A control room chart record, see the figures below, shows a test on a process in which a control valve was opened from 10% to 50% of its range and the effect on the measurement, to be used in a control loop with the valve, recorded. The top and bottom of the chart represent upper and lower range limits on the measurement.
Recommend settings (proportional band and reset time) for
- Proportional Controller
- Proportional Integral Controller
Answers to Question
First of all evaluate the dimensionless process gain:
- Steady state deviation of measurement = 90% - 20% = 70%
- Change in input = 50% - 10% = 40%
- Dimensionless gain = 70 / 40 = 1.75
Next estimate the time delay and first order time constant using the graphical method shown below (drawing the steepest slope tangent):
- Delay time = 2
- Time Constant = 2
Therefore the settings are
- Proportional only controller
- PB = 175%
- PI controller
- PB = 194%
- Reset time = 6.6 minutes