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Flow open and flow closed refer to the direction of fluid flow; it is defined as follows: at the throttle port, when the fluid flows in the direction in which the valve is open (that is, in the same direction as the valve’s opening direction), it is called flow open； Conversely, when flowing in the direction of valve closure (i.e., in the same direction as the valve closing), it is called flow closure. If the diagram shows a valve that allows flow, it is a flow-open valve; otherwise, it is a flow-close valve. The flow-open type is more stable; there is little low-pressure issue. The flow-seal type has better sealing properties and exhibits a negative suction effect, which enhances the resistance of the sealing surface to erosion. Generally, the flow-seal type lasts longer than the flow-open type. Usually, flow-open valves are used in control valves because they require less torque to open, and the packing is not subjected to pressure when the valve is closed. In special cases, such as when there is flashing, flow shutdown should be used. HG/T 20507-2000 Specifications for the Selection and Design of Automation Instruments: (1) For DN 20, the flow direction shall be determined based on the requirement of good stability. (2) For angular valves used with high-viscosity media containing solid particles where good \"self-cleaning\" properties are required, a flow-blocking type should be selected. (3) For single-seat valves and low-flow control valves, a flow-open type is generally selected; when scouring is severe, a flow-close type can be used. (4) Single-seal sleeve valves generally adopt the flow-open type; when self-cleaning is required, the flow-close type can be used. (5) For two-position control valves [single-seat valves, angle valves, sleeve valves, with rapid opening flow characteristics], a flow-closing type should be selected; when water hammer or surging occurs, a flow-opening type should be chosen instead. Selection of the flow direction for control valves: I. Requirements for flow direction: The flow direction can be selected for single-seat valves, angle valves, low-flow valves, high-pressure valves, etc. The flow direction cannot be selected for three-way valves, Venturi angle valves, cage valves, double-seat valves, V-ball valves, eccentric rotary valves, etc. There are no requirements regarding the flow direction for ball valves, disc valves, etc. II. Flow direction selection: Stability – Both flow-open and flow-closed are acceptable. Lifetime – Flow-closed. Self-cleaning – Flow-closed. Sealing – Flow-closed. Output force – Flow-closed. Operating speed: Flow-open for flash evaporation; flow-closed for high-pressure valves, valves with a pressure difference of less than 20 mm, high pressures, large pressure differences, and severe cavitation erosion. *Angle valves: Flow-closed is preferred for high-viscosity fluids, those containing suspended solids or solids, as it offers better self-cleaning properties. *Globe valves are usually chosen for flow-open operation. *Valves for low flow rates are generally selected for flow-open operation. *Anti-surge valves are typically used in flow-open mode. *For valves selected to operate in flow-closed mode, it is important to ensure that the minimum opening degree is between 20–30%, with equal percentage characteristics. Why is a flow opening used for flashing? Our valves are generally designed to allow flow in one direction; only the labyrinth valve is used when handling liquid media such as methanol in a flow-blocking mode. This is done so that, during flashing, the fluid flows against each other within the labyrinth structure, thereby dissipating most of the energy and preventing flashing from damaging the valve core and pipes.