Safety Practices in Chemical and Nuclear Industries

Transcription

1 Lecture 12 Practices in Chemical and Nuclear Industries Hazard Evaluation Dr. Raghuram Chetty Department of Chemical Engineering Indian Institute of Technology Madras Chennai

2 Procedure Petrochemic al Plant A hazard is defined to be a characteristic of a system, process and plant, that represent a potential for an accident. An accident is defined as an unplanned event or sequence of events that has undesirable consequences. The consequence could be a fire or explosion or release of toxic material that could lead to health effects, economic losses and public relation losses. Effective hazard control requires analysis that is systematic, comprehensive, and sufficiently easy to carry out that it may be done by engineers associated with design and operation of a plant. These procedures should amongst other things be used to identify accidents that could occur infrequently but result in serious injury or loss when they do.

3 Procedure Petrochemical Plant Hazard Evaluation - Review/Audit/Inspection - Process/ System Checklists - Dow & Mond Indices - Hazard Analysis (HAZAN) - HAZOP Risk Assessment - What can go wrong & how? - What are the chances? - Consequences? Extremes - Low probability - Minimal consequences Process / System Hazard identification & evaluation Accident probability Scenario identification Accident consequences Risk determination risk & hazard acceptable? Y Accept system N Modify design

4 Layers of Protection Procedure Focus on hazard identification and control Hazard still exist Reduce the risk Petrochemical Plant Layers of protection increase the complexity of the process, and hence the capital and operating cost.

5 Procedure Centre for Chemical Process (CCPS) of the American Institute of Chemical Engineers under Guidelines for Hazard Evaluation Procedures considers the following as most favor evaluation procedures: Petrochemical Plant 1. Review Methods 2. Process/ System Checklists 3. Relative Ranking Techniques (Dow & Mond Indices)

6 Procedure Petrochemical Plant 3. Preliminary Hazard Analysis (PHA) 4. What if Analysis 5. Hazard and Operability studies (HAZOP) 6. Failure Modes Effects Analysis (FMEA) 7. Fault Tree Analysis (FTA) 8. Event Tree Analysis (ETA) 9. Cause-Consequence Analysis (CCA) 10. Human Reliability Analysis (HRA)

7 Qualitative methods The DOW and MOND methods provide a quick and simple way of estimating risks in process plants. Procedure Petrochemical Plant The procedure employed assigns penalties for those processes or operations that can contribute to an accident and assigns credits to the safety features of the plant that can mitigate the effects of an accident. The penalties and credits are combined into an index that indicates the relative ranking of the plant risk.

8 Qualitative methods Procedure Petrochemical Plant Preliminary Hazard Analysis (PHA) is a general, qualitative study that yields a rough assessment of the potential hazards and means of their rectification within a system. It is called preliminary because it is usually refined through additional studies. PHA contains a brief description of potential hazards in system development, operation or disposal. This method focuses on special attention on sources of energy for the system and on hazardous materials that might adversely affect the system or environment.

9 Qualitative methods Procedure Petrochemical Plant HAZOP (HAzard and OPerability studies) is a procedural tool designed to highlight the deficiency and shortcomings in the design and operation of industrial plants. Benefits: The HAZOP technique is a powerful tool for hazard analysis. Its methodical approach ensures that deviations from design intent are detected and acted upon. Limitations: HAZOP utilises a team approach and hence can not be conducted by a single analyst. The team should be skilled and multidisciplinary with good knowledge of the plant, its intended design and operation.

10 Qualitative methods Procedure Failure Modes and Effects Analysis (FMEA) is methodology for analyzing potential reliability problems early in the development cycle where it is easier to take actions to overcome these issues, thereby enhancing reliability through design. Petrochemical Plant FMEA is a useful qualitative tool for failure analysis and identification and can be used extensively with other hazard identification techniques such as HAZOP and fault tree analysis.

11 Quantitative methods Procedure Petrochemical Plant Fault Tree Analysis (FTA) is a reliability and logicbased methodology. It is used for identifying and analysing the events that could lead to an accident or an undesirable event. Benefits and Limitations: FTA is a very useful tool for studying the routes by which a hazard can occur, although its implementation requires skilled analysts. Furthermore, the quantification of the fault tree depends on the accuracy of the failure data and its availability. Event Tree Analysis (ETA) is one of the logic tree methods for hazard identification. Unlike fault tree analysis, event tree analysis is a "forward thinking" process.

12 Qualitative vs. Quantitative Qualitative Quantitative Procedure Petrochemical Plant The aim is a complete, detailed description. Researcher may only know roughly in advance what he/she is looking for. Recommended during earlier phases of research projects. The design emerges as the study unfolds. Researcher is the data gathering instrument. Data is in the form of words, pictures or objects. The aim is to classify features, count them, and construct statistical models in an attempt to explain what is observed. Researcher knows clearly in advance what he/she is looking for. Recommended during latter phases of research projects. All aspects of the study are carefully designed before data is collected. Researcher uses tools, such as questionnaires or equipment to collect numerical data. Data is in the form of numbers and statistics.

13 Hazard Evaluation Procedure Petrochemical Plant Summary The Hazard Evaluation (HE) techniques can be used to identify different types of hazards within the system components and to propose possible solutions to eliminate the hazards. These procedures are extremely useful in identifying system modes and failures that can contribute to the occurrence of accidents; they should be an integral part of different phases of process development from conceptual design to installation, operation and maintenance. The HE techniques vary in sophistication and scope, and no single one will always be the best.

14 Hazard Evaluation Qualitative methods (finds scenario) Procedure Petrochemical Plant Summary These methods help a multi-disciplinary team (1) identify potential accident scenarios and (2) evaluate the scenario in sufficient detail to make a reasonable judgment of risk. If the team is not satisfied on the risk assessment, a scenario identified in a qualitative hazard review may be further analyzed using one or more of the quantitative techniques.

15 Hazard Evaluation Quantitative methods (can help to decide, how best to lower risk for selected scenario) Procedure These do not identify possible accident scenarios, but they instead aid in risk judgment by providing more detailed, statistical evaluations of the risk of a specific scenario. Petrochemical Plant Summary

16 Hazard Evaluation Procedure Petrochemical Plant Different techniques are used at different stages in the development, design, construction, operation and decommissioning of a process. It goes without saying that the outcome of a hazard evaluation depends on the experience, knowledge and intelligence of the team conducting it. The procedures, of themselves, only provide a logical framework that may be used to uncover hidden system failure modes and indicate how they can be rectified.

17 Hazard Evaluation Procedure Petrochemical Plant The team must have a clear understanding of a system being considered, including its intended function, interactions with personnel, other components and the environment, sources of energy, and materials used and produced. Information regarding operating conditions, materials used, processed (toxic, flammable, explosive) moving parts, electrical aspects should be know to the team. Preparation, modelling, evaluation and documentation.

18 Typical Uses for HE Techniques Review Checklist Relative Ranking PHA What-If HAZOP FMEA FTA ETA CCA HRA R&D Conceptual Design Procedure Pilot Plant Operation Detailed Engineering Petrochemical Construction/Start-up Plant Routine Operation Expansion or Modification Incident Investigation Decommissioning

19 Add-on Knowledge of process Procedure Petrochemical Plant Research Conceptual Flowsheet P ID Detailed eng. Constrution Start up Operation Opportunities for installing add-on safety features Opportunities for installing inherently safer features As a process goes through the phases of lifecycle, such as research and development, design, construction, operation, modification, and finally decommissioning, inherent and added on safety have a varying emphasis. The process development and conceptual design phases give the best opportunities of implementing inherent safety.

20 Add-on Knowledge of process Procedure Research Conceptual Flowsheet P ID Detailed eng. Constrution Start up Operation Opportunities for installing add-on safety features Opportunities for installing inherently safer features Petrochemical Plant The possibility of implementing inherent safety decreases as the design proceeds. Thus the inherent safety characteristics should be evaluated as early as possible to gain benefit.

21 Material Property Data Required for Hazard Identification Procedure Petrochemical Plant Acute /Chronic toxicity Inhalation Oral Dermal Carcinogenicity Mutagenicity Teratogenicity Exposure limits TLV PEL STEL IDLH ERPG Biodegradability Aquatic toxicity Persistence in the environment Odour threshold Physical properties Freezing point Coefficients of expansion Boiling point Solubility Physical properties Vapour pressure Density/ Specific volume Corrosivity / erosivity Heat capacity Specific heat Reactivity Process materials Desired /side/decomposed reactions Kinetics Materials of construction Raw material impurity/contaminants Decomposition products Incompatible chemicals Stability Shock Temperature Light Polymerisation Flammability/ Explosivity LEL/ LEF UEL /UFL Dust explosion parameters Minimum ignition energy Flash point Boiling point Solubility

22 SAFETY REVIEWS Procedure Petrochemical Plant A Review typically involves an inspection often performed by a team, that is meant to identify and evaluate plant hazards. The Review is usually a cooperative effort between plant personnel and the inspection team, and the results usually address major risk situation rather than routine housekeeping and morale problems.

23 SAFETY REVIEWS Procedure Petrochemical Plant The Review is usually initiated with a preparatory phase in which a detailed description of the plant and operating procedures is assembled, together with information regarding materials processed and stored, as well as available records regarding accidents and injuries that have occurred. This may be followed by discussions with plant engineers and operators to clarify problems and fill in missing information.

24 SAFETY REVIEWS Procedure Petrochemical Plant Procedure for periodic testing safety-related equipment, and ensuring proper maintenance, should receive particular attention, as should emergency response plan. The second phase is to identity deficiencies and problem areas, and to develop recommendations for remedial action. The final phase consisting of documenting the results and following up with reviews to ensure that the problems have been dealt with.

25 to a Petrochemical Plant Procedure Petrochemical Plant A petrochemical plant has been operating for 30 years and from a financial viewpoint it is desirable that operation continues for another 15 years. A Review is initiated by management to determine whether plant safety considerations would allow this. The Review team assembles all the available information regarding the original plant, modifications in the last 30 years and plant inspections, interviews with personnel and review of specific safety equipment design, maintenance and inspection procedures. Three major deficiencies are identified.

26 to a Petrochemical Plant Three major deficiencies were identified by Review team: 1. Though the plant has been modified substantially to increase capacity and allow more flexibility in operation, the emergency relief systems, and systems to treat the relieved fluids have not been re-evaluated. The Review therefore recommends that all emergency relief and treatment systems be re-evaluated and modifications made as required.

27 to a Petrochemical Plant 2. The control systems and interlocks date back to the origins of the plant and do not meet current standards. The Review therefore recommends that modern control and interlock systems be designed and installed.

28 to a Petrochemical Plant 3. Equipment layout and spacing do not meet currently accepted industry standards. While some of the problems relate to the propagation of plant fires can be addressed by installing heat-activated sprinkler systems, the most serious arise from the location of the control room- it is too exposed to fire and explosion. The Review therefore recommends some modifications to feedstock storage systems, additional sprinkler systems in certain locations, and construction of a new control room at an appropriate distance from hazardous areas.

29 to a Petrochemical Plant Having said this the Review finds the plant in good condition from the viewpoint of maintenance, corrosion, and general wear and tear. It is concluded that the plant could be operated for another 15 years if the deficiency noted were corrected.

30 PROCESS / SYSTEM CHECKLISTS Checklist Checklists are generally used to indicate compliance with standard procedures and identify common hazards. They are easy to use and can be applied at various stages of a project including commissioning and operation. It is a systematic means of communicating what is required and controlling the development of a project from initial design to plant shutdown. The checklist are usually prepared from prior experience using standard procedures, manuals and a knowledge of the system plant.

31 Process/System Checklists Checklist - Design - Construction - Startups - Operation - Shutdown Design Construction Startups Operation Shutdown

32 RELATIVE RANKING TECHNIQUES Relative ranking technique Methods that may be used to quickly estimate risks in process plants can be useful in identifying process areas, conditions, and materials, that contribute most to the overall hazards of a facility. Alternative can be rapidly explored and the impact of remedial actions evaluated.

33 RELATIVE RANKING TECHNIQUES Relative ranking technique Because process areas can be ranked in terms of the hazards they present, methods which allow this to be done without a detailed risk analysis are called Relative Ranking Techniques. 1. Dow Fire and Explosion Index 2. Mond Index 3. Substance Hazard Index 4. Material Hazard Index 5. Chemical Exposure Index 6. Threshold Planning Index

34 Relative Ranking Techniques Dow Fire and Explosion Index Relative ranking technique Considers factors involving material properties, process conditions, operating characteristics, safety and fire protection systems and other aspects, in arriving at an index, for each process unit, that characterizes the fire and explosion risk, Mond Index An extension of the DOW F&EI to enable a wider range of processes and properties, as well as aspects of toxicity to be covered. Substance Hazard Index This index addresses the risks associated with toxic vapour releases.

35 Relative Ranking Techniques Material Hazard Index Relative ranking technique An index based on material vapour pressure and a level of concern related to toxicity, flammability, explosivity etc., used to determine threshold quantities of materials above which a risk management program is required. Chemical Exposure Index An index, developed by Dow Chemicals, that defines the risk of material releases, based on toxicity, volatility, molecular weight, various process parameters and distance to area of concern.

36 Dow Fire and Explosion Index (FEI) FEI is a leading hazard index methodology recognized by the chemical industry. Relative ranking technique Fire and Explosion Index The Dow FEI is a ranking system that gives a relative index to the risk of individual process units due to potential fires and explosions. It serves as a guide for the selection of fire and explosion protection methods.

37 Dow Fire and Explosion Index (FEI) It assists in determining the spacing between adjacent unit process. Relative ranking technique Fire and Explosion Index It is a guide for insurance agencies to set insurance rates. It ranks individual process units where special safety attention can be focused.

38 What Does the FEI Consider? Relative ranking technique Fire and Explosion Index 1. Six general process hazards. 2. Twelve special process hazards. 3. Nine process control credit factors. 4. Four material isolation credit factors. 5. Nine fire protection credit factors.

39 1. General process hazards Relative ranking technique Fire and Explosion Index Exothermic reactions. Endothermic processes. Material handling and transfer. Enclosed or indoor process units. Access. Drainage and spill control.

40 2. Special process hazards Toxic materials. Relative ranking technique Fire and Explosion Index Sub-atmospheric pressure (<500 mmhg). Operating in or near flammable range. Dust explosion. Pressure. Low temperature. Quantity of flammable/unstable material. Liquids or gases in process. Liquids or gases in storage. Combustible solids in storage.

41 2. Special process hazards Relative ranking technique Corrosion and erosion. Leakage joints and packing. Use of fired equipment. Hot oil heat exchanger system. Rotating equipment. Fire and Explosion Index

42 3. Process Control Credit Factors Relative ranking technique Fire and Explosion Index Emergency power. Cooling. Explosion control. Emergency shutdown. Computer control. Inert gas. Operating instruction procedures. Reactive chemical review. Process hazard analysis.

43 4. Material Isolation Credit Factors Relative ranking technique Remote control valves. Dump or blow down control. Drainage. Interlocks. Fire and Explosion Index

44 5. Fire Protection Credit Factors Relative ranking technique Fire and Explosion Index Leak detection. Structural steel. Fire water supply. Special systems. Sprinkler systems. Water curtains. Foam. Hand extinguishers. Cable protection.

45 What-If Analysis Relative ranking technique Fire and Explosion Index What-If Analysis What-If analysis identifies: hazards, possible accidents, qualitatively evaluates the consequences and determines the adequacy of safety levels. Systematic, but loosely structured, assessment: team of experts brainstorming generate a comprehensive review typically performed by one or more teams with diverse backgrounds and experience Applicable to any activity or system

46 What-If Analysis generates qualitative descriptions of potential problems Relative ranking technique Fire and Explosion Index in the form of questions and responses lists of recommendations for preventing problems What-If Analysis

47 What-If Analysis Example of What-If analysis: Line/Vessel of hydrogen fluoride supply system What If Consequences Level Scenario Comments Relative ranking technique Fire and Explosion Index What-If Analysis the pressure relief valve fails? the operator does not valve off the empty cylinder before removing it? Possible rupture of HF cylinder with personnel exposure to HF and blast effect, possible fatalities. HF release with personnel exposure, possible fatalities. None 1 None 2 Add pressure alarm on operator console Review training records to make sure all staff have been trained in current procedures.

48 Summary Hazard Evaluation Techniques Quantitative Qualitative Review Process / System Checklists Dow Fire and Explosion Index What-If Analysis.