Choice of control valve type
It is mainly selected according to the characteristics of the site medium, control requirements, installation environment and other factors and combine with the flow characteristics and structure of the control valve itself.
- The single-seat control valve has simple structure, convenient installation, small leakage, and is not resistant to impact. It is applicable for environments with a diameter of less than 25mm.
- The double-seat regulating valve is not tightly closed, has large leakage, and is impact-resistant. It is applicable in large-diameter environments.
- The valve body of the angle valve is less impacted by the fluid and is not easy to scale. It is especially suitable for occasions with high viscosity, suspended solids and particles, and has strong adjustment and stability.
- The butterfly valve: simple structure, small size and light weight, only needs to rotate 90° to open and close quickly, the operation is simple, and this valve has good fluid control characteristics. Small flow resistance, suitable for service low differential pressure and large flow of gas and environments which contains suspended solid.
- Diaphragm control valve is used for medium of highly corrosive, high viscosity or with fiber.
- The three-way regulating valve is used for three-way flow, it is divided into three-way confluence and three-way diversion, and is particularly effective for confluence and diversion.
- The sleeve control valve has good adjustability, strong versatility and easy maintenance, especially suitable for medium of high viscosity and particle environments.
Choice of upper bonnet
- When the working temperature is -20℃~+250℃, choose the common structure.
- When the working temperature is -50℃~+450℃, choose the dissipation structure that the bonnet has multiple layers of heat sinks.
- The bellows seal valve bonnet is selected for the occasions where the highly toxic, volatile, easily permeable and other important media.
Choice of packing
The commonly used sealing packing for regulating valves are PTFE and asbestos rope, although the former is more expensive than the latter, the sealing effect is much better.
Selection of regulating valve size
The valve size can be considered according to the principle of equal cross-sectional area, and select base on the flow capacity, material flow rate, pressure, etc. When designing a new device, the valve size is usually determined according to the C value of the flow capacity.
There are two common methods.
(1) Calculate the corresponding flow capacity Cmax according to the actual maximum flow Qmax, and then select the valve size corresponding to the C value that is slightly larger than Cmax, the necessary margin shall be left when selecting, and finally the valve opening at the actual maximum flow rate Qmax shall be checked: the opening should not be greater than 90% at Qmax.
(2) A more practical way is to calculate the corresponding flow capacity Cvc according to the common flow rate. The C value of the selected valve should make the Cvc/C be between 0.25 and 0.8, that is, multiply the Cvc value of the common flow rate by (4 to 1.25). Generally, Cvc/C=0.5 is suitable, this ration can be smaller when the operating characteristic is logarithmic.
Choice of flow characteristics
- Analysis of regulating quality from regulating system
According to the characteristics of the regulating valve, the change of the valve amplification factor is used to compensate the change of the amplification factor of the regulating valve object, so that the total amplification factor of the regulating system remains unchanged.
- Analysis of process piping
In the production site, the control valve is always used in conjunction with the pipeline equipments, and there must be resistance. The working flow characteristics of the control valve are different from the ideal flow characteristics.
Select the desired working characteristics, and then consider the process piping conditions to select the corresponding ideal characteristics.
- Analysis of load changes
The flow rate of the direct-travel valve changes greatly when the opening degree is small, and the regulating is too sensitive, which is easy to cause oscillation, therefore, it is not suitable for using in occasions with small load and large changes. The amplification factor of equal percentage valve increases with valve’s travel increasing, the relative change of flow rate is constant, therefore, it is suitable for occasions with large amplitude of load changing. The quick opening characteristic valve is generally used in the occasions of two-position regulating and program control.