Greg McMillan and Stan Weiner bring their wits and more than 66 years of process control experience to bear on your questions, comments, and problems. Write to them at email@example.com.
Stan: The operator is the most underutilized resource in the plant. I think most operators would appreciate a greater understanding of the process and playing a bigger role in improving its performance. This is not to say operators don’t already do a tremendous job in dealing with the inevitable unknowns and problems to keep a plant running nonstop. Downtime in many plants can amount to millions of dollars per day in lost production.
Greg: When we did opportunity assessments, we found the production units at one plant consistently out performed the units at several other plants. The difference was that the operators knew the practical limitations to production better than the technical support engineers and were the initiators of most of the ideas for process control improvement. If the people on the front line who have to resolve problems on a minute-to-minute basis have an understanding of the process relationships, the result can be truly remarkable. The knowledge developed can be put into the automation system. Advanced control is, after all, the embedding of process intelligence.
Stan: The key to unleashing the true capability of a plant is the operator training system (OTS). Most companies realize an OTS is essential for getting the operators to make maximum use of an upgraded instrumentation and control system. The more astute companies realize it offers an ongoing role for exploring and understanding problems and capturing and disseminating knowledge, not only to operations, but also to technical and maintenance support functions. Probably the least recognized opportunity is getting maintenance and operations on the same page. As we said in the March 2010 column, the first question asked when production changes, is what maintenance was done.
Greg: To maximize the performance and benefits of an OTS, we continue our discussion with the president of Mynah, Mart Berutti.
Stan: What are the job functions and skills of people who build and deploy an OTS?
Mart: Operator training systems require process simulations that are dynamic and real-time. Because the purpose of both OTS and testing and system acceptance testing (SAT) is to provide realistic responses at the operator glass, the control system platform is very important in the overall performance of the system. We find that the best developers of these systems are process control engineers that understand the process and process dynamics. If they have advanced control background, they are often very good candidates for developing dynamic simulations. Process design engineers who have an extensive steady-state modeling background sometimes have difficulty understanding the cause and effect and driving forces of a dynamic system. Of course, plant operations involvement is also essential. In many cases, the most experienced operators and operations supervisors can best dictate the use cases and acceptable performance for the dynamic simulator used in the SAT and OTS.
Greg: What type of simulation building environment do your customers find most useful?
Mart: Since we are working with control system engineers more so than process design engineers, we like to use IE1131 programming languages such as function blocks and structured text. This allows the control system designer to make the transition to dynamic simulation developer without learning a completely new configuration environment. The only new paradigm that the user needs to adopt is the use of process equipment objects in the IEC1131 function block environment. In addition, process equipment objects are not connected with wires carrying signal values, but with streams conveying dynamic process information (pressure, flow, temperature, density and composition).
Stan: What are the relative advantages and disadvantages of various methods of communication between the model and the DCS?
Mart: Most offline control systems have an OPC server or Modbus TCP/IP or Ethernet/IP slave interface. The dynamic simulation system needs to have an integrated OPC client or Modbus TCP/IP Master or an Ethernet/IP Scanner. Ideally, the simulator will have all three options, and an I/O service that runs independently of the simulation engine. This allows the user to integrate I/O by tag name and not by the DCS I/O path. Utilities should allow the user to generate the dynamic simulator’s I/O definition and low-level models, such as tiebacks, automatically so that the I/O definition in the dynamic simulation matches the distributed control system automatically
Stan: Do you run your models stand-alone before the configuration is ready, and if so, what control loops do you put in place, and how do you initialize the DCS loops?
Mart: So the OTS simulation can be developed in parallel with the DCS configuration, we run the dynamic model by including the basic control loops in the simulation via IEC1131 control blocks in our library. In order for volumes not to overflow or run dry and for pressures to be in the operating range, the level and pressure loops are immediately put in Auto. Next the temperature loops are put in Auto because these loops are often the key to getting the composition right, in addition to regulating the energy balance. With these loops, the model can be fully explored, tested and documented by a library of operating conditions captured by snapshots. When the configuration is ready, the control can be readily transferred to the actual distributed control system, and a new library of snapshots created for restoring and resuming operating scenarios.