Control valve hunting problem 2

[raingal79]

Hi all,

I have an issue with control valve controlling level in upstream vessel. Deviation of pv value from sv value is approximately 2% and yet the valve output (mv) hunts from 0 to 100%. Is this a valve mechanical problem or controller setting problem (PID control)?

Thanks.

 

[hacksaw]

controller settings,

reduce the gain, turnoff derivative, and increase the integration time setting.

make sure that autotune is switched off

 

[raingal79]

Thanks for the reply. As I understand controller setting helps to bring pv closer to sv and how does it impact the mv of valve then?

 

[smckennz]

Hi Raingal79
it takes too long to explain. If you do what Hacksaw says, and then gradually decrease integral time, then slowly increase gain, you will inwardly learn more than any book can teach you.
Leave derivative alone (off) at this stage.
Then google Ziegler Nicholls, and I am sure you will find a good description of how it all works, and can then adjust in a more scientific manner.

Cheers

Steve

 

[khan101]

There are few more things you can ask your instrument guy to look for;
1)If the valve is equipped with a positioner, check for positioner gain. It may be too high. Or you can simply by-pass the positioner to eliminate one possibility.
2) Do a travel check and see if your valve is sticking. This may be due to stiff packing.
3) See if your air supply pressure is adequately set.
4) If you are using 1/4 inch tube, replace it with 3/8 inch.
5) Put your control in manual and see if hunting is gone.
6) See if your control is too tight.
Then go for controller tuning.