Discussion on various power outage conditions of DCS

My computer froze and I posted two identical posts by mistake; please forgive me

1. At this point, the operator cannot operate the control valve anymore; if it was in automatic mode before, it will adjust itself, while if it was in manual mode previously, the output should be maintained as is. However, standard installations usually have an auxiliary control panel with emergency stop buttons that are hard-wired to the control station. One of the purposes of designing an auxiliary control panel is to enable a safe shutdown in cases where the operation station is unavailable, so that the control valve can be brought into a safe state. 2. When the AO output is 0 Ma, the control valve generally enters the safe state. 3. This relates to the hot start and cold start of the CPU, and a choice can be made.

The signal is lost after a power outage, and 0mA is outputted. As for the position of the control valve after a power outage, it depends on the settings regarding what happens when the signal is lost; in my case, I usually set the control valve to maintain its current position when the signal is lost, though it can also be set to a safe position, with the valve then operating based on the input signal once power is restored.

1. Original position. 2. Initial position (open-circuit and closed-circuit types) 3. Same as 2.

Where in DCS is the lost signal retention setting configured? Which system is it (Is it from Allen-Bradley?)

1. The operation station loses power, while the control station remains powered; the position of the control valve. The on-site process will not change! Keep it as is. 2. The control station and operation station are completely powered off; the position of the control valve is as follows. ESD protection must have been designed, so all process valves should be equipped with ESD protection mechanisms. 3. After a complete power outage, if power is restored immediately, is the control valve in its safe position or in the position it was in before the outage? It depends on how short the time window is available to you. If the device does not detect a power loss, everything remains unchanged; if the controller loses power, this triggers the ESD procedure to be carried out.

It’s a setting built into the intelligent control valve; within the valve, it’s possible to specify how it should react when a signal is lost. There are 3 options: maintain status, fully close, or fully open. If it’s configured through a DCS system, then since no signal is being sent, how is the valve supposed to react?

What you’re saying sounds a bit magical; may I ask which brand’s smart control valve this is? What model? I want to learn* and find out more.

Oh my, what amazing! Modern intelligent electric control valves come with this feature already. The one I’m using is a domestic-made model, and it works very well; both the input and output signals are in the 4–20mA range. If the input signal becomes zero instantly, it is detected as a lost signal or a fault, and the control valve will have an indicator light on its display screen to signal this situation. In such cases, the valve will operate according to the settings you have specified. The default setting by the manufacturer is to keep the valve in its current position. There are three possible modes of operation: 1) remain in the position before the signal was lost, 2) move towards the closed position until it’s fully closed, 3) move towards the open position until it’s fully open. The direction of operation is determined based on the requirements of the specific application environment, in order to ensure safe operation in case of input signal failures. The same principle applies when there is a power outage followed by a power restoration. For example, before the outage the valve was at 50% open (with an input signal of 12mA); after the outage, it will act according to the direction you have set – for instance, if you set it to close completely in the event of a signal loss, then the valve will move to the fully closed position. Once the input signal is restored during this process, the valve will operate based on the value of that input signal.

I have foreign positioners such as Memosens, Yamaki, and Fisher, as well as domestic ones like Hengyang Optoelectronics and Wanxun. Why isn’t that function I mentioned available? I thought it was related to the settings of the DCS system; but upon reflection, when there’s no power, the system becomes useless, and everything depends on the conditions of the hardware and field devices themselves. I think what you’re referring to is simply something I’ve never seen before! Hehe