INHERENTLY SAFE DESIGN OF CHEMICAL PLANTS & DESIGN OF RELIEF DEVICES

1. 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

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

3. 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

4. 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

5. 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

6. 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

7. 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

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

9. 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

10. 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/Hydrazine%20synthesis%20by%20cyclone%20reactor.pdf www.eductor.net

11. 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

12. 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

13. 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

14. 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

15. 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

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

17. 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

18. 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

19. 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

20. 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

21. TYPICAL DESIGN FOR OPERATION ALARMS • HI ALARM ALERTS OPERATOR TO HIGH PROCESS TEMPERATURE • HI/HI ALARM SHUTS OFF VALVE IN STEAM SUPPLY LINE

22. 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.

23. 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

24. SOURCES OF PRESSURE DEVIATIONS • OPERATING UPSET • EQUIPMENT FAILURE • PROCESS UPSET • EXTERNAL SOURCE (FIRE) • UTILITY FAILURE

25. TYPICAL INSTRUMENTATION LAYOUT FOR VESSEL • PRESSURE RELIEF VALVE ALLOWS FOR OVER-PRESSURE AND RESEATS • RUPTURE DISK WILL RELEASE AND NOT RESEAT.

26. SAFETY VALVE SCHEMATIC 1

27. SAFETY VALVE SCHEMATIC 2

28. SAFETY VALVE SCHEMATIC 3

29. RUPTURE DISC MATERIALS OPTIONS • CHEMICALLY COMPATIBLE RUPTURE DISCS • METALS – ALL TYPES • GRAPHITE • COMPOSITE http://www.contdisc.com/products/reverse/Rcsp0101.jpg www.trane.com

30. TYPICAL RELIEF SYSTEM INSTALLATION

31. PHASES PRESENT IN RELIEF INCIDENTS • GAS/VAPOR • LIQUID • TWO PHASE LIQUID/VAPOR

32. 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

33. 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

34. INCIDENTS THAT CANNOT BE RELIEVED • EXPLOSIONS IN OR NEAR VESSELS

35. TYPES OF VESSELS • BASED ON Maximum Allowable Working Pressure (MAWP, PMAWP) • API 650 < 2.5 psig • API 620 2.5 to 15 psig • Pressure Vessels ASME VIII • Normal Maximum Operating Pressure is set at >90% PMAWP • Relief Pressure (PSET) is specified < Normal Maximum Operating Pressure

36. 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

37. PRESSURES IN FLOW PATH • P0 = Stagnation, tank pressure • P1 = Valve inlet • P2 = Nozzle inlet • Pn = Nozzle exit • Pb = Valve exit • PS = Piping exit

38. FLUID VELOCITY DURING RELEASE • BASIC EQUATION THAT APPLIES IS THE BERNOULLI EQUATION • MASS FLOW IS OBTAINED BY INTEGRATION FROM 0 TO n

39. NON-FLASHING LIQUID FLOW • OVER THE SYSTEM

40. VELOCITY IN GAS FLOW • SUBSONIC FOR IDEAL GAS

41. CHOKED FLOW IN GASES • CRITICAL FLOW FOR ANY FLUID IS APPLIED TO IDEAL GAS EQUATIONS

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

43. TWO PHASE FLASH P-V RELATIONSHIP • THIS APPROACH USES THE OMEGA METHOD

44. GENERAL FLASHING MASS FLOW RELATIONSHIP • INTEGRATING THE MASS FLOW EQUATION DERIVED FROM THE BERNOULLI EQUATION, DIMENSIONLESS MASS FLUX IS EVALUATED:

45. 2 PHASE CHOKED FLOW • EQUATIONS ARE BASED ON CHOKED FLOW PRESSURE RATIO

46. 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