Selection and Design of Electric Control Valves

 
Electric control valves play a crucial role in automation control systems, directly impacting the regulation effectiveness and overall efficiency of these systems. Therefore, the rational selection and design of electric control valves are essential not only for ensuring the normal operation of the system but also for optimizing energy use and improving work efficiency. This article delves into the technical parameters of electric control valves and the considerations for their design and selection, assisting engineers and designers in making informed decisions in practical applications.

Technical Parameters of Electric Control Valves

 
Understanding the technical parameters is vital when selecting and designing electric control valves. These parameters not only influence the valve's performance but also determine its applicability and effectiveness in specific applications. Below are detailed descriptions of the main technical parameters of electric control valves, providing a basis for subsequent design and selection.

1. Flow Coefficient (Kv Value)

 
The flow coefficient (Kv) is a significant parameter that describes the flow capacity of the valve. It is defined as the flow rate of water (in cubic meters per hour) that passes through the valve with a pressure differential of 1 bar across the valve. The formula for calculating Kv is Kv = Q / √ΔP, where Q is the flow rate and ΔP is the pressure differential across the valve. The flow coefficient Kvs represents the maximum flow rate when the valve is fully open, while the flow rate is zero when the valve is closed. This characteristic makes Kv a crucial reference for selection.

2. Flow Characteristic Curve

 
The flow characteristic curve illustrates the relationship between the percentage opening of the control valve and the flow rate, primarily categorized into ideal flow characteristics and operational flow characteristics.
Ideal Flow Characteristics: Under constant pressure drop, the flow rate has a linear or equal percentage relationship with the opening. This characteristic allows for more precise regulation.
Operational Flow Characteristics: When the pressure drop across the valve changes, the actual flow characteristics may deviate from the ideal curve, affecting regulation performance.
The shape and nature of the flow characteristic curve directly impact the valve's performance under varying conditions, making it essential to analyze according to specific application scenarios during selection.

3. Valve Authority

 
Valve authority refers to the ratio of the pressure drop across the valve when fully open to the pressure drop when fully closed. A lower valve authority indicates poorer regulation capability, which is especially noticeable under high load or harsh conditions. Thus, designing the valve authority appropriately is crucial for ensuring the system's flexible regulation.

4. Adjustable Ratio

 
The adjustable ratio is the ratio of maximum flow to minimum flow, reflecting the valve's regulation range. In practical applications, the flow variation of the control valve should always be within the adjustable range to ensure system stability and safety.

5. Closing Pressure Differential

 
The closing pressure differential refers to the maximum pressure difference across the valve when it is completely closed. If the closing pressure differential exceeds the valve's allowable range, it may lead to equipment damage or system instability. Therefore, measures such as using a series differential pressure control valve must be taken to ensure normal operation.

Design and Selection of Electric Control Valves

 
When designing and selecting electric control valves, it is essential to consider multiple factors to ensure optimal performance and reliability in specific applications. The following are key elements to focus on during the design and selection process.

1. Flow Requirements

 
Determining the heating load of the system is the first step in selecting an electric control valve. Specific considerations include the area of the heating zone, the thermal performance of the building, the type of radiators, and the set temperature of the heating system. By analyzing these factors, the required valve flow can be calculated, leading to the selection of an appropriate Kv value.

2. Pressure Parameters

 
Pressure parameters are crucial for selecting electric control valves. It is necessary to understand the upstream pressure, downstream pressure, and pressure differential. The side flow of a thermal power plant can be determined through the supply and return water temperatures, combined with the pressure loss in the thermal station to ensure the selection meets actual conditions.

3. Valve Characteristics Selection

 
For systems with parabolic thermal exchange characteristics, such as water-to-water heat exchangers, it is advisable to select control valves with equal percentage flow characteristics. This choice ensures a linear relationship between valve opening and heat exchange, enhancing the system's response speed and regulation accuracy.

4. Valve Body Diameter and Actuator Selection

 
Valve Body Diameter: This should be selected based on the calculated Kvs value to ensure that the valve can meet flow requirements in the fully open state, preventing system instability due to insufficient or excessive flow.
Actuator: It is important to ensure that the actuator meets the maximum closing pressure differential requirements of the system to guarantee the safety and reliability of the valve during operation.

5. Calculation and Verification

 
Kv Value Calculation: Calculate the required Kv value based on the heating load and the supply and return temperatures to ensure flow meets demand.
Sample Lookup: In the selection samples for control valves, find Kvs values close to the calculated Kv value and select the corresponding valve body diameter to ensure system performance.
Pressure Differential and Authority Confirmation: Calculate the pressure drop when the valve is fully open and confirm that the valve authority is within the range of 0.25 to 0.3 to meet practical working requirements.
Allowable Pressure Differential and Temperature Check: Verify that the selected valve's allowable pressure differential and operating temperature conform to actual working conditions to ensure long-term stable operation.

Conclusion

 
The selection and design of electric control valves is a complex yet vital process that involves multiple factors, including flow, pressure, and temperature. Through careful design and selection, not only can the regulation performance of the system be enhanced, ensuring stability and safety, but the lifespan of the valves can also be effectively extended. Therefore, engineers must analyze each parameter in depth, integrating site-specific conditions to make scientifically sound decisions. This approach will provide a competitive edge in a challenging market, leading to higher energy efficiency and improved economic benefits.