Summary
Identification and analysis of equipment failures, incorrect maintenance and repair, misuse, material failure, human error an so on process hazards including design and assembly faults are fundamental principles of the prevention of process accidents in the process safety management system and all possibilities are started to be evaluated in the design process.
When production is targeted, the process details are not considered and design work is usually started by determining the product specification and production capacity according to the market-customer requirements. The minimum production cost is targeted in the determination of production path.
In addition to operating costs, security measures as well as measures required by legal restrictions takes place among the cost inputs.
The risk assessment conducted during the design phase reveals how best to achieve the safety at every stage of the process life cycle. In the determination of the production route and process flow diagram, the risk assessment of the entire life cycle should be accompanied by the identification of chemicals to be used, equipment selection and planning of construction methods.
Hazard identification and risk assessment studies determine the behavior of process chemicals, and the deviations of the equipment from design target. In the determination of critical equipments by effective hazards analysis method during the design phase will govern the identification of risk control processes.
While the process safety is ensured by the identification and installation, the determination of precautions related with safety can be established by the use of tools such as technical standards to provide legal legislation.
Introduction
Each activity or operation that takes inputs and converts them into an output is called a process. While the output of a process can be the end product, it can also be the input of the next process.
We can call the operations where raw materials are converted into the final product with various energy inputs within the equipment under different process parameter values as production process.
The production processes are the plants in which process based flue gas emissions, liquid and solid wastes are produced in addition to the process parameters risks associated with the properties of the chemicals that are found in the process.
The effects of these risks on the technical operating parameters have an important place in the design basis. These risks are due to the uncertainty and lack of information arising from the variables in the process conditions.
Although it is planned to prevent accidents with protective measures in the design of a chemical process, the occurrence of accidents despite all these measures cannot be completely eliminated. Although the accidents as a definition, neither tolerable nor acceptable, damage should be avoided whenever take place.
Process Safety
The main objective of process safety management is to prevent accidents that could harms employees, other people and the environment or minimize the effects as well as to prevent destructive economic losses.
Other than initiating causes such as natural disasters, Etc. the accidents are caused by the deficiencies in the process security. In preventing accidents; the fundamental principle in the process security management system is to identify and analyze equipment defects, improper maintenance, improper use, material failure, metal fatigue, corrosion, welding defects, human error Etc. including design and assembly failures.
Process safety starts with the design concept and is evaluated in detail in the ongoing design calculations. Design problems are poorly defined and do not offer a single solution. When the production is targeted, the process details are not emphasized and the design work is usually started with the determination of the product specification according to the “market data-customer demands”.
The product specification is one of the most important external constraints that limit the designer’s flexibility of choice. Although the production capacity is determined by a healthy market research, there is no information about the production route.
While the route of production requires a combination of technology and capacity relations, the waste specifications determined by the laws limit the design work as another external constraint.
With the evaluation of the effects of these constraints, the process of engineering calculations will be carried out after the completion of which production path will be used, what the process units will be and the determination of the flow conditions between the working conditions and the equipment.
In this selection, the behavior of process chemicals that limit equipment performances (physicochemical, thermodynamic, phase and kinetic data) and even the limitations of equipment design are introduced as internal constraints.
The main purpose of the design study is to reach the cost of economic production in the creation of a technical operable process flow chart by evaluating all the constraints. It should not be forgotten that inputs affecting the cost of production are not the only costs required to operate the process.
The cost effects of the measures taken to ensure process safety can have variable percentages. Hazard information, such as the combustion trends and toxic roperties of chemicals, are introduced as external constraints in the managing of process safety.
When operating parameters are selected in accordance with these limitations, the process efficiency may be low. In this case, the measures for safety and environmental restrictions with high efficiency values are activated at additional costs.
Establishment of Process Flow Diagrams with Safety Approach
The development of the design of the whole factory involves the evaluation of many different subjects. The economic table generally determines whether the proposed facility will receive management approval.
In the plant design, the implementation of engineering principles in a safe and environmentally acceptable manner is related to whether the previous economic objectives can be met. Designers should consider how best to achieve security at every stage of the process life cycle.
Safe design is defined as the integration of hazard identification and risk assessment methods early in the design process to eliminate or minimize accident risks over the life cycle of the designed process.
In the execution of safe design, the properties of process chemicals and hazard information play an important role as external constraints.
Safe design enforces the identification of chemicals to be used, planning of equipment selection and construction methods in the conceptual stage, as well as the consideration of the requirements in the whole life cycle of the process.
In the process life, in addition to operation and maintenance-repair activities, decommissioning, demolition or disassembly and disposal methods should be taken into consideration in the end of the life cycle.
Safety restrictions for chemicals are important in determining the flow chart and operating conditions. Therefore, the process safety requirements for the whole life cycle need to be evaluated at the stage of creating of process flow charts.
Requirements for safe plant operation, is keeping process variables within known safe operating limits, detatecting dangerous situations, providing alarm and automatic shutdown systems, and establishing connections and alarms to prevent dangerous operating methods.
The risk assesment carried out at the design stage shed lights on how how best to achieve security at every stage of the life cycle of the process in addition to determination the risk control proceses.
The basis of the risk assessment starts with hazard identification. In the definition of the hazards, the chemical properties as well as the process conditions to which they are exposed which may lead to turning to risk are also important.
Possible changes in process conditions, causing to temperature, pressure, pH, etc. deviations within the equipment, can lead to changes in the phase or reaction stoichiometry as well as the sudden changes of the parameters that go out of control.
How the function of an equipment under which conditions and time is also a design output and the realibility of equipments can be defined during the installation phase.
“Functional Security” studies consider the preventation of risk evaluating parameter deviations from design target caused by operator errors and, equipment and hardware malfunctions at the design stage.
Depending on the process operating conditions and the designed equipment, the preliminary idea can be obtained about “Reliability Centered Maintanence” for process reability and “Risk Based Maintanence” activities for preventing troubleshooting can be thougt.
When considering the above points, determination of “Critical Equipment” or equipments is of great importance. The determination of critical equipment leads to a plan for plant layout for the prevention of domino effect.
The potential errors of critical equipment, how often they can occur, and the evaluation of their impacts will shed light in planning the “Risk Based Audit” where necessary control activities are determined in the process life cycle.
The basis for the determination of critical equipment is based on the anticipation of the severity of the potential deviations.
This may require the use of realistic approaches such as the “Dow Fire Index”, which takes into account the simple risk determination methods in which the probabilities of failure are evaluated, as well as the chemical content of the equipment and their properties.
With the Hazard and Operability (HAZOP) hazard analysis to be carried out for critical equipment, process deviations can be determined in a healthy way.
The creation of the “Piping&Instrumentation Diagrams-P&ID” is started in a way including the “Basic Process Control System-BPCS” for eliminating deviations from the parameter values is started. After completing the concept process design, BPCS sufficieny is questioned for process security and additional measures are determined.
Layer of Protection Analysis-LOPA, based on accident senerios identifying the weakneas estimated from a process Hazard Analysis (PHA) from a simplified scan to a rigorous HAZOP engineering study, leads the determination of risk mitigation measures to prevent occurrence of Loss of Contaminent-LOC and mitigation actions in case of accidents occurred at the design stage.
The installation of SIS is based on the principle of the detection of a possible undesired dangerous situation that may occur during any specified operation and the activation of the necessary safety elements to prevent this situation. The P&ID is completed by identifying the SIS elements and their position in the process.
P&ID automatic control circuits; include level, flow, pressure, temperature, speed and etc., and deviation controls from the design values. Suitable control elements (P, PID, fuzzy control, Etc.) and computer control routines etc. for the detection of parameter deviation as a result of detection.
It is carried out by the installation of controllers. The last stage of the design is the use of risk mitigation measures to prevent or reduce the frequency of accidents by eliminating deviations that lead to the loss of integrity, and the determination of mitigation actions in the event of an accident.
Safe Design Tools
In the design of the process units, it is necessary to include many protective measures to minimize the dangers caused by toxic emision, fire, explosion or other involuntary events is necessary.
The compulsory emmisions of materials and waste transport based on process operations and leak emmisions from connections and maintanence and repairs operations Etc. are the basic sources.
However, fire protection or explosion-proof design is generally not technically feasible at all times. In these cases, the planning of measures to reduce the impact is activated.
The legal regulations, application rules and standards to be observed and followed in the life cycle of the process takes a compelsatory role or guide duty for managind the process security.
A wide range of standards has been developed to ensure the safety requirements for the installation, operation and maintenance of industrial plants.
It is started by compilation of information on the dangers of chemicals in the process. Material Safety Data Sheets (MSDS) may not be sufficient for physical properties, toxicity, reactivity and combustion.
In this case, internationally accepted CAS (Chemical Abstract Service), ESIS (European Chemical Substance Information System) and similar systems should be used. While implementation of leagislation on permissible exposure information allows for the control of health risks, the assessment of the uncontroleble deviations and hazardous effects of predictable mixing of different chemicals should be also considered.
Carrying out healty calculations in order to ensure prevent undesirable accidents due to the fire and explosion tendencies are carried out with the relevant accepted standarts.
Conclusions
The safety design carried out with the technical design which is planned as early as possible, considering how to establish safety throughout the entire life cycle. Safe design and installation is achieved by integrating hazard identification and risk assessment methods in the design process.
At this stage, it is important to evaluate the chemical hazard information and process deviations. Reaching healthy risk assessment and access to safe design also requires answering relatively simple questions, as much as many engineering questions.
The road map of risk management are presented at the design stage by determination of system description by “What am I investigating?“; identifying hazard identification and fault conditions by “What can go wrong?”; results and impacts analysis by “How bad?”; frequency estimation by How often?”; risk calculation and risk
assessment by “What if?”. With the contribution of these questions, while determination of measures and installation are ensuring, the needs for the use of technical measures such as technical standards and so on, arises. The use of standards compatible with the system is the strongest aid for risk management.
Prof. Dr. Suna Balcı
Gazi University
Engineering Faculty
Chemical Engineering Department
References
• Lipton, S., Lynch, J., “Health Hazard Control in the Chemical Process Industry,” Wiley, New York, 1987.
• McKinnon, G.P., Tower, K., “Fire Protection Handbook,” National Fire Protection Association, Boston, MA, 1986.
• Peters, M.S., Timmerhaus, Klaus D., West, Ronald E. “Plant Design and Economics for Chemical Engineers,” 5th ed., McGraw-Hill, New York, 2003.
• Seider, W.D., Seader, J.D., Lewin, D.R., Widagdo, S., “Product & Process Design Principles”, 3nd ed., Wiley, New York, 2010.
• Sinnott, R.K. “Coulson & Richardson Series: Chemical Engineering Design”, 6th ed., Butterworth-Heinemann, Oxford, 2005.
• Turton, R., Bailie, R.C., Whiting, W.B., Shaeiwitz, J.A., Bhattacharyya, D., “Analysis, Synthesis, and Design of Chemical Processes”, 4th ed., Pearson, New Jersey, 2013.
