INHERENTLY SAFE DESIGN OF CHEMICAL PLANTS

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INHERENTLY SAFE DESIGN OF CHEMICAL PLANTS
DESIGN OF RELIEF DEVICES

• M.B. JENNINGS
• Summary of a report from Center for Chemical
  Process Safety of AIChE by F. Owen Kubias, 1966

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OUTLINE

• Develop concept of Inherently Safe Design (ISD)
• Indicate how control systems are included in ISD
• Present some specific design techniques for
  protection devices

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PRIMARY CONCEPT

• Plants can be designed to prevent the possibility
  of hazardous incidents
• Inherently Safe Design (ISD) is supplemented by
• Control Systems
• Alarms and Interlocks
• Shutdown Systems
• Protection Systems and Devices
• Response Plans

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SAFETY OPTIONS

• PREVENT BY USING INHERENTLY SAFE DESIGN METHODS
• CONTROL BY INCLUDING PRIMARY RESPONSE SYSTEMS
• MITIGATE BY USING SECONDARY RESPONSE SYSTEMS TO
  LIMIT IMPACT
• BUFFER BY ISOLATING FACILITIES AWAY FROM
  POPULATIONS

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CATEGORIES OF ISD

• The following keywords are used for ISD
  categories 1
• Intensification
• Attenuation
• Limitation
• Simplification
• Other means
• 1Kletz, Trevor, Process Plants A Handbook for
  Inherently Safer Design, Taylor Francis, 1998

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ISD CATEGORY DETAILS - 1

• Intensification minimizes inventories of
  hazardous materials.
• Substitution replaces hazardous materials with
  safer materials.
• Attenuation uses hazardous materials under the
  least hazardous conditions.
• Limitation changes designs or conditions to
  reduce potential effects.
• Simplification reduces complexity to reduce the
  opportunity for error.

http//www.ehw.org/Chemical_Accidents/CHEM_RenoLtr
.htm
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ISD CATEGORY DETAILS - 2

• Other means include using designs that
• avoid potential "domino" effects
• make incorrect assembly impossible
• tolerate misuse
• keep controls and computer software easy to
  understand and use
• keep process status clear
• have well-defined instructions and procedures
• employ passive safety
• and minimize hazards throughout the material's
  life-cycle

http//www.ehw.org/Chemical_Accidents/CHEM_RenoLtr
.htm
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INTENSIFICATION

• ATTEMPT TO MINIMIZE THE QUANTITIES OF MATERIALS
  IN THE PROCESS
• REACTORS
• SEPARATION DEVICES
• ENERGY TRANSFER
• STORAGE VESSELS
• MATERIALS TRANSPORT SYSTEMS
• NUMBER OF TRAINS

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INTENSIFICATION EXAMPLE FOR REACTORS PHASE 1

• BATCH REACTORS REQUIRE THE LARGEST VOLUMES OF
  MATERIALS1
• PLUG FLOW REACTORS REQUIRE SMALLER QUANTITIES AND
  MAY HAVE BETTER HEAT TRANSFER

1www.hasbrouckengineering.com
http//www.owlnet.rice.edu/chbe403/hysys/pfex.htm
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INTENSIFICATION EXAMPLE FOR REACTORS PHASE 2

• EDUCTOR OR CYCLONE REACTORS ARE THE SMALLEST
  PRACTICAL VOLUME
• FOR OXIDATIONS AND EXPLOSIVE MIXTURES

http//paniit.iitd.ac.in/chemcon/Hydrazine20synt
hesis20by20cyclone20reactor.pdf
www.eductor.net
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OTHER INTENSIFICATION OPTIONS

• REDUCE INVENTORIES
• REDUCE QUANTITIES IN SUMPS
• USE CENTRIFUGAL MIXERS FOR REACTORS
• USE EDUCTORS FOR OTHER TYPES OF CONTACTORS
• USE PLANT LAYOUT TO MINIMIZE PIPING

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SUBSTITUTION

• USE OF WATER BASED SOLVENTS IN PLACE OF ORGANIC
  SOLVENTS
• ELIMINATION OF CFC REFRIGERANTS
• USE OF CYCLOHEXANE IN PLACE OF BENZENE
• SUPERCRITICAL CO2 IN PLACE OF METHYLENE CHLORIDE
• USE MEMBRANE PROCESS TO PRODUCE Cl2 AND ELIMINATE
  NEED FOR Hg
• CHANGE SEQUENCE OF STEPS FOR REACTION TO AVOID
  TOXIC INTERMEDIATES

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ATTENUATION

• REDUCE TEMPERATURES IN REACTORS
• USE DILUTE REACTANTS IN SOLVENTS
• USE GRAVITY OR GAS PRESSURE TO TRANSPORT UNSTABLE
  LIQUIDS
• USE REFRIGERATED STORAGE INSTEAD OF PRESSURIZED
  STORAGE LOX

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LIMITATION OF EFFECTS

• MINIMIZE DIKED AREAS AROUND STORAGE TANKS
• AVOID HAVING MULTIPLE STAGE REACTIONS IN A SINGLE
  VESSEL
• KEEP CONDITIONS BELOW DECOMPOSITION LEVELS
• USE SUBMERGED PUMPS
• MINIMIZE EQUIPMENT WITH MOVING PARTS
• ISOLATE REACTIVE CHEMICAL STORAGE
• USE SAFE LOCATIONS FOR OPERATING FACILITIES

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SIMPLIFICATION

• INCREASE VESSEL STRENGTH TO AVOID THE NEED FOR
  RELIEF VALVES
• USE MATERIALS THAT CAN FUNCTION OVER THE RANGE OF
  PROCESS CONDITIONS
• ELIMINATE OPPORTUNITIES FOR HUMAN ERROR THROUGH
  SIMPLE INSTRUCTIONS
• ELIMINATE EXTRA EQUIPMENT
• MINIMIZE NUMBERS OF CONTROL LOOPS

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OTHER MEANS

• RIGOROUSLY FOLLOW TAG-OUT PROCEDURES
• AVOID REVERSE FLOW DESIGNS
• KEEP PROCESSES SEPARATED
• HAVE REVIEWS BEFORE THE DESIGN BECOMES FINALIZED

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SAFE DESIGN FOR PRIMARY CONTROL SYSTEMS - 1

• INTENSIFICATION USE THE MINIMUM NUMBER OF LOOPS
  FOR PROCESS CONTROL
• DETERMINE WHICH VARIABLES THAT NEEDS TO BE
  CONTROLLED AND WHICH VARIABLES ARE USED TO MAKE
  ADJUSTEMENTS
• USE INDEPENDENT SENSORS FOR ALARMED VARIABLES
• CONSIDER FEED FORWARD AND CASCADE CONTROL
  OPPORTUNITIES

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SAFE DESIGN FOR PRIMARY CONTROL SYSTEMS - 2

• SPECIALIZED CONTROLS FOR START-UP, PARTIAL
  SHUTDOWN, CONTROLLED SHUTDOWN TO BE ON PLC BASE.
• START-UP SHOULD BE BASED ON STANDARD TIMES AS
  WELL AS ACHIEVING CONDITIONS
• PARTIAL SHUTDOWN NEEDS TO CONSIDER ALL UPSTREAM
  AND DOWNSTREAM UNIT OPERATIONS
• COMPLETE SHUTDOWN SHOULD BE TESTED DURING
  TURNAROUNDS
• EMERGENCY SHUTDOWNS SHOULD ALSO HAVE A PLC FOR
  BACKUP
• ASSUMING THE UNIT IS EVACUATED
• ASSUMING POSSIBLE LOSS OF PRIMARY UTILITIES

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SAFE DESIGN FOR PRIMARY CONTROL SYSTEMS - 3

• CONSIDER ALL INTERACTIONS BETWEEN INTERCONNECTED
  UNIT OPERATIONS
• NEED TO AVOID REVERSE FLOWS
• CONSIDER OVER-PRESSURIZATION DUE TO LOSS OF FLOWS
• CONSIDER IMPACT OF MATERIALS THAT ARE NOT AT
  DESIGN TEMPERATURES

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ALARMS FOR NORMAL OPERATION

• FIRST STAGE ALARMS
• LOW OR HIGH ALARMS
• CAN BE PART OF THE PRIMARY CONTROLLER CARD
• REQUIRE MANUAL INTERVENTION
• OPERATOR HAS SPECIFIC ALARM NOTIFICATION
• SECOND STAGE ALARMS SAFETY INTERLOCKS
• LO/LO OR HI/HI ALARMS
• AUTOMATICALLY ACTIVATE SYSTEM FOR PROTECTION
• OPERATOR HAS SPECIFIC ALARM NOTIFICATION

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TYPICAL DESIGN FOR OPERATION ALARMS

• HI ALARM ALERTS OPERATOR TO HIGH PROCESS
  TEMPERATURE
• HI/HI ALARM SHUTS OFF VALVE IN STEAM SUPPLY LINE

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DESIGNS FOR PRESSURE RELIEF SYSTEMS

• BASED ON INFORMATION FROM
• Grossel Louvar, Design for Overpressure and
  Underpressure Protection, Center for Chemical
  Process Safety, AIChE, 2000.
• Darby, Emergency Relief System Design, Center for
  Chemical Process Safety, AIChE, 1997.

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PROTECTIVE EQUIPMENT DESIGN DEVICE TYPES

• RELIEF SYSTEMS ARE USED TO AVOID OVERPRESSIZATION
  OF VESSELS
• THESE CAN BE TEMPORARY DEVICES THAT RESET AFTER
  THE SYSTEM PRESSURE RETURNS TO NORMAL
• ALTERNATELY THESE DEVICES DO NOT RESET AFTER
  ACTIVATION AND REQUIRE REPLACEMENT
• OTHER SYSTEMS USED FOR VACUUM CONDITIONS IN
  TANKS, ARE NOT IN THIS PRESENTATION

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SOURCES OF PRESSURE DEVIATIONS

• OPERATING UPSET
• EQUIPMENT FAILURE
• PROCESS UPSET
• EXTERNAL SOURCE (FIRE)
• UTILITY FAILURE

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TYPICAL INSTRUMENTATION LAYOUT FOR VESSEL

• PRESSURE RELIEF VALVE ALLOWS FOR OVER-PRESSURE
  AND RESEATS
• RUPTURE DISK WILL RELEASE AND NOT RESEAT.

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SAFETY VALVE SCHEMATIC 1
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SAFETY VALVE SCHEMATIC 2
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SAFETY VALVE SCHEMATIC 3
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RUPTURE DISC MATERIALS OPTIONS
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www.trane.com

• CHEMICALLY COMPATIBLE RUPTURE DISCS
• METALS ALL TYPES
• GRAPHITE
• COMPOSITE

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TYPICAL RELIEF SYSTEM INSTALLATION
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PHASES PRESENT IN RELIEF INCIDENTS

• GAS/VAPOR
• LIQUID
• TWO PHASE LIQUID/VAPOR

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CAPACITY OF RELIEF DEVICES

• THE VOLUMETRIC CAPACITY OF THE DEVICE MUST BE
  EQUAL OR GREATER THAN THE VOLUMETRIC GENERATION
  RATE IN THE VESSEL.
• VESSEL CAN BE RUPTURED IF THE CAPACITY IS TOO LOW

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TYPICAL RELIEF INCIDENTS

• RUNAWAY REACTION
• OVERHEAT DUE TO CONTROL FAILURE (TANK HEATER)
• LINE BLOCKAGE
• OVERPRESSURE DUE TO CONTROL FAILURE (BLANKET)
• OVERFILLING A TANK
• EXTERNAL FIRE

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INCIDENTS THAT CANNOT BE RELIEVED

• EXPLOSIONS IN OR NEAR VESSELS

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TYPES OF VESSELS

• BASED ON Maximum Allowable Working Pressure
  (MAWP, PMAWP)
• API 650 lt 2.5 psig
• API 620 2.5 to 15 psig
• Pressure Vessels ASME VIII
• Normal Maximum Operating Pressure is set at gt90
  PMAWP
• Relief Pressure (PSET) is specified lt Normal
  Maximum Operating Pressure

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RELEASE SEQUENCE

• PRIOR TO RELEASE THE TANK IS AT UNIFORM PRESSURE
• WITH FLOW THERE ARE DIFFERENT PRESSURES THROUGH
  THE FLOW PATH
• THE UPPER LIMIT FOR FLOW IS SONIC VELOCITY
• THIS CONDITION IS CHOKED FLOW
• DOWNSTREAM PRESSURE HAS NO EFFECT ON THE FLOW
  WITH CHOKED FLOW

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PRESSURES IN FLOW PATH

• P0 Stagnation, tank pressure
• P1 Valve inlet
• P2 Nozzle inlet
• Pn Nozzle exit
• Pb Valve exit
• PS Piping exit

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FLUID VELOCITY DURING RELEASE

• BASIC EQUATION THAT APPLIES IS THE BERNOULLI
  EQUATION
• MASS FLOW IS OBTAINED BY INTEGRATION FROM 0 TO n

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NON-FLASHING LIQUID FLOW

• OVER THE SYSTEM

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VELOCITY IN GAS FLOW

• SUBSONIC FOR IDEAL GAS

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CHOKED FLOW IN GASES

• CRITICAL FLOW FOR ANY FLUID IS APPLIED TO IDEAL
  GAS EQUATIONS

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TWO PHASE FLOW

• FLASHING FLOWS CAN RESULT IN CHOKED FLOW AS THE
  LIQUID FLASHES
• VOLUME FOR TWO PHASE FLOW IS

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TWO PHASE FLASH P-V RELATIONSHIP

• THIS APPROACH USES THE OMEGA METHOD

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GENERAL FLASHING MASS FLOW RELATIONSHIP

• INTEGRATING THE MASS FLOW EQUATION DERIVED FROM
  THE BERNOULLI EQUATION, DIMENSIONLESS MASS FLUX
  IS EVALUATED

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2 PHASE CHOKED FLOW

• EQUATIONS ARE BASED ON CHOKED FLOW PRESSURE RATIO

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CONCLUSIONS

• SAFETY IS A FACTOR IN CONTROL DESIGN AT ALL
  LEVELS
• IT IS POSSIBLE TO MINIMIZE RISK TO PROCESS
  HAZARDS BY USING ISD
• PROCESS HAZARDS ANALYSIS MAY INDICATE POTENTIAL
  SOURCES OF PROBLEMS
• FINAL RELIEF DEVICES SHOULD BE THE LAST RESORT
  FOR DESIGN