Process Simulators

Process simulations fall into one of three general categories. The simplest provides only a static simulation while the most complex provides a full dynamic simulation of the process.

Static Simulation

The static simulation is implemented in the control system hardware by connecting the analog output to inputs and connecting motor starter outputs to auxiliary contact inputs. This level of simulation will allow operators to manipulate controller modes, start & stop motors and check the operation of graphic displays, alarms and interlocks. It does not provide the dynamic response of a real operation. Vessel levels and temperatures will make instantaneous changes in response to setpoint changes. The static simulation is adequate for pre-start up check-out of the control system and training operators on the use of the control system, but is of minimum use for process training.

Full Dynamic Simulation

The full dynamic simulation utilizes a dynamic model of the process to provide the real-time feel of the process. The model is based upon energy and mass balance calculations across the entire process including each piece of process equipment. This level of simulation usually requires additional computer hardware to support the model and additional software to interface the model to the operator's graphic displays. Interfacing the model to the control system logic is required for complete checks of the control system logic. Because of the expense of the control system hardware, some full dynamic simulators are implemented as stand-alone systems which simulate the control logic as part of the process simulation. This provides an excellent tool for training the operators on the process dynamics, but provides no checks of the actual control system logic. This level of simulation is usually used for on-going operator training for hazardous operations.

Partial Dynamic Simulation

A partial dynamic simulation utilizes the static simulation method for discrete devices such as motor and solenoid operated valves. First order approximations are used on non-interacting loops such as flows into open vessels and other 1^st order loops of minimal process importance. Energy and mass balances around selected pieces of process equipment are used to simulate the more complex interactions between individual control loops. This provides the same process responses for operator training as the full dynamic simulation. The partial dynamic simulation can usually be accomplished in the control system hardware with minimum additional hardware cost. After initial installation of the control system, the simulator can be loaded into a training system made up from the spare parts on hand. This level of simulation is adequate for process training on non-hazardous process operations only, since the absolute values at any instant in time do not represent an actual balance of the entire process.

Summary

The following table summarizes the strengths and weakness of each level of simulation.

* Assumes use of associated project hardware for pre-startup I/O check-out only.