What exactly does the “arrow” on the valve mean? Some say it’s the “medium flow direction,” others say it’s the “installation direction”; 90% of people don’t know!
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If you work with valves frequently, you must have seen the prominent arrow on the valve body. Some say it’s the \"medium flow direction,\" others say it’s the \"installation direction,\" and still others think \"it doesn’t matter how it’s installed, as long as it can be turned on and off.\" But I have to tell you: if this arrow is installed in the wrong direction, it can cause the valve to fail in mild cases, and lead to safety accidents in severe cases. Today we’re going to figure out once and for all what the arrows on valves actually mean. 1. The most common scenario: The arrow indicates the direction of flow of the medium. https://mmbiz.qpic.cn/mmbiz_png/ATJTETmyyGHq96oVSzNqQibxhIxy5IJ3PSichM6crCibUMicu02DjlDmNlSTwkPfyss8Y9eicznjAqHw04ThhlicjibMdCQMVPgkVn2bfN8el8krlc/640?wx_fmt=png&from=appmsg&watermark=1#imgIndex=1 For most valves – such as globe valves, check valves, butterfly valves, certain ball valves, control valves, etc. – the arrow on the valve body shows the normal direction of flow of the medium. Why mark this? Because many valves have a one-way structure and can withstand pressure in only one direction. Taking a stop valve as an example, its design generally follows the principle of \"low inlet and high outlet\". The medium enters from the tail of the arrow and exits from the head of the arrow. If installed in the wrong direction, the medium pressure will act directly on the valve disc and the sealing mechanism; this can result in poor valve sealing and severe internal leakage in mild cases, while in more serious cases it can cause abnormal stress on the valve stem, failure of the seals, medium leakage, and even lead to safety accidents. Needless to say, a check valve serves the purpose of allowing flow in only one direction, preventing backflow. The direction of the arrow indicates the direction in which flow is allowed. It was installed backwards, causing the check valve to fail completely and failing to prevent backflow at all.
But some arrows do not indicate the flow direction of the medium. https://mmbiz.qpic.cn/mmbiz_png/ATJTETmyyGGbk31iaE2Jk5HDRLVfVqacOib5CX7mqWhwjKlRkn4G5xu82ZnlRX2NNicno3tAiaeXMybMQbkmaRsTjIUzBiaEZAETFWaP0ibq54Pxc/640?wx_fmt=png&from=appmsg&watermark=1#imgIndex=2 There is a common misconception here: not all arrows on valves represent the flow direction of the medium. You need to pay special attention to the following situations: For some ball valves and gate valves, the arrow may indicate the \"pressure-bearing direction.\" For valves that are theoretically usable in both directions (such as certain floating ball valves and gate valves), some manufacturers still mark an arrow on the valve body to show the direction recommended for bearing pressure – that is, the side where the seal performance is better and the service life is longer. If installed in the wrong direction, the valve can generally still be used, but issues such as a reduced sealing life, increased switching torque, and a higher risk of internal leakage may occur. Safety valve – The arrow indicates the “direction of discharge”. The arrow on the safety valve points in the direction of the discharge outlet, that is, the direction in which the fluid is safely released when overpressure occurs. This direction must not be mistaken under any circumstances; otherwise, the release outlet will be facing people, equipment, or electrical installations, and the consequences in the event of an explosion would be disastrous. Thermostat valve – The arrow indicates the \"flow direction of the medium or the installation direction.\" In most cases, the arrow on the thermostat valve body shows the flow direction of the medium; however, some models have specific requirements regarding the installation orientation (horizontal or vertical), and it is necessary to refer to the instructions for clarification. It should not be assumed simply that the arrow only indicates the installation direction. The arrows on pneumatic/electric actuators indicate the \"on/off position.\" Some valves also have arrows on their actuator bodies, which generally show the valve’s on/off position or degree of opening; these arrows have nothing to do with the flow direction of the medium, and they should not be confused with the arrows on the valve body itself.
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