Design for Overpressure and Underpressure Protection
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Design for Overpressure and Underpressure Protection Slide Show Slides with Text Exit Design for Overpressure and Underpressure Protection SLIDE PRESENTATION NEXT HOME Outline Introduction Reliefs Runaways Safeguards • Introduction • Causes of Overpressure and Underpressure • Reliefs • Effluent Handling Systems for Reliefs • Runaway Reactions • Overpressure Protection for Internal Fires and Explosions For Further Information: Refer to the Appendix Supplied with this Presentation NEXT PREVIOUS Text Home Causes of Overpressure • Operating Problem • Equipment Failure • Process Upset • External Fire • Utility Failures NEXT PREVIOUS Text Home Causes of Underpressures • Operating Problem • Equipment Failure NEXT PREVIOUS Text Home Presentation 1 of 3: Reliefs Causes of Overpressure/Underpressure Presentation 2: Runaways NEXT PREVIOUS Presentation 1: Reliefs Presentation 3: Safeguards Text Home Pressure Relief Devices • Spring-Loaded Pressure Relief Valve • Rupture Disc • Buckling Pin • Miscellaneous Mechanical NEXT PREVIOUS Text Home Spring-Loaded Pressure Relief Valve NEXT PREVIOUS Text Home Rupture Disc NEXT PREVIOUS Text Home Buckling Pin Relief Valve Closed Pressure Below Set Pressure Full Open Pressure at or Above Set Pressure (Buckles in Milliseconds at a Precise Set Pressure) NEXT PREVIOUS Text Home Simple Mechanical Pressure Relief NEXT PREVIOUS Text Home Types of Spring-Loaded Pressure Reliefs • Safety Valves for Gases and Vapors • Relief Valves for Liquids • Safety Relief Valves for Liquids and/or Gases NEXT PREVIOUS Text Home Types of Safety Valves • Conventional • Balanced Bellows, and • Pilot-Operated NEXT PREVIOUS Text Home Conventional Safety Valve NEXT PREVIOUS Text Home Balanced Bellows Safety Valve NEXT PREVIOUS Text Home Pilot-Operated Safety Valve NEXT PREVIOUS Text Home Types of Relief Valves • Conventional • Balanced Bellows NEXT PREVIOUS Text Home Types of Rupture Discs • Metal • Graphite • Composite • Others NEXT PREVIOUS Text Home Rupture Disc and Pressure Relief Valve Combination NEXT PREVIOUS Text Home Vacuum Relief Devices • Vacuum Relief Valves • Rupture Discs • Conservation Vents • Manhole Lids • Pressure Control NEXT PREVIOUS Text Home Conservation Vent NEXT PREVIOUS Text Home Pressure or Vacuum Control • Add Air or Nitrogen • Maintain Appropriately NEXT PREVIOUS Text Home Relief Servicing • Inspection • Testing NEXT PREVIOUS Text Home Relief Discharges • To Atmosphere • Prevented • Effluent System NEXT PREVIOUS Text Home Effluent Systems • Knock-Out Drum • Catch Tank • Cyclone Separator NEXT PREVIOUS Text Home Effluent System (continued) • Condenser • Quench Tank • Scrubber • Flares/Incinerators NEXT PREVIOUS Text Home Effluent Handling System NEXT PREVIOUS Text Home Presentation 2 of 3: Runaways Causes of Overpressure/Underpressure Presentation 2: Runaways NEXT PREVIOUS Presentation 1: Reliefs Presentation 3: Safeguards Text Home Runaway Reaction • Temperature Increases • Reaction Rate Increases • Pressure Increases NEXT PREVIOUS Text Home Causes of Runaway Reactions • Self-Heating • Sleeper Characteristics of Runaway • Tempered • Gassy • Hybrid NEXT PREVIOUS Text Home Self-Heating Reaction • Loss of Cooling • Unexpected Addition of Heat • Too Much Catalyst or Reactant • Operator Mistakes • Too Fast Addition of Catalyst or Reactant NEXT PREVIOUS Text Home Sleeper Reactions • Reactants Added But Not Mixed (Error) • Reactants Accumulate • Agitation Started .. Too Late NEXT PREVIOUS Text Home Tempered Reaction • Heat Removed by Evaporation • Heat Removal Maintains a Constant Temperature NEXT PREVIOUS Text Home Gassy System • No Volatile Solvents • Gas is Reaction Product NEXT PREVIOUS Text Home Hybrid System • Tempered • Gassy NEXT PREVIOUS Text Home Reliefs for Runaway Reactions • Two Phase (or Three Phases: Liquid, Vapor, and Solid) Flow • Relief Area: 2 to 10 Times the Area of a Single Gaseous Phase NEXT PREVIOUS Text Home Two Phase Flow NEXT PREVIOUS Text Home Relief Valve Sizing Methodology • Special Calorimeter Data • Special Calculation Methods NEXT PREVIOUS Text Home Characterization of Runaway Reactions • ARC • APTAC • VSP • PHI-TEC • RSST • Dewars NEXT PREVIOUS Text Home Presentation 3 of 3: Safeguards Causes of Overpressure/Underpressure Presentation 2: Runaways NEXT PREVIOUS Presentation 1: Reliefs Presentation 3: Safeguards Text Home Safeguards • Safety Interlocks • Safeguard Maintenance System • Short-Stopping NEXT PREVIOUS Text Home Safety Interlocks • Agitator Not Working: Stop Monomer Feed and Add Full Cooling • Abnormal Temperature: Stop Monomer Feed and Add Full Cooling NEXT PREVIOUS Text Home Safety Interlocks (continued) • Abnormal Pressure: Stop Monomer Feed and Add Full Cooling • Abnormal Heat Balance: Stop Monomer Feed and Add Full Cooling • Abnormal Conditions: Add Short-Stop NEXT PREVIOUS Text Home Safeguard Maintenance System • Routine Maintenance • Management of Change • Mechanical Integrity Checks • Records NEXT PREVIOUS Text Home Short-Stops to Stop Reaction • Add Reaction Stopper • Add Agitation with No Electrical Power NEXT PREVIOUS Text Home Protection for Internal Fires and Explosions • Deflagrations • Detonations NEXT PREVIOUS Text Home Protection Methods for Internal Fires and Explosions • Deflagration Venting • Deflagration Suppression • Containment NEXT PREVIOUS Text Home Protection Methods for Internal Fires and Explosions (continued) • Reduction of Oxidant • Reduction of Combustible • Flame Front Isolation NEXT PREVIOUS Text Home Protection Methods for Internal Fires and Explosions (continued) • Spark Detection and Extinguishing • Flame Detection and Extinguishing • Water Spray and Deluge Systems NEXT PREVIOUS Text Home Deflagration Venting • Vent Area via NFPA 68 • Vent Safely NEXT PREVIOUS Text Home Vent of Gas Deflagration NEXT PREVIOUS Text Home Vent of Dust Deflagration NEXT PREVIOUS Text Home Deflagration Suppression System NEXT PREVIOUS Text Home Containment • Prevent Rupture and Vessel Deformation • Prevent Rupture but Deform Vessel NEXT PREVIOUS Text Home Reduction of Oxidant • Vacuum Purging • Pressure Purging • Sweep-Through Purging NEXT PREVIOUS Text Home Reduction of Combustible • Dilution with Air • NFPA 69 NEXT PREVIOUS Text Home Flame Front Isolation NEXT PREVIOUS Text Home Spark/Flame Detection and Extinguishing NEXT PREVIOUS Text Home Water Spray or Deluge Systems NEXT PREVIOUS Text Home Deluge System NEXT PREVIOUS Text Home Conclusion NEXT PREVIOUS Text Home End of Slide Presentation Causes of Overpressure/Underpressure Presentation 2: Runaways NEXT PREVIOUS Presentation 1: Reliefs Presentation 3: Safeguards Text Home Design for Overpressure and Underpressure Protection SLIDES WITH TEXT Introduction Reliefs Runaways Safeguards This presentation includes technical information concerning the design for overpressure and underpressure protection. The presentation is designed to help students and engineers to: NEXT PREVIOUS Slide Home Design for Overpressure and Underpressure Protection Introduction Reliefs Runaways Safeguards • Understand the technologies, special engineering devices, and methods that are used for the protection against overpressure and underpressure (vacuum) incidents, NEXT PREVIOUS Slide Home Design for Overpressure and Underpressure Protection Introduction Reliefs Runaways Safeguards • Understand the root causes of overpressure and underpressure incidents, and • Design plants with the appropriate features to protect against overpressure and underpressure incidents. NEXT PREVIOUS Slide Home Six Sections 1. Introduction 2. Causes of Overpressure and Underpressure 3. Reliefs 4. Effluent Handling Systems for Reliefs 5. Runaway Reactions, and 6. Overpressure Protection for Internal Fires and Explosions Introduction This presentation is divided into six sections: NEXT Reliefs Runaways Safeguards PREVIOUS Slide Home Six Sections 1. Introduction 2. Causes of Overpressure and Underpressure 3. Reliefs 4. Effluent Handling Systems for Reliefs 5. Runaway Reactions, and 6. Overpressure Protection for Internal Fires and Explosions Introduction Reliefs Runaways Safeguards The “Introduction” button on your left will lead you to this introduction and an explaination of the Causes of Overpressure and Underpressure NEXT PREVIOUS Slide Home Six Sections 1. Introduction 2. Causes of Overpressure and Underpressure 3. Reliefs 4. Effluent Handling Systems for Reliefs 5. Runaway Reactions, and 6. Overpressure Protection for Internal Fires and Explosions Introduction Reliefs Runaways Safeguards The “Reliefs” Button sends you to Sections 3 and 4, covering Reliefs and Effluent Handling Systems for Reliefs NEXT PREVIOUS Slide Home Six Sections 1. Introduction 2. Causes of Overpressure and Underpressure 3. Reliefs 4. Effluent Handling Systems for Reliefs 5. Runaway Reactions, and 6. Overpressure Protection for Internal Fires and Explosions Introduction Reliefs Runaways Safeguards The “Runaways” Button leads to a discussion on Runaway Reactions, and . . . NEXT PREVIOUS Slide Home Six Sections 1. Introduction 2. Causes of Overpressure and Underpressure 3. Reliefs 4. Effluent Handling Systems for Reliefs 5. Runaway Reactions, and 6. Overpressure Protection for Internal Fires and Explosions Introduction Reliefs Runaways Safeguards The “Safeguards” Button will take you to a section on Overpressure Protection fot Internal Fires and Explosions NEXT PREVIOUS Slide Home Appendix Contains Detailed Information Introduction Reliefs Runaways Safeguards This design package includes an appendix with detailed information for each of the sections of this presentation. The appendix also includes an extensive list of relevant references. NEXT PREVIOUS Slide Home Causes of Overpressure • Operating Problem Introduction Reliefs Runaways Safeguards The major causes of overpressure include: • Operating problems or mistakes such as an operator mistakenly opening or closing a valve to cause the vessel or system pressure to increase. An operator, for example, may adjust a steam regulator to give pressures exceeding the maximum allowable working pressure (MAWP) of a steam jacket. NEXT PREVIOUS Slide Home Causes of Overpressure • Operating Problem Introduction Reliefs Runaways Safeguards Although the set pressure is usually at the MAWP, the design safety factors should protect the vessel for higher pressures; a vessel fails when the pressure is typically several times the MAWP. NEXT PREVIOUS Slide Home Causes of Overpressure • Operating Problem • Equipment Failure Introduction Reliefs Runaways Safeguards • Equipment failures; for example a heat exchanger tube rupture that increases the shell side pressure beyond the MAWP. Although the set pressure is usually the MAWP, the design safety factors should protect the vessel for higher pressures; a vessel fails when the pressure is typically several times the MAWP. NEXT PREVIOUS Slide Home Causes of Overpressure • Operating Problem • Equipment Failure • Process Upset • External Fire • Utility Failures Introduction Reliefs Runaways Safeguards • Process upset; for example a runaway reaction causing high temperatures and pressures. • External heating, such as, a fire that heats the contents of a vessel giving high vapor pressures, and • Utility failures, such as the loss of cooling or the loss of agitation causing a runaway reaction. NEXT PREVIOUS Slide Home Causes of Underpressures Introduction Reliefs Runaways Safeguards The causes of underpressure or the inadvertent creation of a vacuum are usually due to operating problems or equipment failures. NEXT PREVIOUS Slide Home Causes of Underpressures • Operating Problem Introduction Reliefs Runaways Safeguards • Operating problems include mistakes such as pumping liquid out of a closed system, or cooling and condensing vapors in a closed system. NEXT PREVIOUS Slide Home Causes of Underpressures • Operating Problem • Equipment Failure Introduction Reliefs Runaways Safeguards • Equipment failures include an instrument malfunction (e.g. vacuum gage) or the loss of the heat input of a system that contains a material with a low vapor pressure. NEXT PREVIOUS Slide Home Part 1 of 3: Reliefs Introduction NEXT Reliefs Runaways Safeguards PREVIOUS Slide Home Pressure Relief Devices Introduction Reliefs Runaways Safeguards Pressure relief devices are added to process equipment to prevent the pressures from significantly exceeding the MAWP (pressures are allowed to go slightly above the MAWP during emergency reliefs). NEXT PREVIOUS Slide Home Pressure Relief Devices • Spring-Loaded Pressure Relief Valve • Rupture Disc • Buckling Pin • Miscellaneous Mechanical Introduction Reliefs Runaways Safeguards The pressure relief devices include spring-loaded pressure relief valves, rupture discs, buckling pins, and miscellaneous mechanical devices. NEXT PREVIOUS Slide Home Spring-Loaded Pressure Relief Valve Introduction Reliefs Runaways Safeguards This is a sketch of a spring-loaded pressure relief valve. As the pressure in the vessel or pipeline at point A exceeds the pressure created by the spring, the valve opens. The relief begins to open at the set pressure which is usually at or below the MAWP; this pressure is usually set at the MAWP. NEXT PREVIOUS Slide Home Rupture Disc Introduction Reliefs Runaways Safeguards This is a sketch of a rupture disc. In this case the disc ruptures when the pressure at A exceeds the set pressure. Recognize, however, that it is actually the differential pressure (A-B), that ruptures the disc. NEXT PREVIOUS Slide Home Buckling Pin Relief Valve Closed Pressure Below Set Pressure Full Open Pressure at or Above Set Pressure (Buckles in Milliseconds at a Precise Set Pressure) Introduction Reliefs Runaways Safeguards This sketch shows a buckling pin pressure relief valve. As shown, when the pressure exceeds the set pressure, the pin buckles and the vessel contents exit through the open valve. The rupture disc and the buckling pin relief valves stay open after they are opened. NEXT PREVIOUS Slide Home Buckling Pin Relief Valve Closed Pressure Below Set Pressure Full Open Pressure at or Above Set Pressure (Buckles in Milliseconds at a Precise Set Pressure) Introduction Reliefs Runaways Safeguards The spring operated valves close as the pressure decreases below the “blowdown” pressure. The blowdown pressure is the difference between the set pressure and closing pressure. NEXT PREVIOUS Slide Home Simple Mechanical Pressure Relief Introduction Reliefs Runaways Safeguards A simple mechanical pressure relief is a weighted man-way cover as shown in this sketch. Another mechanical relief is a Utube filled with water (or equivalent). NEXT PREVIOUS Slide Home Types of Spring-Loaded Pressure Reliefs • Safety Valves for Gases and Vapors • Relief Valves for Liquids • Safety Relief Valves for Liquids and/or Gases Introduction Reliefs Runaways Safeguards There are three types of spring-loaded pressure relief valves: • Safety valves are specifically designed for gases. • Relief valves are designed for liquids, and • Safety relief valves are designed for liquids and/or gases. NEXT PREVIOUS Slide Home Types of Safety Valves • Conventional • Balanced Bellows, and • Pilot-Operated Introduction Reliefs Runaways Safeguards There are three types of safety valves; that is: • Conventional, • Balanced bellows, and • Pilot-operated. NEXT PREVIOUS Slide Home Conventional Safety Valve Introduction Reliefs Runaways Safeguards A conventional safety valve is designed to provide full opening with minimum overpressure. The disc is specially shaped to give a “pop” action as the valve begins to open. NEXT PREVIOUS Slide Home Balanced Bellows Safety Valve Introduction Reliefs Runaways Safeguards A balanced bellows safety valve is specially designed to reduce the effect of the back pressure on the opening pressure. As illustrated in this sketch the differential pressure that is required to open the valve is the pressure inside the vessel minus the atmospheric pressure. NEXT PREVIOUS Slide Home Balanced Bellows Safety Valve Introduction Reliefs Runaways Safeguards The bellows design allows the outside air and pressure to be on the downstream side of the valve seal. Once the relief is open, then the flow is a function of the differential pressure A-B. NEXT PREVIOUS Slide Home Pilot-Operated Safety Valve Introduction Reliefs Runaways Safeguards A pilot-operated safety valve is a spring-loaded valve. As illustrated, the vessel pressure helps to keep the valve closed. When the pressure exceeds the set pressure (or the spring pressure), the pressure on top of the valve is vented and the valve opens. NEXT PREVIOUS Slide Home Pilot-Operated Safety Valve Introduction Reliefs Runaways Safeguards The set pressure of this type of valve can be closer to the operating pressure compared to conventional and balanced bellows valves. The disadvantages, however, are (a) the process fluid needs to be clean, (b) the seals must be resistant to the fluids, and (c) the seals and valves must be appropriately maintained. NEXT PREVIOUS Slide Home Pilot-Operated Safety Valve Introduction Reliefs Runaways Safeguards These disadvantages are also true for spring operated reliefs. Pilot-operated valves are not used in liquid service; they are normally used in very clean and low pressure applications. NEXT PREVIOUS Slide Home Types of Relief Valves • Conventional • Balanced Bellows Introduction Reliefs Runaways Safeguards Relief valves (for liquid service) are either the conventional or the balanced bellows types. NEXT PREVIOUS Slide Home Types of Rupture Discs • Metal • Graphite • Composite • Others Introduction Reliefs Runaways Safeguards As illustrated, there are many different types of rupture discs. They are especially applicable for very corrosive environments; for example: discs made of carbon or Teflon coating are used for corrosive service. NEXT PREVIOUS Slide Home Types of Rupture Discs • Metal • Graphite • Composite • Others Introduction Reliefs Runaways Safeguards A rupture disc that is used for pressure reliefs may need a specially designed mechanical support if it is also used in vacuum service. NEXT PREVIOUS Slide Home Rupture Disc and Pressure Relief Valve Combination Introduction Reliefs Runaways Safeguards Rupture discs, as illustrated, are sometimes used in combination with a spring operated relief device. In this case the disc gives a positive seal compared to the disc-to-seal design of a spring operated valve. NEXT PREVIOUS Slide Home Rupture Disc and Pressure Relief Valve Combination Introduction Reliefs Runaways Safeguards This is useful when handling very toxic materials where even a very small release (through the seal) may be hazardous, or when handling materials that polymerize. The spring operated relief following the rupture disc reseats when the pressure drops below the blow-down pressure. NEXT PREVIOUS Slide Home Rupture Disc and Pressure Relief Valve Combination Introduction Reliefs Runaways Safeguards This design, therefore, stops the discharge from the vessel. The discharge is not stopped if only a rupture disc is used. This design (rupture disc followed by a spring-operated relief) is discouraged by some practitioners. NEXT PREVIOUS Slide Home Rupture Disc and Pressure Relief Valve Combination Introduction Reliefs Runaways Safeguards In this design, as illustrated, a pressure detection device (per ASME Code), e.g., a pressure indicator, needs to be placed between the disc and the spring-operated valve. This pressure reading is checked periodically to be sure the rupture disc has its mechanical integrity. NEXT PREVIOUS Slide Home Rupture Disc and Pressure Relief Valve Combination Introduction Reliefs Runaways Safeguards A pin-hole leak in the rupture disc could increase the pressure on the discharge side of the disc. This is a major problem because it increases the relief pressure, that is: the differential pressure across the disc is the rupturing mechanism. NEXT PREVIOUS Slide Home Rupture Disc and Pressure Relief Valve Combination Introduction Reliefs Runaways Safeguards Another major problem with this design is the possibility that a piece of the rupture disc could plug the discharge orifice of the spring operated relief. This problem is prevented by specifying a rupture disc that will maintain its integrity when it is ruptured; that is, non-fragmenting. NEXT PREVIOUS Slide Home Vacuum Relief Devices • Vacuum Relief Valves • Rupture Discs • Conservation Vents • Manhole Lids • Pressure Control Introduction Reliefs Runaways Safeguards Vacuum relief devices are: vacuum relief valves, rupture discs, conservation vents, manhole lids designed for vacuum relief, and pressure control. NEXT PREVIOUS Slide Home Conservation Vent Introduction Reliefs Runaways Safeguards A conservation vent is illustrated in this sketch. As shown, it is designed to relieve a pressure usually for pressures in the region of 6 inches of water. It is also designed to let air into the vessel to prevent a vacuum, usually a vacuum no more than 4 inches of water. NEXT PREVIOUS Slide Home Pressure or Vacuum Control • Add Air or Nitrogen • Maintain Appropriately Introduction Reliefs Runaways Safeguards Sometimes pressure or vacuum control systems are used to add air or nitrogen to the vessel to maintain a slight pressure. In this case, the system needs to be appropriately maintained because a malfunction could result in an overpressure or underpressure. In either case the consequence could be a ruptured vessel. NEXT PREVIOUS Slide Home Relief Servicing • Inspection • Testing Introduction Reliefs Runaways Safeguards Every relief device needs to be inspected and tested before installation and then at predetermined intervals during its lifetime. The interval depends on the service history, vendor recommendations, and regulatory requirements, but it is usually once a year. NEXT PREVIOUS Slide Home Relief Servicing • Inspection • Testing Introduction Reliefs Runaways Safeguards Operating results and experience may indicate shorter or longer intervals. Records must be carefully maintained for every inspection and test, and for the entire life of the plant. NEXT PREVIOUS Slide Home Relief Discharges • To Atmosphere Introduction Reliefs Runaways Safeguards Discharges from pressure relief devices may be sent directly to the atmosphere if they are innocuous, discharged in a safe manner, and regulations permit it. NEXT PREVIOUS Slide Home Relief Discharges • To Atmosphere • Prevented Introduction Reliefs Runaways Safeguards An additional option is to prevent releases by (a) designing vessels with high MAWPs to contain all overpressure scenarios, or (b) add a sufficient number of safeguards and/or controls to make overpressure scenarios essentially impossible. NEXT PREVIOUS Slide Home Relief Discharges • To Atmosphere • Prevented • Effluent System Introduction Reliefs Runaways Safeguards The third option is to design an effluent system to capture all nocuous liquids and gases. NEXT PREVIOUS Slide Home Effluent Systems • Knock-Out Drum • Catch Tank • Cyclone Separator Introduction Reliefs Runaways Safeguards An effluent system may contain a • Knock-out drum • Catch tank • Cyclone separator NEXT PREVIOUS Slide Home Effluent System (continued) • Condenser • Quench Tank • Scrubber • Flares/Incinerators Introduction Reliefs Runaways Safeguards • Condenser • Quench tank • Scrubber, and/or • Flares or incinerators An effluent handling system may have any combination of the above unit operations. NEXT PREVIOUS Slide Home Effluent Handling System Introduction Reliefs Runaways Safeguards One effluent handling system is illustrated in this sketch. Every element of an effluent system needs to be designed very carefully. The design requires detailed physical and chemical properties, and the correct design methodology for each unit operation. NEXT PREVIOUS Slide Home Effluent Handling System Introduction Reliefs Runaways Safeguards It should also be recognized that it is important to size the relief appropriately, because the size of the entire effluent system is based on this discharge rate. The design methodology is in the references noted in the Appendix of this package. NEXT PREVIOUS Slide Home Part 2 of 3: Runaways Introduction NEXT Reliefs Runaways Safeguards PREVIOUS Slide Home Runaway Reaction • Temperature Increases • Reaction Rate Increases • Pressure Increases Introduction Reliefs Runaways Safeguards A runaway reaction is an especially important overpressure scenario. A runaway reaction has an accelerating rate of temperature increase, rate of reaction increase, and usually rate of pressure increase. The pressure, of course, increases if the reaction mass has a volatile substance, such as, a solvent or a monomer; or if one of the reaction products is a gas. NEXT PREVIOUS Slide Home Causes of Runaway Reactions • Self-Heating • Sleeper Characteristics of Runaway • Tempered • Gassy • Hybrid Introduction Reliefs Runaways Safeguards In general, there are two causes of runaway reactions (selfheating and sleeper) and three characteristics of runaways (tempered, gassy, and hybrid). NEXT PREVIOUS Slide Home Causes of Runaway Reactions • Self-Heating • Sleeper Characteristics of Runaway • Tempered • Gassy • Hybrid Introduction Reliefs Runaways Safeguards When protecting a system for overpressures due to runaway reactions the engineer needs to know the type of runaway and needs to characterize the behavior of the specific runaway with a special calorimeter. This specific methodology is described in this section of this presentation. NEXT PREVIOUS Slide Home Self-Heating Reaction • Loss of Cooling • Unexpected Addition of Heat • Too Much Catalyst or Reactant • Operator Mistakes • Too Fast Addition of Catalyst or Reactant Introduction Reliefs Runaways Safeguards One self-heating scenario occurs when the reaction is exothermic and a loss of cooling gives an uncontrolled temperature rise. A few causes of self-heating scenarios are shown. NEXT PREVIOUS Slide Home Sleeper Reactions • Reactants Added But Not Mixed (Error) • Reactants Accumulate • Agitation Started .. Too Late Introduction Reliefs Runaways Safeguards Sleeper reactions are usually the result of an operator error. Two examples include: (a) the addition of two immiscible reactants when the agitator is mistakenly in the off position, and (b) the addition of a reactant to the reaction mass when the temperature is mistakenly lower than that required to initiate the reaction. NEXT PREVIOUS Slide Home Sleeper Reactions • Reactants Added But Not Mixed (Error) • Reactants Accumulate • Agitation Started .. Too Late Introduction Reliefs Runaways Safeguards In these cases the runaway is initiated by starting the agitator and adding heat respectively. NEXT PREVIOUS Slide Home Tempered Reaction • Heat Removed by Evaporation • Heat Removal Maintains a Constant Temperature Introduction Reliefs Runaways Safeguards Tempered runaway reactions maintain their temperature when the energy exiting the relief device is equal to the energy generated in the reactor due to the exothermic reaction. The reaction heat is absorbed by the evaporation of the volatile components. The vapor pressure in a tempered system can typically be characterized by an Antoine type equation. NEXT PREVIOUS Slide Home Gassy System • No Volatile Solvents • Gas is Reaction Product Introduction Reliefs Runaways Safeguards A system that is characterized as “gassy” has no volatile solvents or reactants. The pressure build-up is due to the generation of noncondensible gas such as N2 or CO2. NEXT PREVIOUS Slide Home Hybrid System • Tempered • Gassy Introduction Reliefs Runaways Safeguards A hybrid system is the combination of a tempered and a gassy system. Under runaway conditions, the pressure increases due to the vapor pressure of the volatile components as well as from the generation of noncondensible gaseous reaction products. NEXT PREVIOUS Slide Home Reliefs for Runaway Reactions • Two Phase (or Three Phases: Liquid, Vapor, and Solid) Flow Introduction Reliefs Runaways Safeguards Under runaway conditions, when the relief device opens, the relief discharge is a foam; that is, the gases are entrained with the liquid. NEXT PREVIOUS Slide Home Reliefs for Runaway Reactions • Two Phase (or Three Phases: Liquid, Vapor, and Solid) Flow Introduction Reliefs Runaways Safeguards To maintain a constant temperature in the reactor (i.e. control the runaway reaction), the relief valve is sized to remove all the heat generated from the exothermic reaction via the heat removed with the discharged mass, which is typically a foam. Detailed information on runaway reactions is found in the appendix. NEXT PREVIOUS Slide Home Reliefs for Runaway Reactions • Two Phase (or Three Phases: Liquid, Vapor, and Solid) Flow • Relief Area: 2 to 10 Times the Area of a Single Gaseous Phase Introduction Reliefs Runaways Safeguards The required relief area to remove this heat with the foam is two to ten times the area that would be required by releasing a single gaseous phase. NEXT PREVIOUS Slide Home Two Phase Flow Introduction Reliefs Runaways Safeguards This is a picture that illustrates the two-phase flow characteristics of a relief discharge due to a runaway reaction. As illustrated, the discharge is similar to the release of foam from a freshly opened bottle of pop after being shakened. If the relief is not designed for two-phase flow, the pressures would increase rapidly and the vessel could rupture. NEXT PREVIOUS Slide Home Relief Valve Sizing Methodology • Special Calorimeter Data • Special Calculation Methods Introduction Reliefs Runaways Safeguards The relief valve sizing methodology for runaway reactions is very complex. It requires the characterization of the runaway reaction using a specially designed calorimeter. Relief valve sizing, additionally, requires special calculation methods that are described in the Appendix of this package. NEXT PREVIOUS Slide Home Characterization of Runaway Reactions Introduction Reliefs Runaways Safeguards The characterization of runaway reactions includes the determination of the rates of rise of the temperature and pressure under adiabatic conditions. The test results also characterize the reaction type, that is, tempered, gassy, and/or a hybrid system. NEXT PREVIOUS Slide Home Characterization of Runaway Reactions • ARC • VSP • RSST Introduction Reliefs Runaways Safeguards Various calorimeters are used for this characterization: • The accelerating rate calorimeter (ARC) • The vent sizing package (VSP) • The reactive system screening tool (RSST) NEXT PREVIOUS Slide Home Characterization of Runaway Reactions Introduction Reliefs Runaways Safeguards • ARC • APTAC • VSP • PHI-TEC • RSST • Dewars • The automated pressure-tracking adiabatic calorimeter (APTAC) • The Phi-Tec, and • Dewars. Each of these calorimeters have advantages and disadvantages that need to be understood when studying a specific system. NEXT PREVIOUS Slide Home Part 3 of 3: Safeguards Introduction NEXT Reliefs Runaways Safeguards PREVIOUS Slide Home Safeguards Introduction Reliefs Runaways Safeguards This section of the presentation covers safeguards. Safeguards include the methods and controls used to prevent runaways. As illustrated previously, a containment system (a safeguard), can be very complex and expensive. Alternatively, a series of safeguards may be justified. NEXT PREVIOUS Slide Home Safeguards • Safety Interlocks • Safeguard Maintenance System • Short-Stopping Introduction Reliefs Runaways Safeguards Safeguards include safety interlocks, safeguard maintenance system, and/or short-stopping. NEXT PREVIOUS Slide Home Safety Interlocks • Agitator Not Working: Stop Monomer Feed and Add Full Cooling • Abnormal Temperature: Stop Monomer Feed and Add Full Cooling Introduction Reliefs Runaways Safeguards The list of alternative interlocks is fairly extensive. Usually more than one interlock and some redundancy and diversity is required for each runaway scenario. As the number of interlocks increases, the reliability of the system increases. These are examples of safety interlocks for a semibatch polymerization reactor. NEXT PREVIOUS Slide Home Safety Interlocks (continued) • Abnormal Pressure: Stop Monomer Feed and Add Full Cooling • Abnormal Heat Balance: Stop Monomer Feed and Add Full Cooling • Abnormal Conditions: Add Short-Stop Introduction Reliefs Runaways Safeguards This is a list of additional interlocks. Other interlocks (manual) that are not on this list include: gages with manual shutdowns, and alarms with manual shutdowns. NEXT PREVIOUS Slide Home Safeguard Maintenance System • Routine Maintenance • Management of Change • Mechanical Integrity Checks • Records Introduction Reliefs Runaways Safeguards A safeguard maintenance system includes routine maintenance, management of change, mechanical integrity checks, and the appropriate records. These are the steps that are required to be sure the safeguards and interlocks perform appropriately under emergency conditions and/or potential runaway reaction scenarios. NEXT PREVIOUS Slide Home Safeguard Maintenance System • Routine Maintenance • Management of Change • Mechanical Integrity Checks • Records Introduction Reliefs Runaways Safeguards The maintenance of safeguard systems is especially important, because: • Safeguards and interlocks do not operate on a day-to-day basis, but • When they are required to operate (emergency conditions) they need to operate flawlessly. See ISA SP 84.01 for details for the design of safety instrumented systems. NEXT PREVIOUS Slide Home Short-Stops to Stop Reaction • Add Reaction Stopper • Add Agitation with No Electrical Power Introduction Reliefs Runaways Safeguards A short-stopping system, stops a runaway reaction by adding a reaction stopper solution to the reacting mass. The reaction-stopper stops the reaction in time to short-circuit the progress of the reaction. A reaction stopper needs to be added when the reaction mass is relatively cold. If the mass is too hot, a short-stopper will not work. NEXT PREVIOUS Slide Home Short-Stops to Stop Reaction • Add Reaction Stopper • Add Agitation with No Electrical Power Introduction Reliefs Runaways Safeguards Good agitation, of course, is required to adequately mix the reaction mass with the inhibitor. Since a power failure is often the initiating event of a runaway, an alternative method of agitation needs to be included in the design. A compressed nitrogen system together with a sparge ring is one alternative. NEXT PREVIOUS Slide Home Protection for Internal Fires and Explosions • Deflagrations • Detonations Introduction Reliefs Runaways Safeguards This section of the presentation covers protection methods for internal fires and explosions. Overpressure protection is needed for process equipment that can potentially explode due to an internal deflagration or detonation. NEXT PREVIOUS Slide Home Protection for Internal Fires and Explosions • Deflagrations • Detonations Introduction Reliefs Runaways Safeguards A deflagration is defined as the propagation of a combustion zone at a velocity in the unreacted medium that is less than the speed of sound. A detonation has a velocity greater than the speed of sound in the unreacted medium. NEXT PREVIOUS Slide Home Protection for Internal Fires and Explosions • Deflagrations • Detonations Introduction Reliefs Runaways Safeguards The burning material can be a combustible gas, a combustible dust, a combustible mist, or a hybrid mixture (a mixture of a combustible gas with either a combustible dust or combustible mist). The reaction actually occurs in the vapor phase between the fuel and the air or some other oxidant. NEXT PREVIOUS Slide Home Protection Methods for Internal Fires and Explosions • Deflagration Venting • Deflagration Suppression • Containment Introduction Reliefs Runaways Safeguards The protection methods used for fires or explosions include • Deflagration venting • Deflagration suppression • Containment NEXT PREVIOUS Slide Home Protection Methods for Internal Fires and Explosions (continued) • Reduction of Oxidant • Reduction of Combustible • Flame Front Isolation Introduction Reliefs Runaways Safeguards • Reduction of the oxidant • Reduction of the combustible • Flame front isolation NEXT PREVIOUS Slide Home Protection Methods for Internal Fires and Explosions (continued) • Spark Detection and Extinguishing • Flame Detection and Extinguishing • Water Spray and Deluge Systems Introduction Reliefs Runaways Safeguards • Spark detection and extinguishing • Flame detection and extinguishing • Water or foam spray deluge systems NEXT PREVIOUS Slide Home Deflagration Venting • Vent Area via NFPA 68 Introduction Reliefs Runaways Safeguards The technology required for venting deflagrations is given in NFPA 68. Deflagration venting is usually the simplest and least costly means of protecting process equipment against damage due to the internal pressure rise from deflagrations. NEXT PREVIOUS Slide Home Deflagration Venting • Vent Area via NFPA 68 • Vent Safely Introduction Reliefs Runaways Safeguards If equipment is located inside a building, the vents must be discharged through a vent duct system to a safe location outside of the building. The design of the vent duct system is critical to avoid excessive pressures developed during the venting process. See NFPA 68 for details. NEXT PREVIOUS Slide Home Deflagration Venting • Vent Area via NFPA 68 • Vent Safely Introduction Reliefs Runaways Safeguards A safe location will avoid injury to personnel and minimize damage to equipment outside of the building. The next two pictures illustrate that the “safe venting” may not be trivial. NEXT PREVIOUS Slide Home Vent of Gas Deflagration Introduction Reliefs Runaways Safeguards This is a picture of the venting of a gas deflagration. As illustrated, the flame propagates a significant distance from the vessel. The length of the flame is estimated using an equation found in NFPA 68. The main purpose of venting is to protect the mechanical integrity of the equipment. As illustrated, even when it is vented safely, this is a major event. NEXT PREVIOUS Slide Home Vent of Dust Deflagration Introduction Reliefs Runaways Safeguards This is a picture of the venting of a dust deflagration. As illustrated, the burning dust continues to burn at great distances from the vent. With dusts, this burning zone is larger because the container has a larger fuel-to-air ratio compared to the gas deflagration scenario. These pictures clearly illustrate the problems with venting deflagrations. NEXT PREVIOUS Slide Home Deflagration Suppression System Introduction Reliefs Runaways Safeguards One alternative to venting a deflagration is suppression. This sketch illustrates a deflagration suppression system that includes (a) a flame or pressure detector, (b) a quick opening valve, and (c) the addition of a flame suppressant. NEXT PREVIOUS Slide Home Deflagration Suppression System Introduction Reliefs Runaways Safeguards The commonly used suppression agents include water, potassium acid phosphate, sodium bicarbonate, and Halon substitutes. The technology for deflagration suppression is described in NFPA 69. NEXT PREVIOUS Slide Home Containment • Prevent Rupture and Vessel Deformation • Prevent Rupture but Deform Vessel Introduction Reliefs Runaways Safeguards The thickness of vessel walls may be increased to contain the pressure of a deflagration. • The wall thickness can be large enough to prevent the deformation of the vessel, or • The wall thickness may be large enough to prevent a rupture, but allow the vessel to deform. NEXT PREVIOUS Slide Home Reduction of Oxidant • Vacuum Purging • Pressure Purging • Sweep-Through Purging Introduction Reliefs Runaways Safeguards Protection for overpressures is also provided with an inert gas blanket to prevent the occurrence of a deflagration. Before introducing a flammable substance to a vessel, the vessel must also be purged with an inert gas to reduce the oxidant concentration sufficiently so that the gas mixture cannot burn. NEXT PREVIOUS Slide Home Reduction of Oxidant • Vacuum Purging • Pressure Purging • Sweep-Through Purging Introduction Reliefs Runaways Safeguards The purging methods include vacuum purging, pressure purging, and sweep-through purging. See NFPA 69 and the book by Crowl and Louvar for more details. NEXT PREVIOUS Slide Home Reduction of Combustible • Dilution with Air • NFPA 69 Introduction Reliefs Runaways Safeguards A deflagration can also be prevented by reducing the concentration of the combustible material so that the concentration is below the lower flammability limit (LFL). This is usually accomplished by dilution with nitrogen. The specifications for this type system are given in NFPA 69. NEXT PREVIOUS Slide Home Flame Front Isolation Introduction Reliefs Runaways Safeguards As illustrated, isolation devices are used in piping systems to prevent the propagation of a flame front. The method illustrated has a fast-acting block valve. This isolation system prevents the propagation of the flame front; more importantly it prevents deflagration transitions to detonations. NEXT PREVIOUS Slide Home Spark/Flame Detection and Extinguishing Introduction Reliefs Runaways Safeguards Another method of preventing the propagation of deflagrations in pipelines is the early detection and extinguishment of sparks or flames. In this type system, a detector activates an automatic extinguishing system that sprays water or other extinguishing agents into the fire. This system is similar to the deflagration suppression system discussed previously. NEXT PREVIOUS Slide Home Water Spray or Deluge Systems Introduction Reliefs Runaways Safeguards Process equipment and structures are very effectively protected against fire by water spray or deluge systems. They can be activated manually or automatically. They are designed to cool the equipment or structural members so that the heat from a fire will not weaken them. NEXT PREVIOUS Slide Home Deluge System Introduction Reliefs Runaways Safeguards This picture shows a typical deluge system in operation. In this example, the deluge system is automatically activated when the concentration of the flammable gas below the vessel is detected to be at or over 25% of the lower flammability limit. NEXT PREVIOUS Slide Home Conclusion Introduction Reliefs Runaways Safeguards This concludes our technology package covering overpressure and underpressure protection. The appendix of this package contains more detailed information. The enclosed references contain the state-of-the-art technology to assist engineers and students with their detailed designs. NEXT PREVIOUS Slide Home End of Slide Presentation (with text) Causes of Overpressure/Underpressure Presentation 2: Runaways HOME PREVIOUS Presentation 1: Reliefs Presentation 3: Safeguards Slide Exit
