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Design forOverp ressure and
UnderpressureProtect ion
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SLIDE PRESENTATION
Design for
Overp ressu re andUnderpressure
Protect ion
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Outline• Introduction
• Causes of Overpressure andUnderpressure
• Reliefs• Effluent Handling Systems for
Reliefs
• Runaway Reactions
• Overpressure Protection forInternal Fires and Explosions
Introduction
Reliefs
Runaways
Safeguards
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For Further Information:Refer to the Appendix
Supplied with this Presentation
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Causes of Overpressure
• Operating Problem
• Equipment Failure
• Process Upset
• External Fire
• Utility Failures
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Causes of Underpressures
• Operating Problem
• Equipment Failure
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Presentation 1 of 3: Reliefs
Causes of
Overpressure/Underpressure
Presentation 1: Reliefs
Presentation 2: Runaways
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Pressure Relief Devices
• Spring-Loaded Pressure Relief Valve
• Rupture Disc
• Buckling Pin
• Miscellaneous Mechanical
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Spring-LoadedPressure Relief Valve
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Rupture Disc
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Buckling Pin Relief Valve
ClosedPressure Below
Set Pressure
Full OpenPressure at or Above
Set Pressure(Buckles in Milliseconds at a Precise Set Pressure)
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Simple Mechanical
Pressure Relief
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Types of Spring-Loaded
Pressure Reliefs• Safety Valves for Gases and Vapors
• Relief Valves for Liquids
• Safety Relief Valves for Liquids
and/or Gases
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Types of Safety Valves
• Conventional
• Balanced Bellows, and• Pilot-Operated
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Conventional Safety Valve
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Balanced Bellows Safety Valve
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Pilot-Operated Safety Valve
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Types of Relief Valves
• Conventional
• Balanced Bellows
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Types of Rupture Discs
• Metal
• Graphite
• Composite
• Others
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Rupture Disc and PressureRelief Valve Combination
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Vacuum Relief Devices
• Vacuum Relief Valves
• Rupture Discs
• Conservation Vents
• Manhole Lids
• Pressure Control
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Conservation Vent
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Pressure or Vacuum Control
• Add Air or Nitrogen
• Maintain Appropriately
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Relief Servicing
• Inspection
• Testing
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Relief Discharges
• To Atmosphere
• Prevented
• Effluent System
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Effluent Systems
• Knock-Out Drum
• Catch Tank
• Cyclone Separator
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Effluent System (continued)
• Condenser
• Quench Tank
• Scrubber
• Flares/Incinerators
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Effluent Handling System
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Presentation 2 of 3: Runaways
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Causes of
Overpressure/Underpressure
Presentation 1: Reliefs
Presentation 2: Runaways
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Runaway Reaction
• Temperature Increases
• Reaction Rate Increases
• Pressure Increases
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Causes of Runaway Reactions
• Self-Heating• Sleeper
• Tempered
• Gassy
• Hybrid
Characteristics of Runaway
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Self-Heating Reaction
• Loss of Cooling
• Unexpected Addition of Heat
• Too Much Catalyst or Reactant
• Operator Mistakes
• Too Fast Addition of Catalyst or Reactant
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Sleeper Reactions
• Reactants Added But Not Mixed
(Error)
• Reactants Accumulate
• Agitation Started .. Too Late
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Tempered Reaction
• Heat Removed by Evaporation
• Heat Removal Maintains a Constant
Temperature
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Gassy System
• No Volatile Solvents
• Gas is Reaction Product
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Hybrid System
• Tempered
• Gassy
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Reliefs for Runaway Reactions
• Two Phase (or Three Phases:
Liquid, Vapor, and Solid) Flow
• Relief Area: 2 to 10 Times theArea of a Single Gaseous Phase
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Two Phase Flow
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Relief Valve Sizing
Methodology• Special Calorimeter Data
• Special Calculation Methods
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Characterization of
Runaway Reactions• ARC
• VSP• RSST
• APTAC
• PHI-TEC• Dewars
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Presentation 3 of 3:Safeguards
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Causes of
Overpressure/Underpressure
Presentation 1: Reliefs
Presentation 2: Runaways
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Safeguards
• Safety Interlocks
• Safeguard Maintenance System
• Short-Stopping
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Safety Interlocks
• Agitator Not Working: Stop Monomer
Feed and Add Full Cooling
• Abnormal Temperature: StopMonomer Feed and Add Full Cooling
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Safety Interlocks
(continued)• Abnormal Pressure: Stop Monomer Feed
and Add Full Cooling
• Abnormal Heat Balance: Stop MonomerFeed and Add Full Cooling
• Abnormal Conditions: Add Short-Stop
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Safeguard Maintenance
System• Routine Maintenance
• Management of Change
• Mechanical Integrity Checks
• Records
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Short-Stops to Stop Reaction
• Add Reaction Stopper
• Add Agitation with No Electrical
Power
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Protection for Internal
Fires and Explosions
• Deflagrations
• Detonations
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Protection Methods for
Internal Fires and Explosions
• Deflagration Venting
• Deflagration Suppression
• Containment
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Protection Methods for
Internal Fires and Explosions(continued)
• Reduction of Oxidant
• Reduction of Combustible
• Flame Front Isolation
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Protection Methods for
Internal Fires and Explosions(continued)
• Spark Detection and Extinguishing
• Flame Detection and Extinguishing
• Water Spray and Deluge Systems
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Deflagration Venting
• Vent Area via NFPA 68
• Vent Safely
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Vent of Gas Deflagration
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Vent of Dust Deflagration
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Deflagration SuppressionSystem
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Containment
• Prevent Rupture and Vessel
Deformation
• Prevent Rupture but Deform
Vessel
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Reduction of Oxidant
• Vacuum Purging
• Pressure Purging
• Sweep-Through Purging
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Reduction of Combustible
• Dilution with Air
• NFPA 69
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Flame Front Isolation
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Spark/Flame Detectionand Extinguishing
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Water Spray orDeluge Systems
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Conclusion
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End of Slide Presentation
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Causes of
Overpressure/Underpressure
Presentation 1: Reliefs
Presentation 2: Runaways
Presentation 3: Safeguards
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Design forOverp ressure and
UnderpressureProtect ion
This presentation includes technical information
concerning the design for overpressure and
underpressure protection. The presentation is designed
to help students and engineers to: Slide
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Design forOverp ressure and
UnderpressureProtect ion
• Understand the root causes of overpressure and
underpressure incidents, and
• Design plants with the appropriate features to protect
against overpressure and underpressure incidents. Slide
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Six Sections
1. Introduction2. 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
This presentation is divided into six sections:
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Six Sections
1. Introduction2. 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
The “Introduction” button on your left will lead you to this
introduction and an explaination of the Causes of
Overpressure and Underpressure
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Six Sections
1. Introduction2. 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
The “Reliefs” Button sends you to Sections 3 and 4,
covering Reliefs and Effluent Handling Systems for
Reliefs
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Six Sections
1. Introduction2. 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
The “Runaways” Button leads to a discussion on
Runaway Reactions, and . . .
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Six Sections
1. Introduction2. 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
The “Safeguards” Button will take you to a section on
Overpressure Protection fot Internal Fires and
Explosions
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Appendix ContainsDetailed Information
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. Slide
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Causes of Overpressure
• Operating Problem
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 toincrease. An operator, for example, may adjust a steam regulator togive pressures exceeding the maximum allowable working pressure
(MAWP) of a steam jacket. Slide
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Causes of Overpressure
• Operating Problem
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.
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Causes of Overpressure
• Operating Problem
• Equipment Failure
• 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. Slide
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Causes of Overpressure
• Operating Problem
• Equipment Failure
• Process Upset
• External Fire
• Utility Failures• Process upset; for example a runaway reaction causing high
temperatures and pressures.
• External heating, such as, a fire that heats the contents of a vesselgiving high vapor pressures, and
• Utility failures, such as the loss of cooling or the loss of agitation
causing a runaway reaction. Slide
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Causes of Underpressures
The causes of underpressure or the inadvertent creation of avacuum are usually due to operating problems or equipmentfailures.
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Causes of Underpressures
• Operating Problem
• Operating problems include mistakes such as pumping liquidout of a closed system, or cooling and condensing vapors in aclosed system.
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Causes of Underpressures
• Operating Problem
• Equipment Failure
• Equipment failures include an instrument malfunction (e.g.vacuum gage) or the loss of the heat input of a system thatcontains a material with a low vapor pressure.
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Part 1 of 3: Reliefs
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Pressure Relief Devices
Pressure relief devices are added to process equipment toprevent the pressures from significantly exceeding the MAWP(pressures are allowed to go slightly above the MAWP duringemergency reliefs).
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Pressure Relief Devices
• Spring-Loaded Pressure Relief Valve
• Rupture Disc
• Buckling Pin
• Miscellaneous Mechanical
The pressure relief devices include spring-loaded pressure reliefvalves, rupture discs, buckling pins, and miscellaneousmechanical devices.
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Spring-LoadedPressure Relief Valve
This is a sketch of a spring-loaded pressure relief valve. As thepressure in the vessel or pipeline at point A exceeds thepressure created by the spring, the valve opens. The reliefbegins to open at the set pressure which is usually at or belowthe MAWP; this pressure is usually set at the MAWP.
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Rupture Disc
This is a sketch of a rupture disc. In this case the disc ruptureswhen the pressure at A exceeds the set pressure. Recognize,however, that it is actually the differential pressure (A-B), thatruptures the disc.
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Buckling Pin Relief Valve
ClosedPressure Below
Set Pressure
Full OpenPressure at or Above
Set Pressure
(Buckles in Milliseconds at a Precise Set Pressure)
This sketch shows a buckling pin pressure relief valve. Asshown, when the pressure exceeds the set pressure, the pinbuckles and the vessel contents exit through the open valve.
The rupture disc and the buckling pin relief valves stay open
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Buckling Pin Relief Valve
ClosedPressure Below
Set Pressure
Full OpenPressure at or Above
Set Pressure
(Buckles in Milliseconds at a Precise Set Pressure)
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.
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Si l M h i l
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Simple MechanicalPressure Relief
A simple mechanical pressure relief is a weighted man-waycover as shown in this sketch. Another mechanical relief is a U-tube filled with water (or equivalent).
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T f S i L d d
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Types of Spring-Loaded
Pressure Reliefs
• Safety Valves for Gases and Vapors
• Relief Valves for Liquids
• Safety Relief Valves for Liquids
and/or Gases
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.
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Conventional Safety Valve
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.
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Balanced Bellows Safety Valve
A balanced bellows safety valve is specially designed to reducethe effect of the back pressure on the opening pressure. Asillustrated in this sketch the differential pressure that is requiredto open the valve is the pressure inside the vessel minus theatmospheric pressure.
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Balanced Bellows Safety Valve
The bellows design allows the outside air and pressure to be onthe downstream side of the valve seal. Once the relief is open,then the flow is a function of the differential pressure A-B.
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Pilot-Operated Safety Valve
A pilot-operated safety valve is a spring-loaded valve. Asillustrated, the vessel pressure helps to keep the valve closed.When the pressure exceeds the set pressure (or the springpressure), the pressure on top of the valve is vented and thevalve opens.
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Pilot-Operated Safety Valve
The set pressure of this type of valve can be closer to theoperating pressure compared to conventional and balancedbellows valves. The disadvantages, however, are (a) theprocess fluid needs to be clean, (b) the seals must be resistantto the fluids, and (c) the seals and valves must be appropriately
maintained. Slide
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Pilot-Operated Safety Valve
These disadvantages are also true for spring operated reliefs.Pilot-operated valves are not used in liquid service; they arenormally used in very clean and low pressure applications.
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Types of Relief Valves
• Conventional
• Balanced Bellows
Relief valves (for liquid service) are either the conventional or
the balanced bellows types.
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Types of Rupture Discs
• Metal
• Graphite
• Composite
• Others
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.
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Types of Rupture Discs
• Metal
• Graphite
• Composite
• Others
A rupture disc that is used for pressure reliefs may need a
specially designed mechanical support if it is also used in
vacuum service.
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Rupture Disc and PressureRelief Valve Combination
Rupture discs, as illustrated, are sometimes used incombination with a spring operated relief device. In this casethe disc gives a positive seal compared to the disc-to-sealdesign of a spring operated valve.
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Rupture Disc and PressureRelief Valve Combination
This is useful when handling very toxic materials where even avery small release (through the seal) may be hazardous, orwhen handling materials that polymerize.
The spring operated relief following the rupture disc reseatswhen the pressure drops below the blow-down pressure.
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Rupture Disc and PressureRelief Valve Combination
This design, therefore, stops the discharge from the vessel.The discharge is not stopped if only a rupture disc is used. Thisdesign (rupture disc followed by a spring-operated relief) isdiscouraged by some practitioners.
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Rupture Disc and PressureRelief Valve Combination
In this design, as illustrated, a pressure detection device (per ASME Code), e.g., a pressure indicator, needs to be placedbetween the disc and the spring-operated valve. This pressurereading is checked periodically to be sure the rupture disc hasits mechanical integrity.
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Rupture Disc and PressureRelief Valve Combination
A pin-hole leak in the rupture disc could increase the pressureon the discharge side of the disc. This is a major problembecause it increases the relief pressure, that is: the differentialpressure across the disc is the rupturing mechanism.
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Rupture Disc and PressureRelief Valve Combination
Another major problem with this design is the possibility that apiece of the rupture disc could plug the discharge orifice of thespring operated relief. This problem is prevented by specifyinga rupture disc that will maintain its integrity when it is ruptured;that is, non-fragmenting.
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Vacuum Relief Devices
• Vacuum Relief Valves
• Rupture Discs
• Conservation Vents
• Manhole Lids
• Pressure ControlVacuum relief devices are: vacuum relief valves, rupture discs,
conservation vents, manhole lids designed for vacuum relief,
and pressure control.
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Conservation Vent
A conservation vent is illustrated in this sketch. As shown, it isdesigned to relieve a pressure usually for pressures in theregion of 6 inches of water. It is also designed to let air into thevessel to prevent a vacuum, usually a vacuum no more than 4inches of water.
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Pressure or Vacuum Control
• Add Air or Nitrogen
• Maintain Appropriately
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.Slide
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Relief Servicing
• Inspection
• Testing
Every relief device needs to be inspected and tested beforeinstallation and then at predetermined intervals during itslifetime. The interval depends on the service history, vendorrecommendations, and regulatory requirements, but it is usuallyonce a year.
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Relief Servicing
• Inspection
• Testing
Operating results and experience may indicate shorter or longerintervals.
Records must be carefully maintained for every inspection andtest, and for the entire life of the plant.
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Relief Discharges
• To Atmosphere
• Prevented
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.
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Effluent System (continued)
• Condenser
• Quench Tank
• Scrubber
• Flares/Incinerators
• Condenser• Quench tank• Scrubber, and/or• Flares or incinerators
An effluent handling system may have any combination of the above unit
operations. Slide
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Effluent Handling System
One effluent handling system is illustrated in this sketch. Everyelement of an effluent system needs to be designed verycarefully. The design requires detailed physical and chemicalproperties, and the correct design methodology for each unitoperation.
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Effluent Handling System
It should also be recognized that it is important to size the reliefappropriately, because the size of the entire effluent system isbased on this discharge rate. The design methodology is in thereferences noted in the Appendix of this package.
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Part 2 of 3: Runaways
Slide
R R i
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Runaway Reaction
• Temperature Increases
• Reaction Rate Increases
• Pressure Increases
A runaway reaction is an especially important overpressure scenario. Arunaway reaction has an accelerating rate of temperature increase, rateof reaction increase, and usually rate of pressure increase. Thepressure, of course, increases if the reaction mass has a volatilesubstance, such as, a solvent or a monomer; or if one of the reaction
products is a gas. Slide
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Causes of Runaway Reactions• Self-Heating
• Sleeper
• Tempered
• Gassy
• Hybrid
Characteristics of Runaway
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.
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Self-Heating Reaction
• Loss of Cooling
• Unexpected Addition of Heat
• Too Much Catalyst or Reactant
• Operator Mistakes
• Too Fast Addition of Catalyst or Reactant
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.
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Sleeper Reactions
• Reactants Added But Not Mixed
(Error)
• Reactants Accumulate• Agitation Started .. Too Late
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.
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Sleeper Reactions
• Reactants Added But Not Mixed
(Error)
• Reactants Accumulate• Agitation Started .. Too Late
In these cases the runaway is initiated by starting the agitatorand adding heat respectively.
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Tempered Reaction
• Heat Removed by Evaporation
• Heat Removal Maintains a Constant
Temperature
Tempered runaway reactions maintain their temperature when the energyexiting the relief device is equal to the energy generated in the reactordue to the exothermic reaction. The reaction heat is absorbed by theevaporation of the volatile components. The vapor pressure in atempered system can typically be characterized by an Antoine type
equation. Slide
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Gassy System
• No Volatile Solvents
• Gas is Reaction Product
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.
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Reliefs for Runaway Reactions
• Two Phase (or Three Phases:
Liquid, Vapor, and Solid) Flow
Under runaway conditions, when the relief device opens, the
relief discharge is a foam; that is, the gases are entrained with
the liquid.
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Reliefs for Runaway Reactions
• Two Phase (or Three Phases:
Liquid, Vapor, and Solid) Flow
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.
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Two Phase Flow
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Two Phase Flow
This is a picture that illustrates the two-phase flowcharacteristics of a relief discharge due to a runaway reaction.
As illustrated, the discharge is similar to the release of foamfrom a freshly opened bottle of pop after being shakened. If therelief is not designed for two-phase flow, the pressures would
increase rapidly and the vessel could rupture. Slide
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g
Methodology
• Special Calorimeter Data
• Special Calculation Methods
The relief valve sizing methodology for runaway reactions isvery complex. It requires the characterization of the runawayreaction using a specially designed calorimeter.
Relief valve sizing, additionally, requires special calculationmethods that are described in the Appendix of this package.
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Runaway Reactions
The characterization of runaway reactions includes thedetermination of the rates of rise of the temperature andpressure under adiabatic conditions. The test results alsocharacterize the reaction type, that is, tempered, gassy, and/ora hybrid system.
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Runaway Reactions
• ARC
• VSP
• RSST
Various calorimeters are used for this characterization:
• The accelerating rate calorimeter (ARC)
• The vent sizing package (VSP)
• The reactive system screening tool (RSST)
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Runaway Reactions
• ARC
• VSP
• RSST
• APTAC
• PHI-TEC
• Dewars
• The automated pressure-tracking adiabatic calorimeter (APTAC)
• The Phi-Tec, and
• Dewars.
Each of these calorimeters have advantages and disadvantagesthat need to be understood when studying a specific system.
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Part 3 of 3: Safeguards
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Safeguards
This section of the presentation covers safeguards. Safeguardsinclude the methods and controls used to prevent runaways. Asillustrated previously, a containment system (a safeguard), canbe very complex and expensive. Alternatively, a series ofsafeguards may be justified.
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Safeguards
• Safety Interlocks
• Safeguard Maintenance System
• Short-Stopping
Safeguards include safety interlocks, safeguard maintenancesystem, and/or short-stopping.
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Safety Interlocks
• Agitator Not Working: Stop Monomer
Feed and Add Full Cooling
• Abnormal Temperature: StopMonomer Feed and Add Full Cooling
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.
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System
• Routine Maintenance
• Management of Change
• Mechanical Integrity Checks
• Records
A safeguard maintenance system includes routine maintenance,management of change, mechanical integrity checks, and theappropriate records. These are the steps that are required tobe sure the safeguards and interlocks perform appropriatelyunder emergency conditions and/or potential runaway reactionscenarios.
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System
• Routine Maintenance
• Management of Change
• Mechanical Integrity Checks
• Records
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 instrumentedsystems.
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Short-Stops to Stop Reaction
• Add Reaction Stopper
• Add Agitation with No Electrical
Power
A short-stopping system, stops a runaway reaction by adding areaction stopper solution to the reacting mass. The reaction-stopperstops the reaction in time to short-circuit the progress of thereaction. A reaction stopper needs to be added when the reactionmass is relatively cold. If the mass is too hot, a short-stopper willnot work.
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Short-Stops to Stop Reaction
• Add Reaction Stopper
• Add Agitation with No Electrical
Power
Good agitation, of course, is required to adequately mix the reactionmass with the inhibitor. Since a power failure is often the initiatingevent of a runaway, an alternative method of agitation needs to beincluded in the design. A compressed nitrogen system together witha sparge ring is one alternative.
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Fires and Explosions
• Deflagrations
• Detonations
This section of the presentation covers protection methods forinternal fires and explosions.
Overpressure protection is needed for process equipment thatcan potentially explode due to an internal deflagration ordetonation.
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Fires and Explosions
• Deflagrations
• Detonations
A deflagration is defined as the propagation of a combustionzone at a velocity in the unreacted medium that is less than thespeed of sound. A detonation has a velocity greater than thespeed of sound in the unreacted medium.
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Fires and Explosions
• Deflagrations
• Detonations
The burning material can be a combustible gas, a combustibledust, a combustible mist, or a hybrid mixture (a mixture of acombustible gas with either a combustible dust or combustiblemist). The reaction actually occurs in the vapor phase betweenthe fuel and the air or some other oxidant.
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Internal Fires and Explosions
• Deflagration Venting
• Deflagration Suppression• Containment
The protection methods used for fires or explosions include
• Deflagration venting
• Deflagration suppression
• Containment
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Protection Methods for
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Internal Fires and Explosions
(continued)
• Reduction of Oxidant
• Reduction of Combustible• Flame Front Isolation
• Reduction of the oxidant
• Reduction of the combustible
• Flame front isolation
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Protection Methods for
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Internal Fires and Explosions
(continued)
• Spark Detection and Extinguishing
• Flame Detection and Extinguishing
• Water Spray and Deluge Systems
• Spark detection and extinguishing
• Flame detection and extinguishing
• Water or foam spray deluge systems
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Deflagration Venting
• Vent Area via NFPA 68
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.Slide
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Deflagration Venting
• Vent Area via NFPA 68
• Vent Safely
If equipment is located inside a building, the vents must bedischarged through a vent duct system to a safe locationoutside of the building. The design of the vent duct system iscritical to avoid excessive pressures developed during theventing process. See NFPA 68 for details.
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Deflagration Venting
• Vent Area via NFPA 68
• Vent Safely
A safe location will avoid injury to personnel and minimizedamage to equipment outside of the building. The next twopictures illustrate that the “safe venting” may not be trivial.
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Vent of Gas Deflagration
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Vent of Gas Deflagration
This is a picture of the venting of a gas deflagration. As illustrated,the flame propagates a significant distance from the vessel. Thelength of the flame is estimated using an equation found in NFPA68. The main purpose of venting is to protect the mechanicalintegrity of the equipment. As illustrated, even when it is ventedsafely, this is a major event. Slide
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System
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.Slide
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System
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.Slide
Containment
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Containment
• Prevent Rupture and Vessel
Deformation
• Prevent Rupture but DeformVessel
The thickness of vessel walls may be increased to contain thepressure of a deflagration.
• The wall thickness can be large enough to prevent thedeformation of the vessel, or
• The wall thickness may be large enough to prevent a rupture, butallow the vessel to deform. Slide
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Reduction of Oxidant
• Vacuum Purging
• Pressure Purging
• Sweep-Through Purging
Protection for overpressures is also provided with an inert gasblanket to prevent the occurrence of a deflagration. Beforeintroducing a flammable substance to a vessel, the vessel mustalso be purged with an inert gas to reduce the oxidantconcentration sufficiently so that the gas mixture cannot burn.
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Reduction of Oxidant
• Vacuum Purging
• Pressure Purging
• Sweep-Through Purging
The purging methods include vacuum purging, pressurepurging, and sweep-through purging. See NFPA 69 and thebook by Crowl and Louvar for more details.
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Reduction of Combustible
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educt o o Co bust b e
• Dilution with Air
• NFPA 69
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. Slide
Flame Front Isolation
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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.Slide
Spark/Flame Detectionand Extinguishing
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g g
Another method of preventing the propagation of deflagrations inpipelines is the early detection and extinguishment of sparks or flames.In this type system, a detector activates an automatic extinguishingsystem that sprays water or other extinguishing agents into the fire. Thissystem is similar to the deflagration suppression system discussedpreviously.
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Water Spray orDeluge Systems
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.
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Deluge System
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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.Slide
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Conclusion
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. Slide
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Exit
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