Post-Training Process Analysis
Unfortunately, myriad factors can negatively impact even the most tightly designed processes. Material holdup is one such factor. Devices such as condenser drums, distillation columns and reactors that tend to have significant amounts of material holdup can pinch process flows. Dynamic simulation can effectively assess how these holdups will vary over time and therefore affect overall process stability.
While steady-state simulation can indicate where a process should settle out when controls are adjusted, it can't account for how stable the process will be as it transitions. In contrast, dynamic simulation can predict how the process will behave as it moves from one operating state to another over a wide range of time. This can allow pushing values beyond normal operating parameters to potentially further increase production while staying ahead of process upsets.
Dynamic simulation also can serve as a valuable tool for testing process workarounds, as a new methyl-mercaptan-producing facility discovered. Every time operations personnel there tried to draw water off the H2S-stripper overhead accumulator water boot the resulting sour water (H2S and water mixture) would immediately freeze downstream of the level control valve. So, operators used the custom model provided for training to assess what would happen if they restarted the unit and blocked the water draw from the overhead accumulator. They ran the model for 24 hours to see if the presence of excess water would affect downstream vessels. The model showed that the process remained "on spec" with no adverse reactions downstream.
Simulators tend to fall into disuse after initial training. However, formal ongoing training programs can provide significant benefits.
Analysis indicates that approximately 90% of plant incidents are preventable. And the majority — by some estimates the vast majority — result from the actions or inactions of people. Human beings always will figure in plant operations decision-making; therefore, opportunities will continue for human error to contribute to abnormal situations.
Modern process automation systems have allowed operators to assume responsibility for a larger scope of a plant's operation. However, as systems become more comprehensive, we create the potential to place the operator in an untenable position.
This "paradox of automation" arises because as systems become more complex they become more difficult to operate. One solution is to add more automation — but this increases complexity. Moreover, automation hinders operators' ability to maintain their expertise. The skills lost are precisely the ones most needed when automated systems can't handle a problem and the operator must intervene. That's why it's important to use simulators for training throughout the life of the process, not just at the initial startup phase.
Practical tips for establishing a formal training process include:
1. Identify a simulator "champion." This person should be responsible for developing the actual training curriculum, scheduling operators for training, keeping records and ensuring the simulator is kept up-to-date. The champion should serve as a gatekeeper of sorts for the simulator and should be tasked with identifying opportunities to improve its accuracy. (We'll look more at the champion later.)
2. Develop training programs focused on the most critical processes. This requires prioritizing processes and plant operators for training. So:
• Review and assess riskiest operations (for example, those with largest ripple effects throughout the plant).
.> • Pinpoint processes with advanced control applications. Because some advanced controls essentially run the process, operators have less opportunity to put their knowledge and skills to work. Therefore, it's critical to keep these operators fresh via simulators.
A simulator is just like any other critical subsystem within a plant: the better it's maintained, the more benefits it provides long-term. So, a company should adopt several best practices to maximize its simulation investment.
The biggest reason why simulators fall into disuse after initial training is that companies fail to keep them up-to-date with the actual state of their plants. In other words, changes such as revised control configurations aren't incorporated into the simulator. Thus, operators find the simulators rather useless.
So, it's crucial to implement a change-management system for the simulator. Its golden rule is quite simple: Incorporate all changes in the plant.
The choice of simulator champion is a strategic decision. The champion should be (or report to) a senior-level person within the plant — a senior-level champion may carry more clout in justifying the cost of simulator upkeep and have a better perspective on balancing those costs against others needed to keep the plant running efficiently. The champion should at least have the ear of the plant manager, believe in the role of the simulator, and see the operational and financial benefits the technology reaps.
The champion must ensure the simulator isn't simply thought of as a supporting tool that streamlines operations and helps operators practice their procedures — but is considered an asset that adds value by helping the plant reach production goals while keeping staff and equipment safe.
That person's first critical task is making certain a mechanism is in place for recording all process changes — everything from piping replacement and control configuration alterations to basic control loop tuning. A plant with a change control board should utilize that group to approve changes as well as to ensure they're forwarded to the appropriate simulation engineer. It's best if both champion and simulation engineer sit on the board.
Larger companies may be able to justify having the champion update the simulator. However, this approach requires the champion to possess extensive knowledge of the process and the simulator and be willing to make a longer-term commitment. Smaller companies instead may simply elect to purchase services from their simulation vendors. These arrangements typically call for annual visits specifically to update the simulator based on a running list compiled by the plant's simulator champion.
(In an ideal world, though, sites would streamline this process by validating any changes in the simulator before implementing them in the plant. The benefits here are two-fold: confidence in implementing changes and an up-to-date simulator.)
There's a cost for maintaining a simulator and ongoing training programs. However, it pales against that of unexpected downtime, equipment replacement expenses and lost production from a plant incident. Indeed, in most cases, avoiding a single incident, especially if it carries worker safety consequences, provides a strong economic justification.
Grant Stephenson is an engineering fellow and global simulation architect for Honeywell Process Solutions, London, Ont. Peter Henderson is senior product manager, simulation, for Honeywell in London, Ont. Henry Schindler is a principal consultant for Honeywell Process Solutions in London, Ont. E-mail them at firstname.lastname@example.org, mailto:email@example.com and firstname.lastname@example.org.