5. Causes of Overpressure
• Operating Problem
• Equipment Failure
• Process Upset
• External Fire
• Utility Failures
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11. 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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13. 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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14. Types of Safety Valves
• Conventional
• Balanced Bellows, and
• Pilot-Operated
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29. Presentation 2 of 3: Runaways
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Causes ofCauses of
Overpressure/UnderpressureOverpressure/Underpressure
Presentation 1: ReliefsPresentation 1: Reliefs
Presentation 2: RunawaysPresentation 2: Runaways
Presentation 3: SafeguardsPresentation 3: Safeguards
30. Runaway Reaction
• Temperature Increases
• Reaction Rate Increases
• Pressure Increases
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31. Causes of Runaway Reactions
• Self-Heating
• Sleeper
• Tempered
• Gassy
• Hybrid
Characteristics of Runaway
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32. 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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33. Sleeper Reactions
• Reactants Added But Not Mixed
(Error)
• Reactants Accumulate
• Agitation Started .. Too Late
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34. Tempered Reaction
• Heat Removed by Evaporation
• Heat Removal Maintains a Constant
Temperature
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35. Gassy System
• No Volatile Solvents
• Gas is Reaction Product
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37. 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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43. 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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44. 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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48. Protection Methods for
Internal Fires and Explosions
• Deflagration Venting
• Deflagration Suppression
• Containment
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49. Protection Methods for
Internal Fires and Explosions
(continued)
• Reduction of Oxidant
• Reduction of Combustible
• Flame Front Isolation
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50. 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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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. Slide
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Causes of Overpressure
• Operating Problem
• Equipment Failure
• Equipment failures; for example a heat exchanger tube rupture that
increases the shell side pressure beyond the MAWP. Although the set
pressure is usually the MAWP, the design safety factors should protect
the vessel for higher pressures; a vessel fails when the pressure is
typically several times the MAWP.
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Causes of Overpressure
• Operating Problem
• Equipment Failure
• Process Upset
• External Fire
• Utility Failures
• Process upset; for example a runaway reaction causing high
temperatures and pressures.
• External heating, such as, a fire that heats the contents of a vessel
giving high vapor pressures, and
• Utility failures, such as the loss of cooling or the loss of agitation
causing a runaway reaction. Slide
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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)
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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Types of Spring-Loaded
Pressure Reliefs
• Safety Valves for Gases and Vapors
• Relief Valves for Liquids
• Safety Relief Valves for Liquids
and/or Gases
There are three types of spring-loaded pressure relief valves:
• Safety valves are specifically designed for gases.
• Relief valves are designed for liquids, and
• Safety relief valves are designed for liquids and/or gases.
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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 under-pressure. In either case the
consequence could be a ruptured vessel.
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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. Slide
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Causes of Runaway Reactions
• Self-Heating
• Sleeper
• Tempered
• Gassy
• Hybrid
Characteristics of Runaway
When protecting a system for overpressures due to runaway reactions
the engineer needs to know the type of runaway and needs to
characterize the behavior of the specific runaway with a special
calorimeter. This specific methodology is described in this section of this
presentation.
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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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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. Slide
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Reliefs for Runaway Reactions
• Two Phase (or Three Phases:
Liquid, Vapor, and Solid) Flow
To maintain a constant temperature in the reactor (i.e. control the
runaway reaction), the relief valve is sized to remove all the heat
generated from the exothermic reaction via the heat removed with the
discharged mass, which is typically a foam. Detailed information on
runaway reactions is found in the appendix.
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Two Phase Flow
This is a picture that illustrates the two-phase flow
characteristics of a relief discharge due to a runaway reaction.
As illustrated, the discharge is similar to the release of foam
from a freshly opened bottle of pop after being shakened. If the
relief is not designed for two-phase flow, the pressures would
increase rapidly and the vessel could rupture. Slide
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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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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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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. Slide
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Short-Stops to Stop Reaction
• Add Reaction Stopper
• Add Agitation with No Electrical
Power
Good agitation, of course, is required to adequately mix the 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 spare ring is one alternative.
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Vent of Gas Deflagration
This is a picture of the venting of a gas deflagration. As illustrated,
the flame propagates a significant distance from the vessel. The
length of the flame is estimated using an equation found in NFPA
68. The main purpose of venting is to protect the mechanical
integrity of the equipment. As illustrated, even when it is vented
safely, this is a major event. Slide
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Vent of Dust Deflagration
This is a picture of the venting of a dust deflagration. As illustrated,
the burning dust continues to burn at great distances from the vent.
With dusts, this burning zone is larger because the container has a
larger fuel-to-air ratio compared to the gas deflagration scenario.
These pictures clearly illustrate the problems with venting
deflagrations. Slide
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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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