With Siemens control valves, what impact does a large change in pressure within the pipeline have on the valve’s operation?
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If a control valve is equipped with a Siemens positioner, and the positioner is tuned when the pipeline is empty, will there be any impact on the valve’s operation when a fluid is present in the pipeline and the pressure is high? Will the output air pressure of the locator change?
It’s not a problem related to the absolute pressure inside the pipeline; it’s a matter of the pressure difference before and after the valve. Since the area of the medium in front of the valve that the valve core faces is different from the area of the medium behind the valve, different pressure differences result. Even when the valve is in the same position, the air pressure output by the actuator varies. For air-actuated shut-off valves, the greater the pressure difference, the higher the output air pressure. Of course, this is on the condition that the locator is equipped with a balance rod for the valve position. A positioner without a balance rod doesn’t take into account whether the actual valve position is correct or not; the same air pressure output can correspond to different actual valve positions.