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The flow direction of a control valve is the direction indicated by the arrow on the valve body. Changhui Instruments reminds users that when installing control valves, it is necessary to ensure that the direction of the arrow on the valve body 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. http://yunrun.com.cn/upload/201809/13/201809132224564572.png Control valves are divided into two types based on the direction of flow of their medium: \"flow-open type\" and \"flow-close type\". When the flow direction of the control valve changes, the forces acting on the valve core also change. For example, the pressure in front of the valve acts on the valve core, exerting a force that tends to push it open; this is known as the \"flow-open type\"” ； Conversely, there is a force that presses the valve core against the valve seat, causing the valve to tend to close, which is known as the \"flow-closing type\". In terms of the flow direction around the valve core caused by the medium, a \"flow-open\" medium causes flow from the smaller end of the valve core toward the larger end, while a \"flow-close\" medium causes flow from the larger end of the valve core toward the smaller end. The impact of the flow direction of a control valve on its performance is shown in the table below: Table of the Impact of Control Valve Flow Direction on Its Performance. Flow Direction, Control Valve Performance: Stability, Service Life, Flow Characteristics, Capacity, Allowable Pressure Difference, Stem Sealing, Self-cleaning Ability. For the flow-open type, performance is good, service life is long, characteristics are standard, allowable pressure difference is small, sealing performance is good, and self-cleaning ability is also good; however, the stability is poor. For the flow-close type, performance is poor, service life is short, characteristics change, capacity is reduced, allowable pressure difference is large, sealing performance is poor, and self-cleaning ability is also poor. Below are some specific applications where changing the flow direction of a control valve is useful: In small-diameter, high-pressure valves, changing from a flow-open type to a flow-close type can **extend the service life**. This is because in the flow-open type, the fluid flows in the open direction, causing cavitation and erosion mainly on the sealing surfaces, which leads to rapid damage to the sealing surfaces at the base of the valve element and between the valve element and its seat ； In the case of a flow-blocking medium, the flow moves in the direction toward closure; cavitation and erosion occur after throttling, below the valve seat sealing surface, which protects the sealing surface and the root of the valve element, 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, it is possible to change the operating mode of the control valve from flow-through to flow-blocking, which allows the valve to handle 10%-15% more flow. http://yunrun.com.cn/upload/201605/29/201605292111332920.jpg How to choose the flow direction of a control valve? “As can be seen from the \"Table of the Impact of Control Valve Flow Direction on Performance,\" both the flow-open and flow-close orientations have their advantages and disadvantages; therefore, it is essential to choose the control valve flow direction correctly. The selection should be made based on the actual operating conditions. As many devices require the control valve to be able 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 service life, it is advisable to choose the \"flow-blocking type\", as this can increase the valve’s service life. In applications where strict requirements are placed on valve stem sealing, to prevent leakage, a \"flow-open type\" should be selected. For suspensions, highly viscous media, and those containing solid particles, a \"flow-closing type\" should be selected to prevent sedimentation and blockages. For the same type of control valve, the flow direction is not necessarily the same in all cases. For example, for small-diameter high-pressure valves that operate under high pressure differences, suffer from severe cavitation, and have a short lifespan, a \"flow-closing\" type should be chosen. However, for larger-diameter high-pressure valves where the operating pressure is high but the pressure difference is not too great, it is more appropriate to use 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 changed 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 when operating at low opening degrees, it is necessary to keep the minimum opening degree of the valve at 30%-40%. 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 flow direction of a control valve will alter its capacity and flow characteristics, which inevitably has an impact on the control system; therefore, when changing the valve’s flow direction, appropriate consideration should also be given to the issues mentioned above. Source: Changhui Instruments http://yunrun.com.cn/