how to replace the butterfly valve
At the chemical company the Butterfly Valve was used to control the bottom level of a distillation column. The distillation column was the last one in a train of three columns, of which each column had a progressively smaller diameter. Moderate increases in feed rate to the first column easily caused high-level alarms when they propagated to the small final column. The level controller originally seemed to be responding too slowly to handle these upsets, so the loop tuner increased the controller gain to achieve fast response at high flow rates. However, at normal flow rates, where the process gain was 15 times higher, the loop was unstable and oscillated continuously.
The correct solution to this problem would have been to replace the butterfly valve with a Control Valve that has a linear flow characteristic and then retune the control loop. However, this could only be done during the plant’s annual maintenance shutdown. In the mean time we installed a characterizer to linearize the Butterfly Valve . The characterizer compensated for the butterfly valve’s nonlinearity and made the flow through the valve follow the controller output in a reasonably linear fashion.
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