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The flow direction of a control valve is indicated by the arrow on its body; when installing such a valve, it is necessary to ensure that the direction of the arrow matches the flow direction of the medium being controlled. However, due to differences in production sites, the requirements for using control valves vary as well. Practice has shown that the flow direction of a control valve affects its operating characteristics and directly impacts whether the control system can function properly. Control valves are divided into two types, “flow-open type” and “flow-close type”, depending on the flow direction of the medium. When the flow direction changes, the forces acting on the valve spool also change. For example, the pressure in front of the valve acts on the spool, exerting a force that tends to push the spool open, resulting in a \"flow-open type\" configuration” ； Conversely, there is a force that presses the valve core against the valve seat, causing the valve to tend to close, resulting in a \"flow-blocking type\". From the perspective of the flow direction around the valve core, a \"flow-opening\" medium flows from the smaller end of the valve core to the larger end, while a \"flow-closing\" medium flows from the larger end to the smaller end; the flow direction of the control valve affects its performance. For small-diameter high-pressure valves, changing the flow pattern from open-flow to closed-flow can **extend their service life. This is because in the open-flow pattern, the medium flows in an outward direction; cavitation and erosion mainly affect the sealing surfaces, causing rapid damage to the sealing surfaces at the base of the valve core as well as those on the valve seat ； In the case of a flow-closed type medium, the flow moves in a closed direction; cavitation and erosion occur after throttling, below the valve seat sealing surface, which protects the sealing surface and the root of the valve stem, thereby extending the service life of the control valve. Sometimes, due to an increase in production or errors in calculation or selection that result in a low flow coefficient of the control valve, the required flow rate cannot be met even with the valve fully open. In such cases, the flow pattern can be changed from open to closed, which allows the control valve to handle 10% to 15% more flow. Both the flow-open and flow-closed types have their respective advantages and disadvantages; therefore, it is essential to choose the correct flow direction for the control valve. The selection should be made based on the actual operating conditions. As many devices require the control valve to completely shut off the working medium, in which case the valve’s sealing performance becomes a key requirement; therefore, a \"flow-blocking type\" valve should be selected. If the selected valve is a single-seat valve, the flow direction is exactly opposite to the arrow indicated on the valve body. For angular high-pressure valves that suffer from severe cavitation and have a short lifespan, it is advisable to choose the \"flow-blocking type\" variant, as this can increase the valve’s service life. In applications where strict requirements are placed on valve stem sealing, to prevent leaks, a \"flow-open type\" should be selected. For suspensions, highly viscous media, and those containing solid particles, a \"flow-sealing type\" should be selected to prevent leakage and clogging. For the same type of valve, the flow direction is not necessarily the same. For example, in the case of small-diameter high-pressure valves, which operate under high pressure differences, suffer from severe cavitation, and have a short lifespan, it is advisable to choose a \"flow-closing\" type. However, for larger-diameter high-pressure valves where the operating pressure is high but the pressure difference is not too great, it is better to opt for a \"flow-opening\" type, as this type offers better stability and eliminates the need for measures to prevent valve oscillation. Since the flow capacity of the \"flow-closing type\" is 10%–15% higher than that of the \"flow-opening type,\" when the full-open flow rate of a control valve installed in the \"flow-opening type\" direction still does not meet the process requirements, it can be switched to the \"flow-closing type\" to temporarily resolve the issue. Due to the poor stability of the \"flow-closed type,\" stability becomes a major issue when this type is used. Since the valve tends to oscillate at low opening degrees, the minimum opening degree of the valve should be kept at 30%–40% as much as possible. An actuator with high spring stiffness should be selected, and it should be used in conjunction with a valve positioner if necessary. Any change in the valve’s flow direction will alter its capacity and flow characteristics, which inevitably affects the control system; therefore, when changing the valve’s flow direction, appropriate consideration should also be given to the issues mentioned above.