Control Valve Selection

The selection of control valve is a complex and critical process that involves multiple parameters to ensure that the valve can operate efficiently and safely under specific operating conditions.

The following are the main parameters to consider when selecting a control valve:

Process parameters

The viscosity, density, corrosiveness, solid content, and other characteristics of different fluids have a significant impact on the selection of control valves. For example, high viscosity fluids may require larger valve diameters and special valve core designs to ensure good flow control; For corrosive fluids, corrosion-resistant valve body and valve core materials such as hastelloy, titanium alloy, etc. should be selected. Pressure and temperature: Determine the working pressure and temperature range of the system, and the rated pressure and temperature rating of the selected control valve must meet the process requirements and have a certain margin. High temperature working conditions may require high-temperature resistant materials.

Valve characteristics

According to different flow and opening relationships, the common inherent flow characteristics of control valves include linear, Equal percentage, and fast opening etc.

Linear characteristic: When the control valve has a small opening, the flow rate is small, but the relative change in flow rate is large, indicating high sensitivity; Due to the fact that slight changes in travel can cause significant fluctuations in flow rate, oscillations are prone to occur at small openings. When the opening is large, the flow rate is high, but the relative change in flow rate is small, that is, the sensitivity is very low; Due to the need for significant changes in travel to alter the flow rate, the control becomes sluggish when the opening is large, resulting in delayed adjustment of the control valve. See the flow characteristic diagram 1(2).

Equal percentage characteristic: The change in relative flow caused by the same stroke variation of the control valve at different opening degrees is equal, hence it is called the equal percentage flow characteristic. The equal percentage flow characteristic has the same control accuracy throughout the entire travel range. The equal percentage flow characteristic control valve has a small amplification factor when the opening is small, so the adjustment is smooth; When the opening is large, although the amplification factor is large, it can still be effectively adjusted and adjusted in a timely manner.See the flow characteristic diagram 1(3).

Fast opening characteristic:The disc form of the fast opening flow characteristic is flat, and when the displacement increases further, the flow area of the valve no longer increases, losing its regulating effect. The fast opening flow characteristic control valve is suitable for fast opening and closing shut-off valves or dual position regulating systems. See the flow characteristic diagram 1(1).

Valve structure

Valve disc type: Common valve disc type include single seat valves, double seat valves, sleeve valves, ball valves, butterfly valves, etc. The leakage of a single seat valve is small, but the allowable pressure difference is small; The double seat valve allows for a large pressure difference, but the leakage is relatively large; Sleeve valves have good regulating performance and low leakage; Ball valves and butterfly valves have greater flow capacity and faster opening and closing speeds. The appropriate valve disc type should be selected according to the process requirements and fluid characteristics.

Connection type: The connection type for control valves include flange connection, threaded connection, welding connection, etc. The flange connection is easy to install, convenient for maintenance and replacement of valves, but it occupies a large area; Threaded connections are suitable for small-diameter pipelines, but the connection strength is relatively low; Welding connections have good sealing, but installation and maintenance are inconvenient. The appropriate connection method should be selected based on factors such as pipeline size, pressure rating, and installation environment.

Actuator Type

Type selection: Actuators mainly include pneumatic, electric, hydraulic, and other types. Pneumatic actuators have a simple structure, reliable action, and low cost, making them suitable for flammable and explosive environments; Electric actuators have high control accuracy, fast response speed, and can achieve remote control, but special explosion-proof measures need to be taken in situations with high explosion-proof requirements; Hydraulic actuators have a high output force, but require specialized hydraulic systems, resulting in higher costs. The appropriate actuator should be selected based on factors such as process requirements, control accuracy, explosion-proof requirements, and cost. Output force and stroke: The output force of the actuator should be able to overcome resistance such as friction and medium pressure during the opening and closing process of the valve, ensuring that the valve can open and close normally. At the same time, the appropriate stroke should be selected according to the opening requirements of the valve. A stroke that is too large or too small will affect the regulating performance of the valve. When selecting, it is necessary to calculate or determine the output force and stroke of the actuator based on factors such as valve specifications, working pressure, and required opening range.

Other Parameters

Installation space: Consider the installation position and space size of the regulating valve to ensure smooth installation and operation of the valve. Cost factor: On the premise of meeting the process and performance requirements, the procurement cost, installation cost, operation cost, and maintenance cost of the regulating valve should be comprehensively considered, and the valve with high cost-effectiveness should be selected.

In summary, when selecting a control valve, it is necessary to comprehensively consider the above parameters and combine them with specific operating conditions and usage requirements for selection.