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The selection and design of electric control valves are very important; whether the right choice is made or not will affect the outcome of system regulation. Furthermore, to address the issue of excessive pressure drop occurring during the operation of electric control valves, a manual control valve or a differential pressure control valve can be connected in series to reduce the operating pressure drop of the electric control valve, thereby maintaining its regulating performance. I. Technical parameters of electric control valves (1) Flow capacity The flow capacity of an electric control valve indicates its ability to allow fluid to pass through it. It is defined as the flow rate through the valve when the pressure difference across the valve is 1 bar, and it is expressed in units of Kv. Kv = Q/ΔP, where Q represents the flow rate through the control valve, measured in m3/h ； ΔP refers to the pressure difference before and after the control valve, measured in bars. When the valve is fully open, the flow capacity is at its maximum, and the Kv value is also at its maximum; this value is referred to as Kvs ； When the valve is closed, the flow capacity is 0. (II) Flow characteristic curve The flow characteristic curve of an electric control valve shows the relationship between the flow rate through the valve and the percentage of the rated stroke, as the rated stroke varies from 0 to 100%. It also reflects the relationship between the relative flow rate and the relative opening degree of the control valve. When the pressure drop across the valve remains constant, the flow characteristic through the valve is referred to as an ideal flow characteristic ； When the pressure drop across the valve changes, the flow characteristic through the valve is referred to as the operating flow characteristic. (III) Valve authority The valve authority of an electric control valve refers to the ratio of the pressure drop across the valve when it is fully open to the pressure drop across the control system when the valve is fully closed. The degree of control of an electric control valve is related to the system’s regulation capability. The smaller the valve authority, the poorer the system’s regulation capability ； The opposite is better. (IV) Adjustable ratio and shut-off pressure difference The adjustable ratio of an electric control valve refers to the ratio between the upper limit flow rate and the lower limit flow rate that can be controlled. During operation, flow variations should remain within the controllable range of the control valve. The shut-off pressure difference is the maximum pressure difference across the valve when it is fully closed. If this shut-off pressure difference exceeds the allowable range, measures should be taken immediately (such as using a pressure difference control valve in series) to bring it back within the normal range. II. Design and Selection of Electric Control Valves (1) Design and Selection Parameters The parameters that need to be considered when designing and selecting electric control valves include flow rate, pressure before the valve, pressure difference, pressure after the valve, and temperature. Firstly, factors such as the heating area within the coverage of the heat station, the thermal insulation performance of buildings, the types of radiators, and the heating temperature in rooms determine the heating load of the heat station ； Secondly, the primary-side flow rate of the heat station can be determined based on the supply and return water temperatures in the primary loop, which in turn allows the flow rate of the control valve to be determined ； Finally, the pressure before the control valve, the pressure difference, or the pressure after the valve can be determined using the water pressure diagram of the primary network in the heating system and the resistance losses in the heat plant; these values must be determined based on the actual conditions of the heating system. (II) Principles for design selection The optimal principle for regulating a heating system is to ensure that there is a linear relationship between the change in the opening degree of the control valve and the change in the heat transfer rate of the heat exchanger. The heat transfer characteristic of the water-to-water heat exchanger in the thermal power station is a rising curve; therefore, equal percentage flow control valves should be selected. Furthermore, in order to ensure regulatory performance in practical applications, the valve coefficient of the control valve should be no less than 0.25~0.3. The bore size of the body of the electric control valve should be selected based on the Kvs value related to its flow capacity, while the actuator chosen must meet the requirements regarding the maximum closing pressure difference. (III) Design and selection calculations: The flow rate of the electric control valve is calculated based on the heating load of the thermal station and the supply and return water temperatures on the primary side ； Determine the pressure drop of the electric control valve based on the water pressure diagram of the pipeline, the resistance in the heat station, and the valve coefficient ； Calculate the required Kv value ； Find the selection guide for electric control valves, select a Kvs value that is greater than the Kv value and belongs to the same category, and choose the diameter of the control valve ； Calculate the pressure drop when the control valve is fully open in practice, and then determine the actual valve coefficient, which should not be less than 0.25~0.3 ； Check the allowable pressure difference and allowable temperature in the selection examples, and select the valve type ； Select an actuator that matches the valve body based on the selected sample, ensure it meets the closing pressure difference requirements, and determine the type of control signal. III. Solutions for Excessive Head Pressure (1) Series connection of manual control valves Manual control valves act as resistance elements; by connecting them in series, it is possible to overcome the excess head pressure provided by the heating system, allowing the electric control valve to operate at an appropriate pressure difference. This ensures that the pressure drop across the valve is greater than 0.25–0.3 times the operating pressure difference, thereby improving the control performance. Strictly speaking, a series-connected manual control valve does not change the control characteristic of the control valve; it merely alters the relative opening degree of the electric control valve during the control process, allowing it to operate within an appropriate range of openings. When the flow rate on the primary side of the thermal plant decreases, it is necessary to adjust the manual control valve in order to reduce the pressure drop at the valve outlet during operation of the electric control valve, thereby keeping its opening within the allowable range. (II) Series-connected differential pressure control valve: If the available head at the thermal power station is too high, a differential pressure control valve can be connected in series to provide a constant differential pressure for the electric control valve. The differential pressure control valve can absorb additional head pressure, ensuring that the electric control valve can operate under stable conditions, free from the effects of changes in the head pressure provided by the heating system as well as from adjustments made by other thermal stations. Due to the many advantages of series differential pressure control valves, they should be installed in series with electric control valves on the primary side of the thermal station.