Emerson Exchange 365

Hello.

I started a new thread to for opinions or recommendations for the following Level Application:

Existing Setup

The design is currently setup like:

Level Controller maintain levels of dry pellets with level transmitter and valve below tank. DCS is setup to open and close valve at predefined open % and close %, (lets say 40% open and 10% close) for time period (seconds) based on a time calculation from the PID output. We want this setup to keep the valve opening and closing so pellets want get plugged. For example, based on the PID (in AUTO) output the valve may be open for at 40% for 60 seconds, and close fat 10% for 15 seconds.

The current PID is setup for PID structure, Gain 2.2, reset 1.5 seconds. Level is currently being maintained (PV equal SP) but PID output oscillates between 15% (for about 30 minutes) valve at 10%. Then PID output goes to 45% (for about 20 minutes) valve at 40%. Even though level is controlling, the repeated open and close of the valve causes pellets to build up and then dump out which effects issues downstream..

For two days, i have tried to tune this loop using Ondemand Tuning but because its intergrating level it takes a longggg time to get a process gain response time even with a 10 step size. Maybe I am not waiting long enough for the Ondemand tuning to finish. I know its level and its a while. But i believe this opening and closing time of the valve makes its very complex to tune the level like a normally level feedback control. 

If I can get this loop tuned to decrease the opening and closing time, it would be great. 

Is it possible to tune this loop with the existing configuration? I am still researching why we even need the open and closing times, and if possible to just set up like a regular level feedback controller.

Thank you