Gasification is a process that utilizes coal to produce gas through pressurized gasification. The entire process boasts a high level of automation. In process control, the quality of actuator control is closely linked to the stability of production conditions. Therefore, the selection of actuators is particularly important. For chemical enterprises with high safety and explosion-proof requirements, pneumatic control valves are widely used.
The selection of pneumatic control valves generally involves the following considerations:
1. Choose the appropriate valve structure and type based on process conditions.
2. Select the appropriate flow characteristic based on the process object.
3. Calculate the flow coefficient based on process parameters to choose the valve size.
4. Select materials and auxiliary devices based on process requirements.
The structure and type of pneumatic control valves:
Pneumatic control valves consist of pneumatic actuators and valves. The pneumatic actuator receives the input air signal, generates corresponding thrust, displaces the push rod, and actuates the valve. The valve refers to the valve body component connected to the pipeline. It accepts the thrust of the actuator's push rod, changes the valve stem displacement, thereby altering the valve opening, and ultimately controls the fluid flow rate change.
Pneumatic control valves can be divided into linear stroke and angular stroke according to their strokes. They include direct single-seat valves, direct double-seat valves, high-pressure valves, angle valves, sleeve valves, diaphragm valves, butterfly valves, and eccentric rotary valves. Single-seat valves are more common, with low leakage rates, but cannot withstand large pressure differentials before and after the valve. Double-seat valves are the opposite. High-pressure valves are suitable for media measurement with high static pressure and pressure differentials. In the case of high-pressure differentials, the material of the valve core and valve seat should be considered to improve their service life. Angle valves are suitable for controlling fluids with high-pressure differentials, high viscosity, suspended particles, and granular substances. Diaphragm valves are more suitable for controlling highly corrosive media such as strong acids and alkalis. Butterfly valves are suitable for gas media with large flow rates and low pressure differentials. Sleeve valves adopt a balanced valve core structure, characterized by low noise and are one of the widely used valves.
Pneumatic control valves can be of two types: air-to-open and air-to-close. The principle for determining the valve switch mode is that when the signal pressure is interrupted, the safety of the process equipment and production should be ensured. If it is safest for the fluid not to flow after the signal pressure interruption, air-to-close valves should be used. Conversely, if it is safest for the fluid to continue flowing after the signal pressure interruption, air-to-open valves should be used. For example, for regulating valves on fuel gas or fuel oil pipelines of heating furnaces, air-to-open valves should be selected. After the signal interruption, the valve automatically closes, cutting off the fuel to prevent accidents due to excessive furnace temperature. For regulating valves on boiler water inlet pipelines, air-to-close valves should be selected. After the signal interruption, the valve automatically opens, continuing to supply water to the boiler, which can prevent the boiler from drying out.
Flow characteristics of pneumatic control valves in gasification process:
The flow characteristic of pneumatic control valves in the gasification process refers to the functional relationship between the relative flow rate Q of the medium passing through the valve and the relative stroke of the valve core (the relative opening of the valve) L: Q = f (L).
When the pressure difference △P across the regulating valve remains unchanged, the flow characteristic of the valve is called inherent flow characteristic. Inherent flow characteristics mainly include linear, equal percentage (logarithmic), parabolic, and quick-opening types.
In production, valve inherent flow characteristics mainly include linear, equal percentage, and quick-opening types. Parabolic characteristics fall between linear and equal percentage, and equal percentage characteristics are generally used instead. Quick-opening characteristics are mainly used for two-position control.
In general, the pressure difference between the two ends of the valve cannot remain constant indefinitely. This leads to distortion of the valve's inherent flow characteristic, known as the operating flow characteristic. This requires the introduction of a coefficient called the valve resistance ratio (S).
S = ΔP / ΣΔP
Where ΣΔP is the total pressure difference in the system, which is the sum of all pressure differences across the valve, all process equipment, and the pipeline system.
Analysis of the working flow characteristics of the valve can be done from the following three aspects:
Analysis from the perspective of control system quality
For a simple control system, it consists of several basic components: the controlled object, transmitter, regulator, and control valve. The total amplification factor K of the system is given by K = K1, K2, K3, K4, K5, where K1 K3 are fixed, only the amplification factor X5 of the object varies with the load changes. Therefore, selecting an appropriate flow characteristic to compensate for the changes in the object's characteristics ensures that the product of K4K5 remains constant, thereby ensuring that the total amplification factor K of the system is a stable value.
Analysis from the perspective of process piping
Regulating valves are always used in conjunction with pipelines and equipment. The existence of pipeline resistance will inevitably cause the valve's operating characteristics to differ from its inherent characteristics. Therefore, the appropriate working characteristics should be selected according to the object's characteristics, and then the corresponding inherent flow characteristics of the valve should be selected based on the piping conditions.
Analysis from the perspective of load changes
Linear characteristic regulating valves have a relatively large relative change in flow at small openings, making them too sensitive and prone to oscillation, leading to damage to the valve core and seat. Therefore, they are not suitable for use in situations with small S values and large fluctuations in load. The amplification factor of equal percentage valves increases with the increase in valve stroke, with a constant relative change in flow.
Calculation of the diameter of pneumatic control valves The determination of the diameter of a pneumatic control valve is based on the calculation of the flow coefficient CV. The definition of the flow coefficient refers to the fluid volume flow through the valve when the pressure difference ΔP between the two ends of the valve is 100 kPa and the fluid density ρ is 1g/cm3 under full open conditions.
Its throttle 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, and the calculation of C value is divided into two cases:
When the pressure difference ΔP before and after the valve is less than 0.5 times the pressure before the valve P1, that is, ΔP <0.5P1, in addition, when the medium is superheated steam, the calculation of the C value should consider the degree of superheat of the steam.
After determining the CV value, it is necessary to calculate the valve opening degree of the pneumatic control valve. It is required that the valve opening is not greater than 90% at maximum flow and not less than 10% at minimum flow. Under normal operating conditions, the valve opening should be between 15% and 85%. Finally, the valve diameter is determined based on the CV value.
The selection of pneumatic control valves generally involves the following considerations:
1. Choose the appropriate valve structure and type based on process conditions.
2. Select the appropriate flow characteristic based on the process object.
3. Calculate the flow coefficient based on process parameters to choose the valve size.
4. Select materials and auxiliary devices based on process requirements.
The structure and type of pneumatic control valves:
Pneumatic control valves consist of pneumatic actuators and valves. The pneumatic actuator receives the input air signal, generates corresponding thrust, displaces the push rod, and actuates the valve. The valve refers to the valve body component connected to the pipeline. It accepts the thrust of the actuator's push rod, changes the valve stem displacement, thereby altering the valve opening, and ultimately controls the fluid flow rate change.
Pneumatic control valves can be divided into linear stroke and angular stroke according to their strokes. They include direct single-seat valves, direct double-seat valves, high-pressure valves, angle valves, sleeve valves, diaphragm valves, butterfly valves, and eccentric rotary valves. Single-seat valves are more common, with low leakage rates, but cannot withstand large pressure differentials before and after the valve. Double-seat valves are the opposite. High-pressure valves are suitable for media measurement with high static pressure and pressure differentials. In the case of high-pressure differentials, the material of the valve core and valve seat should be considered to improve their service life. Angle valves are suitable for controlling fluids with high-pressure differentials, high viscosity, suspended particles, and granular substances. Diaphragm valves are more suitable for controlling highly corrosive media such as strong acids and alkalis. Butterfly valves are suitable for gas media with large flow rates and low pressure differentials. Sleeve valves adopt a balanced valve core structure, characterized by low noise and are one of the widely used valves.
Pneumatic control valves can be of two types: air-to-open and air-to-close. The principle for determining the valve switch mode is that when the signal pressure is interrupted, the safety of the process equipment and production should be ensured. If it is safest for the fluid not to flow after the signal pressure interruption, air-to-close valves should be used. Conversely, if it is safest for the fluid to continue flowing after the signal pressure interruption, air-to-open valves should be used. For example, for regulating valves on fuel gas or fuel oil pipelines of heating furnaces, air-to-open valves should be selected. After the signal interruption, the valve automatically closes, cutting off the fuel to prevent accidents due to excessive furnace temperature. For regulating valves on boiler water inlet pipelines, air-to-close valves should be selected. After the signal interruption, the valve automatically opens, continuing to supply water to the boiler, which can prevent the boiler from drying out.
Flow characteristics of pneumatic control valves in gasification process:
The flow characteristic of pneumatic control valves in the gasification process refers to the functional relationship between the relative flow rate Q of the medium passing through the valve and the relative stroke of the valve core (the relative opening of the valve) L: Q = f (L).
When the pressure difference △P across the regulating valve remains unchanged, the flow characteristic of the valve is called inherent flow characteristic. Inherent flow characteristics mainly include linear, equal percentage (logarithmic), parabolic, and quick-opening types.
In production, valve inherent flow characteristics mainly include linear, equal percentage, and quick-opening types. Parabolic characteristics fall between linear and equal percentage, and equal percentage characteristics are generally used instead. Quick-opening characteristics are mainly used for two-position control.
In general, the pressure difference between the two ends of the valve cannot remain constant indefinitely. This leads to distortion of the valve's inherent flow characteristic, known as the operating flow characteristic. This requires the introduction of a coefficient called the valve resistance ratio (S).
S = ΔP / ΣΔP
Where ΣΔP is the total pressure difference in the system, which is the sum of all pressure differences across the valve, all process equipment, and the pipeline system.
Analysis of the working flow characteristics of the valve can be done from the following three aspects:
Analysis from the perspective of control system quality
For a simple control system, it consists of several basic components: the controlled object, transmitter, regulator, and control valve. The total amplification factor K of the system is given by K = K1, K2, K3, K4, K5, where K1 K3 are fixed, only the amplification factor X5 of the object varies with the load changes. Therefore, selecting an appropriate flow characteristic to compensate for the changes in the object's characteristics ensures that the product of K4K5 remains constant, thereby ensuring that the total amplification factor K of the system is a stable value.
Analysis from the perspective of process piping
Regulating valves are always used in conjunction with pipelines and equipment. The existence of pipeline resistance will inevitably cause the valve's operating characteristics to differ from its inherent characteristics. Therefore, the appropriate working characteristics should be selected according to the object's characteristics, and then the corresponding inherent flow characteristics of the valve should be selected based on the piping conditions.
Analysis from the perspective of load changes
Linear characteristic regulating valves have a relatively large relative change in flow at small openings, making them too sensitive and prone to oscillation, leading to damage to the valve core and seat. Therefore, they are not suitable for use in situations with small S values and large fluctuations in load. The amplification factor of equal percentage valves increases with the increase in valve stroke, with a constant relative change in flow.
Calculation of the diameter of pneumatic control valves The determination of the diameter of a pneumatic control valve is based on the calculation of the flow coefficient CV. The definition of the flow coefficient refers to the fluid volume flow through the valve when the pressure difference ΔP between the two ends of the valve is 100 kPa and the fluid density ρ is 1g/cm3 under full open conditions.
Its throttle 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, and the calculation of C value is divided into two cases:
When the pressure difference ΔP before and after the valve is less than 0.5 times the pressure before the valve P1, that is, ΔP <0.5P1, in addition, when the medium is superheated steam, the calculation of the C value should consider the degree of superheat of the steam.
After determining the CV value, it is necessary to calculate the valve opening degree of the pneumatic control valve. It is required that the valve opening is not greater than 90% at maximum flow and not less than 10% at minimum flow. Under normal operating conditions, the valve opening should be between 15% and 85%. Finally, the valve diameter is determined based on the CV value.
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