…That’s an easy answer! It’s because you’re using your ball valve to throttle your flow. Ball valves are designed for one thing: to turn flow off and on. In fact, the ball inside the valve that does the work is optimized so that it doesn’t restrict flow or cause pressure to drop through the valve if it’s open, and stop flow when it’s closed.
By trying to throttle the flow by partially opening the ball, you are adding an abrupt restriction, causing the liquid to accelerate to turbulent levels as it moves around the ball and through the smaller opening. This acceleration and turbulence is the noise that you hear: a beautiful singing ball valve.
Unfortunately, that beautiful song comes with a price. This type of oil flow tends to target the weakest part of your valve and start wearing at seals and balls, causing the valve to become an on- or almost-off valve. A better way to throttle flow would be to use a flow control, needle valve, gate valve or any other device that has an orifice designed to throttle flow without abrupt geometry changes in the flow path. Choosing a product that is designed to throttle flow will stop your ball valve from making noise, and more accurately control your liquid’s flow.
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