An incident at a gas processing plant centered around the loss of liquid level and breakthrough of a high pressure two-phase mixture into low pressure equipment (HP/LP Interface) leading to cold temperatures and subsequent brittle failure of a vessel. The case team needed to confirm or rule out operator actions that could have led to the incident. The individual events and actions that lead to the incident happened over a period of several hours. The process was so complex that careful study by many experts was unable to clearly identify the cause-and-effect of each of the events and actions.
To study the events that lead to the incident, a dynamic computer simulation was created by ioMosaic for an entire process. The events and actions to be simulated included: feed rate changes, pump outages and restarts, and controller set point changes. The model outputs (thick lines) are compared to the actual measured flows (thin lines). To match the measured flows, controllers were set using the process simulators event scheduler.
The process consisted of two parallel reboiled absorber columns with an absorber solvent recovery system. The solvent recovery system consisted of three columns, two reboiled strippers and one fractionation column. The process was highly energy integrated and had six major recycle loops. To insure the validity of the model, the first step was to construct models of each of the individual columns. The steady-state operation of four of the five columns had been studied. The data from the studies was used to validate the operation of each column in a steady-state mode before even attempting the dynamic simulation. When the individual steady-state column models had been validated, a dynamic model was constructed using HYSYS dynamics.
Because of the extensive heat recovery, reboilers on two of the columns had to be split out as separate unit operations. They could not be included in a “column” simulation and solved directly using the normal techniques. That made the initial convergence to a starting (steady-state) solution a little difficult. But once convergence of the initial solution was achieved, the conversion to a dynamic simulation continued in a straight forward manner. Modeling showed the dynamic response of one of the reboilers. The simulation was started from a steady-state condition at about 11:00 pm. The dynamic and steady-state simulators are identical and some time (about two hours in this case) was required to obtain a “stable” dynamic solution after the dynamic simulation was initiated.
One event of particular interest was overfilling in the sumps of the parallel absorbers. The sumps had to be included as separate unit operations to be able to closely monitor them during the dynamic simulation. The temperature, pressure and level in the absorber sumps were part of the crucial output from the dynamic model. Modeling showed the carryover from the sumps of each absorber and the effects on the feed tray (the tray just above the sump). The impact of the carryover showed up as large temperature and level excursions.
Overfilling in the absorber sumps was directly tied to the capacity of a particular heat exchanger. Therefore, one of the crucial things to simulate was a fouled heat exchanger and the temperature controller tied to it. The temperature controller was a complex computer control, not a standard DCS. A special program was written and incorporated into the commercial process simulator to simulate the controller action. In addition, there were six pressure controllers, 11 flow controllers, 10 level controllers and nine temperature controllers included in the simulation. They were each tuned to simulate the process operation. The computer model was validated against actual controller traces and strip chart recordings of temperature, pressure and flow from the time leading up to the incident.
The validated dynamic model, prepared by ioMosaic and delivered to the client, enabled the case team to reliably and quickly confirm or rule out specific scenarios and/or combinations of operator actions that could have led to the incident.