Sep.20.2024
In the dynamic world of fluid control, electric control valves and pumps are key components, coordinating the seamless movement of liquids and gases in various industrial applications. Their symbiotic relationship plays a pivotal role in ensuring the precision, efficiency, and reliability of fluid systems. Let's delve into the world of electric control valves and pumps, unraveling their workings, characteristics, and methods of coordinating their design.
I. Characteristics and Working Principles of Electric Control Valves
Electric control valves are mainly composed of an electric actuator and a control valve body. By receiving signals from industrial automation control systems, they drive the valve to change the cross-sectional area between the valve core and the valve seat, controlling the flow, temperature, pressure, and other process parameters of pipeline media to achieve remote automatic control. The equal percentage characteristic is considered optimal, providing stability and excellent control performance.
(1) Structural Features:
1. The servo amplifier adopts deep dynamic negative feedback to improve automatic control accuracy.
2. The electric actuator comes in various forms, suitable for 4-20mA DC or 0-10mA DC signals.
3. Large adjustable range with an inherent adjustable ratio of 50, featuring linear and equal percentage flow characteristics.
4. Electronic electric control valves can be directly controlled by the current signal without the need for a servo amplifier.
5. The valve body is designed based on fluid mechanics principles, featuring a low-resistance flow channel with a 30% increase in rated flow coefficient.
(2) Classification of Electric Control Valve Structures:
Electric control valves are generally classified into single-seated and double-seated structures. Single-seated electric control valves are suitable for applications with strict leakage requirements, low pressure differentials before and after the valve, and working conditions with a certain viscosity and fibrous media. Double-seated electric control valves have the advantages of low unbalanced force, allowing for large pressure differentials and high flow capacities, making them suitable for applications with less stringent leakage requirements.
(3) Working Principles of Electric Control Valves:
Electric control valves automatically control the valve opening based on signals from the control position, achieving regulation of medium flow, pressure, and liquid level. Using the commonly used 4-20mA current signal as an example, when the control system sends a 4mA signal to the electric control valve, the valve is in the fully closed state. When a 20mA signal is sent, the valve is in the fully open state. Different signal values in the 4-20mA range correspond to different valve opening degrees, allowing the control system to achieve precise regulation based on the specific process parameters.
II. Conditions and Characteristics of Electric Pumps and Applications
Electric pumps, driven by electricity, play a crucial role in various industries. They consist of a pump body, lifting pipe, pump base, submersible motor (including cables), and starting protection device. The pump body is the working part of the submersible pump, composed of an inlet pipe, guide shell, check valve, pump shaft, and impeller. Impellers can be fixed on the shaft in two ways.
The impeller is installed inside the pump casing and securely fastened to the pump shaft. The pump shaft is directly driven by the motor. In the center of the pump casing, there is a liquid suction pipe. Liquid enters the pump through the check valve and suction pipe. The liquid discharge outlet on the pump casing is connected to the discharge pipe.
Before starting the pump, the pump casing is filled with the liquid to be transported. After starting, the impeller is driven to rotate at high speed by the shaft, and the liquid between the blades must also rotate with the impeller. Under the action of centrifugal force, the liquid is thrown from the center of the impeller to the outer edge, gaining energy and leaving the impeller at high speed to enter the spiral pump casing. In the spiral casing, the liquid slows down due to the gradual enlargement of the passage, and part of the kinetic energy is converted into static pressure energy. Finally, it flows into the discharge pipe with higher pressure and is sent to the required location. As the liquid flows from the center to the outer edge of the impeller, a certain vacuum is formed in the center of the impeller. Due to the pressure above the liquid level in the storage tank being greater than the pressure at the pump inlet, liquid is continuously pressed into the impeller. As long as the impeller keeps rotating, liquid will be continuously sucked in and discharged.
(1) Conditions for Use:
1. The temperature should not exceed 20°C.
2. The mass fraction of solid particles in the liquid should not exceed 0.01%.
3. The pH value of the liquid should be between 6.5 and 8.5.
4. The chloride ion content should not exceed 400 milligrams per liter.
5. Frequent switching between "on" and "off" states of the electric pump should be avoided.
(2) Applications:
In the production of the chemical and petroleum industries, raw materials, semi-finished products, and finished products are mostly liquids. The production process of turning raw materials into semi-finished and finished products involves complex processes. Electric pumps play a role in transporting liquids and providing pressure and flow for chemical reactions in these processes. Additionally, electric pumps are used to regulate temperature in many installations.
In the mining and metallurgical industries, electric pumps are also the most widely used equipment. Mines need pumps for drainage, and pumps are used for water supply in processes such as ore dressing, smelting, and rolling.
In the power industry, nuclear power plants require main pumps, secondary pumps, tertiary pumps, and thermal power plants require a large number of boiler feedwater pumps, condensate pumps, oil and gas mixed transport pumps, circulating water pumps, and ash slurry pumps.
In defense construction, pumps are needed for various purposes, such as adjusting the flaps and rudders of aircraft, rotating the turrets of warships and tanks, and controlling the buoyancy of submarines. Pumps may handle high-pressure and radioactive liquids, with some requiring leak-free operation.
In summary, whether it's aircraft, rockets, tanks, submarines, drilling, mining, trains, ships, or daily life, electric pumps are needed everywhere and are in operation everywhere. This is why pumps are classified as general machinery, representing a major product category in the mechanical industry.
III. Coordinating the Design of Electric Control Valves and Pumps
The inherent flow characteristics of control valves indicate how the effective flow area of the valve changes with the opening. Different types, such as quick opening, linear, equal percentage, and parabolic, provide various control responses. In engineering, the first three are most common, and selecting the appropriate valve is crucial for stable control.
(1) Features and Applications:
1. Quick opening feature: Rapid response to valve opening is required in situations where quick changes are needed within a small opening range.
2. Linear characteristics: Constant flow change with opening within the 0-100% opening range, suitable for control loops with a system gain of several, such as liquid level control. Preferred relative opening at normal flow is 50%-60%.
3. Equal percentage characteristics: Small flow rate increase with opening in a small opening, but as the valve opening increases, the change rate increases rapidly. Mainly used in pressure, flow, and temperature control occasions. Preferred relative opening at normal flow is 70%-80%.
(2) Role of Control Valves in Pump Circuits:
A typical pump circuit includes a main flow control valve, branch temperature or flow control valve, and minimum return line control valve.
The main circuit flow control valve adjusts the processing capacity of the pump according to different working conditions. Calculations consider normal operating conditions, maximum operating conditions, and parking conditions for the pump.
The branch temperature or flow control valve meets downstream user requirements and process needs by adjusting the flow rate of the regulating valve installed on each branch.
The minimum return line control valve is installed on the minimum return line of the pump and protects the pump or fulfills backflow requirements when the pump's flow rate reaches the minimum return flow set value.
(3) Process Calculation of Control Valves in Pump Circuits:
All control valves in the pump circuit need to first meet the requirements of the main circuit, calculate the parameters of the control valves on the main circuit, and then calculate the parameters of the control valves on other circuits based on the process parameters of the main circuit at each node. The usual calculation steps are as follows:
1. Determine the main loop according to the system process characteristics. At the maximum flow rate of the pump, based on experience or project requirements, given the pressure drop value of the regulating valve on the main loop, calculate the process parameters of the pump and select an appropriate working curve of the pump.
2. On the main circuit, based on the working curve of the selected pump and the hydraulic equation, calculate the regulating valve parameters under normal working conditions and shutdown conditions.
3. On the main circuit of the pump, establish the hydraulic equation of the sub-circuit of the pump and calculate the process parameters of each regulating valve on the sub-circuit under different working conditions.
4. Establish the hydraulic equation of the minimum return line of the pump and calculate the process parameters of the return control valve under the minimum return flow based on the pump operating curve.
(4) Process Characteristics of Control Valves in Pump Circuits:
For control valves in pump circuits, they usually have the following characteristics:
1. The regulating valve on the main road requires a large change rate of flow rate with opening and usually needs to bear a large pressure drop. Quick opening characteristic valves are preferred.
2. The regulating valve on the branch needs to control the flow more accurately. A valve with equal percentage characteristics is preferred to control the operating range of the regulating valve within a small opening range.
3. The regulating valve on the minimum return line usually has a small flow rate and does not have high requirements for precise flow control. The upstream pressure and pressure difference are large, protecting the pump from damage.
4. Usually, the pressure drop of the regulating valve on the pump circuit will not reach the condition of causing blocked flow. However, for some low vapor pressure situations, careful analysis of physical properties and status before and after the regulating valve is required and should be noted in the regulating valve data sheet.
5. For control valves on the pump circuit, the noise level is generally not high, and noise prevention is not required.
In essence, the coordination of electric control valves and pumps is crucial for achieving optimal performance in fluid control systems. Engineers need to carefully consider the characteristics of the system, pump operating curves, and specific requirements to ensure precise and stable fluid control. As industries evolve, the integration of these components continues to be a cornerstone in achieving operational excellence and reliability in fluid dynamics.
