Deep analysis of the selection and installation of the regulating valve type

At present, in the field of automatic control, whether a process can meet various process control indicators, whether the control process is stable, whether the overshoot, attenuation ratio, and disturbance are within the specified range are stable, fast, and accurate, except for the rational design of the process. In addition to advanced equipment, the important point is whether the regulating valve can accurately act according to the subject's consciousness, so that the control consciousness of the process subject is reflected in the precise change of material energy and flow. In many automatic control, because the quality of the regulating valve is not good, the flow characteristics are poor, the leakage is large, and the action is unreliable, the automatic control loses the high quality regulation, which brings significant economic loss to the production and increases the labor intensity.

Because the regulating valve is installed on the process pipe and directly contacts the operating medium, it is affected by harsh working conditions such as high temperature, high pressure, corrosion and friction. Therefore, the choice of the regulating valve is very important for the control of the system. Therefore, it is especially important to choose and install the regulating valve correctly and reasonably.

1 mediation valve selection

1.1 The choice of the type of mediation valve

Mainly according to the characteristics of the field medium, control requirements, installation environment and other factors combined with the flow characteristics and structure of the regulating valve itself, as shown in Table 1.

The choice of the valve should also take into account the form of the bonnet and the packing used. When the working temperature is -20 ° C ~ +250 ° C, only the ordinary structure is used; when the working temperature is -50 ° C ~ +450 ° C, the heat dissipation structure with multi-layer heat sink on the valve cover should be adopted. The corrugated chamber sealing bonnet of the bellows is mainly used for occasions such as highly toxic, volatile, and permeable. The commonly used sealing packings for regulating valves are Teflon and Asbestos rope packing, although the former is more expensive than the latter, but the sealing effect is much better. The selection of the air-opening and air-closing regulating valve shall be determined according to the size of the damage caused by the process at the position where the regulating valve is opened or closed when the pressure of the air source signal is interrupted. If the valve is in the open position, the air is off when the hazard is small. The action process is as follows: if there is gas, it will be closed, if there is no gas, it will be opened; if the valve is in the closed position, the harm is small, then the gas-opening type is selected, and the action process is: if the gas is open, the gas is closed.

1.2 The choice of mediation valve caliber

In the process of automation technology transformation, the valve diameter can be considered according to the principle of equal cross-sectional area. According to the large diameter of the valve when manual operation, and the opening area is estimated, the control valve with the same opening area under normal working conditions is selected. . In the design of a new device, the valve diameter is usually determined according to the flow capacity C value.

There are two common methods.

1 Calculate the corresponding flow capacity Cmax according to the actual maximum flow rate Qmax, and then select the C value slightly larger than Cmax and the corresponding valve diameter from the product series. The necessary space should be left when selecting. Finally, the valve at the actual maximum flow rate Qmax is opened. Check the degree: the opening should be no more than 90% at Qmax.

2 A more practical way is to calculate the corresponding circulation capacity Cvc according to the common flow rate. The C value of the selected valve should be such that Cvc/C is between 0.25 and 0.8, that is, multiplied by the usual flow rate Cvc value (4~1.25), and generally Cvc/C=0.5 is appropriate. It can be smaller when the working characteristics are logarithmic.

1.3 The choice of the flow characteristics of the mediation valve

1.3.1 Analysis of the adjustment quality from the regulation system

The principle is considered: the characteristics of the regulating valve are appropriately selected, and the change of the amplification factor of the valve is compensated by the change of the valve amplification factor, so that the total amplification factor of the adjustment system remains unchanged.

1.3.2 Analysis of process piping

At the production site, the regulating valve is always used in conjunction with the pipeline equipment, and there must be resistance. The working flow characteristics of the regulating valve are different from the ideal flow characteristics. Therefore, first of all, according to the characteristics of the control system

Table 2 Characteristics of the regulating valve in the process piping

Note: S indicates the value of the differential pressure when the valve is fully open and the total pressure difference of the system.

Select the desired operating characteristics and then consider the process piping to select the appropriate ideal characteristics. Table 2 is for reference.

1.3.3 Analysis of load changes

The linear valve has a large flow change at a small opening degree, the adjustment is too sensitive, and it is easy to cause oscillation. Therefore, it is not suitable to use in the case where the S small load changes greatly; the amplification factor of the equal percentage valve increases as the valve stroke increases, and the relative flow change is constant. It is constant, so it is suitable for applications where the load varies greatly. The quick-opening characteristic valve is generally used for two-position adjustment and program control.

2 mediation valve installation

Proper installation of the regulating valve will ensure that he has good control in the control system. For the installation of the regulating valve, the following points should be noted.

1 The regulating valve should be installed vertically on the horizontal pipe. If it must be tilted or installed horizontally, support should be added. There should be a straight pipe with a length greater than 10 pipe diameters before and after the valve.

2 The flow direction of the fluid should be the same as the mark on the valve body.

3 The installation site should be easy to maintain and have a bypass.

4 When the diameter of the regulating valve is smaller than the diameter of the pipe, the conical pipe should be connected (generally, the control effect of the valve is obvious, and the pipe diameter is larger than the diameter of the regulating valve.)

5 When working under low ambient temperature, the regulating valve should be equipped with a heat pipe.

Because the selection and installation of the regulating valve is a very closely related process, not independent of each other. In order to achieve a regulating effect of a regulating valve, it must be properly installed on the basis of a reasonable selection of the regulating valve, in order to achieve the desired effect. Since the on-site environment is very complex and variable, in order to make the regulating valve to meet the adjustment requirements to the greatest extent. Be sure to carefully analyze the site conditions, reasonably select and install the regulating valve. This will ensure the smooth production. It can be seen that in the field use, the installation of the regulating valve is as important as the choice of the regulating valve.