Selection method of pneumatic control valve for coal gasification process
Coal gasification is a process that uses coal pressure gasification to produce gas. The whole process has 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 requirements for safety and explosion protection, most actuators use pneumatic control valves.
The selection of pneumatic control valves should generally be considered from the following aspects:
1. Select the appropriate structure and type of control valve according to the process conditions.
2. Select appropriate flow characteristics based on the process object.
3. Calculate the flow coefficient and select the valve diameter based on the process parameters.
4. Select materials and auxiliary devices according to process requirements.
Structure and type of pneumatic control valve
The pneumatic control valve is composed of two parts: a pneumatic actuator and a valve. The pneumatic actuator receives the input air source signal and generates corresponding thrust, causing the push rod to shift and pushing the valve to move; A valve refers to the part of the valve body component connected to the pipeline, which receives the push rod thrust of the actuator, changes the displacement of the valve stem, and thus changes the valve opening, ultimately controlling changes in fluid flow.
Pneumatic control valves can be divided into two types based on their stroke: straight stroke and angular stroke. According to their structural types, they can be divided into straight single seat valves, straight double seat valves, high-pressure valves, angle valves, sleeve valves, diaphragm valves, butterfly valves, eccentric rotary valves, etc. Among them, straight through valves are more common, with a single seat valve having less leakage, but the pressure difference before and after the valve should not be too large, while a double seat valve is the opposite. High pressure valves are suitable for medium measurement with high static pressure and high pressure difference. However, under high pressure difference conditions, the fluid can cause severe material erosion and cavitation. Generally, the material of the valve core and valve seat should be considered to improve their service life. Angle valves can be used for controlling fluids with high pressure difference, high viscosity, suspended solids, and particulate matter. Diaphragm valves are more suitable for controlling highly corrosive media such as strong acids and alkalis. Butterfly valves are suitable for gas media with high flow rate and low pressure difference. The sleeve valve adopts a balanced valve core structure and has the characteristic of low noise, making it 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 the 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 signal interruption and the fluid is safest without interruption, an air shut-off valve should 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 position, an air operated valve is selected. For example, the regulating valve on the fuel gas or fuel pipeline of the heating furnace should be selected as the gas opening valve. When the signal is interrupted, the valve will automatically close and the fuel will be cut off to avoid accidents caused by excessive furnace temperature; The regulating valve on the boiler inlet pipeline should be an air shut-off valve. When the signal is interrupted, the valve will automatically open and still supply water to the boiler, which can prevent the boiler from burning dry.
Flow characteristics of pneumatic control valve for coal gasification process
The flow characteristic of the pneumatic control valve for coal gasification 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 (the relative opening of the valve): Q=f (L).
When the pressure difference △ P between the two ends of the regulating valve remains constant, the flow characteristic of the valve is called the inherent flow characteristic. The inherent flow characteristics mainly include four types: straight line, equal percentage (logarithmic), parabolic, and fast opening.
There are three inherent flow characteristics of valves in production: straight line, equal percentage, and quick opening. The parabolic characteristic is between a straight line and an equal hundred, and is generally replaced by the equal hundred characteristic. The quick opening feature is mainly used for two position control.
In general, the pressure difference between the two ends of the valve cannot remain constant forever, and the inherent flow characteristics of the valve will undergo distortion. The characteristics of the valve under actual working conditions are called the working flow characteristics. At this point, a coefficient S called valve resistance ratio should be introduced when determining the flow characteristics.
Analyze the working flow characteristics of the valve from the following three aspects:
1. Analysis from the control quality aspect of the control system
For a simple control system, it is composed of several basic links: the control object, transmitter, regulator, and regulating valve. The total amplification coefficient of the system K=K1K2K3K4K5
K1~K5 respectively represent the amplification coefficients of the transmitter, regulator, actuator, valve, and control object. In the case of load changes, it is necessary to ensure that the control system can maintain the predetermined control indicators and the total amplification coefficient remains unchanged throughout the entire operating range of the control system. Generally speaking, in a certain system, the coefficients of K1~K3 are fixed and unchanged, and only the amplification coefficient X5 of the object changes with the load. Therefore, selecting appropriate flow characteristics to compensate for the changes in the object characteristics and ensuring that the product of K4K5 is a constant ensures that the total amplification coefficient K of the system is a stable value.
2. Analysis of process piping situation
Regulating valves are always used together with pipelines and equipment, and the existence of pipeline resistance will inevitably make the working characteristics and inherent characteristics of the valve different. Therefore, appropriate working characteristics should be selected according to the characteristics of the object, and then the inherent flow characteristics of the corresponding valve should be selected according to the piping situation.
3. Analysis from load changes
The linear characteristic regulating valve has a relatively large change in flow rate when the opening is small, which is too sensitive and can easily cause oscillation. The valve core and seat are easily damaged, so it is not suitable to use it in situations with small S values and large load changes. The amplification coefficient of the equal percentage valve increases with the increase of valve stroke, and the relative change in flow remains constant.
Caliber calculation of pneumatic control valves
The determination of the diameter of the pneumatic control valve is based on the calculation of the valve flow coefficient CV. The definition of flow coefficient refers to the pressure difference Δ P between the two ends of the valve at 100 kPa and the fluid density under the condition of fully open valve ρ When it is 1g/cm3, the volume flow rate of the fluid passing through the valve is Q (m3/h),
The throttling formula is: CV=mC.
C is a proportional coefficient, which is m times the flow coefficient. When the flow characteristic is linear, m=1.63, and when the flow characteristic is equal percentage, m=1.97.
CV=mC is the calculation method when the measuring medium is liquid. When the measuring medium is gas, the influence of temperature and pressure on the volume of the medium should be considered. The calculation of its C value can be divided into two situations:
When the pressure difference Δ P between the front and rear of the valve is less than 0.5 times the valve front pressure P1, that is, Δ P<0.5P1. In addition, when the medium is superheated steam, the calculation of the C value should consider the superheat degree of the steam.
After determining the CV value, the opening of the pneumatic control valve should be checked and calculated. It is required that the valve opening should not exceed 90% at maximum flow rate and not be less than 10% at minimum flow rate. Under normal operating conditions, the valve opening should be between 15% and 85%. Finally, determine the diameter of the regulating valve based on the CV value.
