The dry goods shared with you by Xiao Valve today are the matters needing attention in the selection of regulating valves. Because the control valve is installed on the process pipeline and directly contacts with the operating medium, it has been affected by harsh working conditions such as high temperature, high pressure, corrosion and friction for a long time. Therefore, the selection of the regulating valve has a great relationship with the control function of the system. Therefore, it is particularly important to choose and install the regulating valve correctly and reasonably. The following small valves are collected and sorted out to pay attention to knowledge points when purchasing regulating valves. For your reference when purchasing a regulating valve!
Selection of regulating valve type
The selection is mainly based on the characteristics of the on-site medium, control requirements, installation environment and other factors combined with the flow characteristics and structure of the regulating valve itself, as shown in the table.
When selecting a regulating valve, the form of the upper bonnet and the packing used should also be considered. When the operating temperature is -20°C~+250°C, only the ordinary structure should be used; when the operating temperature is -50°C~+450°C, the heat dissipation structure with multi-layer heat sinks on the valve cover should be used. The corrugated chamber sealing valve cover of the bellows is mainly used in the occasion of important media such as highly toxic, volatile, and easy to penetrate. Commonly used sealing packings for control valves include polytetrafluoroethylene and asbestos rope packing. Although the former is more expensive than the latter, the sealing effect is much better. The selection of air-opening and air-closing control valves should be based on the degree of harm caused by the open or closed position of the control valve to the process when the pressure of the air source signal is interrupted. If the hazard is small when the valve is in the open position, the air-to-close type is selected. The action process is: close when there is air, and open when there is no air; if the valve is in the closed position, the hazard is small, choose the air-open type, and the action process is: open when there is air, and close when there is no air.
Selection of regulating valve caliber
When carrying out automation technology transformation, the valve caliber can be considered based on the principle of equal cross-sectional area. According to the size of the valve caliber and the number of opening turns during manual operation, the opening area is estimated, and then the regulating valve with the same opening area under normal working conditions is selected. . When designing a new device, the valve caliber is usually determined according to the flow capacity C value.
There are two common methods.
①Calculate the corresponding flow capacity Cmax based on the actual maximum flow Qmax, and then select the C value slightly larger than Cmax and the corresponding valve caliber from the product series. The necessary room should be left when selecting, and finally, when the actual large flow Qmax Check the opening of the valve: the opening should not be greater than 90% at Qmax.
②A more practical way is to calculate the corresponding flow capacity Cvc according to the common flow. The C value of the valve should be selected so that Cvc/C is between 0.25 and 0.8, that is, the Cvc value of the common flow is multiplied by (4~1.25), and generally Cvc/C=0.5 is suitable. It can be smaller when the working characteristic is logarithmic.
Selection of flow characteristics of regulating valve
1. From the adjustment quality analysis of the adjustment system
Consideration principle: properly select the characteristics of the regulating valve, use the change of the valve amplification coefficient to compensate the change of the amplification coefficient of the regulating valve object, and keep the total amplification coefficient of the regulating system unchanged.
2. Analysis of process piping
In the production site, the control valve is always used together with the pipeline equipment, there must be resistance, and the working flow characteristics of the control valve are different from the ideal flow characteristics.Therefore, first of all, according to the characteristics of the control system
Table 2 Characteristics of regulating valves in process piping
Note: S represents the value of the differential pressure when the valve is fully open and the total differential pressure of the system.
Select the desired working characteristics, then consider the process piping conditions to select the corresponding ideal characteristics. Table 2 is for reference.
3. Analysis of load changes
When the linear valve has a small opening, the flow rate changes greatly, and the adjustment is too sensitive, which is easy to cause oscillation, so it is not suitable for use in occasions where the S small load changes greatly; the amplification factor of the equal percentage valve increases with the increase of the valve stroke, and the relative change of the flow rate is constant. Unchanged, so it is suitable for occasions with large load changes, and quick-opening characteristic valves are generally used for two-position adjustment and program control occasions.