(58e) Using a "Level Control" Experiment to Demonstrate Operational Safety Concepts

A level control experiment was modified and used to educate undergraduate chemical engineering students about four aspects of operational safety at chemical processing facilities.  First, students were taught that safety systems should be based on dedicated instruments separate from the basic control system.  For the level experiment, a pressure switch and associated alarms were added to provide an independent measure of the tank level in addition to the pressure transmitter used for level feedback control.  Second, students were taught about safety instrumented systems (SIS) which are used to provide additional layers of protection against incidents.  To simulate a SIS for the level control experiment, logic was added to protect the outlet pump from being damaged in the event that the level tank ran dry during its operation.   The control logic was configured to only activate the relay that supplied the pump with power if the level was high enough to activate the pressure switch.  Third, students were made aware of the difference between an indicated process value and the actual process state.  They were able to observe during the course of the experiment that the level’s percentage full value obtained from the pressure transmitter output was different from the level obtained by visual inspection.  Finally, students were exposed to the possibility that the configured range may be exceeded for a particular process instrument.  The process point used to display the level’s percentage full value was configured to show a “bad value” if the level was out-of-range.  Lack of knowledge of how a level instrument responds to over-range and other process conditions was apparently a contributing factor in the BP Texas City Refinery explosion.