The trick is to determine whether the problem is with the valve or is a tuning problem. Step one is to put the controller in the manual mode, then you start making small changes in the output (1/2%) in either direction. Document each change and continue in the same direction until you see a clear process response, continue to make 1/2% steps in the same direction until you see the second process response. Then start to make 1/2% steps in the opposite direction until you see a clear process response, you will have a good picture of how the valve tracks the controller output. If there was more than 1/2% output change between the first process response and the second process response (same direction) the valve exhibits high friction that is influencing your process control. If there is more than 1/2% output change between the second process response and the third process response (change in direction) the valve has hysteresis, slack. Either will cause process control issues and cannot be corrected by retuning the controller. The only option is to repair or replace the valve. There are valve testers that will analyze your valves for performance issues but the process must be down to use them. Benchmark by Black Diamond Engineering is one, Flowscan by Fisher Valve Co. is another.what should i need to check in the Masonielan and fisher valves , if Copntrol valve have hunting problem ? please share your valuable thoughts.
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by Seth Price