HAZARD AND OPERABILITY STUDY • • • • Brainstorming, Multidisciplinary Team Approach Structured Using Guide Words Problem Identifying Cost Effective When to Use? Optimal from a cost viewpoint 1. when applied to new plants at the point where the design is nearly firm and documented or 2. to existing plants where a major redesign is planned. It can also be used for existing facilities. Results Types: The results are the team findings. Which include: (1) identification of hazards and operating problems, (2) recommended changes in design, procedure, etc., to improve safety; and (3) recommendations for follow-on studies where no conclusion was possible due to lack of information. Nature: Qualitative. Requirements Data: The HazOp requires detailed plant descriptions, such as drawings, procedures, and flow charts. A HazOp also requires considerable knowledge of the process, instrumentation, and operation, and this information is usually provided by team members who are experts in these areas. Staff: The HazOp team is ideally made up of 5 to 7 professionals, with support for recording and reporting. For a small plant, a team as small as two or three could be effective. Time and Cost The time and cost of a HazOp are directly related to the size and complexity of the plant being analyzed. In general, the team must spend about three hours for each major hardware item. Where the system analyzed is similar to one investigated previously, the time is usually small. Additional time must be allowed for planning, team coordination, and documentation. This additional time can be as much as two three times the team effort as estimated above HAZOP STUDY - TEAM COMPOSITION A Team Leader, an expert in the HAZOP Technique Technical Members, for example New Design Existing Plant Design or Project Engineer Plant Superintendent Process Engineer Process Supervisor (Foreman) Commissioning Manager Maintenance Engineer Instrument Design Engineer Instrument Engineer Chemist Technical Engineer Principles of HAZOP Concept •Systems work well when operating under design conditions. •Problems arise when deviations from design conditions occur. Basis •a word model, a process flow sheet (PFD) or a piping and instrumentation diagram (P&ID) Method •use guide words to question every part of process to discover what deviations from the intention of design can occur and what are their causes and consequences may be. PRINCIPLES OF HAZOPS GUIDE WORDS* NONE MORE OF LESS OF PART OF MORE THAN OTHER CAUSE DEVIATION (from standard condition or intention) CONSEQUENCES (trivial, important, catastrophic) -hazard -operating difficulties *COVERING EVERY PARAMETER RELEVANT TO THE SYSTEM UNDER REVIEW: i.e. Flow Rate. Flow Quantity, Pressure, Temperature, Viscosity, Components STUDY NODES The locations (on P&ID or procedures) at which the process parameters are investigated for deviations. These nodes are points where the process parameters (P, T, F etc.) have an identified design intent. INTENTION The intention defines how the plant is expected to operate in the absence of deviations at the study nodes. DEVIATIONS These are departures from the intension which can be discovered by systematically applying the guide words. •Process conditions •activities •substances •time •place GUIDE WORDS Guide Words Meaning No, None Negation of Intention More Of Quantitative Increase Less Of Quantitative Decrease As Well As (More Than) Qualitative Increase Part Of Qualitative Decrease Reverse Logical Opposite of Intention Other Than Complete Substitution Deviations Generated by Each Guide Word Guide word NONE MORE OF LESS OF PART OF MORE THAN OTHER THAN Deviations No forward flow when there should be, i.e. no flow. More of any relevant physical property than there should be, e.g. higher flow (rate or total quantity), higher temperature, higher pressure, higher viscosity, etc. Less of any relevant physical property than there should be, e.g. lower flow (rate or total quantity), lower temperature, lower pressure, etc. Composition of system different from what it should be, e.g. change in ratio of components, component missing, ect. More components present in the system than there should be, e.g. extra phase present (vapour, solid), impurities (air. Water, acids, corrosion products), etc. What else can happen apart from normal operation, e.g. start-up, shutdown, uprating, low rate running, alternative operation mode, failure of plant services, maintenance, catalyst change, etc. REVERSE: reverse flow B A B EXAMPLE C The flowsheet shows that raw material streams A and B are transferred by pump to a reactor, where they react to form product C. Assume that the flow rate of B should not exceed that of A. Otherwise, an explosion may occur. Let’s consider the flow of A in line 1: FB FA NONE MORE LESS AS WELL AS PART OF REVERSE OTHER THAN No flow of A Flow of A greater than design flow Flow of A less than design flow Transfer of some component additional to A Failure to transfer a component of A Flow of A in a direction opposite to design direction Transfer of some material other than A Beginning End 1 Select a vessel 2 Explain the general intention of the vessel and its lines 3 Select a line 4 Explain the intention of the line 5 Apply the first guide words 6 Develop a meaningful deviation 7 Examine possible causes 8 Examine consequences 9 Detect hazards 10 Make suitable record 11 Repeat 6-10 for all meaningful deviations derived from first guide words 12 Repeat 5-11 for all the guide words 13 Mark line as having been examined 14 Repeat 3-13 for each line 15 Select an auxiliary system (e.g. Heating system) 16 Explain the intention of the auxiliary system 17 Repeat 5-12 for auxiliary system 18 Mark auxiliary as having been examined 19 Repeat 15-18 for all auxiliaries 20 Explain intention of the vessel 21 Repeat 5-12 22 Mark vessel as completed 23 Repeat 1-22 for all vessels on flow sheet 24 Mark flow sheet as completed 25 Repeat 1-24 for all flow sheets Figure 8.9 Hazard and operability studies : detailed sequence of examination (Chemical Industry Safety and Health Council, 1977 Item 6) HAZOP DISPLAY Guide Word Deviation Possible Causes Consequences Action Required No System Over- Shutdown Heated System No Flow Pump Fail Line Blockage Operator Stops Pump More More Flow Excessive Over-Cooled Pump Speed Product (Control System) (Incomplete Reaction) Product Unacceptable; Dump EXAMPLE An alkene/alkane fraction containing small amounts of suspended water is continuously pumped from a bulk intermediate storage tank via a half-mile pipeline into a buffer/settling tank where the residual water is settled out prior to passing via a feed/product heat exchanger and preheater to the reaction, is run off manually from the settling tank at intervals. Residence time in the reaction section must be held within closely defined limits to ensure adequate conversion of the alkene and to avoid excessive formation of polymer. Results of hazard and operability study of proposed olefine dimerization unit: results for line section from intermediate storage to buffer/settling tank Guide word NONE Deviation No flow Possible causes (1)No hydrocarbon available at intermediate storage. Consequences Loss of feed to reaction section and reduced output. Polymer formed in heat exchanger under no flow conditions. Action required (a)Ensure good communications with intermediate storage operator (b)Install low level alarm on settling tank LIC. (2)J1 pump fails (motor fault, loss of drive, impeller corroded away etc.) As for (1) (3)Line blockage, isolation valve closed in error, or LCV fails shut. As for (1) J1 pump overheats. Covered by (b) Covered by (b) (c)Install kickback on J1 pump. (d)Check design of J1 pump strainers. (4)Line fracture As for (1) Hydrocarbon discharged into area adjacent to public highway. (1) Covered by (b) (e)Institute regular patrolling & inspection of transfer line. Results of hazard and operability study of proposed olefine dimerization unit: results for line section from intermediate storage to buffer/settling tank Guide word MORE OF Deviation More flow Possible causes (5)LCV fails open or LCV bypass open in error. Consequences Settling tank overfills. Action required (f)Install high level alarm on LIC and check sizing of relief opposite liquid overfilling. (g)Institute locking off procedure for LCV bypass when not in use. More pressure More temperature (6)Isolation valve closed in error or LCV closes, with J1 pump running. Incomplete separation of water phase in tank, leading to problems on reaction section. (h)Extend J2 pump suction line to 12’’ above tank base. Transfer line subjected to full pump delivery or surge pressure. (j)Covered by (c) except when kickback blocked or isolated. Check line. FQ and flange ratings and reduce stroking speed of LCV if necessary. Install a PG upstream of LCV and an independent PG on settling tank. (7)Thermal expansion in an Line fracture or flange leak. isolated valved section due to fire or strong sunlight. (k)Install thermal expansion relief on valved section (relief discharge route to be decided later in study). (8)High intermediate storage temperature. (l)Check whether there is adequate warning of high temperature at intermediate storage. If not, install. Higher pressure in transfer line and settling tank. (2) Results of hazard and operability atudy of proposed olefine dimerization unit: results for line section from intermediate storage to buffer/settling tank Guide word LESS OF Deviation Less flow Possible causes (9)Leaking flange of valved stub not blanked and leaking. Consequences Material loss adjacent to public highway. Action required Covered by (e) and the checks in (j). Less temperature (10)Winter conditions. Water sump and drain line freeze up. (m)Lag water sump down to drain valve and steam trace drain valve and drain line downstream. High water concentration in stream. (11)High water level in intermediate storage tank. Water sump fills up more quickly. Increased chance of water phase passing to reaction section. (n)Arrange for more frequent draining off of water from intermediate storage tank. Install high interface level alarm on sump. High concentration of lower alkanes or alkenes in stream. (12)Disturbance on distillation columns upstream of intermediate storage. Higher system pressure. (p)Check that design of settling tank and associated pipework, including relief valve sizing, will cope with sudden ingress of more volatile hydrocarbons. MORE THAN Organic acids present (13)As for (12) Increased rate of corrosion of tank base, sump and drain line. (q)Check suitability of materials of construction. OTHER Maintenance (14)Equipment failure, flange leak, etc. Line cannot be completely drained or purged. (r)Install low-point drain and N2 purge point downStream of LCV. Also N2 vent on settling tank. PART OF (3) C HAZOP PREPLANNING ISSUES Preplanning issues addressed in a typical refinery unit HAZOP include the following: • Verification of as-built conditions shown on the P&IDs • Line segment boundaries set; markup of P&IDs • List of support documents compiled • P&IDs (base study document) • Process flow diagrams (PFDs) • Process description • Operating manuals/procedures • Processing materials information • Equipment and material specifications • Tentative schedules of time to be spent per P&IDs sheet • Recording technique (computer program or data sheet) determination • List of standard abbreviations and acronyms compiled • Criticality rankings devised • HAZOP training given to all team members (one day) • Arrange for system or process briefings for team before work begins. HAZOP STUDY LOGISTICS Logistical development of this refinery unit HAZOP included the following: • • • • • • • • • • Preplanning issues were addressed the prior week. The team include three core team members and four part-time members. The study included 16 moderately busy P&Ids. The study took three and one-half weeks. The team met 4 hours per day in morning review sessions and spent 2 hours per day on individual efforts for reviews, follow-ups, and field checks. Dedicated space was required for storing the large number of documents. The study resulted in 170 data sheets. The team recorder used a personal computer to record, sort, and retrieve data. The Stone & Webster proprietary program PCHAZOPa was used. The plant operator was the key contribution plant member of the team. Key operating procedures were reviewed relative to the P&Ids and safe engineering practices.
