Presentation on theme: "Chemical Process Safety"— Presentation transcript:
1 Chemical Process Safety Read Chapter 24: Turton’s Design Book(Crowl & Louvar)Chapter 11: Hazard IdentificationChapter 12: Risk AssessmentFor an outstanding safety program we need to prevent the existence of hazards in the first place. This requires a hazard identification method.Guidelines for Hazard Evaluation Procedures, 3rd Ed., CCPS (John Wiley), 2008:5.3; ; ch 9
2 Introduction SAFE PROCESS DEVELOPMENT SyntheticorganicchemistryideasDesktopscreening,databases,calculationsDiscoveryresearch andmultipleexperimentsAutomatedlaboratoryreactors.ProcessoptimizationPilot plantstudiesScale upand designIndustrial production.Debottlenecking.Optimization ofmature processes.Retrofits.SAFE PROCESS DEVELOPMENTMicro and mini scale reactorsAdiabatic calorimetersKinetics,modelling,simulationDesign reappraisal,relief systems, dump and quench tanksDesktopstudiesObjective is to move from the earliest phases of research and development through tofull scale production in a confident, safe and cost effective mannerAutomatedCalorimetersand reactorsHAZOP,HAZAN,HAZIDThe procedures to identify hazards are well-developed in the chemical industry.C H E M I C A L P R O C E S S L I F E C Y C L ED. Crowl, notes
3 This is a flowchart of the overall risk assessment procedure This is a flowchart of the overall risk assessment procedure. It includes hazard identification.Figure 11-1 Hazards identification and risk assessment procedure. (Adapted from Guidelines for Hazards Evaluation Procedures (New York: American Institute of Chemical Engineers, 1985), pp. 1–9.)
4 Process Hazard Analysis – Many Options What-IfChecklistWhat-If/ChecklistFMEA – Failure Mode & Effects AnalysisFTA – Fault Tree AnalysisHazards SurveysHAZOP – Hazards & Operability studyThe checklist is the simplest method. It works better at reminding the engineer of hazards to consider.
5 Process Hazard Analysis – Many Options What-IfChecklistWhat-If/ChecklistFMEA – Failure Mode & Effects AnalysisFTA – Fault Tree AnalysisHazards SurveysHAZOP – Hazards & Operability study
6 Unstructured method for considering results of unexpected events 1. What-If AnalysisUnstructured method for considering results of unexpected eventsUses questions beginning with "what-if“Not concerned with "how" failures occurPurpose is to identify problems that could lead to accidentsResults in a list of potential problem areas and suggested mitigation methodsWhat if…Answer…Recommendations…
7 What-If Example LNG Vaporizer What if: (a) Water flow is stopped? (b) LNG flow is stopped?(c) Natural gas temperature is too low?(d) Water flow is too low?(e) Water pressure is too high?(f) A tube leaks into the shell?(g) Inlet water temperature is too low?D. Crowl, notes
8 What-If Example LNG Vaporizer What-If Consequence/ Hazard RecommendationWater flow is stopped?Water in shell freezes andmay rupture shell; naturalgas temperature too low.Automatic interlock tostop LNG flow if waterflow is stopped.LNG flow is stopped?Not HazardousNoneNatural gas temperature isDownstream piping maybecome embrittled.Monitor gas temperature;low temperature alarm.Water flow is too low?Natural gas temperaturemay be too low; water mayfreeze in tubes.Monitor flow rate; lowtoo low?For the process flow diagram or system shown we perform the what-if analysis and record the analysis using a work sheet as shown.For example we could ask the question what id the water flow stopped on the heat exchanger. The consequence/hazard resulting from this could be that water in the shell side could freeze and even rupture it. Another consequence is that because of the lack of heat exchange the temperature of LNG could be too low than required. To prevent the consequences the team could recommend installation of automatic interlocks to stop LNG flow if the flow of water in the heat exchanger stopped. Similarly we ask other question such as What-if the LNG flow is stopped?, What-if the LNG temperature is too low, What-if the water flowing for heat exchange is too low, and so on. For each of these questions the team brainstorms further to come up with credible consequences/hazards and make appropriate recommendations.Even for such a simple process you could come up with about a hundred what-if analysis about buildings, raw materials, material handling etc. The beauty of this method is that you can be very creative and come up with many types of questions. However it is not used for complex or high hazard processes because this method is not systematic and so its very easy to miss the analysis of certain sections or not think about certain types of issues while we are brainstorming.flow alarm.D. Crowl, notes
9 Can be simple like inventory of hazardous chemicals 6. Hazards SurveysCan be simple like inventory of hazardous chemicalsMore rigorous procedures:- Dow Fire & Explosion Index- Dow Chemical Exposure IndexHazard surveys is a method of PHA which has the inventory of hazardous chemicals. Rigorous procedures in this are the Dow Fire & Explosion Index (Dow FEI)s and Dow Chemical Exposure Index (Dow CEI). Hazards surveys are suitable for identifying hazards associated with equipment design, layout, material storage, and so forth.They are not suitable for identifying hazards resulting from improper operation or upset conditions. On the other hand, this approach is fairly rigorous, requires little experience, is easy to apply, and provides a quick result (Crowl and Louvar).
10 6. Hazards Surveys: Dow Fire & Explosion Index Complex and detailed procedure carried out by an individualRates relative hazards of storing, handling, processing flammableand explosive materialsSystematic approach independent of judgmental factorsBreak the process down into units or sections, e.g. the reactor,storage tank or a pumpUse experience to select the units or sections that have the highestlikelihood of a significant hazard (too many to cover all); may usechecklist approach to chooseDefine the material factor (what chemicals are being used); in general, higherthe value the more flammable / explosiveAdjust this with various penalties based on conditions such as storage abovenormal boiling point, exothermic reaction, etcThen take credits for safety procedures and safety systemsFinally arrive at a number that rates the hazard; compare with table /experienceDow fire and explosion index is a complex and detailed procedure carried out by individuals. It is a method to rate the relative hazards of storing, handling, processing flammable and explosive materials.The first step in the procedure is to conceptually divide the process into separate process units. A process unit is a single pump, a reactor, or a storage tank. The usual approach is to select only the units that experience shows to have the highest likelihood of a hazard. A process safety checklist or hazards survey is frequently used to select the most hazardous units for further analysis.The next step is to determine the material factor (MF) in Figure 10-3 and table 10-1 in the text. In general higher the MF the more flammable/ toxic the chemical.Penalties are given based on condition of the storage. Finally a number or rating of the hazard is determined.
11 6. Hazards Surveys: Dow Fire & Explosion Index Dow Fire & Explosion Index standard form; C&L Fig 11-3PenaltiesMaterial factorPenalty factorsSpecial ProcessHazards FactorMFF1General ProcessThe Dow F&EI form is as shown in the figure. The MF can be acquired from the table and give penalties for base factor, special process hazards to get the process unit hazards factor and F&EI value.F2D. Crowl, notes
12 6. Hazards Surveys: Dow Fire & Explosion Index Dow F&EI - Determining the degree of hazard,Table 11-2F&EI index value Degree of hazard1 – Light61 – Moderate97 – Intermediate128 – Heavy> SevereOnce we get the F&EI index value we can estimate the degree of level of hazard as shown in this table. If you get an FEI index is between and greater than 158, the hazard level should be further studied for proper mitigation measures.All the FEI procedures are available in the complete Dow reference handbook. The Dow index is also useful for determining equipment spacing requirements. The F&EI uses an empirical correlation based entirely on the F&EI value to estimate the radius of exposure.D. Crowl, notes
13 7. Hazard and Operability (HAZOP) Study HAZOP is a Structured "What If" Type of StudyObjectives- Identify Hazards- Identify Operability ProblemsHAZOPs Use Team ApproachMulti-DisciplinaryGuide word basedStructured and SystematicHAZOP is a very structured and exhaustive “What-If” type of PHA study. The objectives of the HAZOP study are to identify hazards and operability problems. For example, if there is a hazard due moisture in the line when chlorine is flowing in the line then in addition to consequences the team also studies whether the line will be operable in the presence of the hazard.HAZOP is an exhaustive PHA method with greater emphasis on brainstorming and analyzing the sections/nodes of the process/system as a team (usually people with much experience/expertise). The team must be multi-disciplinary with mechanics, operators, computer aided draftsmen, etc. The team must have an understanding of performing a HAZOP. It is exhaustive because it is highly structured and systematic wherein various guidewords are used to study possible deviations-consequences and mitigation measures.
14 Hazards and Operability Study Investigative ProcessSelect study nodesMajor process vesselsMajor process lines connected to process vesselsPumps and compressorsHeat exchangersMajor support systemsPick a process parameter- Flow, level, temperature, pressure, volume, pH, concentration, agitation, etcHAZOP study is an exhaustive investigative process where the whole system is divided into study nodes. The nodes could be major process vessels, major lines connected to process vessels, pups and compressors, heat exchangers or major support systems. Once the system is divided into nodes the team chooses a process parameter such as flow, level, temperature and so on for further study of the node.
15 Hazards and Operability Study Investigative Process (Cont’d)Apply guide words to process parametersDetermine deviation from designDetermine consequences of deviationsEvaluate consequencesTypical causes of deviationsHardware failuresHuman errorOutside forcesUnanticipated process stateOnce the process parameter is chosen, the team applies guidewords to process parameters to determine the deviations from the intended design, their consequences and their evaluation to determine the causes of such potential deviations.Typically, deviations are due to: a) hardware failures such as pump failure, valve failure, etc, b) human error such as, say the operator accidentally opens the wrong valve, c) Outside forces – example is say there is a large external fire to cause the failure of pipelines running in the plant, and d) unanticipated process state.
16 Hazards and Operability Study Investigative Process (Cont’d)Suggested actionsChange in designChange in equipmentAlter operating proceduresImprove maintenanceInvestigate furtherHAZOP Follow-upAssign responsibility for carrying out recommendations with agreed timetableRefer recommendations to appropriate managersEvaluate and reviewRecord keepingCopy of all data usedCopy of all working papersHAZOP worksheetsNext the team has to provide suggestions to prevent the consequence or mitigate the potential risk. The team could make a suggestion to change the design, equipment, alter existing operating procedures, improve maintenance or even suggest further study/investigation of the node/process under study if it is too complex.HAZOP team must provide a follow-up procedure as well. The team should assign responsibility for carrying out recommendations within some timeframe. This is important because many a times the team makes valuable suggestion in the light of an existing risk/hazard but, those suggestions are never implemented or implemented in time. This completely defeats the idea of performing a HAZOP study in order to help mitigate risk and improve safety. The best way to ensure HAZOP suggestions are implemented is to refer them to appropriate managers in the plant. For example if the recommendation is to put in a new relief device or a better controller, then these suggestions is best given to the process design manager in the particular plant itself. Once the suggestions are implemented the team should evaluate the system and review it again in the next HAZOP study or whenever some new addition is being made in that particular node.Record keeping is very valuable. All the HAZOP sheets must be recorded and working papers properly documented for future studies or review. Proper documentation will also enable newer HAZOP team members to understand the history or background of the system on which they are to perform HAZOPs in the future. Proper documentation of HAZOP provides a wealth of knowledge to engineers and operators on certain changes made in the plant and why those changes/additional safety devices were implemented.
17 Hazards and Operability Study Guide Words and Their Meanings Simple words or phrases used to qualify the intention and associated parameters in order to discover deviations.When you do the HAZOP it is important to ensure that you cover the entire process plant and also that the deviating scenarios the team considers are realistic deviations or credible deviations. For example if you are looking at pressure for some analysis. Say you have N2 supply which is pressurized and you are looking at the possible ways in which the pressure could exceed 100psi in the line. If the storage pressure in the vessel before the line is only 100 psi then there is no way the pressure in the line could exceed 100psi.On the other hand if you have a storage tank with fluid stored at 500psi and you want the pressure in the line downstream to be 100psi then you place a throttle valve after the vessel on the line to decrease the pressure in the line to 100psi. Now if you are doing an analysis on this node and you say can the pressure in the line be greater than 100psi then now that is a possibility because there is chance of the throttle valve failure. If the throttle valve fails then we can have pressure in the line exceeding the desired 100psi. Now because this scenario is realistic we want to look at the problem and check if the process could cause some other upset, etc. If the deviation causes significant consequence then we need to consider mitigation measures to curtail that by say placing some safety device etc.With that in in mind, here are the different guidewords to use in the HAZOP study along with their meanings, its application to the input material, application to desired activity and further applications (if any).
19 HAZOP ExampleChemistry is such that concentrations of B must not exceed that of AFirst Study Node - pipeline from suction side of pump that delivers A to the reaction vesselFirst Guide Word - No to design intent of transfer ACauses of DeviationSupply tank is emptyPumps fail to runPipeline is fracturedIsolation valve is closedConsequencesExcess of B over A could lead to an explosionRecommendationInstall interlock device on pump B into reactorLets say we have a simple reactor with agitators and we feed chemical A from storage and chemical B from another storage. Both have valving before the reactor. You react these two chemicals and chemical C comes into another storage tank. So if you want to do the HAZOP on this, first get the team and assign the facilitator, the scribe, etc. The facilitator and the scribe come in with some pre-HAZOP work/preparation already done. Once the team meets you choose the node. Pre-HAZOP is description of the process, select nodes, each design intent of node 1, all information of the process chemistry.The team chooses the first node as say, the pipeline from suction side of the pump delivers chemical A to the reaction vessel then, the team uses guidewords on the node to come with various cause of deviation, its respective consequences and appropriate recommendations for each of the consequences. The design intent is such that the concentration of B does not exceed A.So if the first guideword is No to design intent of transfer A then, the causes of deviation is that the supply tank is empty, pumps fail to run, pipeline is fractured or isolation valve is closed.If there is no flow of A into reactor then B will eventually exceed that of A, then it could lead to an explosion. So you must next make recommendation is to install interlock device directly to pump B into reactor. So if there is loss of A then automatically supply of B is shut also.
20 HAZOP Example Worksheet While performing any PHA it is important to properly document the analysis. Here there is an example of the format for HAZOP study. For each node you give a node number and for each guideword, the scribe records the cause-consequence-deviation, safeguards, recommendations. If desired the team could also give a rating for consequence and risk ( R) qualitatively. Any additional comments can also be recorded. Risk ranking is important because it enables the team to scrutinize only those high risk scenarios to implement the recommendations made.D. Crowl, notes
22 Risk Matrix Risk = F x C Consequence Frequency Unacceptable UndesirableRisk =F x CMarginalAcceptableB.K. Vaughen, PSM Overview, SACHE, 2012
23 Risk EquationFrequency How often the event may occur - its likelihood is a “probability”Consequence How severe the event may be - an undesired result of the eventB.K. Vaughen, PSM Overview, SACHE, 2012
24 Operational Discipline The personal commitment of everyone to ensure their personal and process safety by 1) performing their tasks correctly, and 2) recognizing, responding to and seeking help, as needed, to unanticipated situations or conditions.ODB.K. Vaughen, PSM Overview, SACHE, 2012
26 Design Phase: the best time to use ISP Risk ReductionF Frequency Engineering and Administrative ControlsDesign Phase: the best time to use ISPC Consequence Inherently Safer Processes Emergency ResponseB.K. Vaughen, PSM Overview, SACHE, 2012
27 Risk ReductionOD Operational Discipline Safety Culture Organizational OD Safety Behavior and Personal OD Commitment CharacteristicsB.K. Vaughen, PSM Overview, SACHE, 2012
28 Effect of Poor OD on Risk B.K. Vaughen, PSM Overview, SACHE, 2012
30 There is always some level of risk PSM SystemsDesigned to minimize process safety risk:There is always some level of riskOur PSM-related risk reduction efforts are compared and evaluated against other potential business risks (i.e., environmental, operational, maintenance, quality and financial)B.K. Vaughen, PSM Overview, SACHE, 2012