SAFETY IN CHEMICAL ENGINEERING

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SAFETY IN CHEMICAL ENGINEERING

• ChE 417
• Chemical Engineering Design
• Tülin Taskin
• Sadik Demirci
• Pinar Uysal
• Hakan
  Savasturk

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OUTLINE

• IDENTIFICATION OF PROCESS SAFETY
• IDENTIFICATION OF HAZARD
• RISK ASSESSMENT TECHNIQUE
• FIRES AND EXPLOSIONS
• REQUIREMENTS
• DESIGN TO PREVENT THEM
• Inerting
• Controlling Static Electricity
• Ventilation
• Sprinkler System
• Relief Systems
• CASE EXAMPLES

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What is process safety?

• It is prevention of accidents by use of
  appropriate technologies
• It starts with hazards identification of
  a chemical plant
• It ends with elimination them before
  accident occurs

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Components Of A Succesful Safety Program
Safety Knowledge and Experience
A succesful safety program
Technical Competence
Safety Management Support
Commitment
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HAZARDS

• Mechanical hazards worker injuries
  from tripping, falling or moving
  equipment
• fire and explosion hazards
• Chemical Hazards reactivity
  hazards
• 
  toxic hazards

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Chemical Hazards

• A chemical reaction that goes out of control
  and runs away can create a serious incident with
  the risk of injury to people and damage to
  property and the environment.

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Chemical Plant Accidents
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RISK ASSESSMENT

• We need to carry out a risk assessment of our
  process to have a safe chemical plant.

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A typical chemical process risk assessment

• defining the process, operating conditions and
  plant
• identifying the hazards
• evaluating the risks arising from the hazards
• deciding whether existing precautions are
  adequate
• selecting and specifying appropriate safety
  measures
• implementing and maintaining the selected safety
  measures.

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Evaluating reaction hazards

• Determination of the hazards of a reaction
  includes search on
• the possibility of thermal decomposition of raw
  materials, intermediates, products and
  by-products
• whether exothermic runaway can occur
• the rate and quantity of heat and gas produced
  by the reaction.

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FIRES AND EXPLOSIONS

• REQUIREMENTS
• Explosive Substance
• Oxygen
• Start of fire
• Ignition Source
• Auto Ignition Temperature
• Suitable Conditions
• UFL
• LFL

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UFL AND LFL

• UFL (upper flammability limit)
• Maximum concentration of fuel that will support
  combustion
• LFL (lower flammability limit)
• Minimum concentration of fuel that will support
  combustion

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IGNITION SOURCES

• Mainly heat and sparks,flames,static
  electricity
• Ignition sources are normally the easiest to
  eliminate.

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DESIGN TO PREVENT FIRES AND EXPLOSIONS

• Inerting
• Controlling Static Electricity
• Ventilation
• Sprinkler System
• Relief Systems

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Inerting

• Inerting is the process of adding an inert gas to
  a combustible mixture to reduce the concentration
  of oxygen below the minimum oxygen concentration.
• Nitrogen,carbon dioxide and sometimes steam can
  be used as inert gas.
• For many gases the minimum oxygen concentration
  is 10 and for many dusts 8.

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Inerting Procedure

• begins with an initial purge of the vessel with
  inert gas to bring the oxygen concentration.
• Then,the flammable material is charged.
• An automatic inert gas addition system maintains
  the oxygen concentration below the MOC.

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Static Electricity Formation
Grounded thermowell grounds all metal
Lightening Like Discharge
Solids
Propagating Brush Discharge
Conical Pile Discharge
Brush Discharge
Tramp Metal
Metal to Metal Spark
Metal Shell
Solids
Solids
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Prevention

• Natural grounding and static electricity
  prevention mechanisms
• Avoiding use of highly insulator material

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Ventilation

• Ventilation is a common protection against
  exposure to inhalation. Ventilation is based on
  two principles.
• Dilute the contaminant below the target
  concentration.
• Remove the contaminant before workers are
  exposed.

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Why ventilation?

• can quickly remove dangerous concentrations of
  flammable and toxic materials.
• can be highly localized, reducing the quantity of
  air moved and the equipment size.
• can be easily installed.
• can be added to an existing facility.

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• Ventilation systems are composed of fans and
  ducts
• As a disadvantage, they have high operating costs

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Types of ventilation

• There are two types of ventilation techniques
• local ventilation
• dilution
  ventilation.

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Local ventilation

• the most common example is the hood.
• A hood is a device that either completely
  encloses the source of contaminant or moves the
  air in such a fashion to carry the contaminant to
  an exhaust device.
• Fume hoods provide a flow of air away from the
  person.

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Dilution ventilation

• if the contaminant cannot be placed in a hood and
  must be used in an open area or room, dilution
  ventilation is necessary
• requires more airflow than the local ventilation

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• When ventilation system is designed occupational
  health standards such as TLV must be considered.
• TLV (Threshold Limit Values)
• Below this dose the body is able to detoxify.

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SPRINKLER SYSTEMS

• These systems are used to contain fires.
• They are consist of an array of splinkler heads
  connected to a water supply.
• The heads are put in a high location and they
  disperse a fine spray of water over an area.
• Altough splinkler systems may cause water damage
  depending over the building or process
  structure,this water damage is never as great as
  the damage from fires,statistically.

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Methods activate the sprinkler heads

• Closed Head Area SystemActivates the heads
  individually by melting of fusible link holding a
  plug in the haed.This system is used for storage
  areas,laboraties,control rooms.
• Open Head Area SystemActivates the entire
  sprinkler array from a common control point.the
  control point is connected to an array of heat or
  smoke detectors.This system is used for plant
  process areas and larger pilot plants.

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Why relief systems are required?

• to protect personnel from the dangers of
  overpressurizing equipment
• to minimize chemical losses during pressure upset
• to prevent damage to equipment
• to prevent damage to adjoining property
• to reduce insurance premiums

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Relief Types

• 1- Spring operated
• Conventional
• Balanced bellow
• 2-Rupture discs

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What is relief system?

• A relief system protects the process from the
  damaging effects of high or low pressure
• A relief system removes energy from a process by
  discharging mass with an energy content

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How can high pressures develop?

• Overheating or freezing
• Failure of regulator
• External fire
• Runaway reaction
• Combustion of gases and dusts

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In order to relieve the high pressure

• Spring loaded reliefs

Set Presure Adjust screw
Spring
Bonnet
Body
Disc Holder
Seat Disc
Blowdown Adjustment Ring
Nozzle
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• Set pressure Pressure at which the device begins
  to open
• Back pressure The pressure downstream of the
  device during the relief

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In spring loaded reliefs

• Set pressure and flow are affected by back
  pressure
• - Set pressure increases
• - Flow decreases

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• Advantages
• - Most reliable type if properly sized and
  operated
• - Versatile -- can be used in many service
• Disadvantages
• - Relieving pressure affected by back pressure

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Rupture Discs

• a thin diaphragm
• a weak element to protect vessels and piping
  against excessive pressure
• Primary pressure relief device
• - in runaway reactions,
• - when PR valve can not respond quickly

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Disc
Carrier Assembly
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• Advantages
• - Protect against rapid pressure rise
• - Less expensive
• - Provide secondary protective device for large
  relief areas
• Disadvantages
• - Dont reclose after relief
• - Burst pressure cannot be tested
• - Greater sensitivity to temperature

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Piston Type Pilot Operated Safety Relief Valve
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• Advantages
• - Relief pressure is not affected by
  backpressure
• - Can operate at up to 98 of set pressure
• Disadvantages
• - Limited temperature range
• - Potential for back flow

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RUPTURE PINS

• non re-closing similar to rupture disc

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• Advantages
• - can operate closer to its set point
• - suitable for liquid service
• - replacement of pins are 1/3 to 1/4 the cost of
  replacement of discs
• Disadvantages
• - maximum operating temperature is about 230 ºC,
  elastomer o-ring seals
• - cost of installation is greater than for a
  rupture disc

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CASE STUDY

• Bhopal Accident
• in India
• Leakage of an intermediate,Methyl
  Isocyanate(MIC)

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ERROR 1
    

• The scrubber and flare system should have been
  fully operational to prevent the release.

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ERROR 2

• The tank should have been tested to be sure it
  is safe. It cannot be known that it is safe until
  after the testing.  

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ERROR 3

• An alternative reaction scheme that involves a
  less dangerous intermediate than methly
  isocyanate such as chloroformate could have been
  used.

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ERROR 4

• One such plant that produces or consumes
  dangerous materials could have been constructed
  in residential areas.

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bhopal
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2000 people died in Bhopal disaster
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More than 20000 civilians were injured.
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FINAL REMARK

• After reviewing these incidents, as seen it is
  very essential to see how knowing what caused
  past accidents can prevent future accidents from
  happening

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• THANK
• YOU
• FOR
• LISTENING
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