Air-to-close valve

This point can easily get lost in the long explanation An air-to-open valve has a positive gain and is failed-closed. An air-to-close valve has a negative gain (-Kv) and is failed-open. 

If we use a controller with positive gain (+KC), the controller output increases as the liquid level drops. We can only reduce the flow if we use an air-to-close valve (-Kv). In the case of a power outage, the valve will stay open. This fail-open valve can drain the entire tank, an event that we may not want to happen. 

If we were cooling instead ofheating, we would want the coolant flow to increase when the temperature increased. But the controller action would still be reverse because the control valve would be an air-to-close valve, since we want it to fail wide... 

Most often, the control valve gets stuck. If a steel nut or welding rod is jammed into the valve seat, maximum air pressure (assuming an air-to-close valve) will certainly not close it. However, the control room instrument, having signaled maximum air pressure, will show the control valve to be shut. 

The Signal and Output dials should have the same reading. Let s assume that this is an "air-to-close valve." That means that more air pressure closes the valve. Typically, the supply air pressure is 30 psig. If the air output pressure to the control valve is 3 psig, the valve should be 100 percent open. If the air output pressure to the control valve is 15 psig, the valve would be shut. We say the valve operates... 

The operation of a control valve involves an air supply that positions its movable part (i.e. plug, ball, or vane) relative to the stationary seat of the valve. A valve actuator accurately locates the valve plug in a position determined by the pneumatic control signal and operates to move the valve to either fully open or fully closed positions. The actuators may be either piston or diaphragm types. Air-to-open valves require air to open and therefore automatically close in the event of fail closure. They are therefore used on fuel lines to furnaces. Air-to-close valves fail to open on a loss of air pressure and are used on air lines into fuel burners. In general, fail-to-open and fail-to-close valves operate when the supplied air pressure drops below a minimum value. 

Fig. 9. Control valve and actuator (a) nomenclature (b) fail-open (F/O) or air-to-close (A/C) action and (c) fail-closed (F/C) or air-to-open (A/O)...

Consistency in controls and response is important for example, upward movement on a control panel always causes the valve to open, no matter whether the valve is air to open or air to close. ... 

A process waste heat boiler was damaged when the control operator inadvertently closed the cooling water valve when he intended to open it. The manual control knob turned in the opposite direction from most of the other valves in the plant—the L, cooling water valve was air to close—most control valves were air... 

A pneumatic air-to-open valve also means that energy is required to keep it open. Under a system failure where power is lost, the valve closes and prevents flooding the tank. 

Cascade control, along with ratio control, is used to control the temperature. The cold-water line is to have an air-to-close control valve. In case of failure in the air supply, the valve would open fully and a runaway reaction would be prevented. The hot-water line will have an air-to-open valve for similar reasons. After the two streams are mixed, the temperature will be measured. If it is above the desired temperature, the amount of air supplied to the valves will be reduced. This will increase the cold-water flow rate, and decrease the hot-water throughput. The result will be a reduction in the inlet water temperature. The desired temperature will be determined from a measurement of the reactor temperature. A deviation from the desired temperature will cause the set point of the second controller to be changed. This will result in a change of the inlet water temperature. 

An important safety feature is provided by the spring in an actuator. It can be designed to position a control valve in a safe position if a loss of supply air occurs. On a loss of supply air, the actuator in Figure 36 will fail open. This type of arrangement is referred to as "air-to-close, spring-to-open" or simply "fail-open." Some valves fail in the closed position. This type of actuator is referred to as "air-to-open, spring-to-close" or "fail-closed." This "fail-safe" concept is an important consideration in nuclear facility design. 

The fail-safe condition is to not feed air into the regenerator. Therefore, the suction valve is air-to-open and the vent valve is air-to-close. What action should the flow controller have, direct or reverse ... 

Figure 7.120 is a simplified view of a spring diaphragm actuator. The actuator receives a pneumatic signal from the controller via a booster flow enlarger or a valve positioner and can be adapted in the form of an air-to-open or an air-to-close mechanism. 

Discuss some of the factors you should take into account before deciding whether to use an air-to-close or air-to-open pneumatic control valve. 

As opposed to the air line, the nitrogen line has an air-to-close solenoid valve (ATC). In the case of an emergency, it is preferable to sweep the lines with an inert gas to reduce the possibility of forming a flammable mixture. 

Engineers select and evaluate process equipment. The selected equipment must be technically sound, rehable, safe and easy to operate, serviceable, economically attractive and environmentally sound, e.g. minimal noise pollution and no mechanical and fire hazards. For example, all pneumatic valves, except for the modulating valves, are air-to-open/ spring-to-close valve type for safe shutdown unless the valve is a divert drain valve as... 

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