Working principles and proper selection of various flow control valves

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Metered charging contributes to macro-level energy savings through three main approaches: Firstly, flow control valves are installed to achieve flow balance, thereby overcoming the issue of uneven temperature distribution； Secondly, through the action of the temperature control valve, the free heat from solar energy, household appliances, lighting, and other devices is utilized ； Third, it has raised the energy-saving awareness among residents who use heat, reducing unnecessary heat loss through things like opening windows. Of these three energy-saving approaches, two are achieved through flow control valves. It is evident how important flow control valves are in heating systems that use metered billing. Therefore, it is very important to know how to properly select and design flow control valves. I. Temperature control valves 1. Structure and working principle of radiator temperature control valves The temperature in a user’s indoor space is controlled through the radiator thermostatic control valve. The radiator thermostatic control valve consists of a thermostatic controller, a flow control valve, and a pair of connectors; the core component of the thermostatic controller is the sensor unit, namely the temperature bulb. The thermal bulb can change its volume in response to changes in the surrounding temperature, which in turn causes the valve element to move, thereby adjusting the amount of water flowing through the radiator and thus altering its cooling capacity. The set temperature of the thermostatic valve can be adjusted manually; it automatically controls and regulates the water flow to the radiator according to the set parameters, thereby achieving the goal of controlling the indoor temperature. Thermostatic valves are generally installed in front of the radiator, and by automatically adjusting the flow rate, they help to achieve the room temperature desired by the residents. Thermostatic valves are divided into two-way thermostatic valves and three-way thermostatic valves. Three-way temperature control valves are mainly used in single-tube systems with bypass pipes; their diversion coefficient can vary within the range of 0 to 100%, offering a large degree of flow regulation. However, they are relatively expensive and have a more complex structure. Two-way temperature control valves are used in some dual-pipe systems and in some single-pipe systems. The two-way temperature control valve used in dual-tube systems has high resistance ； The resistance for single-tube systems is low. The temperature sensing element of the thermostatic valve and the valve body are generally assembled as a single unit, with the temperature sensing element itself serving as the sensor for the indoor temperature at the site. If necessary, a remote temperature sensor can be used ； The remote temperature sensor is placed in the room where temperature control is required, while the valve body is located at a certain point in the heating system. 2. Selection and design of thermostatic valves: Thermostatic valves are the primary devices used for regulating flow in heating systems; other control valves serve only as auxiliary devices. Therefore, thermostatic valves are essential. A heating system cannot be considered a heat metering billing system if it does not have a temperature control valve. In the design of temperature control valves, proper selection is very important. The purpose of selecting a temperature control valve is to determine the KV value (flow coefficient) based on the design flow rate (under a known heat load) and the allowable pressure drop ； Then, the diameter (model) of the temperature control valve is determined by the KV value. Therefore, design catalogs or manufacturer samples must provide the relationship between KV values and diameter; otherwise, it is not convenient for designers to use them. In the selection and design of temperature control valves, it is by no means sufficient to simply choose a valve with the same diameter as the pipeline. Rather, it is necessary to create ideal pressure difference operating conditions for the selected temperature control valve during the selection process. The normal operating pressure difference for a thermostatic valve is between 2 and 3 mH2O, with a maximum of no more than 6 to 10 mH2O. To this end, it is necessary to specify the range of preset values for the temperature control valve in order to prevent noise generation and ensure its proper operation. When there are two or more valve sizes available for the same KV value, it is preferable to choose the valve with the smaller diameter, in order to improve the control performance of the temperature control valve. II. Electric control valve: The electric control valve is a device used for flow regulation in computer-based monitoring systems. It is generally used in unattended heat stations. An electric control valve consists of a valve body, a driving mechanism, and a transmitter. A temperature control valve is a device that performs self-acting flow regulation through a temperature sensor, and it does not require an external power supply ； Electric control valves generally require a single-phase 220V power supply, and are typically used as actuators in computer-based monitoring systems to regulate flow. Electric control valves or temperature control valves are the main devices for flow regulation in heating systems, with all others serving as auxiliary equipment. III. Balance valves: Balance valves are divided into manual balance valves and self-acting balance valves. Whether it is a manual balance valve or a self-acting balance valve, their function is to increase the resistance at the upstream end of the heating system, thereby preventing the actual flow rate from exceeding the designed flow rate ； In other words, its function is to overcome the excess head pressure at the upstream end of the heating system, allowing the electric control valve or temperature control valve to operate under an allowable head pressure. Therefore, both manual balance valves and self-acting balance valves are auxiliary flow control devices for thermostatic valves or electric control valves; yet they are extremely important. If not selected properly or if the design is inadequate, neither the electric control valve nor the thermostatic valve can function properly. 1. Manual balance valve 1.1. Working principle of the manual balance valve: The manual balance valve is adjusted manually once, and it is unable to change its resistance coefficient automatically in response to changes in the system conditions; hence it is called a static balance valve. The manual balance valve acts on resistance; it functions as a manually adjustable orifice to balance the resistance in the piping network system, thereby achieving resistance equilibrium across various circuits. It is capable of addressing the issue of steady-state imbalance in the system: when the operating conditions differ from those designed, the flow rate of water may be higher or lower than the value specified in the design. Since balance valves are used to balance the system’s resistance, they can distribute the new water flow in proportion to the design requirements, ensuring that the flow rate in each branch increases or decreases in a proportional manner, thus still meeting the flow requirements under the current load. 1.2 Issues to consider in the selection and design of manual balance valves (2) (1) The valve’s characteristic curve determines its regulating capabilities; for example, in the case of globe valves, if it is considered that flow rate changes within the range of 95%–100% are insignificant, then a flow rate variation from 0% to 5% already covers the entire range of possible flow rates. Such valves cannot be used for balancing flow rates in hydraulic systems. Since the theoretical characteristic curve of a valve is determined under high pressure differences, in actual operating conditions, as long as the valve’s coefficient is not 1, the pressure difference across the valve is large when it is open to a small degree, while it is smaller when it is open to a large degree. This results in the value of dG/dC increasing at small opening degrees and decreasing at large opening degrees, causing the actual operating curve of the valve to shift in the direction of faster opening. The smaller the valve’s coefficient, the greater this shift. For valves with linear characteristics, such shifts in performance lead to a reduction in the effective range of opening degrees for precise regulation; therefore, it is better to use a characteristic curve that follows a lower chord arc, such as an equal percentage characteristic. For valves with equal percentage characteristic curves, the actual operating curve may approach a linear characteristic when the valve authority is between 0.3 and 0.5. (2) Usually, when a valve is opened to a small degree, the flow velocity through the valve is too high, resulting in vigorous turbulent vortex areas behind the valve. The pressure in these vortex areas is very low; when this pressure drops below the saturation pressure corresponding to the water temperature, vaporization occurs, leading to steam hammering: severe noise, vibration of the valve and pipes, and damage to the valve, pipes, and pipe supports. To prevent such accidents, it is necessary first to consider, in the design of the valve flow channels, the formation of a narrow throttling passage between the valve plug and the valve seat at low opening degrees, thereby restraining the formation of intense turbulent vortices ； Secondly, when selecting valves, try to increase their valve coefficient as much as possible to avoid operating the valves at low opening degrees. Additionally, whenever possible and without involving pressure-related issues, the alkali balance valve should be installed on the return water pipe with lower water temperature. 2. Self-acting balance valve 2.1 Principle of operation of the self-acting balance valve: A self-acting balance valve can automatically achieve flow balance in a system without the need for an external power source. A self-acting balance valve regulates flow by maintaining a constant pressure difference before and after the orifice (with a fixed aperture); therefore, it can also be referred to as a constant-flow valve. A constant flow valve operates on flow rate and is capable of locking the amount of water passing through it, rather than focusing on balancing resistance. He is able to address the issue of dynamic imbalance in systems: to ensure the efficient operation of individual devices such as refrigerators, boilers, cooling towers, and heat exchangers, it is necessary to maintain their flow rates at rated values ； From the perspective of the system’s end points, in order to avoid mutual interference from dynamic adjustments, it is also necessary to limit the flow rate at the end devices or branches. It should be noted in the design that the drawback of self-acting flow control valves is the requirement for a minimum operating pressure difference; typical products require a minimum operating pressure difference of 20 KPa. If such valves are installed in the most unfavorable circuit, it will necessitate an additional head of 2 meters of water column for the circulation pump, so it is advisable to install them near the source rather than at the far end. Such self-acting flow control valves should not be installed when the user is more than 80% of the heating radius away from the heat source. IV. Differential Pressure Control Valve 1. Principle of the differential pressure control valve The principle of the differential pressure control valve is essentially the same as that of a self-acting balance valve. In self-acting balance valves, however, the orifice plate exists as a component within the valve body ； In a differential pressure control valve, there is no orifice plate; instead, the system downstream of the control valve is regarded as an orifice plate. Therefore, the differential pressure value of the control valve actually refers to the pressure difference between the inlet and outlet of the system downstream of it. From the structure of the differential pressure control valve, it can be seen that the purpose of this type of control valve is to maintain a constant pressure difference between the inlet and outlet of the system it is connected to. The basic function is to automatically adjust the operating flow rate of heat users according to their heat load requirements. When a building requires lower room temperatures due to some heat users, the opening degree of the temperature control valves in those rooms decreases. This leads to an increase in the pressure difference across the differential pressure control valve, which exceeds the set value. At this point, the differential pressure control valve automatically reduces the opening of its valve element, thereby increasing the throttling effect and reducing the system’s pressure difference until it returns to the set value. The ultimate result is a reduction in flow rate, enabling adaptation to the heat demands of heavy users and thereby reducing the frequent operation of the temperature control valve. When hot users request an increase in room temperature, the function of the pressure difference control valve is exactly the opposite (3). 2. Issues to consider during design: Some believe that pressure differential control valves should be installed on each household’s system or riser pipes. According to simulation calculations, if balance valves (including manual and self-acting types) or differential pressure control valves are installed at the thermal inlet of the building (with proper design), then within any adjustment range, the pressure difference before and after the indoor temperature control valve will not exceed 6–10 mH2O; thus, the temperature control valve can operate under reasonable conditions. Therefore, installing too many differential pressure control valves is unnecessary and uneconomical. V. Selection of flow control valves when the circulating water pump operates at variable flow rates. This mainly refers to the selection of manual balance valves, self-acting balance valves, and differential pressure control valves. When the circulating water pump operates at variable flow rates, the manual balance valve is out of balance in a proportional manner, which is most favorable for the operation of the temperature control valve ； However, its drawback is that it requires too much manual operation, making it difficult to achieve ideal adjustment. The circulating water pump operates with variable flow, and the ideal set pressure difference at the inlet of each heat user should vary according to outdoor temperatures. In this regard, both self-acting balance valves and differential pressure control valves are not ideal, but regulation loss of control does not occur. Therefore, this type of control valve can be used, which is beneficial for improving the control performance of the heating system.