Design for Overpressure and Underpressure Protection Slide ShowExitSlides with Text

Design for Overpressure and

Underpressure Protection



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SLIDE PRESENTATIONSLIDE PRESENTATION





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OutlineOutline• Introduction

• Causes of Overpressure and Underpressure

• Reliefs

• Effluent Handling Systems for Reliefs

• Runaway Reactions

• Overpressure Protection for Internal Fires and Explosions

• Introduction

• Causes of Overpressure and Underpressure

• Reliefs

• Effluent Handling Systems for Reliefs

• Runaway Reactions

• Overpressure Protection for Internal Fires and Explosions

IntroductionIntroduction

ReliefsReliefs

RunawaysRunaways

SafeguardsSafeguards

For Further Information:Refer to the Appendix

Supplied with this Presentation

For Further Information:Refer to the Appendix

Supplied with this Presentation

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Causes of OverpressureCauses of Overpressure• Operating Problem

• Equipment Failure

• Process Upset

• External Fire

• Utility Failures

• Operating Problem


• Process Upset

• External Fire


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Causes of UnderpressuresCauses of Underpressures





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Presentation 1 of 3: ReliefsPresentation 1 of 3: Reliefs

Causes of Causes of Overpressure/UnderpressureOverpressure/Underpressure

Presentation 1: ReliefsPresentation 1: Reliefs

Presentation 2: RunawaysPresentation 2: Runaways

Presentation 3: SafeguardsPresentation 3: SafeguardsHome

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Pressure Relief DevicesPressure Relief Devices

• Spring-Loaded Pressure Relief Valve

• Rupture Disc

• Buckling Pin

• Miscellaneous Mechanical


• Rupture Disc

• Buckling Pin


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Spring-Loaded Pressure Relief Valve


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Rupture DiscRupture Disc

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Buckling Pin Relief ValveBuckling 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-LoadedPressure 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 ValvesTypes of Safety Valves

• Conventional

• Balanced Bellows, and

• Pilot-Operated

• Conventional


• Pilot-Operated

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Conventional Safety ValveConventional Safety Valve

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Balanced Bellows Safety ValveBalanced Bellows Safety Valve

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Pilot-Operated Safety ValvePilot-Operated Safety Valve

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Types of Relief ValvesTypes of Relief Valves

• Conventional

• Balanced Bellows

• Conventional


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Types of Rupture DiscsTypes of Rupture Discs

• Metal

• Graphite

• Composite

• Others

• Metal

• Graphite

• Composite

• Others

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Rupture Disc and Pressure Relief Valve Combination


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Vacuum Relief DevicesVacuum Relief Devices• Vacuum Relief Valves

• Rupture Discs

• Conservation Vents

• Manhole Lids

• Pressure Control

• Vacuum Relief Valves

• Rupture Discs


• Manhole Lids


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Conservation VentConservation Vent

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Pressure or Vacuum ControlPressure or Vacuum Control

• Add Air or Nitrogen

• Maintain Appropriately



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Relief ServicingRelief Servicing

• Inspection

• Testing

• Inspection

• Testing

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Relief DischargesRelief Discharges

• To Atmosphere

• Prevented

• Effluent System

• To Atmosphere

• Prevented

• Effluent System

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Effluent SystemsEffluent Systems

• Knock-Out Drum

• Catch Tank

• Cyclone Separator

• Knock-Out Drum

• Catch Tank


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Effluent System (continued)Effluent System (continued)

• Condenser

• Quench Tank

• Scrubber

• Flares/Incinerators

• Condenser

• Quench Tank

• Scrubber


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Effluent Handling SystemEffluent Handling System

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Presentation 2 of 3: RunawaysPresentation 2 of 3: Runaways

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Presentation 3: SafeguardsPresentation 3: Safeguards

Runaway ReactionRunaway Reaction

• Temperature Increases

• Reaction Rate Increases

• Pressure Increases




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Causes of Runaway ReactionsCauses of Runaway Reactions• Self-Heating

• Sleeper

• Tempered

• Gassy

• Hybrid

• Self-Heating

• Sleeper

• Tempered

• Gassy

• Hybrid

Characteristics of RunawayCharacteristics of Runaway

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Self-Heating ReactionSelf-Heating Reaction

• Loss of Cooling

• Unexpected Addition of Heat

• Too Much Catalyst or Reactant

• Operator Mistakes

• Too Fast Addition of Catalyst or Reactant

• Loss of Cooling





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Sleeper ReactionsSleeper Reactions

• Reactants Added But Not Mixed (Error)

• Reactants Accumulate

• Agitation Started .. Too Late




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Tempered ReactionTempered Reaction

• Heat Removed by Evaporation

• Heat Removal Maintains a Constant Temperature



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Gassy SystemGassy System

• No Volatile Solvents

• Gas is Reaction Product



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Hybrid SystemHybrid System

• Tempered

• Gassy

• Tempered

• Gassy

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Reliefs for Runaway ReactionsReliefs 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



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Two Phase FlowTwo Phase Flow

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Relief Valve Sizing Methodology


• Special Calorimeter Data

• Special Calculation Methods



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Characterization of Runaway ReactionsCharacterization of Runaway Reactions

• ARC

• VSP

• RSST

• ARC

• VSP

• RSST

• APTAC

• PHI-TEC

• Dewars

• APTAC

• PHI-TEC

• Dewars

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Presentation 3 of 3: Safeguards

Presentation 3 of 3: Safeguards

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Presentation 3: SafeguardsPresentation 3: Safeguards


• Safety Interlocks

• Safeguard Maintenance System

• Short-Stopping



• Short-Stopping

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Safety InterlocksSafety Interlocks

• Agitator Not Working: Stop Monomer Feed and Add Full Cooling

• Abnormal Temperature: Stop Monomer 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 Monomer Feed 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




• Records

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Short-Stops to Stop ReactionShort-Stops to Stop Reaction

• Add Reaction Stopper

• Add Agitation with No Electrical Power



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Protection for InternalFires and ExplosionsProtection for InternalFires and Explosions

• Deflagrations

• Detonations

• Deflagrations

• Detonations

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Protection Methods forInternal Fires and Explosions


• Deflagration Venting

• Deflagration Suppression

• Containment



• Containment

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Protection Methods for Internal Fires and Explosions

(continued)


(continued)

• Reduction of Oxidant

• Reduction of Combustible

• Flame Front Isolation




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(continued)


(continued)

• Spark Detection and Extinguishing

• Flame Detection and Extinguishing

• Water Spray and Deluge Systems




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Deflagration VentingDeflagration Venting

• Vent Area via NFPA 68

• Vent Safely


• Vent Safely

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Vent of Gas DeflagrationVent of Gas Deflagration

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Vent of Dust DeflagrationVent of Dust Deflagration

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Deflagration Suppression System


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ContainmentContainment

• Prevent Rupture and Vessel Deformation

• Prevent Rupture but Deform Vessel



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Reduction of OxidantReduction of Oxidant

• Vacuum Purging

• Pressure Purging

• Sweep-Through Purging

• Vacuum Purging



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Reduction of CombustibleReduction of Combustible

• Dilution with Air

• NFPA 69


• NFPA 69

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Flame Front IsolationFlame Front Isolation

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Spark/Flame Detectionand Extinguishing


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Water Spray or Deluge SystemsWater Spray or Deluge Systems

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Deluge SystemDeluge System

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ConclusionConclusion

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End of Slide PresentationEnd of Slide Presentation

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Presentation 3: SafeguardsPresentation 3: SafeguardsText

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SafeguardsSafeguards Home

SLIDES WITH TEXTSLIDES WITH TEXT





This presentation includes technical information concerning the design for overpressure and underpressure protection. The presentation is designed to help students and engineers to:

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• Understand the technologies, special engineering devices, and methods that are used for the protection against overpressure and underpressure (vacuum) incidents,

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• Understand the root causes of overpressure and underpressure incidents, and

• Design plants with the appropriate features to protect against overpressure and underpressure incidents.

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Six SectionsSix Sections1. 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

1. Introduction


3. Reliefs




This presentation is divided into six sections:

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3. Reliefs




1. Introduction


3. Reliefs




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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3. Reliefs




1. Introduction


3. Reliefs




The “Reliefs” Button sends you to Sections 3 and 4, covering Reliefs and Effluent Handling Systems for Reliefs

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3. Reliefs




1. Introduction


3. Reliefs




The “Runaways” Button leads to a discussion on Runaway Reactions, and . . .

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3. Reliefs




1. Introduction


3. Reliefs




The “Safeguards” Button will take you to a section on Overpressure Protection fot Internal Fires and Explosions

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Appendix Contains Detailed InformationAppendix Contains Detailed 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.

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Causes of OverpressureCauses of Overpressure• Operating Problem• 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 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. Slide

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Causes of OverpressureCauses of Overpressure• Operating Problem• 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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• 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.

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• Process Upset

• External Fire




• Process Upset

• External Fire


• 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. Slide

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The causes of underpressure or the inadvertent creation of a vacuum are usually due to operating problems or equipment failures.

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• Operating Problem• Operating Problem

• Operating problems include mistakes such as pumping liquid out of a closed system, or cooling and condensing vapors in a closed system.

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• 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.

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Part 1 of 3: ReliefsPart 1 of 3: Reliefs

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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).

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• Rupture Disc

• Buckling Pin



• Rupture Disc

• Buckling Pin


The pressure relief devices include spring-loaded pressure relief valves, rupture discs, buckling pins, and miscellaneous mechanical devices.

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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.

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Rupture DiscRupture Disc

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.

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Set Pressure


Set Pressure


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.

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Set Pressure


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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A simple mechanical pressure relief is a weighted man-way cover as shown in this sketch. Another mechanical relief is a U-tube filled with water (or equivalent).

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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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Types of Safety ValvesTypes of Safety Valves

• Conventional


• Pilot-Operated

• Conventional


• Pilot-Operated

There are three types of safety valves; that is: • Conventional, • Balanced bellows, and • Pilot-operated.

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Conventional Safety ValveConventional 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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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.

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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.

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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.

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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. Slide

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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.

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Types of Relief ValvesTypes of Relief Valves

• Conventional


• Conventional


Relief valves (for liquid service) are either the conventional or the balanced bellows types.

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• Metal

• Graphite

• Composite

• Others

• 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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• Metal

• Graphite

• Composite

• Others

• 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 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.

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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.

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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.

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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.

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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.

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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.

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Vacuum Relief DevicesVacuum Relief Devices• Vacuum Relief Valves

• Rupture Discs


• Manhole Lids


• Vacuum Relief Valves

• Rupture Discs


• Manhole Lids


Vacuum relief devices are: vacuum relief valves, rupture discs, conservation vents, manhole lids designed for vacuum relief, and pressure control.

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Conservation VentConservation Vent

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.

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Pressure or Vacuum ControlPressure or Vacuum Control





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.

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• Inspection

• Testing

• Inspection

• Testing

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.

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• Inspection

• Testing

• Inspection

• Testing

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.

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• To Atmosphere• To Atmosphere

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.

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• To Atmosphere

• Prevented

• 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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• To Atmosphere

• Prevented

• Effluent System

• To Atmosphere

• Prevented

• Effluent System

The third option is to design an effluent system to capture all nocuous liquids and gases.

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Effluent SystemsEffluent Systems

• Knock-Out Drum

• Catch Tank


• Knock-Out Drum

• Catch Tank


An effluent system may contain a • Knock-out drum• Catch tank• Cyclone separator

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Effluent System (continued)Effluent System (continued)

• Condenser

• Quench Tank

• Scrubber


• Condenser

• Quench Tank

• Scrubber


• 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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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.

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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.

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Part 2 of 3: RunawaysPart 2 of 3: Runaways

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Runaway ReactionRunaway Reaction







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. Slide

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• Sleeper

• Tempered

• Gassy

• Hybrid

• Self-Heating

• Sleeper

• Tempered

• Gassy

• Hybrid


In general, there are two causes of runaway reactions (self-heating and sleeper) and three characteristics of runaways (tempered, gassy, and hybrid).

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• Sleeper

• Tempered

• Gassy

• Hybrid

• Self-Heating

• Sleeper

• Tempered

• Gassy

• Hybrid


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 ReactionSelf-Heating Reaction

• Loss of Cooling





• Loss of Cooling





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 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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In these cases the runaway is initiated by starting the agitator and adding heat respectively.

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Tempered ReactionTempered Reaction





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. Slide

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Gassy SystemGassy System





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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Hybrid SystemHybrid System

• Tempered

• Gassy

• Tempered

• Gassy

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.

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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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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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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.

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Two Phase FlowTwo Phase Flow

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. Slide

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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.

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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.

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• ARC

• VSP

• RSST

• 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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• ARC

• VSP

• RSST

• ARC

• VSP

• RSST

• APTAC

• PHI-TEC

• Dewars

• APTAC

• PHI-TEC

• 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.

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Part 3 of 3: SafeguardsPart 3 of 3: Safeguards

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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.

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• Short-Stopping



• Short-Stopping

Safeguards include safety interlocks, safeguard maintenance system, and/or short-stopping.

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Safety InterlocksSafety Interlocks





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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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.

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• Records




• Records

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. Slide

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• Records




• 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 instrumented systems. Slide

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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. Slide

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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.

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• Deflagrations

• Detonations

• Deflagrations

• Detonations

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.

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• Deflagrations

• Detonations

• Deflagrations

• Detonations

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.

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• Deflagrations

• Detonations

• Deflagrations

• Detonations

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.

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• Containment



• Containment

The protection methods used for fires or explosions include• Deflagration venting• Deflagration suppression• Containment

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(continued)


(continued)







• Reduction of the oxidant• Reduction of the combustible• Flame front isolation

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(continued)


(continued)







• Spark detection and extinguishing• Flame detection and extinguishing• Water or foam spray deluge systems

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• Vent Area via NFPA 68• 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.

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• Vent Safely


• Vent Safely

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.

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• Vent Safely


• Vent Safely

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.

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Vent of Gas DeflagrationVent 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. 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. Slide

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Vent of Dust DeflagrationVent of Dust Deflagration

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. Slide

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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.

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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.

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ContainmentContainment





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. Slide

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• Vacuum Purging



• Vacuum Purging



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.

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• Vacuum Purging



• Vacuum Purging



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.

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Reduction of CombustibleReduction of Combustible


• NFPA 69


• 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

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Flame Front IsolationFlame Front Isolation

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.

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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.

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Water Spray or Deluge SystemsWater Spray or Deluge 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 SystemDeluge System

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.

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ConclusionConclusion

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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