Seeking advice on issues related to the control of electric cut-off ball valves
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Situation: An electric shut-off valve is installed on the oil inlet pipeline of the tank; when the liquid level in the tank is high, the valve closes automatically to stop oil inflow； When the oil tank level is slightly low, the valve opens automatically to allow oil to flow in. The valve is usually in the open position; it is only closed when the oil level is high during oil drainage. Question: 1. How are the electric open and electric close states of an electric cut-off valve understood? As I understand it, with electric operation: the valve is usually closed, and when power is applied (the open signal), the valve opens. Electric shutdown: Normally the valve remains open; when power is applied (a shutdown signal), the valve closes. In the above situation, I should choose the electric switch type, right? 2. Control: 2.1: If the understanding in point 1 is correct, then does it mean that I just need to send a signal to close the valve; once that closing signal is removed, the valve will automatically return to the open state? : 2.2: Signals are still required for both valve opening and closing – a signal to open the valve causes it to open, while a signal to close it causes it to close. Below are the electrical wiring diagrams I found online; switch signals must be sent to the valve in order to control it. Which of these two controls, 2.1 and 2.2, is the correct one? I’d like to ask everyone for advice, thank you.
Electric opening refers to the valve opening as the control signal increases. The valve is not necessarily in a closed position when there is no power.
The last edit to this post was made by chengf on 2017-7-19 at 08:21. For electric valves, the open signal and the close signal are generally two separate signals； According to you, the shutdown signal is triggered when the oil level is high, while the startup signal is triggered when the oil level is low. As for oil leakage, logically, an oil leakage condition can be added. However, since the oil level is still high when it shuts off, that means it actually follows the same switching conditions as mentioned above. If the valve remains open at all times, it indicates that the oil level has never reached the high level.
Thank you everyone. In terms of control, both an on and an off signal are required; if only the relevant signal is present, when that signal disappears, it doesn’t necessarily return to the on position. Is that correct? Since there are switch signals in all cases, then doesn’t that mean there is no such thing as electrically-operated or manually-operated types? There’s still a different understanding regarding turning it on and off electrically. I might not have explained clearly the oil unloading part; oil unloading refers to the process of transferring oil from tank trucks, and the valves are also installed on the oil unloading pipelines. The valve is closed only when the oil level is close to the height of the tank during oil unloading.
Electric valves require a separate power supply to operate; they are controlled to open and close via switch signals. Normally, unless special measures are taken, the valve remains in its original position and does not move when the power is cut off.
The switch is used to control the forward and reverse rotation of the motor, right?
First of all, it is important to note a key difference between electric valves and pneumatic valves: due to their power source, electric valves remain in their previous state when there is a failure (that is, when the power is cut off). In other words, they retain the valve position they were in before the power was lost (of course, this does not apply to eccentric butterfly valves where the pressure difference before and after the valve is too high, forcing the valve body to move)； On the other hand, pneumatic valves (single-acting) have a faulty state (i.e., when there is no air supply) due to the action of their return spring, which corresponds to the FO and FC states of the valve. Secondly, both the opening and closing actions of electric valves require motor drive; therefore, electric valves need at least two control signals, one for opening and one for closing. Finally, the common signal wiring for electric valves is attached. (On, Off, On feedback, Off feedback, Fault, Remote/Local, 4–20mA valve position setpoint, 4–20mA valve position feedback); among these, Remote/Local is used in cases where there is a on-site valve knob, to send the status of that knob back to the DCS, while the 4–20mA signal is generally used for electric control valves.
The electric valve is driven by an electric servo motor; it opens when a signal is sent to it and closes when another signal is sent. It remains in its current position in the event of a power outage. During an ESD situation, it can be set to open or close as required, and there is no issue of it remaining in an open or closed state due to a fault, as is the case with pneumatic valves
Interlock control scheme for oil depot liquid levels; the example is for reference only