Design for Overpressure and Underpressure Protection

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