Selection method of pneumatic control valve in control loop

A pneumatic control valve consists of a pneumatic actuator and a valve. The pneumatic actuator receives an input air signal, generates a corresponding thrust, causes the push rod to displace, and drives the valve to actuate. The valve refers to the valve body assembly connected to the pipeline, which receives the thrust from the actuator's push rod, changes the valve stem displacement, thereby altering the valve opening, and ultimately controlling the change in fluid flow.

Pneumatic control valves can be divided into two types based on their travel: linear and angular. According to their structural types, they can be further classified into straight-through single-seat valves, straight-through double-seat valves, high-pressure valves, angle valves, sleeve valves, diaphragm valves, butterfly valves, eccentric rotary valves, etc. Among them, straight-through valves are relatively common. Single-seat valves have a small leakage rate, but the pressure difference before and after the valve cannot be too large, while double-seat valves are just the opposite. High-pressure valves are suitable for measuring media with high static pressure and high pressure difference. However, in the case of high pressure difference, the fluid scours and cavitates the material severely, so it is generally necessary to consider the materials of the valve core and valve seat to improve their service life. Angle valves can be selected for the control of fluids with high pressure difference, high viscosity, suspended solids, and particulate matter. Diaphragm valves are more suitable for the control of strong corrosive media such as strong acids and alkalis. Butterfly valves are suitable for gas media with large flow rates and low pressure differences. Sleeve valves adopt a balanced valve core structure and have the characteristic of low noise, making them one of the most widely used valves.

Pneumatic control valves come in two types: air-to-open and air-to-close. The principle for determining the switching mode of a control valve is to ensure the safety of process equipment and production when signal pressure is interrupted. If the valve is in the open position after signal interruption and the fluid is safest if not interrupted, an air-to-close valve should be selected; if the valve is in the closed position after signal pressure interruption and the fluid is safest if not passing through, an air-to-open valve should be selected. For example, a control valve on the fuel gas or fuel oil pipeline of a heating furnace should be an air-to-open valve. When signal pressure is interrupted, the valve automatically closes, cutting off the fuel to prevent accidents caused by excessive furnace temperature. A control valve on the boiler's water inlet pipeline should be an air-to-close valve. When signal pressure is interrupted, the valve automatically opens, still supplying water to the boiler, which can prevent the boiler from drying out.

3 Flow characteristics of regulating valve

The flow characteristic of a regulating valve refers to the functional relationship between the relative flow rate Q of the medium flowing through the valve and the relative travel of the valve core (i.e., the relative opening of the valve):

¡¡¡¡Q=f(L)

When the pressure difference ¡÷P across the regulating valve remains constant, the flow characteristic of the valve is referred to as the inherent flow characteristic. There are primarily four types of inherent flow characteristics: linear, equal percentage (logarithmic), parabolic, and quick-opening, as illustrated in Figure 3-1:

In production, the inherent flow characteristics of valves include linear, equal percentage, and quick-opening types. The parabolic characteristic falls between the linear and equal percentage types and is generally replaced by the equal percentage characteristic. The quick-opening characteristic is mainly used for two-position control.

Under normal circumstances, the pressure difference across the valve cannot remain constant indefinitely. At this point, the inherent flow characteristics of the valve will undergo distortion. The characteristics of the valve under actual operating conditions are referred to as the working flow characteristics. When determining the flow characteristics, a coefficient called the valve resistance ratio, denoted as S, is introduced.

¡¡¡¡S=¡÷P/¡Æ¡÷P

¡Æ¡÷P represents the total pressure difference of the system, which is the sum of all pressure differences across the valve, all process equipment, and the pipeline system.

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

1) Analyze from the perspective of control quality of the control system

For a simple control system, it consists of several basic components: the control object, transmitter, regulator, and control valve. The overall amplification factor of the system is K=K1K2K3K4K5

K1~K5 represent the amplification factors of the transmitter, regulator, actuator, valve, and control object, respectively. In the case of load variation, to ensure that the control system can maintain predetermined control indicators, it is desirable for the total amplification factor to remain constant throughout the entire operating range of the control system. Generally speaking, in a given system, the coefficients of K1~K3 are fixed and unchanged, and only the amplification factor X5 of the object varies with load changes. Therefore, selecting appropriate flow characteristics to compensate for changes in object characteristics and ensuring that the product of K4K5 is a constant ensures that the total amplification factor K of the system is a stable value.

2) Analysis from the perspective of process piping

Regulating valves are always used in conjunction with pipelines and equipment. The presence of pipeline resistance will inevitably cause the operating characteristics of the valve to differ from its inherent characteristics. Therefore, it is necessary to select the appropriate operating characteristics based on the characteristics of the object, and then choose the corresponding inherent flow characteristics of the valve according to the piping conditions. When considering the process piping conditions, Table 3-1 can be referred to for selecting the inherent characteristics of the valve. Fluorine-lined Butterfly Valve

Table 3-1 Piping Condition Table

The working characteristics of valves with piping conditions of S=1~0.6, S=0.6~0.3, and S<0.3 are straight-line equal percentage, straight-line equal percentage, and straight-line equal percentage, respectively. It is not suitable to control the inherent characteristics of valves with equal percentage, equal percentage, and equal percentage

From Table 3-1, it can be seen that when S ranges from 1 to 0.6, the selected inherent flow characteristic is consistent with the operating flow characteristic. When S is less than 0.3, the characteristic curve is severely distorted and difficult to control.

3) Analyze from the perspective of load changes

The flow rate of a linear characteristic regulating valve changes significantly at small openings, making it overly sensitive and prone to oscillation. The valve core and seat are easily damaged, so it is not suitable for use in situations where the S value is small and the load changes greatly. The amplification factor of an equal percentage valve increases with the valve stroke, while the relative change in flow rate remains constant.

4. Diameter of regulating valve

The determination of the regulating valve's diameter is based on the calculation of the valve flow coefficient CV. The definition of the flow coefficient refers to the fluid volume flow rate Q (m3/h) passing through the valve when the pressure difference ¡÷P across the valve is 100 kPa and the fluid density ¦Ñ is 1g/cm3 under fully open conditions. The throttling formula is as follows:

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.

Formula (4-1) is the calculation method when the measuring medium is a liquid. When the measuring medium is a gas, the influence of temperature and pressure on the medium's volume should be considered. The calculation of its C value falls into two cases:

When the pressure difference ¡÷P between the upstream and downstream of the valve is less than 0.5 times the upstream pressure P1, that is, ¡÷P < 0.5P1

In addition, when the medium is superheated steam, the degree of superheat of the steam should be considered when calculating the C value.

After determining the CV value, it is necessary to verify the opening of the regulating valve. The valve opening should not exceed 90% at maximum flow and should not be less than 10% at minimum flow. Under normal operating conditions, the valve opening should be between 15% and 85%. Finally, the diameter of the regulating valve should be determined based on the CV value.

5 Materials and auxiliary devices of regulating valve

The valve body of a regulating valve is generally made of cast iron, and its pressure rating, operating temperature range, and corrosion resistance should not be inferior to the requirements for process pipelines. However, special considerations should be made when the medium is flammable or explosive. The selection of valve core and valve seat materials should take into account whether there are solid particles in the fluid that may cause wear and erosion, which may impact the valve internals. Generally, stainless steel is commonly used for non-corrosive fluids; Hastelloy alloy can be selected for highly corrosive fluids; and for fluids with severe impact, vibration, and wear, hardfacing or spray-coated materials can be chosen. Additionally, when the operating medium temperature is higher than +200¡æ, an upper valve cover with heat sinks should be selected.

The auxiliary devices of the regulating valve mainly include valve positioners, solenoid valves, return switches, filter regulators, and holding valves.

The valve positioner is an important accessory of the regulating valve, commonly classified into pneumatic valve positioners and electric valve positioners. It is primarily used for:

1) Situations with high pressure differences; 2) Situations involving high-pressure, high-temperature, or low-temperature media; 3) Situations where the medium contains solid suspended matter or viscous fluids; 4) Situations where the regulating valve has a larger diameter; 5) To achieve split-range control; 6) To improve the flow characteristics of the regulating valve.

The function of a solenoid valve is to quickly cut off and connect the air source, allowing the valve to be in a fully open or closed position. It is commonly used in safety vent valves. A return switch is selected based on the need to monitor the valve position signal in real time from the control room. The selection of a holding valve depends on the process conditions. When the air source is cut off, the valve position is required to remain in a certain position, but this situation is rare.

6 Conclusion

The regulating valve is a crucial component in automatic control systems. Inaccurate selection and calculation, as well as improper use and maintenance, can directly affect the control quality of the system and even lead to serious production accidents. Therefore, great importance must be attached to the correct selection, installation, and maintenance of regulating valves.

With the development of industrial automation, the requirements for pneumatic control valves are becoming increasingly stringent. These requirements encompass new structures, novel materials, high performance, computer communication capabilities, and superior dynamic performance. Therefore, researching and studying new types of control valves is an arduous yet crucial task.