Direct-acting and reverse-acting pneumatic valves?
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As the title suggests, 1. What does positive action for pneumatic valves mean? What does \"reaction\" mean? 2. What do DA and RA represent respectively? 3. Since they are both air shut-off valves, why is one labeled DA while the other is labeled RA? Thank you.
#Here’s a quick reply: #In a positive-acting actuator, the push rod moves downward when the signal pressure increases; in a negative-acting actuator, the push rod moves upward when the signal pressure increases. The signal pressure of the reaction actuator is applied below the diaphragm; as the signal pressure increases, the diaphragm moves downward. Determining whether to open or close the valve also depends on whether the control mechanism is installed in the normal or reversed orientation. When the required quantity to be controlled increases while the regulator output needs to decrease, this serves as a counteraction ； When the required load increases and thus an increase in the regulator’s output is needed, it operates in a positive manner ； A control valve consists of a control circuit, an on-site valve positioner, an actuator, and a valve body. The logic of the control loop determines whether the output value increases or decreases after a change in the variable. The locator can also be configured to determine the output of instrument air based on electrical signals. The pressure from the actuator is applied above or below the diaphragm; as the pressure increases, the valve stem moves upward or downward. Valves also come in two types: normal installation and reverse installation. These are determined during the programming selection and set during DCS configuration. In the case of no factory wind or a communication failure, the safe state of the valve is closed. When the wind is shut off, the valve is in an open safe state. There is also another option in which, in the event of a failure, the valve maintains its original position for safety reasons. To give an example: a pneumatic control valve is normally closed (by \"normally closed\" it is meant that in the event of a failure in the air supply, the valve remains in the closed position). Positive action: When a 20MA signal is applied, the valve is in the fully open position; when a 4MA signal is applied, the valve is in its initially fully closed position. Reaction: When a 20MA signal is applied, the valve is in its initial position (fully closed); when a 4MA signal is applied, the valve moves to the fully open position.
This post was last edited by che*ao* on 2016-4-1 at 16:57. Take a look at the relevant material – the basics aren’t well understood here. DA stands for direct action, while RA stands for reverse action. Third question: For gas control valves, the settings within the DCS are determined based on the control requirements; for example, if an instruction specifies an opening range of 0-100, the output current will be 4-20mA. This can be understood as direct action； The other one is for a command range of 0-100 degrees, with an output current of 20-4 mA; this can be considered as a reactive effect.
An increase in the input signal to the control valve causes the valve stem to extend, which is a direct-acting effect; the opposite is a reverse-acting effect. As the input signal increases, the valve opening increases, resulting in a valve that opens to allow air flow; otherwise, it is a valve that closes to prevent air flow. The positive (negative) effect is not the same as whether the wind is on (off). DA is a positive action, while RA is a reactive action.
The more complex something is, the easier it is to forget
DA and RA represent positive action and negative action respectively; in principle, this is different from the concept of \"open when current increases\" and \"close when current increases\". Positive action means that an increase in current leads to an increase in the opening degree, while negative action means that an increase in current results in a decrease in the opening degree. In the case of open airflow, there is airflow when it is open but no airflow when it is closed; the opposite is true for closed airflow. Generally, for normal operation a pneumatic actuator is used, so that it shuts off in the event of a power loss or air supply interruption; the same principle applies in reverse. In very rare cases, the operation of valves that are powered off differs from that of those with air supply cut off; in other words, as you mentioned, it’s the same type of air-actuated valve, but one uses a DA signal while the other uses a RA signal. It is recommended that you study the process in detail, as this situation is quite uncommon.
The answer is upstairs; that’s right, haha
The direct and reverse action of a pneumatic control valve refers to the following: when the input signal to the control valve increases (for example, in the case of an electrical valve positioner, the input signal changes from 4mA to 20mA), the opening degree of the control valve increases, resulting in an increase in the flow rate of the medium being controlled – this is what is known as the direct action of the control valve. Conversely, when the input signal to the control valve increases (for example, in the case of an electric valve positioner, where the input signal changes from 4 mA to 20 mA), the opening degree of the control valve decreases, and as a result the flow rate of the controlled medium is reduced; this is known as the reverse action of the control valve.
Remember, the gas-on/gas-off mechanism relates to the valve opening degree and gas flow, while the reverse action is related to the valve opening degree and electrical signals. Once you understand these two points, you’ll be successful in anything you do.
Remember, the gas-on/gas-off mechanism relates to the valve opening degree and gas flow, while the reverse action is related to the valve opening degree and electrical signals. Once you understand these two points, you’ll be successful in anything you do.