Does DCS have positive and negative effects? How is it defined? What is the difference between the direct and reverse action of a valve?

DCS has positive and negative effects, but this is relative to the controller and has nothing to do with the valves on site. (http://bbs.hcbbs.com/thread-666251-1-1.html) With regard to the forward/reverse operation of the valve on-site and its fault position, the DCS output is generally 4-20mA. In the case of a valve that needs to be opened due to a fault, if the actuator is not set to reverse operation, its action will be completely opposite to the control signal; therefore, it must be set to reverse operation. For fault shutdown, no setting is required; the default setting is sufficient, as it operates in a positive manner.

For a valve that is stuck open, it is not necessarily necessary to change the positive/negative action of the positioner; by setting the DCS to use inverted AO signals, it is possible to ensure that the 0-100% range on the DCS corresponds to the 0-100% range of the actual valve. This is the common practice nowadays.

DCS is the controller, which is determined by taking into account the characteristics of the circuit as well as whether the valves operate on air opening or air closing principle.

In the control module of DCS, the forward and reverse action can be set, and together with the air-open/air-close functionality of the valves, negative feedback in the control loop can be achieved

The DCS has positive and negative outputs; the positive output corresponds to an opening degree of 0-100 for a 4-20 signal, while the negative output corresponds to 100-0 for the same 4-20 signal. The direct and reverse action of the valve (actuator) refers to the case where the valve stem moves downward when the signal increases, which is reverse action; whereas when the signal increases, the valve stem moves upward, which is direct action. You can look at the diaphragm control valve: when the air supply enters from the top, it operates in reverse mode, while when it enters from the bottom, it operates in direct mode.

For fault shutdown, no setting is required; the default setting is sufficient, as it operates in a positive manner. What a professional question.

DCS has positive and negative effects; a control system must operate on a closed-loop negative feedback basis. Generally, the mode of operation of the valve is determined first, and then the control system is arranged to form a closed loop in order to determine whether the DCS has positive or negative effects.

The operator’s permissions allow for adjusting the reaction; in other words, it involves matching the magnitude of the signal to the extent of valve movement

I understand it this way: the so-called positive and negative effects of DCS are essentially determined by the type of soft regulator present in the DCS system. In the window of the soft regulator, the bar graph representing the MV output valve control signal shows 0% at the bottom and 100% at the top; the percentage values indicate the degree to which the valve is open, that is, the percentage of the opening signal. For example, if the output of the soft regulator is 25%, it means that the valve on site should be opened to 25%, which corresponds to a control signal output of 8 milliamps. However, the default state of the control valves on-site, that is, their fault condition, is determined based on the safety requirements of process control; FO/FC settings are chosen with regard to the safety of the equipment in the event of a failure. Therefore, the control valves on-site are divided into two types: air-operated open valves and air-operated closed valves. Although the ultimate FO/FC of control valves results in four modes due to the combination of the actuator and the regulating mechanism, in most control valves today the regulating mechanism, namely the valve spool, adopts a normal installation configuration; as a result, the FO/FC of these control valves essentially corresponds to that of their actuators – where FO refers to air-shut valves and FC refers to air-open valves. Since the current signal output by the DCS has only one corresponding relationship – 4-20 milliamps corresponding to the valve opening – the valves on site operate in two modes; air-operated valves are fully closed at 4 milliamps, which is in line with the signal sent from the control room. The air shut-off valve is fully open at 4 milliamps, which is the exact opposite of the current signal coming from the control room; hence there is a matching issue. At a valve opening degree of 25% as desired by the DCS, the current signal generated is 8 milliamps; the on-site air control valve matches this value exactly, allowing for a 25% opening degree. However, the signal indicating that the on-site flow control valve is at 75% open indicates that it is in wind-off mode, which does not match the control requirements from the control room; therefore, improvements are necessary. In such situations, we could previously adjust the valve positioners on site to achieve this, but using the forward/reverse action settings of those positioners to establish such a relationship does not work well in practical applications. It’s not that the valve positioners are unable to perform this function; rather, it is because maintenance work can lead to incorrect operations. If the positioners are set to forward/reverse action, inexperienced maintenance personnel may, due to improper settings, prevent the control valves from functioning properly when replacing or adjusting them on site, which further complicates maintenance tasks. In short, the versatility of on-site valve positioners is poor. By changing the settings within the DCS, it is possible to alter the output current profile of the soft regulator, thereby changing the control signal for the control valve – that is, switching between forward and reverse action. This approach provides a permanent solution; there is no need to make adjustments on the field locator, and failures in the control valve can be avoided due to improper settings of the field valve locator, thus increasing the safety factor.