time constant of the final control element is either at least 3 times higher , or less than one third of the time constant of the system . The system time constant is defined as the time in second it takes for the transmitter signal to reach 63 % towards the set point at the controller , following corrective actions of the controller following an upset .

12 www . valve-world . net Valve World October 2024