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Control valve Q & A
Q : I have assembled an apparatus for bench testing control valve performance to make sure that the valve will perform satisfactorily when installed in the process . Could you describe what tests I should perform ?
A : I have been retired for some years now . When I was bench testing valves , I performed two types of tests . The first was a static resolution and dead band test . The sticky behavior of valves is often referred to as “ Stiction .” It is the result of the interaction between static friction and dynamic friction . Static friction is usually much higher than dynamic friction . As a result , a valve tends to stick in place until enough pressure builds up in the actuator to break the static friction , and then the valve moves quickly to the new position . Resolution is a measure of the smallest movement in the same direction that a valve is capable of . This is called a “ static ” test because we always wait long enough after each step for any possible movement to take place . We don ’ t make any measurements while the valve is moving but only concern ourselves with the valve ’ s ( static ) position after it has come to rest . The control signal is stepped in one direction in very small steps . After each step , there is a waiting period to make sure that there is time for the valve to make any move it is going to make before the next step is initiated . Observing the number of control signal steps that are required to make the move tells us how sensitive the valve is , and the term used to describe this is “ resolution .” After several steps in the same direction , the direction of the steps is reversed . Observing the number of steps required to initiate the reversal of valve motion tells us what the “ dead band ” is . Figure 1 represents a typical static resolution and dead band test . In this example the step size is 1 ⁄ 4 %. In the same direction , this valve responds to each 1 ⁄ 4 % step , so it has a sensitivity or “ resolution ” of at least 1 ⁄ 4 %. Upon reversal , it took two of the 1 ⁄ 4 % steps before the valve started moving in the reverse direction , so this valve has a dead band of no more than ∞ %. Note that the scales for the input and position are different so that the two graphs will be easier to differentiate from each other . The result of excessive stiction in a closed loop control system is a limit cycle , a situation where the process variable oscillates in a more or less square wave from above set point to below set point . My acceptance criterion for the resolution and dead band tests was less than or equal to 0.5 percent . The second test I performed was a speed of response test . This is a dynamic test because we are concerned with what the valve is doing throughout the test . Figure 2 is a typical speed of response test . My acceptance criteria were based on the recommendation of a process control specialist that I knew at the time . Fast loops : 1 ) Valve dead time , Td ≤ 20 % of desired closed loop process time constant .
About the author
Jon F . Monsen , PhD , PE , was a control specialist with over 45 years of experience in the control valve industry . He lectured nationally and internationally on the subjects of control valve application and sizing . Jon ' s website , www . Control-Valve- Application-Tools . com freely shares articles , training and professional development materials , and Excel worksheets that might be of interest to those who use or specify control valves . Jon passed away in December 2023 and his series of columns is being published in accordance with his wishes .
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Figure 1 . Typical static resolution and dead band test . Note that the scales for the input and position are different so that the two graphs will be easier to differentiate from each other .
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14 www . valve-world . net Valve World February 2024