Brief: several factors for the selection of pneumatic control valve are introduced. According to the process conditions of chemical production, select the appropriate structure and type of regulating valve; Select appropriate flow characteristics according to the process object; Calculate the flow coefficient and select the diameter of the regulating valve according to the process parameters; Select materials and auxiliary devices according to process requirements.

Key words: control loop; Pneumatic control valve; choice

Modern chemical enterprises have a high level of automation. In process control, the control quality of actuators is closely related to the stability of production conditions, so the selection of actuators is particularly important. For chemical enterprises with high safety and explosion-proof requirements, most actuators use pneumatic control valves.

The selection of pneumatic control valve shall generally be considered from the following aspects:

(1) Select the appropriate structure and type of regulating valve according to the process conditions.

(2) Select the appropriate flow characteristics according to the process object.

(3) According to the process parameters, calculate the flow coefficient and select the diameter of the valve.

(4) Select materials and auxiliary devices according to process requirements.

The following discusses the above four points.

1 Structure and type of regulating valve

The pneumatic control valve is composed of a pneumatic actuator and a valve. The pneumatic actuator receives the input air source signal and generates corresponding thrust to displace the push rod and push the valve action; The valve refers to the valve body component connected with the pipeline. It receives the push rod thrust of the actuator to change the valve stem displacement, so as to change the valve opening and finally control the change of fluid flow. Pneumatic control valve can be divided into straight stroke and angular stroke according to its stroke. According to its structural type, it can be divided into straight through single seat valve, straight through double seat valve, high-pressure valve, angle valve, sleeve valve, diaphragm valve, butterfly valve, eccentric rotary valve, etc.

Among them, the through valve is relatively common, and the leakage of single seat valve is small, but the pressure difference between the front and back of the valve can not be too large, while the double seat valve is just the opposite. High pressure valve is suitable for medium measurement of high static pressure and high pressure difference. However, under the condition of high pressure difference, the material erosion and cavitation of fluid are serious. Generally, the material of valve core and valve seat should be considered to improve its service life. Angle valve can be used in the control of fluid with high pressure difference, high viscosity and suspended and granular substances. Diaphragm valve is more suitable for the control of strong corrosive media such as strong acid and alkali. Butterfly valve is suitable for gas medium with large flow and low pressure difference. Sleeve valve adopts balanced valve core structure and has the characteristics of low noise. It is one of the widely used valves.

There are two types of pneumatic control valves: air on and air off. The principle for determining the switching mode of regulating valve is to ensure the safety of process equipment and production when the signal pressure is interrupted. If the valve is in the open position after the signal is interrupted and the fluid is the safest without interruption, the air shut-off valve shall be selected; If the valve is in the closed position after the signal pressure is interrupted, and the fluid does not pass through the safest, the gas open valve shall be selected. For example, the fuel gas of the heating furnace or the regulating valve on the fuel oil pipe shall be gas open valve. When the signal is interrupted, the valve will automatically close and the fuel will be cut off to avoid accidents due to excessive furnace temperature; The regulating valve on the water inlet pipeline of the boiler shall be an air shut-off valve. When the signal is interrupted, the valve will open automatically and still send water to the boiler, which can prevent the boiler from drying up.

2 flow characteristics of regulating valve

The flow characteristic of the regulating valve refers to the functional relationship between the relative flow Q of the medium flowing through the valve and the relative stroke of the valve core (relative opening of the valve):

Q=f(L)

When the differential pressure △ P at both ends of the regulating valve remains unchanged, the flow characteristic of the valve is called the inherent flow characteristic. Natural flow characteristics mainly include straight line, equal percentage (logarithm), parabola and quick opening, as shown in Figure 1.

Natural flow characteristic diagram of valve

Fig. 1 inherent flow characteristics of valve

In production, the inherent flow characteristics of the valve include straight line, equal percentage and quick opening. Parabola characteristic is between straight line and equal percentage, which is generally replaced by equal percentage characteristic. The quick opening feature is mainly used for two position control.

In general, the differential pressure at both ends of the valve cannot remain unchanged forever. At this time, the inherent flow characteristics of the valve will be distorted. The characteristics of the valve under actual working conditions are called working flow characteristics. At this time, a coefficient s called valve resistance ratio should be introduced when determining the flow characteristics.

S=△P/ Σ △P

Σ △ P is the total differential pressure of the system, which is the sum of all differential pressures on the valve, all process equipment and pipeline system.

The working flow characteristics of the valve are analyzed from the following three aspects:

(1) The control quality of the control system is analyzed. For a simple control system, it is composed of several basic links: control object, transmitter, regulator and regulating valve. The total amplification factor of the system is k = k1k2k3k4k5.

K1 ~ K5 respectively represent the amplification factors of transmitter, regulator, actuator, valve and control object. When the load changes, the control system should maintain the predetermined control index, and the total amplification factor is expected to remain unchanged in the whole operation range of the control system. Generally speaking, in a certain system, the coefficients of K1 ~ K3 are fixed, and only the amplification factor K5 of the object changes with the change of load. Therefore, select appropriate flow characteristics to compensate for the change of object characteristics, and ensure that the product of k4k5 is a constant, so as to ensure that the total amplification factor K of the system is a stable value.

(2) From the analysis of process piping, the regulating valve is always used together with pipeline and equipment. The existence of pipeline resistance will inevitably make the working characteristics and inherent characteristics of the valve different. Therefore, the appropriate working characteristics should be selected according to the characteristics of the object, and then the natural flow characteristics of the corresponding valve should be selected according to the piping situation. When considering the process piping, refer to table 1 to select the inherent characteristics of the valve.

Table 1 piping condition

Piping condition s = 1 ~ 0.6 s = 0.6 ~ 0.3 s ＜ 0.3

The working characteristics of the valve should not be controlled by straight line equal percentage

The inherent characteristics of the valve should not be controlled by equal percentage of straight line

From table 1, when s = 1 ~ 0.6, the selected natural flow characteristics are consistent with the working flow characteristics. When s < 0.3, the characteristic curve is seriously distorted and should not be controlled.

(3) From the analysis of load change, the linear characteristic regulating valve has large flow relative change at small opening, is too sensitive, is easy to cause oscillation, and the valve core and valve seat are easy to be damaged. Therefore, it should not be used in the occasions with small s value and large load change range. The amplification factor of equal percentage valve increases with the increase of valve stroke, and the relative change of flow is constant.

3 diameter of regulating valve

The diameter of the regulating valve is determined on the basis of calculating the valve flow coefficient Cv. The definition of flow coefficient refers to the differential pressure △ P at both ends of the valve is 100KPA and the fluid density under the condition of full opening of the valve ρ= When 1g / cm3, the fluid volume flow through the valve is Q (m3 / h), and the throttling formula is:

Formula (1)

C is a proportional coefficient, and its relationship with the flow coefficient is m times, that is, CV = MC. When the flow characteristic is linear, M = 1.63; when the flow characteristic is equal percentage, M = 1.97. Equation (1) is the calculation method when the measuring medium is liquid. When the measuring medium is gas, the influence of temperature and pressure on the medium volume shall be considered. The calculation of C value can be divided into two cases: when the pressure difference before and after the valve △ P ＜ 0.5 times the pressure P1 in front of the valve, i.e. △ P ＜ 0.5p1:

Formula (2)

P2 in formula (2) is the pressure behind the valve.

△ P ≥ 0.5p1:

Formula (3)

In addition, when the medium is superheated steam, the superheat of steam shall be considered in calculating the C value. After determining the CV value, the opening of the regulating valve shall be checked. It is required that the valve opening shall be ≯ 90% at the maximum flow and ≮ 10% at the minimum flow. Under normal working conditions, the valve opening shall be between 15% ~ 85%. Finally, the diameter of the regulating valve is determined according to the CV value.

4 materials and auxiliary devices of regulating valve

The valve body of the regulating valve is generally made of cast iron, and its pressure resistance grade, service temperature range and corrosion resistance shall not be lower than the requirements for the process pipeline, but special consideration shall be given when the medium is flammable and explosive fluid. The selection of valve core and valve seat materials shall take into account the impact of wear and cavitation caused by solid particles in the fluid on the valve internals. Generally, stainless steel is often used for non corrosive fluids; For highly corrosive fluids, Hastelloy can be selected; For the fluid with serious impact, vibration and wear, surfacing cemented carbide or spraying material can be selected. In addition, when the temperature of the medium on the valve cover is greater than 200 ℃, the temperature of the medium on the valve cover shall be selected.

The auxiliary devices of the regulating valve mainly include valve positioner, solenoid valve, return switch, filter pressure reducer, holding valve, etc. Valve positioner is an important accessory of regulating valve. There are two common types: pneumatic valve positioner and electrical valve positioner. It is mainly used for:

Occasions with high differential pressure;

Occasions with high pressure, high temperature or low temperature medium;

Occasions where the medium contains solid suspended solids or viscous fluid;

Occasions with large diameter of regulating valve;

Realize split range control;

Improve the flow characteristics of the regulating valve.

The function of solenoid valve is to quickly cut off and connect the air source to make the valve in the fully on and off position. It is mostly used for safety vent valve. The return switch is selected only when the valve position signal needs to be monitored in real time in the control room. The selection of holding valve is based on the process conditions. When the air source is cut off, the valve position is required to be kept at a certain position, but this situation is rare.

5 Conclusion

The regulating valve is an important link in the automatic control system. If the selection and calculation are inaccurate and the use and maintenance are improper, it will directly affect the control quality of the control system and even cause serious production accidents. Therefore, great attention must be paid to the correct selection, installation and maintenance of control valves.

With the development of industrial automation, the requirements for pneumatic control valve are higher and higher, including new structure, new materials, high performance, communication with computer and superior dynamic performance. Therefore, the research and study of the new control valve is an arduous and important task.