I’d like to ask gently: what if there’s no instrument air?

Sure it won’t work – when the wind-on valve is closed, everything else is open, and when the wind-off valve is open, everything else is closed. Setting the DCS to manual mode only means that the valves don’t change according to the values you measure

When the compressed air pressure is zero, a pneumatic control valve can only be in either fully open or fully closed position, depending on whether the actuator is of the air-close or air-open type. Therefore, even when operating manually, it will not move.

Without instrument air, on-site actuators cannot operate

You still haven’t fully understood this: in automatic mode, it’s the set parameter values that cause the control valve to operate on its own, while in manual mode, you are directly adjusting the parameters that control the valve! If there’s no power wind, then it’s useless to try to move anything; the valve will return to either the open or closed position!

This post was last edited by ylb913 on 2016-1-14 06:38. Control valves can have either a diaphragm type or a cylinder type; those with a diaphragm feature a transverse diaphragm inside. On one side of this diaphragm is the process air, while on the other side is a spring. When the pressure of the process air drops, the spring expands, thereby closing (in the case of an air-operated valve that opens when air is applied) or opening (in the case of an air-operated valve that closes when air is applied) the valve. It’s no use trying to operate it manually at this point; even the manual control of the valves is driven by instrument air. The correct approach is to use the control valve bypass to make adjustments for the air inlet valve, while for the air outlet valve, it is necessary to reduce the flow by adjusting the manual valves located before and after it. It is driven by a cylinder; inside there is a piston. On one side of this piston is instrument air, and on the other side is a spring – it’s the same as what was described earlier. In some of them, there are no springs; both sides are filled with instrument air. When the instrument air is stopped, the position of the valve remains unchanged. ——Some models are equipped with an air storage tank; after the instrument power is cut off, the instrument air stored in this tank can be used to open or close the valves.

Put simply, first of all, there are the manual and automatic concepts. Manual operation means providing a drive signal, while automatic operation involves the valve adjusting itself based on the values you set. In both cases, an electrical signal is sent to the valve controller, which then controls the opening and closing of the valve. However, the controller acts as a converter between electricity and gas; without gas supply, the controller loses its ability to function, and the valve cannot be controlled manually from a distance. The manual you mentioned is not the one that involves turning a handwheel on-site.

Instrument air is the power source for control valves; without it, the control valves lose their functionality.

Operating manually definitely means it cannot be automatically adjusted according to working conditions

Without instrument air, the valves will only operate in fully closed, fully open, or hold position mode (determined by process requirements), regardless of whether the control valve is operated manually or automatically