Questions regarding the operating modes of pneumatic control valves

The mode of operation of a control valve is primarily related to the driving method of the valve, the working principle of the actuator, and the changes in the control signal. Based on the information you provided, we can analyze the functioning mode of the valve step by step. 1. **Valve stem movement mechanism**: If, in a straight-stroke control valve, the valve closes when the valve stem moves downward and opens when it moves upward, this indicates that the valve stem is directly connected to the valve core; the valve core moves downward to block the valve seat and thus close the valve, while moving upward to move away from the valve seat and thereby open the valve. 2. **Actuator drive method**: Your mention of \"the air inlet valve on the cylinder closing\" usually refers to a pneumatic actuator. In this case, if the valve closes when air enters the upper part of the cylinder, it means the cylinder is of the \"reverse-acting\" type (fail-close or air-to-raise-action). That is, when the air pressure or air supply is lost, the spring or own weight of the actuator will push the valve stem downward, causing the valve to close. Such a design is generally considered reasonable from a safety perspective, as it causes the system to automatically close the valve in the event that the control signal is lost (such as a loss of air supply), thereby preventing potential hazards. 3. **Signal range of the locator**: The locator is used to receive control signals and convert them into actions of the actuator. Based on your description, \"4 mA to close the valve and 20 mA to open it\" represents a standard analog signal range, indicating that the actuator is configured to be controlled by 4-20 mA signals. At 4 mA, the positioner causes the control valve to close ； At 20mA, the positioner will fully open the control valve. This signal range is very common in the industry. Based on the above analysis, this valve can be described as an air-to-open or direct-acting control valve, as it is customary to consider the opposite scenario: since the valve closes due to the spring or its own weight when the air supply fails, gas pressure is required to open the valve under normal operating conditions. In this installation configuration, 4 mA corresponds to the closed state, 20 mA corresponds to the fully open state, and signals in between correspond to a partially open state, thereby enabling regulation of flow rate. Such a mode of operation is based on safety considerations. In industrial automation, especially when dealing with hazardous materials, it is often desirable for the system to shut down automatically in the event of a control failure, in order to protect equipment and personnel. .

When the current signal increases, the push rod moves downward; this is known as a positive-acting actuator； Conversely, when the current signal increases and the push rod moves upward, it is called a reaction actuator. The valve body components are further divided into positive and reverse types. When the valve stem moves downward, the flow cross-sectional area between the valve core and the valve seat decreases; this is known as the normal installation type ； The opposite is called reverse installation. Gas open and gas close are determined by the reverse actions of the actuator and the reversible installation of the valve body components.

It is possible to determine whether it operates on air open or air close based on the membrane head, as well as the type of sealing surface; I’m not sure how to judge this visually

A comprehensive judgment can be made based on the location of air intake at the membrane head and the indicator markings on the scale plate; for example, if the air supply enters from the lower side of the membrane head and the \"off\" indication is located on the upper side of the scale plate, then this valve should be of the air-shut type, otherwise it is of the air-open type

This post was last edited by Agelxx on 2023-12-18 08:32. First, it relates to the actuator, that is, the first two points you mentioned: the diaphragm head and the force applied to the valve stem. When the gas source pressure increases and the valve stem moves downward, we define it as a positive-acting actuator. The opposite is reaction! Next is the positioner; when the input current signal increases, the air pressure delivered to the diaphragm head increases, which we call direct action, while the opposite case is known as reverse action ； These two points merely define the direction and magnitude of his action, but they cannot determine the actual regulatory effect. Whether the valve stem moves upward or downward, and whether the air supply pressure increases or decreases, it is not possible to determine whether the valve’s opening degree increases or decreases. It is necessary to consider the installation orientation of the valve core – whether it is installed in the correct direction or reversed – in order to make such a determination. Whether to use the normal orientation or the reversed orientation is usually determined by practicality in manufacturing. The accident location of the valve is determined by process safety. For example, media such as feed and heating generally use an emergency shut-off function, while cooling and venting usually employ an emergency open function to ensure safety. Another important point is the positive and negative feedback of the regulator, that is, the PID control module in DCS (this is just an example; other types of regulators such as PLCs are also included). The direct action of a regulator is that its output increases as the positive deviation increases; conversely, this is the reverse action. For example, we use control valves to regulate the liquid level in the water tank; whether it is the inlet pipeline or the outlet pipeline that is being controlled, as long as the liquid level exceeds the set value, the output of the regulator increases – this is what is referred to as positive action. The opposite is reaction. And discussing the significance of positive and negative operations is about how we can achieve the goal of automatic control. For example, in the case mentioned above, if the control valve regulates the water inlet pipeline. We usually use a reactive regulator in combination with an emergency shut-off valve. When the liquid level exceeds the set value, the output of the regulator decreases, and the valve opening shrinks, thereby turning the system into a control system with closed-loop negative feedback. If it is to control outlet water management, we choose a positively acting regulator ； If it is to control the pressure in the reactor, we usually use a feedback regulator in combination with an emergency relief valve. When the pressure exceeds the set value, the regulator’s output decreases, and the valve opening increases. This is my personal understanding; please let me know if there are any mistakes