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Hazard Operability Analysis Politecnico di Milano Dipartimento di Energia

HAZOP Qualitative Deductive (search for causes) Inductive (consequence analysis) AIM: Identification of possible process anomalies and their associated causes and consequences

HAZOP Initially developed to analyze chemical process systems; later extended to complex operations and other types of systems (e.g., software) It is a qualitative, structured and systematic examination of a planned or existing process or operation in order to identify and evaluate problems that may represent risks to personnel or equipment, or prevent efficient operation Deductive (search for causes) Inductive (consequence analysis)

HAZOP: When? Impact of results on Design Completeness of HAZOP study HAZOP as a final check upon detailed design Design Phase HAZOP study: better later than never. It may also be performed on an existing facility (improvement of the operability, risk reduction)

HAZOP: Who? HAZOP team members (multidisciplinary): Team Leader (HAZOP experienced) Project engineers Process engineers Instrument/electrical engineers Safety engineers Maintenance Engineers.

HAZOP: How? 1. Clearly identify the study nodes, i.e., the locations (e.g., on piping) at which the process parameters are investigated 2. Identify the functionally independent process units (pumps, vessels, heat exchangers, etc.) that are between the nodes (which cause changes in the parameters between nodes) 3. For each node identify its operation modes (start-up, regime, shut-down, maintenance, etc.) and the corresponding plant configurations (valves open or closed, pumps on or off, etc.) 4. For each node in each of its operation modes, identify the possible deviations from nominal behavior, by compiling an HAZOP table

HAZOP: Procedure steps 1. Decompose the system in functionally independent process units 2. For each process unit identify its operation modes (start-up, regime, shut-down, maintenance, etc.) and configurations (valves open or closed, pumps on or off, etc.) 3. For each process unit in each of its operation modes, identify the possible deviations from nominal behavior, by compiling an HAZOP table

HAZOP: Procedure steps specify unit in/out fluxes (energy, mass, control signals, etc.) and process variables (temperature, flow rate, pressure, concentrations, etc.) write down unit functions (heating, cooling, pumping, filtering, etc.) apply keywords (low, high, no, reverse etc.) to the unit process variables and functions => process deviations for each process deviation (qualitatively) identify its possible causes and consequences

HAZOP TABLE Source: IEC 61882

HAZOP: A synoptic Taken from Rausand, M. and Høyland, A.: "System Reliability Theory: Models, Statistical methods, and Applications" (2nd ed.), Wiley, Hoboken, 2004

HAZOP: Details of procedure steps Specify elements: in/out fluxes (e.g., energy, mass, control signals, etc.), process variables (e.g., temperature, flow rate, pressure, concentrations, etc.), etc. Flow Composition ph Pressure Addition Sequence Temperature Separation Signal Mixing Time Start/stop Stirring Phase Operate Transfer Speed Level Maintain Particle size Services Viscosity Measure Communication Reaction Control

HAZOP: Details of procedure steps Specify elements: in/out fluxes (e.g., energy, mass, control signals, etc.), process variables (e.g., temperature, flow rate, pressure, concentrations, etc.), etc. Apply Guide-Words to the unit process variables and in/out fluxes => process deviations Guide-word Meaning Example No (not, none) None of the design intent is achieved No flow when production is expected More (more of, higher) Quantitative increase in a parameter Higher temperature than designed Less (less of, lower) Quantitative decrease in a parameter Lower pressure than normal As well as (more than) An additional activity occurs Other valves closed at the same time (logic fault or human error) Part of Reverse Other than (other) Early /late Only some of the design intention is Achieved Logical opposite of the design intention occurs Complete substitution - another activity takes place The timing is different from the intention Only part of the system is shut down Back-flow when the system shuts down Liquids in the gas piping

HAZOP TABLE UNIT : OPERATION MODE: Keyword Deviation Cause Consequence Hazard Actions needed More More Temperature Additional Thermal Resistance Higher pressure in tank Release due to Overpressure Install high temperature warning and pressure relief valve 1. Identify the deviation (install an alarm) 2. Compensate for the deviation (automatic control system) 3. Prevent the deviation from occurring 4. Prevent further escalation of the deviation (plant shut-down) 5. Relieve the process from the hazardous deviation (pressure safety valve)

EXAMPLE: SMALL EXTERNAL POOL The water of the pool is in re-circulation through pumps that aspire the water of the compensation tank making it pass through the treatment organs before throw it again in the pool. SYSTEM TO FILTER THE WATER SYSTEM TO CORRECT WATER PH SYSTEM TO DISINFECT THE WATER

HAZOP TABLE The objective of the HAZOP analysis is to identify the possible deviations to the normal operation that can contribute to the reduction of the quality of the water.

Hazop: example SYSTEM: shell & tube heat exchanger Study Node: 1 Operational Mode: Nominal Conditions Design Intent: P= 2bar, T=20 C, Flow=1l/sec Process fluid 3 4 2 1 Cooling water

Solution Guide Word Element Deviation Causes Consequences Action LESS FLOW Less flow of cooling water Pipe blockage NONE FLOW No cooling water flow Failure of inlet cooling water valve to open MORE FLOW More cooling flow Failure of cooling water valve REVERSE FLOW Reverse s cooling fluid flow MORE PRESSURE More pressure on tube side Temperature of process fluid remains constant Process fluid temperature is not lowered accordingly Temperature of process fluid decrease High Temperature Alarm Install Temperature indicator before and after the process fluid line Install TAH Low Temperature Alarm Not credible Not credible Not credible Failure of process fluid valve Bursting of tube Install high pressure alarm..............

HAZOP results Improvement of system or operations Reduced risk and better contingency More efficient operations Improvement of procedures Logical order Completeness General awareness among involved parties

HAZOP: Strength 1. Simple and systematic (computer tools available) 2. Include consequence effects also on other units: domino effects. 3. Covers human errors 4. Covers safety as well as operational aspects 5. It gives good identification of cause and excellent identification of critical deviations. 6. HAZOP is an excellent well-proven method for studying large plant in a specific manner.

HAZOP: weakness 1. Very time consuming and laborious (boredom for analysts) 2. Tends to generate many failure events with insignificance consequences and generate many failure events which have the same consequences 3. Does not identify all causes of deviations (it may omit some scenarios) 4. Does not allow to consider with multiple-combination events 5. Gives little account to the probabilities of events or consequences (meaningfulness of deviations are expert judgment based)

HAZOP: comments 1. Include consequence effects also on other units: domino effects. 2. Simple and systematic (computer tools available) 3. Subjective (relies on analyst s expertise) 4. Often used in support to the construction of FT and for RCM

Questions: 1. What is the different between FMEA and HAZOP? 2. Who should take the job of making FMEA and HAZOP? 3. How to improve the FMEA and HAZOP?