Controllers: Direct vs. Reverse-Acting Control

Valve Failure Position and Controller Actions Are Independently Determined. In This Article our Experts Tackle Them Separately

By Bela Liptak

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Q: As I understand it, if the controller output increases when the measurement value rises, it is a direct-acting controller, and if controller output decreases when the measurement rises, it is a reverse-acting controller. In addition, at minimum output of the controller, the desired fail-safe action must be achieved. For example, if we have two controllers on a distillation column—reflux and net overhead—where the reflux control valve is fail-open, and net overhead control valve is fail-close, the reflux controller will be reverse-acting, and the net overhead controller will be direct-acting. Is my understanding correct? - Vasant Warke / vasant.warke@ril.com

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A: Valve failure position and controller actions are independently determined, therefore, let's talk about them separately.

Valve failure position is determined by safety considerations. If, in case of failure, you want your column to go on full reflux, your selection is right (Figure 1). Assuming that the valve actuators are spring-operated pneumatic ones (you did not say what they were), and assuming that you define "failure" as the loss of air supply, the spring will act to open a fail-open (FO) valve and to close a fail-closed (FC) valve, regardless what the controller actions are. (I neglect to mention the role of positioners because I don't like to use them on flow control valves because they can be slower than the flow process and, therefore, they can cause cycling.)

Now let's turn to the subject of controller action. Assuming that your system is as shown in the figure, an increase in column temperature should result in increased cooling, which is accomplished by returning more cold reflux into the column. This means that the temperature controller (TC) has to reduce the reflux flow setpoint, which in turn will increase the level in the accumulator, and to overcome that, the level control (LC) increases the setpoint of the reflux flow control (FC). So a measurement increase requires the TC to reduce its output (reverse action, R/A), while the LC increases its output upon a level increase (D/A). The response of the slave FC controllers is to increase the flow through their valves as the master raises their setpoint, so with the FC valve, it will be D/A, and with the FO valve, it will be R/A.

Naturally, the control system shown in the figure is a very simple one (does nothing about interactions, etc.), but it is sufficient for explaining the issue at hand.

Béla Lipták
liptakbela@aol.com

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  • <p>I have a query here - if the response of the reflux FC controllers is to increase the reflux flow through their valves as the master (TC or LC) raises their setpoint, then are we allowed to have the reflux Flow valve to be Fail-close? </p> <p>The reason I am asking this question is - the prime intention of reflux control here is to maintain the temperature within the distillation column, and if we choose to have our reflux flow valve to be Fail-close, that could be unsafe for the process. Please indicate if my understanding is correct?</p> <p>Also, in the arrangement given, can we use ratio controller for the Flow valves on either side of downstream of reflux drum. Because anyway temperature control would require more of reflux back into the column, and on the other hand any excess of reflux going to the column will be the throughput of column.</p>

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  • <p>i had my control class today and it was quite different &gt;&gt; if the output signal increases and the input signal increases it's calles direct &gt;&gt; but if the output signal increases and the input signal decreases it's called reverse &gt;&gt; but i really need help understanding these definitions by simple examples if there is any please</p>

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  • <p>In response to the question (khaled nabilsy) , Taking the a controller in isolation and not using the example discussed above. the most basic example is a Heating Controller Vs a Cooling controller. In a Heating Controller as Temperature increases you need to turn DOWN the heating and vice versa - Reverse acting... In a Cooling Controller as Temperature Increases you need to Turn UP the Cooling and vice versa - Direct Acting... Of course life is not always so simple so in a control loop you must take into consideration the reverse or direct action of the actuator, its positioner if it has one, and any other control equipment of the control loop as a whole. This is often defined by the failsafe status of each individual component. In certain circumstances I have known this to mean that perhaps even in a cooling control loop a reverse acting control was required to give a direct action to the control loop as a whole entity......</p>

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  • <p>it is valuable informationof direct/reverse controller and example mentioned.</p>

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  • <p>it is simple and valuable explanation. thank you so much.</p>

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