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Design for Overpressure and Underpressure Protection. Slide Show. Slides with Text. Exit. SLIDE PRESENTATION. Design for Overpressure and Underpressure Protection. NEXT. HOME. Outline. Introduction. Introduction Causes of Overpressure and Underpressure Reliefs

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slide presentation

SLIDE PRESENTATION

Design for Overpressure and Underpressure Protection

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outline
Outline

Introduction

  • Introduction
  • Causes of Overpressure and Underpressure
  • Reliefs
  • Effluent Handling Systems for Reliefs
  • Runaway Reactions
  • Overpressure Protection for Internal Fires and Explosions

Reliefs

Runaways

Safeguards

causes of overpressure
Causes of Overpressure
  • Operating Problem
  • Equipment Failure
  • Process Upset
  • External Fire
  • Utility Failures

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causes of underpressures
Causes of Underpressures
  • Operating Problem
  • Equipment Failure

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presentation 1 of 3 reliefs

Presentation 1 of 3: Reliefs

Causes of Overpressure/Underpressure

Presentation 2: Runaways

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Presentation 3: Safeguards

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pressure relief devices
Pressure Relief Devices
  • Spring-Loaded Pressure Relief Valve
  • Rupture Disc
  • Buckling Pin
  • Miscellaneous Mechanical

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rupture disc
Rupture Disc

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buckling pin relief valve
Buckling Pin Relief Valve

ClosedPressure Below Set Pressure

Full OpenPressure at or AboveSet Pressure

(Buckles in Milliseconds at a Precise Set Pressure)

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types of spring loaded pressure reliefs
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 valves
Types of Safety Valves
  • Conventional
  • Balanced Bellows, and
  • Pilot-Operated

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conventional safety valve
Conventional Safety Valve

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balanced bellows safety valve
Balanced Bellows Safety Valve

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pilot operated safety valve
Pilot-Operated Safety Valve

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types of relief valves
Types of Relief Valves
  • Conventional
  • Balanced Bellows

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types of rupture discs
Types of Rupture Discs
  • Metal
  • Graphite
  • Composite
  • Others

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vacuum relief devices
Vacuum Relief Devices
  • Vacuum Relief Valves
  • Rupture Discs
  • Conservation Vents
  • Manhole Lids
  • Pressure Control

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conservation vent
Conservation Vent

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pressure or vacuum control
Pressure or Vacuum Control
  • Add Air or Nitrogen
  • Maintain Appropriately

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relief servicing
Relief Servicing
  • Inspection
  • Testing

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relief discharges
Relief Discharges
  • To Atmosphere
  • Prevented
  • Effluent System

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effluent systems
Effluent Systems
  • Knock-Out Drum
  • Catch Tank
  • Cyclone Separator

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effluent system continued
Effluent System (continued)
  • Condenser
  • Quench Tank
  • Scrubber
  • Flares/Incinerators

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effluent handling system
Effluent Handling System

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presentation 2 of 3 runaways

Presentation 2 of 3: Runaways

Causes of Overpressure/Underpressure

Presentation 2: Runaways

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Presentation 1: Reliefs

Presentation 3: Safeguards

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runaway reaction
Runaway Reaction
  • Temperature Increases
  • Reaction Rate Increases
  • Pressure Increases

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causes of runaway reactions
Causes of Runaway Reactions
  • Self-Heating
  • Sleeper
  • Tempered
  • Gassy
  • Hybrid

Characteristics of Runaway

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self heating reaction
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
Sleeper Reactions
  • Reactants Added But Not Mixed (Error)
  • Reactants Accumulate
  • Agitation Started .. Too Late

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tempered reaction
Tempered Reaction
  • Heat Removed by Evaporation
  • Heat Removal Maintains a Constant Temperature

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gassy system
Gassy System
  • No Volatile Solvents
  • Gas is Reaction Product

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hybrid system
Hybrid System
  • Tempered
  • Gassy

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reliefs for runaway reactions
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

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two phase flow
Two Phase Flow

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relief valve sizing methodology
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

Characterization of Runaway Reactions

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presentation 3 of 3 safeguards

Presentation 3 of 3: Safeguards

Causes of Overpressure/Underpressure

Presentation 2: Runaways

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Presentation 1: Reliefs

Presentation 3: Safeguards

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safeguards
Safeguards
  • Safety Interlocks
  • Safeguard Maintenance System
  • Short-Stopping

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safety interlocks
Safety 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
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
Safeguard Maintenance System
  • Routine Maintenance
  • Management of Change
  • Mechanical Integrity Checks
  • Records

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short stops to stop reaction
Short-Stops to Stop Reaction
  • Add Reaction Stopper
  • Add Agitation with No Electrical Power

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protection for internal fires and explosions
Protection for InternalFires and Explosions
  • Deflagrations
  • Detonations

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protection methods for internal fires and explosions
Protection Methods forInternal Fires and Explosions
  • Deflagration Venting
  • Deflagration Suppression
  • Containment

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protection methods for internal fires and explosions continued
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 continued1
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
Deflagration Venting
  • Vent Area via NFPA 68
  • Vent Safely

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vent of gas deflagration
Vent of Gas Deflagration

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vent of dust deflagration
Vent of Dust Deflagration

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containment
Containment
  • Prevent Rupture and Vessel Deformation
  • Prevent Rupture but Deform Vessel

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reduction of oxidant
Reduction of Oxidant
  • Vacuum Purging
  • Pressure Purging
  • Sweep-Through Purging

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reduction of combustible
Reduction of Combustible
  • Dilution with Air
  • NFPA 69

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flame front isolation
Flame Front Isolation

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water spray or deluge systems

Water Spray or Deluge Systems

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deluge system
Deluge System

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conclusion

Conclusion

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end of slide presentation

End of Slide Presentation

Causes of Overpressure/Underpressure

Presentation 2: Runaways

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Presentation 3: Safeguards

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slides with text

SLIDES WITH TEXT

Design for Overpressure and Underpressure Protection

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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Design for Overpressure and Underpressure Protection

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

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Design for Overpressure and Underpressure Protection

  • 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 sections
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

This presentation is divided into six sections:

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six sections1
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

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Reliefs

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Runaways

Safeguards

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six sections2
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

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

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Runaways

Safeguards

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six sections3
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

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

Reliefs

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Runaways

Safeguards

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six sections4
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

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

Reliefs

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Runaways

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appendix contains detailed information

Appendix 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 overpressure1
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 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.

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causes of overpressure2
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 overpressure3
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.

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causes of overpressure4
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 vessel giving high vapor pressures, and
  • Utility failures, such as the loss of cooling or the loss of agitation causing a runaway reaction.

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causes of underpressures1
Causes of Underpressures

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

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causes of underpressures2
Causes of Underpressures
  • 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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causes of underpressures3
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 that contains a material with a low vapor pressure.

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pressure relief devices1
Pressure Relief Devices

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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pressure relief devices2
Pressure Relief Devices
  • Spring-Loaded Pressure Relief Valve
  • Rupture Disc
  • Buckling Pin
  • Miscellaneous Mechanical

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

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spring loaded pressure relief valve1
Spring-Loaded Pressure Relief Valve

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 disc1
Rupture 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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buckling pin relief valve1
Buckling Pin Relief Valve

ClosedPressure Below Set Pressure

Full OpenPressure at or AboveSet Pressure

(Buckles in Milliseconds at a Precise 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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buckling pin relief valve2
Buckling Pin Relief Valve

ClosedPressure Below Set Pressure

Full OpenPressure at or AboveSet 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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simple mechanical pressure relief1
Simple Mechanical Pressure Relief

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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types of spring loaded pressure reliefs1
Types of Spring-LoadedPressure 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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types of safety valves1
Types of Safety Valves
  • Conventional
  • Balanced Bellows, and
  • Pilot-Operated
  • There are three types of safety valves; that is:
    • Conventional,
    • Balanced bellows, and
    • Pilot-operated.

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conventional safety valve1
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 valve1
Balanced Bellows Safety Valve

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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balanced bellows safety valve2
Balanced Bellows Safety Valve

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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pilot operated safety valve1
Pilot-Operated Safety Valve

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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pilot operated safety valve2
Pilot-Operated Safety Valve

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.

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pilot operated safety valve3
Pilot-Operated Safety Valve

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 valves1
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 discs1
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 discs2
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 pressure relief valve combination1
Rupture Disc and Pressure Relief Valve Combination

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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rupture disc and pressure relief valve combination2
Rupture Disc and Pressure Relief Valve Combination

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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rupture disc and pressure relief valve combination3
Rupture Disc and Pressure Relief Valve Combination

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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rupture disc and pressure relief valve combination4
Rupture Disc and Pressure Relief Valve Combination

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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rupture disc and pressure relief valve combination5
Rupture Disc and Pressure Relief Valve Combination

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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rupture disc and pressure relief valve combination6
Rupture Disc and Pressure Relief Valve Combination

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 devices1
Vacuum Relief Devices
  • Vacuum Relief Valves
  • Rupture Discs
  • Conservation Vents
  • Manhole Lids
  • Pressure Control

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 vent1
Conservation 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 control1
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.

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relief servicing1
Relief Servicing
  • 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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relief servicing2
Relief Servicing
  • 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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relief discharges1
Relief Discharges
  • 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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relief discharges2
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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relief discharges3
Relief Discharges
  • 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 systems1
Effluent Systems
  • Knock-Out Drum
  • Catch Tank
  • Cyclone Separator
  • An effluent system may contain a
    • Knock-out drum
    • Catch tank
    • Cyclone separator

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effluent system continued1
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.

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effluent handling system1
Effluent Handling System

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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effluent handling system2
Effluent Handling System

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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runaway reaction1
Runaway Reaction
  • Temperature Increases
  • Reaction Rate Increases
  • Pressure Increases

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.

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causes of runaway reactions1
Causes of Runaway Reactions
  • Self-Heating
  • Sleeper
  • Tempered
  • Gassy
  • Hybrid

Characteristics of Runaway

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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causes of runaway reactions2
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 reaction1
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 reactions1
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 reactions2
Sleeper Reactions
  • Reactants Added But Not Mixed (Error)
  • Reactants Accumulate
  • Agitation Started .. Too Late

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

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tempered reaction1
Tempered Reaction
  • Heat Removed by Evaporation
  • Heat Removal Maintains a Constant Temperature

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.

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gassy system1
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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hybrid system1
Hybrid System
  • 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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reliefs for runaway reactions1
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 reactions2
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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reliefs for runaway reactions3
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

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 flow1
Two 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.

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relief valve sizing methodology1
Relief Valve Sizing Methodology
  • Special Calorimeter Data
  • Special Calculation Methods

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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characterization of runaway reactions1
Characterization of Runaway Reactions

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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characterization of runaway reactions2
ARC

VSP

RSST

Characterization of Runaway Reactions
  • 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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characterization of runaway reactions3
ARC

VSP

RSST

APTAC

PHI-TEC

Dewars

Characterization of Runaway Reactions
    • 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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safeguards1
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.

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safeguards2
Safeguards
  • Safety Interlocks
  • Safeguard Maintenance System
  • Short-Stopping

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

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safety interlocks1
Safety Interlocks
  • Agitator Not Working: Stop Monomer Feed and Add Full Cooling
  • Abnormal Temperature: Stop Monomer 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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safety interlocks continued1
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

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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safeguard maintenance system1
Safeguard Maintenance System
  • Routine Maintenance
  • Management of Change
  • Mechanical Integrity Checks
  • 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.

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safeguard maintenance system2
Safeguard Maintenance 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 instrumented systems.

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short stops to stop reaction1
Short-Stops to Stop Reaction
  • Add Reaction Stopper
  • Add Agitation with No Electrical Power

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.

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short stops to stop reaction2
Short-Stops to Stop Reaction
  • Add Reaction Stopper
  • Add Agitation with No Electrical Power

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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protection for internal fires and explosions1
Protection for InternalFires and Explosions
  • 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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protection for internal fires and explosions2
Protection for InternalFires and Explosions
  • 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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protection for internal fires and explosions3
Protection for InternalFires and Explosions
  • 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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protection methods for internal fires and explosions1
Protection Methods forInternal 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 internal fires and explosions continued2
Protection Methods for 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 internal fires and explosions continued3
Protection Methods for 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 venting1
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.

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deflagration venting2
Deflagration Venting
  • Vent Area via NFPA 68
  • 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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deflagration venting3
Deflagration Venting
  • Vent Area via NFPA 68
  • 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 deflagration1
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. 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.

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vent of dust deflagration1
Vent 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.

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deflagration suppression system1
Deflagration Suppression 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.

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deflagration suppression system2
Deflagration Suppression 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.

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containment1
Containment
  • Prevent Rupture and Vessel Deformation
  • Prevent Rupture but Deform Vessel
  • 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.

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reduction of oxidant1
Reduction of Oxidant
  • Vacuum Purging
  • Pressure Purging
  • Sweep-Through 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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reduction of oxidant2
Reduction of Oxidant
  • Vacuum Purging
  • Pressure Purging
  • Sweep-Through 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 combustible1
Reduction of Combustible
  • 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.

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flame front isolation1
Flame 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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spark flame detection and extinguishing1
Spark/Flame Detectionand Extinguishing

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 systems1

Water 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 system1
Deluge 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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conclusion1

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.

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end of slide presentation with text

End of Slide Presentation(with text)

Causes of Overpressure/Underpressure

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