How to Select Electric Regulating Valves for Small Power Plants
Introduction
The regulating mechanism is a tool to change the fluid flow, which can be divided into two categories according to its regulating function:
variable voltage regulation and throttle adjustment. Variable voltage regulation adjusts the flow by changing the pressure head, such as variable speed water pump, variable speed fan, etc. Throttle adjustment adjusts the flow by changing the flow resistance of the medium, by regulating valves and baffles.
The variable pressure regulating mechanism is more complex, and there are fewer types of equipment, so its application range is less. In the systems of water supply regulation and air volume regulation, the variable pressure regulating mechanism is often used because it can reduce power loss and achieve great economic effects. On the occasion of large flow and high pressure, the throttle regulating mechanism is difficult to work, and the variable pressure regulating mechanism is often used.
In addition, most regulating systems adopt a throttling regulating mechanism, that is, regulating valves. There are two kinds of regulating valves, pneumatic and electric. In small and medium-sized power plants, because of the limited number of regulating valves, if the pneumatic mode is adopted, a set of instrument air compressor stations should be equipped with high investment, so electric regulating valves are usually adopted. The following is a brief introduction to the selection and calculation of electric regulating valves.
1. The choice of electric regulating valves
The electric regulating valve is a very important link in the flow regulating system. Whether its structure type, caliber and flow characteristics are properly selected will directly affect the regulating quality and working safety and reliability of the automatic regulating system.
1.1 The choice of the structure type of electric regulating valves
In this article, the following suggestions are put forward only for the selection of the structure type of domestic electric regulating valves:
1) When the processing system requires that the leakage of the regulating valve be small (< 0.01% of rated capacity), the single-seat regulating valve is preferred because of its good tight performance.
2) When the pressure difference between the two ends of the regulating valve is large, it is recommended to choose a double-seat regulating valve to make it easier to open than a single-seat regulating valve when the pressure difference is large, but the disadvantage is that the leakage is large.
1.2 Selection of flow characteristics of electric regulating valves
The flow characteristics of the control valve refer to the relationship between the flow of the medium flowing through the control valve and the opening of the control valve. What the manufacturer provides is the ideal flow characteristics of the regulating valve, that is, the flow characteristics when the differential pressure before and after the valve is constant in the whole opening range, which represents the characteristics of the regulating valve.
At present, the control valves produced are divided into the following four types according to the ideal flow characteristics:
1. Linear flowThe flow of the regulating valve has a linear relationship with the opening.
2. Equal percentage (logarithmic) flowThe flow change caused by the change of unit stroke of the regulating valve is proportional to the flow at this point.
3. Fast-opening flow has a large flow at a small opening, and the flow quickly reaches the maximum when the opening increases.
4. Parabolic flowThe flow change caused by the change of unit stroke of the regulating valve is proportional to the square root of the flow at this point.
Because parabola is very close to equal percentage, and the quick opening is only used for on/off control, in fact, there are only two kinds of regulating valves with linear characteristics and equal percentage characteristics as throttling regulation.
For general regulating system, it is hoped that the amplification factor of regulating valves will be smaller at small load to avoid overshoot and oscillation, while it will be larger at large load, so it is advisable to choose to regulate valves with equal percentage characteristics for the system with large load fluctuation.
2. Calculation of regulating valves diameter
For the selection of regulating valve diameter, firstly, calculate the nominal flow capacity C (or Kv) of the regulating valve according to the flow Q required by the production process and the pressure difference ΔP before and after the regulating valve. Then select the regulating valve with the corresponding caliber from the product series. The so-called nominal flow capacity C refers to the flow through the regulating valve every hour, measured in m3/h or t/h. When the regulating valve is fully open, the pressure difference before and after the valve is 0.1MPa and the fluid weight is 1g/cm3. For high viscosity liquid, the viscosity coefficient should be corrected according to Reynolds number. Because it is rarely encountered in small and medium-sized power plants, it can be omitted in this article.
3. Calculation example
Take the hydrophobic regulating valve of the high-pressure heater in a thermal power plant in a high-tech zone as an example. Its technological diameter is D108×4; the medium is water; the maximum flow rate is 22t/h; the medium temperature is 200℃; the working pressure is 1.5MPa; the pressure before the valve is 1.5MPa and the pressure after the valve is 1.0MPa. Water is a general liquid, and its nominal circulation capacity is C = 9.84 so that two-seat regulating valves of DN32 should be selected.
4. Conclusion
In the design of small and medium-sized power plants, the most common one is the selection and caliber calculation of regulating valves for general liquid and superheated steam. It is mainly used in the selection of deaerator pressure and water level regulating valves, high-pressure heater drain regulating valves and condenser makeup regulating valves. When the leakage requirement is very small, single-seat regulating valves should be selected. When the differential pressure at both ends of the regulating valve is large or the tightness requirement is not high, two-seat regulating valves can be used. Usually, the calculated diameter of the regulating valve is smaller than that of the pipeline, so the two ends of the regulating valve need to be equipped with big and small heads.
The regulating mechanism is a tool to change the fluid flow, which can be divided into two categories according to its regulating function:
variable voltage regulation and throttle adjustment. Variable voltage regulation adjusts the flow by changing the pressure head, such as variable speed water pump, variable speed fan, etc. Throttle adjustment adjusts the flow by changing the flow resistance of the medium, by regulating valves and baffles.
The variable pressure regulating mechanism is more complex, and there are fewer types of equipment, so its application range is less. In the systems of water supply regulation and air volume regulation, the variable pressure regulating mechanism is often used because it can reduce power loss and achieve great economic effects. On the occasion of large flow and high pressure, the throttle regulating mechanism is difficult to work, and the variable pressure regulating mechanism is often used.
In addition, most regulating systems adopt a throttling regulating mechanism, that is, regulating valves. There are two kinds of regulating valves, pneumatic and electric. In small and medium-sized power plants, because of the limited number of regulating valves, if the pneumatic mode is adopted, a set of instrument air compressor stations should be equipped with high investment, so electric regulating valves are usually adopted. The following is a brief introduction to the selection and calculation of electric regulating valves.
1. The choice of electric regulating valves
The electric regulating valve is a very important link in the flow regulating system. Whether its structure type, caliber and flow characteristics are properly selected will directly affect the regulating quality and working safety and reliability of the automatic regulating system.
1.1 The choice of the structure type of electric regulating valves
In this article, the following suggestions are put forward only for the selection of the structure type of domestic electric regulating valves:
1) When the processing system requires that the leakage of the regulating valve be small (< 0.01% of rated capacity), the single-seat regulating valve is preferred because of its good tight performance.
2) When the pressure difference between the two ends of the regulating valve is large, it is recommended to choose a double-seat regulating valve to make it easier to open than a single-seat regulating valve when the pressure difference is large, but the disadvantage is that the leakage is large.
1.2 Selection of flow characteristics of electric regulating valves
The flow characteristics of the control valve refer to the relationship between the flow of the medium flowing through the control valve and the opening of the control valve. What the manufacturer provides is the ideal flow characteristics of the regulating valve, that is, the flow characteristics when the differential pressure before and after the valve is constant in the whole opening range, which represents the characteristics of the regulating valve.
At present, the control valves produced are divided into the following four types according to the ideal flow characteristics:
1. Linear flowThe flow of the regulating valve has a linear relationship with the opening.
2. Equal percentage (logarithmic) flowThe flow change caused by the change of unit stroke of the regulating valve is proportional to the flow at this point.
3. Fast-opening flow has a large flow at a small opening, and the flow quickly reaches the maximum when the opening increases.
4. Parabolic flowThe flow change caused by the change of unit stroke of the regulating valve is proportional to the square root of the flow at this point.
Because parabola is very close to equal percentage, and the quick opening is only used for on/off control, in fact, there are only two kinds of regulating valves with linear characteristics and equal percentage characteristics as throttling regulation.
For general regulating system, it is hoped that the amplification factor of regulating valves will be smaller at small load to avoid overshoot and oscillation, while it will be larger at large load, so it is advisable to choose to regulate valves with equal percentage characteristics for the system with large load fluctuation.
2. Calculation of regulating valves diameter
For the selection of regulating valve diameter, firstly, calculate the nominal flow capacity C (or Kv) of the regulating valve according to the flow Q required by the production process and the pressure difference ΔP before and after the regulating valve. Then select the regulating valve with the corresponding caliber from the product series. The so-called nominal flow capacity C refers to the flow through the regulating valve every hour, measured in m3/h or t/h. When the regulating valve is fully open, the pressure difference before and after the valve is 0.1MPa and the fluid weight is 1g/cm3. For high viscosity liquid, the viscosity coefficient should be corrected according to Reynolds number. Because it is rarely encountered in small and medium-sized power plants, it can be omitted in this article.
3. Calculation example
Take the hydrophobic regulating valve of the high-pressure heater in a thermal power plant in a high-tech zone as an example. Its technological diameter is D108×4; the medium is water; the maximum flow rate is 22t/h; the medium temperature is 200℃; the working pressure is 1.5MPa; the pressure before the valve is 1.5MPa and the pressure after the valve is 1.0MPa. Water is a general liquid, and its nominal circulation capacity is C = 9.84 so that two-seat regulating valves of DN32 should be selected.
4. Conclusion
In the design of small and medium-sized power plants, the most common one is the selection and caliber calculation of regulating valves for general liquid and superheated steam. It is mainly used in the selection of deaerator pressure and water level regulating valves, high-pressure heater drain regulating valves and condenser makeup regulating valves. When the leakage requirement is very small, single-seat regulating valves should be selected. When the differential pressure at both ends of the regulating valve is large or the tightness requirement is not high, two-seat regulating valves can be used. Usually, the calculated diameter of the regulating valve is smaller than that of the pipeline, so the two ends of the regulating valve need to be equipped with big and small heads.
Next: What Are Water Hammers?
