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Working principle of 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 plate 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 the balance valve is used to balance the system’s resistance, it can distribute the new flow rate in accordance with the calculated proportions, ensuring that the flow rates in each branch increase or decrease proportionally, 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 performance; 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 maximum pressure differences, in actual operating conditions, as long as the valve’s authority factor is not 1, there is a large pressure difference across the valve at low opening degrees, while this pressure difference is smaller at high opening degrees. As a result, the value of dG/dC increases at low opening degrees and decreases at high opening degrees, causing the actual operating curve of the valve to shift in the direction of faster opening. The smaller the valve’s authority factor, the greater this shift. For valves with linear characteristics, such shifts in actual performance lead to a reduction in the effective range of opening degrees for 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 vortical regions behind the valve. The pressure in these vortical regions is very low; when this pressure falls below the saturation pressure corresponding to the water temperature, vaporization occurs, leading to steam hammer: 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, creating 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. Furthermore, whenever pressure conditions are not a concern, it is advisable to install the alkaline balance valve on the return water pipe where the water temperature is lower. 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 supply. A self-acting balance valve limits flow by maintaining a constant pressure difference before and after the orifice (with a fixed diameter); therefore, it can also be called a constant-flow valve. A constant flow valve operates on flow rate and is capable of locking in the amount of water flowing through it, rather than focusing on balancing resistance. He is able to address the issue of dynamic imbalance in systems: in order to maintain the efficient operation of individual devices such as refrigerators, boilers, cooling towers, and heat exchangers, it is necessary to keep 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.