The air opening-closing of the control valve is relative to the whole control valve machine. With the increase of the gas pressure of the membrane head, the valve gradually opens as a gas opening valve, and as the gas pressure of the membrane head increases, the valve gradually closes as a closed valve. When there is no signal, the air-open valve is closed, and the air-closed valve is fully open.
The positive action-reaction of the control valve is relative to the actuator of the pneumatic membrane control valve, the upper part of the membrane head takes in the air, and the downward movement of the push rod is called the positive action. The lower air intake, the upward movement of the push rod is called reaction.
Flow-on-flow closure is for the medium. When the flow direction of the medium at the throttle port flows towards the opening direction of the valve, it is called the flow open type, and conversely, when it flows towards the closed direction of the valve, it is the flow closed type.
First, the action form of pneumatic actuator
(1) The positive and negative effects of the pneumatic actuator. When the input air pressure of the pneumatic actuator increases, the push rod moves downward, which is called positive action; Conversely, when the input air pressure increases, the pushrod moves upward, which is called a reaction (see Figure 9-16).
(2) Formal and reverse installation of the adjustment mechanism. The spool is available in both positive and reverse versions. If the valve core moves down and the flow cross-sectional area between the valve core and the valve seat decreases, it is called a full-mounted valve; On the contrary, when the spool is moved downward, the flow cross-sectional area increases is called a reverse valve (see Figure 9-16). For double-guided full-mount valves, as long as the stem is connected to the lower end of the spool, it is a reverse valve. Valves with a nominal diameter of Dg<25mm are generally single-pilot, so only full-size valves are available.
(3) The action form of pneumatic actuator. Pneumatic actuators are available in two forms: air-open and air-closed. When the signal pressure increases, the valve opens, which is called air open; On the contrary, when the signal pressure increases, the valve is closed, which is called the gas shutdown type. Since the actuator has positive and negative effects, and the regulating valve (with a double guide spool) also has positive and negative effects, the air opening or air closing of the pneumatic actuator is combined, as shown in Figure 9-16.
For small-caliber control valves, the positive and negative actions of changing the actuator are usually used to achieve air opening or air closing; For large-diameter control valves, it is usually necessary to change the positive and negative actions of the control valve to achieve air opening or air closing.
Second, the locator
The positioner is used in conjunction with a pneumatic film actuator.
1) The positive effect of the valve positioner: when the input signal increases, the air pressure output to the membrane head increases;
2) Reaction of valve positioner: when the input signal increases, the air pressure output to the membrane head decreases;
The positive-acting actuator cooperates with the positive-acting positioner to realize the function of the positive-acting actuator;
The positive-acting actuator cooperates with the reaction positioner to realize the function of the reaction-acting actuator;
The reaction actuator cooperates with the positive action positioner to realize the function of the reaction actuator;
The reaction actuator cooperates with the reaction positioner to realize the function of the positive acting actuator;
3. FC (gas open or fault closed) or FO (air closed or fault open) of the control valve
The choice of gas switch is considered from the safety point of view of process production.
When the air supply is cut off, whether the valve is safe in the closed position or in the open position. For example, in the combustion control of a furnace, a regulating valve is installed on the fuel gas pipeline to control the fuel supply according to the temperature of the furnace chamber or the temperature of the material being heated at the furnace outlet. At this time, it is safer to choose an air-opening valve.
The air-open control valve means that the valve is completely closed in the case of gas cut-off, and the valve will be opened when there is gas. When there is no signal, the valve is closed, and the input signal valve is opened. And the larger the signal, the greater the valve opening. The valve is wide open when the signal is maximum.
Air to open type means that when the air pressure on the membrane head increases, the valve moves in the direction of increasing the opening, and when the upper limit of the input air pressure is reached, the valve is in a fully open state. Conversely, when the air pressure decreases, the valve moves in the direction of closing, and when no air is being fed, the valve is fully closed. Therefore, sometimes the air-open valve is also called Fail to Close (FC).
Air to Close moves in the opposite direction to Air Open. When the air pressure increases, the valve moves in the closed direction, and when the air pressure decreases or is absent, the valve moves in the open direction or until it is fully open. Therefore, it is sometimes referred to as Fail to Open (FO). Then in the process of use, there are usually several fault locations (FO, FC, FL), and the fault in the valve fault closing/fault opening refers to: the action of the valve when the air source is faulty.
For the fault location of pneumatic valves, it is mainly divided into several situations:
1. In the case of interlocking action of pneumatic valve device, the valve position should have the following situations:
FC - The air supply is lost and the valve is in the closed position
FO - The air supply is lost and the valve is in the open position
FL - The air supply is lost, and the valve is in the constant position and remains there
FLC - Loss of air supply, valve held in position but tends to close, valve in closed position (gas in the cylinder is depleted)
FLO - Loss of air supply, valve held in position but tends to open, valve in open position (gas depleted in cylinder)
2. When the regulating valve or switch valve participates in the interlocking action of the device, the valve position should have the following situations:
FC - loss of air source or loss of power of solenoid valve, valve in closed position;
FO - loss of air source or loss of power of solenoid valve, valve in open position;
AFL/EFC-
1) The solenoid valve is not lost when the air source is lost, and the valve is in place;
2) Regardless of whether the air source is lost, the solenoid valve is powered, and the valve is in the closed position;
AFL/EFO
1) The solenoid valve is not lost when the air source is lost, and the valve is in place;
2) Regardless of whether the air source is lost or not, the solenoid valve loses power, and the valve is in the open position.
Pneumatic valve through the output signal to achieve the valve cut-off, on, adjust and other functions, its opening and closing speed is relatively fast, often used for rapid two-position cut-off, can also be used to adjust the flow of use, with different accessories, can not achieve a variety of different control methods.
The flow area of the air-open control valve increases with the increase of the signal pressure, while the air-closed type decreases with the increase of the signal pressure.