TABLE OF CONTENTS There are four major practices for starting up a loop that has a large process time constant or slow ramp time compared to the dead time [3-15]:
Loop Practices for Fast Batch and Startup Response
Switch controller to auto with the final set point.
Switch controller to auto with an initial set point and then switch to final set point.
Put controller in manual or output tracking with final set point, set valve to its normal position, wait, and switch to auto.
Put controller in manual or output tracking with final set point, set valve to extreme position, wait, switch valve to normal position, wait, and switch to auto.
Figure 3-5a shows the batch or startup response of a pressure loop that has an integrating response for practices 1, 2, and 4. Practice 3 is not shown because it is not viable for integrating processes. Other practices exist, such as ramping the set point, for unit operations in which it is desirable that the process variable's approach to set point and the output to its final resting value are moderated or that a profile is enforced [3-15].
| Note | The controller output must be positioned beyond the final resting value (balance point) in an integrating process in order to get the CV to move toward set point. |
In the first and second practices, the controller output is at its initial value at one end or the other of the output...
