2t
Column
2 ) Valve T86 ≤ 40 % of the desired closed loop process time constant ( this is equivalent to saying that the valve should be 5 times faster than the desired closed loop process response time .) T86 is defined as the time it takes the valve to reach 86 % of it ’ s final position , including any dead time .
3 ) Valve settling time ≤ desired closed loop process time constant . Slow loops : Not important Step overshoot : 20 % maximum . Figure 2 demonstrates why the recommended speed of response criteria that I suggest make sense . It meets the requirements for a process where the desired closed loop time constant is 5 seconds . Note that when I talk about the valve response I do not use the term “ time constant ” because control valve response is rarely first order . The dead time is just under the recommended 20 % of the desired closed loop time constant . The dead time is over in time to have little impact on the overall process response . The valve reaches 86 % of its full travel after only 40 % of the desired closed loop time constant ( T86 ). The valve is way ahead of when the process needs to reach 63 % of its final value , and even farther ahead of when the process needs
86 % of total response
Figure 2 . Speed of response test .
to reach its 2 time constant ( 86 %) value . Because the valve reaches 86 % of its total response in two seconds , and the desired process response should reach 86 % of its total response in ten seconds , it is equivalent to saying that the valve is 5 times faster than the desired process response time . At the early stage of the full response , a small overshoot will
In memory of Jon Monsen
Percent of span
52
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50
Td
T86
Overshoot
0 5 20 % 40 % 100 %
Settling time , Tss
Input Signal Response
10 Time ( seconds )
2t
Saying that the valve ’ s T86 should be ≤ 40 % of desired closed loop time constant is equivalent to saying that the valve should be at least 5 times faster than the desired process response time .
Desired closed loop time constant = 5 sec .
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contribute very little to an overshoot in the process . The valve response has settled to its final value after one desired process time constant , long before the process is expected to reach its final value . For an in depth presentation of this topic , I recommend reading ISA Standard , ISA S75.25.01 and the associated technical report , ISA TR 75.25.02 .
This month , Valve World celebrates the life of Jon Monsen , a friend to our global community who generously gave his time and expertise as a control valve technology specialist . Jon recently passed away after a long illness . Readers will know Jon from his many columns and articles . Jon said he “ was born to be an engineer ,” and even as a child , he loved dismantling and reassembling electronic equipment . He became fascinated by radios and became a proficient short-wave radio operator , obtaining his
Jon generously shared his knowledge during a career which spanned over 45 years .
An early passion : Jon the Ham radio operator .
‘ Ham ’ amateur radio license at just 14 . By 20 , Jon had his first-class radiotelephone operator license , which stood him in good stead when he was drafted into the army and sent to the Panama Canal Zone in his mid-twenties . After returning , the GI Bill allowed Jon to enroll in college to study engineering , which led to a job as a control system engineer supervising the specification of control valves at Bechtel Power Corp . Later , he joined Jamesbury ( later Neles ) and also obtained his PhD . On retiring from Neles , Jon took up consulting and established his website www . controlvalve-application-tools . com to freely share his vast technical knowledge on the application of control valves . As agreed with Jon , Valve World will continue to publish his series of columns . The Valve World team is grateful to Jon on behalf of our community and knows he will be missed by readers around the globe , his friends , and his family . www . valve-world . net Valve World February 2024
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