Can a control valve cut off flow?

If the control valve is closed completely, doesn’t that cut off the medium? However, a manual valve is usually installed in front of a control valve in design.

Function of the control valve: As the final actuator, the control valve plays a key role in control systems. A proper selection and accurate calculations form the foundation for the stable long-term operation of valves. When a control valve is in operation, it changes the flow resistance coefficient of the pipeline by altering the flow area, thereby achieving the purpose of regulating flow rate. Self-acting pressure control valves are used to regulate the flow rate, pressure, and level of a medium. Based on the signals from the control point, the opening degree of the valve is automatically adjusted, thereby enabling the regulation of the flow rate, pressure, and level of the medium. Control valves include electric control valves, pneumatic control valves, and hydraulic control valves, among others. A control valve consists of an electric actuator or a pneumatic actuator, along with the control valve itself. They are adjusted and generally divided into straight-through single-seat and straight-through double-seat types; the latter features high flow capacity, low imbalance, and stable operation, making it particularly suitable for applications with high flow rates, large pressure drops, and low leakage. The flow capacity Cv is one of the main parameters for selecting control valves. The flow capacity of a pneumatic diaphragm control valve is defined as the volume of fluid that passes through the valve per hour, when the valve is fully open, with a pressure difference of 0.1 MPa across it and a fluid density of 1 g/cm3. This value is referred to as the flow capacity, or flow coefficient, and is denoted by Cv; its unit is t/h. The Cv value for liquids is calculated using the following formula. By referring to a table based on the value of the flow capacity Cv, it is possible to determine the nominal diameter DN of the control valve. The flow characteristic of a control valve is the relationship between the relative flow rate of the medium passing through the valve and its opening degree, under the condition that the pressure difference across the valve remains constant. The inherent flow characteristic of a control valve refers to the rate of change of the flow rate through the valve as the opening degree changes, when the pressure difference across the valve remains constant. In a physical sense, the inherent flow characteristics of a valve indicate how its effective flow area changes as the valve opens; there are several types such as rapid opening, linear, proportional, and parabolic. The meanings of the three injection characteristics are as follows: Equal percentage characteristic. For the equal percentage characteristic, the relative stroke and the relative flow rate do not follow a linear relationship; at each point along the stroke, the change in flow rate resulting from a unit change in stroke is proportional to the flow rate at that point, with the percentage change in flow rate remaining constant. Therefore, its advantage is that the flow rate changes little when it is low, but changes significantly when it is high; in other words, it maintains the same adjustment accuracy at different opening degrees. Linear characteristic: The relative stroke and relative flow rate have a linear relationship in the case of a linear characteristic. The change in flow rate caused by a change in unit stroke remains constant. When the flow rate is high, the change in the relative value of the flow rate is small; when the flow rate is low, the change in the relative value of the flow rate is large. Parabolic characteristic: The flow varies proportionally to the square of the stroke, exhibiting an intermediate behavior that is roughly between linear and equal percentage characteristics. From the analysis of these three characteristics, it can be seen that in terms of regulation performance, the equal percentage characteristic is the best, offering stable regulation and excellent performance. Moreover, the parabolic characteristic offers better adjustment performance than the linear one, allowing any of these flow characteristics to be chosen depending on the requirements of the application.

Most control valves are not designed to have a shut-off function. I have seen a type of diaphragm control valve whose internal structure allows for rapid shutdown to ensure safety; there is also another type, namely a pneumatic butterfly valve with an internal rotating mechanism, that can enable rapid shutdown as well.

Control valves generally only serve to regulate flow rate, and not to shut off flow. If the schedule is reduced, it may also serve to cut off things, but it is not the main effect. For reliable truncation, check valves also need to be installed before and after the control valve.

Underutilizing such talent.............