RASM Chapter 8

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Information about RASM Chapter 8

Published on July 23, 2014

Author: pthomp

Source: authorstream.com

PowerPoint Presentation: Risk Assessment and Safety Management Hazard and Operability Study (HAZOP) Dr Pauline Thompson PowerPoint Presentation: HAZOP overview Normally performed when a plant/system is first designed Systematic technique for identifying accident scenarios Only hazard identification Normally then combined with quantitative risk assessment Quantitative methods can be called Hazard Analysis (HAZAN ) HAZOP development usually attributed to ICI/Military More commonly used in process safety but increasingly others Team of individuals with different expertise – limitation Brainstorming technique with recorded results Prompts thought about less obvious accident causation mechanisms but time consuming PowerPoint Presentation: HAZOP procedure Continuous Process performed in turn for each part Define INTENTION and then ways deviations could occur Define intention. Determine deviations using application of seven guide words. Determine causes. Brainstorming . Relationship to other plant. Determine consequences. Consequences of deviations deduced by examination of the flowsheet /lateral thinking. Determine hazards. Consequences may be injury to personnel, environmental damage and/or financial loss. Formulate actions. Actions could range from minor modifications to a re-design of the section of plant concerned. Pre-emptive actions not reactions to deviations. PowerPoint Presentation: HAZOP – seven guide words Seven guide words to find deviations from intentions are: No or Not . More . Less . As well as . Part of. Reverse. Other than . PowerPoint Presentation: Application of guide words Can be used a guide word first application Each guide word used for all parameters then move to next Alternative is a parameter first application Each parameter is subjected to all the guide words in turn Guide words are those specified by ICI but other can be used HAZOP is an approach not an official method May change more in other than process industries E.g. could use ‘sooner’ and ‘later’ too HAZARD used loosely in HAZOP Some teams combine hazard identification and consequences into one step PowerPoint Presentation: HAZOP Combinations Guide Word/Parameter Combinations – some more appropriate than others e.g.: No or not. Works with flow but not really meaningful with temperature or pressure More/Less . Works with flow, temperature and pressure, could work with reaction and concentration As well as. Works with contaminant/composition. e.g. rust flakes from corroded pipes/unexpected side reactions Part of. Works with composition - missing stream component. Works with phase to for single phase when two are expected. Reverse . Works with flow, or reaction if it runs in opposite direction to that intended. Not for temperature or pressure. Other than. Intended to cover the unexpected! PowerPoint Presentation: Batch processes Processes that are non-steady state over time May need to change guide deviations such as: A not charged More A charged Less A charged As well as A charged Part of A charged Reverse A charged Other than A charged A charged early A charged late A charged too quickly A charged too slowly PowerPoint Presentation: High Level and Low Level High Level and Low Level HAZOP ‘Low level’ HAZOP - detailed examination of each line and piece of process equipment in turn ‘High level’ HAZOP looks at the plant as a whole. Guide words used may include : Fire Explosion Toxic release Loss of containment High level wont identify root causes as well as a low level one PowerPoint Presentation: HAZOP Example pH balancing tank - part of the wastewater treatment system Wastewater leaving plant must have pH 6-8 Raw waste water pH 3 Mixed with NaOH from resevoir via pump to modify pH Volume adjusted by level controller (LC) pH controller adjusts valve on alkali supply line PowerPoint Presentation: HAZOP example solution INTENTION is to carry treated wastewater at pH 6-8 from the tank to a discharge point, at a flow rate that maintains a constant level of liquid in the tank Results in following table Can be long but OK to avoid repetition by referring to previous lines via code letters Results show possible environmental damage/prosecution PowerPoint Presentation: HAZOP example solution Guide Word Parameter Cause Consequences Hazard Action No or Not Flow No feed to tank. Discharge valve fails shut, faulty level controller. Blockage in pipe. Liquid in tank reaches neutrality, pH controller closes valve on alkali supply line. Tank fills with liquid and overflows. Discharge of wastewater through storm drain system? Pump develops high discharge pressure, possible leakage of alkali. Flooding of local area, slip hazard, lost time during shutdown to solve problem. Possible environmental damage and prosecution. Pump cut-off linked to flow or pressure measurement. Prevent large solids entering wastewater, consider formation of solids by precipitation or agglomeration, clean regularly if scaling a problem. Regular maintenance of valve and controller. High level alarm. Bund around tank. More Flow Increased feed to tank, level controller opens discharge valve to compensate. Discharge valve fails open, faulty level controller. Reduced retention time in tank. pH controller opens valve on alkali supply line. Increased alkali usage. Level falls in tank. Wastewater pH not properly neutralised. None, as long as discharge valve has sufficient capacity to maintain level in tank. None, as long as alkali supply and tank retention time are sufficient for target pH to be achieved. Environmental damage, Possible prosecution. High level alarm to prevent tank overflowing. Emergency bypass system for wastewater? Ensure alkali tank contains sufficient liquid (low level alarm or visible indicator). Regular maintenance of valve and controller. Low level alarm. PowerPoint Presentation: HAZOP problems Pitfalls and problems ‘Brainstorming ’ technique - relies upon knowledge/imagination of the HAZOP team Time consuming technique. HAZOP studies cost money/delays Temptation to overdesign protective devices, with increasing levels of redundancy. Quantitative evaluation will help prioritise Tendency to concentrate on ‘hardware ’/overlook human factors particularly if the HAZOP team is dominated by engineers HAZOP usually based on a Piping and Instrumentation diagram so will not pick up additional risks caused by the proximity of plant items arising from layout. Additional guide words ‘Passing throug’h or ‘Nearby’ may be applied. PowerPoint Presentation: Summary HAZOP is a systematic qualitative team brainstorming technique for identifying potential accident scenarios. Sets out the intentions of the design and then determines deviations using seven guide words. Causes and consequences explored . Actions should be formulated. Seven common guide words are: No or Not/More/Less/As well as/Part of/Reverse/Other than. Guide word or parameter first. A low level HAZOP considers each line and piece of process in turn whereas a high level one looks at the whole system. Example of uses of the guide words in a batch process and a HAZOP of a wastewater treatments system are given. Problems with HAZOP include: lack of team skill, time consuming, excessive safety devices, overlook human aspects, schematic diagrams may miss layout problems etc. PowerPoint Presentation: Discussion Questions Now we recommend: Read through these sections in the notes Tackle the discussion the questions at the end Work with your fellow students on the discussion boards PowerPoint Presentation: Any Questions ? If so – post them in the discussion boards

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