Process simulation and plant safety How Dynamic Simulation Can Be Employed to Help Managing Safety Issues
The safety of industrial plants is the most important concern of everyone related to the process industry. Accidents in chemical plants make headline news, especially when there is loss of life, or the general public is affected in the slightest way. To prevent any possibility that lives of people or property are put to danger, each company is expected to develop and enforce its own practices in the design, installation, testing, and maintenance of safety systems and procedures. Also, the general guidance and documents are produced by governments, industrial groups, and professionals.
Process control and safety
The progress of automation over the last decade and installation of most developed control systems resulted in significantly improved safety environment. When automated procedures replace manual procedures for routine operations, the probability of human errors leading to hazardous situations is lowered. The enhanced capability for presenting information to the process operators in a timely manner and in the most meaningful form increases the operator’s awareness of current conditions in the process. Moreover, as operator training simulators are gaining more and more attention from industry professionals and management, their contribution in improving operators’ level of knowledge and readiness to handle all dangerous scenarios has become enormous.
By looking into a typical industrial unit, process safety can be divided into a few levels. As shown on the picture, process design is representing the core. It is representing all included chemicals, all operating conditions and technological procedures for normal operation, start-ups and shut-downs. Control and safety systems are applied based on process characteristics and technology defined by process design. Physical protection is the last point of defense and should be directed by the procedures and standards defined for the particular unit or section.
How a dynamic simulation contributes to keeping the plants safe
Dynamic simulation is very important in gaining the knowledge about process safety for all mentioned safety topics and can be applied from two different aspects:
- Operator training system (OTS),
- General dynamic simulation.
Certainly, application of OTS has the invaluable role in exploring, analyzing and training of process safety.
Original process design in a form of a mathematical model is connected to original control and safety system to represent the original unit in virtual reality.
Therefore, operators and engineers can train all hazardous situations of their original unit on a daily basis. Moreover, engineers can test and evaluate control strategies of original control and safety system. Also, skills of operators for handling dangerous scenarios can be evaluated.
However, OTS systems are quite expensive and the cost of their implementation cannot always be justified.
But, dynamic simulation can still enhance to level of knowledge and therefore the level of safety during unexpected situations through application of a general dynamic simulation, as can be done using simulation software such as Hysys, Dynsim, Chemcad etc.
The key difference between general dynamic simulation and OTS is that general dynamic simulation will not be connected to original control and safety system; instead, it will be defined in a more general way. It can be replicated to some extent, dependent on the simulation software used, its definitions of process control elements and depth of the user definitions.
So, general dynamic simulation cannot be used to test and analyze the behavior of a control system but it can be used to test, analyze, explore and learn about all safety issues related to process design, processed chemicals, technological characteristics and procedures of the unit.
Behavior and interaction of process variables in the cases of hazardous situations and operating conditions which are far from steady state can be explored to upgrade overall safety condition of the unit.
Examples of scenarios which can be analyzed using general dynamic simulation
When new plants are built, operating and safety procedures related to process design and technology are usually produced by the licensor. However, this might not be always the case and moreover, this type of documentation most often does not exist for older plant.
Therefore, procedures and facts that can be produced from performing dynamic simulation tests should be implemented into overall safety procedures of a particular unit.
Typical examples which can be explored using dynamic simulation and can significantly contribute to internal level of knowledge and standard procedures are:
- Failure of the feed pump: for many chemical, refining and petrochemical processes, this is a dangerous situation, especially if there is a heater or reactor one of the first downstream process units. With this exercise performed using a dynamic simulation tool, behaviors of all downstream process operations is explored, all consequences can be explored and caution measures defined
- Shortage of one reaction component: similar as for the previous example: usually the loss of one reactant of the reaction can cause a dangerous situation. Dynamic simulation should be used to evaluate danger, analyze possible consequences and define caution measures
- Failure of cooling water: loss of the ability for cooling is serious safety issues which can have influence on reactor operation, columns operation and cause serious uncontrolled temperature increase. With dynamic simulation, all risky spots are identified. So that standard procedures can be checked, evaluated and complemented.
Dynamic simulation as the tool for improving safety standards will be applied in a similar way for the cases of:
- Entrance of the unexpected component in feed stream,
- Catalyst deactivation,
- Shortage of one reaction component,
- Failure of cooling water.
Being able to predict all the possible risks is already a halfway to maintaining the safety of industrial plants. Dynamic simulation has an important role in estimating possible consequences and describing the incident outcome. This knowledge should be used to evaluate and continuously improve safety standards and procedures to keep safe impacts on people, environment, and property.