Positioner positive and negative feedback

This post was last edited by Xiaoyao Tenglong on February 27, 2016, at 12:23. I don’t quite understand the phrase “reverse action of the positioner control valve”; however, in general terms, when referring to the reverse action of a valve, it means an air-close valve. “\"FO\" means: valve failure, the valve is in the open position. However, when the actuator feedback stem comes loose, it is not possible to determine the current state of the valve (normally, the valve actuator feedback stem does not come loose). When determining the conditions for a fault switch, we generally do not take into account the possibility of the valve stem coming loose; instead, we focus on what state the valve is in in the event of a sudden power loss or a sudden loss of air supply. So, determining the faulty switch also serves to determine the forward and reverse action of the valve ； Example: Fault open = on/off = reaction. Appendix: FC ； FO ； FL ； FLC ； The fault switch states for these FLO valves are all specified by the process requirements, and they must comply with the requirements outlined in the safety design documents; our instruments do not have any discretion in making choices regarding this. FC: Valve failure, valve in closed position. FO: Valve failure, valve in open position. FL: Valve failure, valve in its final position and remains there. FLC: Valve failure, valve holds its position but tends to close, eventually ending up in the closed position. FLO: Valve failure, valve holds its position but tends to open, eventually ending up in the open position

This is a question from our exam: A positive-action actuator is one in which an increase in the actuator’s input results in an increase in its output; this is what is meant by positive action； A reactive positioner: an increase in the input to the positioner results in a decrease in its output, which is what is meant by reactivity ； The feedback rod of the locator isn’t going to fall off, is it? Does FO refer to a gas supply failure or to failures of all valves?

When the feedback rod comes loose, the valve is out of control, and this has nothing to do with FO and FC. FO and FC are based on the scenarios of loss of air pressure and power outage

Generally, if the feedback rod of a valve comes loose, the valve will not operate and will remain in its original state. FC and FO situations only occur when there is no power supply or oxygen supply.

The control valve and the valve positioner form a negative feedback loop; if the valve stem comes loose, there is no longer any feedback, the amplification factor becomes infinite, and the output of the positioner reaches its extreme value, either the maximum or the minimum. For the reaction locator, its output should be “0”. The correct answer should be determined by analyzing the schematic diagram of the positioner.

You yourself mentioned two issues. First, to determine the open or closed state of a valve in an emergency situation, it is necessary to know whether the valve is double-acting or single-acting. In an emergency, double-acting control valves generally maintain their current position, while only single-acting valves can be in either fully open or fully closed state. However, the decision regarding whether the valve should be in a faulty open or faulty closed state does not lie with the instruments; it is determined by the process design. The definition of an emergency also does not include a detached feedback rod – emergencies are defined as power loss or air supply failure, or as an emergency response mode specifically designed within the DCS system. Secondly, it is indeed possible for the feedback rod of a positioner to come loose. This problem occurs with both Korean YTC mechanical positioners and Siemens’ intelligent positioners. When the feedback rod of a YTC positioner comes loose, the valve will stop at the position where the rod has come loose. If a control signal is sent, the valve will operate based on the current value corresponding to that position; if this value is higher, the valve will open fully, and if it is lower, the valve will close fully (of course, the valve’s positive or negative action must also be taken into account). In the case of Siemens’ intelligent positioners, if the feedback rod comes loose, an error indicator in the form of a lightning symbol will appear in the lower right corner of the device’s LCD screen, and the control valve will temporarily stop functioning I have also repaired a regulating valve for a domestically produced imitation smart locator; when the air supply was cut off, the valve closed. By sending a signal of “0%” to the locator from any valve position other than 1%, the locator would lock up, and the valve remained in the position set by that signal. It was necessary to cut power and then restart it in order to use the device again. Therefore, the situation you described needs to be analyzed in detail!

Therefore, the action and reaction correspond to the states of gas opening and gas closing; they are related to process safety, not to control. The feedback rod has come loose – I’ve never seen that happen before; maybe it does occur, but I’ve only seen the retaining spring of the feedback rod come loose… That’s not considered a fault condition; it’s rather a situation of loss of control, equivalent to damage to the actuator, making it impossible to determine the output. FO refers to a gas supply failure, but I believe that a signal loss condition can also result in a return to a fault state; therefore, I define the fault state as: gas supply failure or signal loss.

The technician above has already explained it in great detail. I’ll only mention what I know; I’ve never encountered smart locators. Mechanical types of valves did exist; currently, the ordinary control valves used in our factory are cage-type valves (the valve core cannot change between FC and FO – this can only be done through the actuator. When air enters from above the diaphragm, it results in FO, while air entering from below results in FC). Whether the valve is in FC or FO mode already determines the directional operation of the valve positioner, and this cannot be changed. Since the travel of the feedback rod and the valve must create negative feedback, when there is no feedback left, the amplification factor becomes infinite; in other words, the output of the positioner is at its extreme value, either the maximum or the minimum. After the feedback rod detaches, due to gravity, the feedback rod moves downward, causing the valve stem to move upward; as a result, both the FO and FC valves open fully.

I think I still prefer what the guy on the 6th floor said – that everyone is welcome to discuss it

Your control valve is of the FO type, which means it will open when the air is lost. Well, the locator is a reactive type of locator: as the input increases, the output decreases. For example, at 20 mA, the output air pressure is 0, which means there is no air pressure and the valve is open. During operation, due to the detachment of the feedback rod, negative feedback is not established, and at this point the control valve opens fully.