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7/26/2019 Plant Layout Considerations
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2. Plant Layout Design
for
Safety & Inherent Safety
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The Lady or the Tiger?
A king offered a challenge to three young men. Each would be
put in a room with two doors and could open both. If he openedone, a hungry tiger would come outthe fiercest and most cruelthat could be obtainedthat would tear him to pieces. If he
opened the other, a young lady would come out, the most suitableto his years and station that His Majesty could select from
amongst his fair subjects.
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The Lady or the Tiger?
The first young man refused the challenge.
He lived safe and died chaste.
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The Lady or the Tiger?
The second young man hired risk-assessment consultants. They collected all the availabledata on lady and tiger populations. They brought in sophisticated equipment to listen for
growling and detect the faintest whiff of perfume. They completed checklists. Theydeveloped a utility function and assessed the young mans risk aversion. Naturally thistook time (and money). The young man, now no longer quite so young, began to worry that
he would soon be no longer able to enjoy the lady. Finally, he asked the consultants torecommend a course of action.
He opened the optimal door and was eaten by a low-probability tiger.
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The Lady or the Tiger?
The third young man took a course in tiger handling..!!
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The moral of the storyThe young men represent us all, the tiger the hazards of industry, and the lady
the benefits industry brings to humanity. Like the first young man, societycan leave the game. We can manage without chemical plants, the benefits
they bring, and the risks they carry.
Like the second young man, we can (and do) try to assess the risks and openthe safest doors, but we can never be completely sure that our assessments are
correct and that an accident will not occur.
When possible, we should try, like the third young man, to change the worksituation and to choose designs or methods of working that minimize the hazard.
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Why Inherently Safer Design?
ISD has received increased attention from the chemical industry since the 1970s as aresult of a number of major industrial accidents.
Flixborough, England (1974) a large release of cyclohexane. killed 28 workers, injured 36, destroyed the plant 53 casualties off site
Pasadena, Texas (1989) a leak of flammable vapor, 23 fatalities and hundreds of injuries, and
destroying the plant. Bhopal, India (1984) water entered a storage tank containing methyl isocyanate (MIC)
Exact casualty figures are disputed, Official Indian government estimate of fatalities was 4000 in 1994.
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Inherent safety Inherent - Something which exist as a permanent and inseparable element.
Safety based on physical and chemical properties of the system, not based on added safetydevices and systems.
The major principle in Inherent Safety is to remove the hazards altogether rather than controlling.
The best method to achieve this is to reduce the inventories of hazardous substances such that amajor hazard is no longer presented.
However, this is not often readily achievable. Other possible methods to achieve an InherentlySafer design are:
Intensification of Processes
Substitution of hazardous substances by less hazardous alternatives
Attenuation of inventories; Reduction of hazardous process conditions i.e. temperature, pressure;
Simpler systems/processes
Fail-safe design e.g. valve position on failure.
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12
COMMUNITY EMERGENCY REPSONSE
PLANT EMERGENCY REPSONSE
PHYSICAL PROTECTION (DIKES)
PHYSICAL PROTECTION (RELIEF DEVICES)
AUTOMATIC ACTION SIS OR ESD
CRITICAL ALARMS, OPERATOR
SUPERVISION, AND MANUAL INTERVENTION
BASIC CONTROLS, PROCESS ALARMS,
AND OPERATOR SUPERVISION
PROCESS
DESIGN
LAH
1
I
Layers of Protection
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Potential Incidents
Layers
ofProtectio
n
Actual Risk
Multiple Layers of Protection
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14
Degraded Layers of Protection
Potential Incidents
Layers
ofProtectio
n
Higher Actual Risk
Degraded
Degraded
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15
PROCESS
DESIGN
LAH
1
I
Inherently Safe Process
No additional layers of protection needed
Probably not possible if you consider ALL potential hazards
But, we can be Inherently Safer
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Inherently Safer Process Risk
Potential Incidents
Actual Risk
N
oLayers
ofProtection
Needed
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Inherently Safer Design
The possibility for affecting the inherent safety of a process decreases asthe design proceeds and more and more engineering and financialdecisions have been made.
It is much easier to affect the process configuration and inherent safety inthe conceptual design phase than in the later phases of process design.
For instance the process route selection is made in the conceptual designand it is many times difficult and expensive to change the route later.
Time and money is also saved when fewer expensive safety modifications
are needed and fewer added-on safety equipment are included to the finalprocess solution.
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Inherently Safer Design Opportunities
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Comparison with other ways of Reducing Risk
Inherent or Intrinsic eliminating the hazard by using materials and process conditions that are
nonhazardous.
Passive eliminating or minimizing the hazard by process and equipment design features thatdo not eliminate the hazard, but do reduce either the frequency or consequence of the hazard
without the need for any device to function actively (e.g., the use of higher pressure-rated
equipment).
Active using controls, safety interlocks, and emergency shutdown systems to detect potentially
hazardous process deviations and take corrective action. These are commonly referred to as
engineering controls.
Procedural using operating procedures, administrative checks, emergency response, and other
management approaches to prevent incidents or to minimize the effects of an incident. These are
commonly referred to as administrative controls.
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Illustrative Example
Case - Example Morton International, Paterson, NJ runaway reaction in 1998,
injured 9 people. The process is a simple exothermic batch reaction in which two or more reactants
are added to a reactor, along with other materials such as solvents, and reactedto produce a desired product.
The reaction is exothermic therefore there is a potential for a runaway reaction the temperature and pressure of the reactor cannot be controlled by the
cooling system and the reactor could rupture due to high pressure an explosionof the reactor.
Hazard of concern runaway reaction causing high temperature and pressureand potential reactor rupture.
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By Inherent Safety Method - Develop chemistrywhich is not exothermic, or mildly exothermic
Maximum adiabatic reactortemperature is less than the boilingpoint of all ingredients and onset
temperature of any decompositionor other reactions, and no gaseousproducts are generated by thereaction.
The reaction does not generate anypressure, either from confined gas
products or from boiling of thereactor contents
TI
PI
VENT
REACTANT FEEDS
COOLING
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By Passive Safety Method
Maximum adiabatic pressure forreaction determined to be 150psig
Carryout the reaction in a vesselwith design pressure as 250 psig.
Hazard (pressure) still exists, butpassively contained by thepressure vessel.
TI
PI
VENT
PRV
REACTANT FEEDS
COOLING
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By Active Safety Method
Maximum adiabatic pressure for100% reaction is 150 psig, reactordesign pressure is 50 psig.
Gradually add limiting reactant withtemperature control to limitpotential energy from reaction.
Use high temperature and pressureinterlocks to stop feed and applyemergency cooling.
Provide emergency relief system.
PA
H
VENT
REACTANT FEEDS
COOLING
RUPTURE DISK WITH DISCHARGE
TO SAFE PLACE
TA
H
SAFETY SYSTEM
LOGIC ELEMENT
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By Procedural Safety Method
Maximum adiabatic pressurefor 100% reaction is 150 psig,reactor design pressure is 50psig
Gradually add limitingreactant with temperaturecontrol to limit potentialenergy from reaction
Train operator to observetemperature, stop feeds and
apply cooling if temperatureexceeds critical operatinglimit
PA
H
VENT
REACTANT FEEDS
COOLING
RUPTURE DISK WITH DISCHARGE
TO SAFE PLACE
TA
H
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Which strategy should we use?
Generally, in order of robustness and reliability: Inherent
Passive
Active Procedural
But - there is a place and need for ALL of these strategies in acomplete safety program
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Site Evaluation
Credible worst case scenarios Major Accident Hazards
Reasonable definition of local meteorological conditions and possibleextremes
Population density and numbers of people likely to be involved
General planning and development guidelines for the region
Ability to control movement of people in an emergency
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Meet quality and capacity
requirement in the most
economical manner.
Minimize unit costs and optimizequality.
Promote effective use of people,
equipment, space and energy.
Provide for employee safety and
comfort.
Control project costs.
Achieve production deadlines.
Type of building and building code
requirements.
Guidelines related to health andsafety.
Waste-disposable problems.
Space available and space
requirement. Auxiliary equipment.
Roads and railroad.
General Goals of Plant Layout Design
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Typical Safety BasedSite Layout Decision-Making Process
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Minimal explosion damage, sinceexplosion overpressure falls off rapidlywith distance from the center of theexplosion
Minimal thermal radiation damage, asthe intensity also falls off with thedistance
Less property damage(containment ofaccidents)
Easier access for emergency services such
as fire fighting Easier access to equipment for
maintenance and inspection
Efficient and safe construction (drainageand grade sloping)
Optimum balance among loss control,maintenance and operation requirements
Future expansions
Impact on plant economics (more spacesincreased capital costs but also enhancedsafety need to optimize)
Inherent Safety Concerns on Plant Layout
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Methods & Techniques
Process industry has used the Dow Fire and Explosion Hazard Index(DOW, 1987) and the Mond Index (ICI, 1985) for many years.
These indices deal with fire and explosion hazard rating of processplants.
Dow and Mond Indices are rapid hazard-assessment methods for useon chemical plant, during process and plant development, and in thedesign of plant layout.
They are best suited to later design stages when process equipment,chemical substances and process conditions are known.
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Evaluation Methods & Techniques
Edwards and Lawrence (1993) have developed a Prototype Index of InherentSafety (PIIS) for process design.
The inherent safety index is intended for analysing the choice of process route;
i.e. the raw materials used and the sequence of the reaction steps.
This method is very reaction oriented and does not consider properly the otherparts of the process even they usually represent the majority of equipment.
The PIIS has been calculated as a total score, which is the sum of a chemical scoreand a process score. The chemical score consists of inventory, flammability,explosiveness and toxicity. The process score includes temperature, pressure andyield.
Some of the scores are based on similar tables in the Dow and Mond Indices.Others have been constructed by dividing the domain of values of a parameterinto ranges and assigning a score to each range.
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Evaluation Methods & Techniques
Safety analysis
methodsElements of the method
Dow Fire and
Explosion
Index
Material factor: flammability and reactivity
General process hazards: exothermic chemical reactions, endothermic processes,
material handling and transfer, enclosed or indoor process units, access to the area,
drainage and spill control
Special process hazards: Such as toxic materials, sub-atmospheric pressure, operation in
or near flammable range, dust explosion, relief pressure, low temperature, quantity of
flammable and unstable materials, corrosion and erosion, leakage in the cases of joints
and packing, use of fired heaters, hot oil exchange systems, rotating equipment
Mond Index Material factor / Special material hazards / General process hazards / Special process
hazards / Quantity factor / Layout hazards / Toxicity hazards
Prototype Index of
Inherent
Safety (PIIS)
Chemical score: inventory, flammability, explosiveness and toxicity
Process score: temperature, pressure and yield
Total score: sum of the chemical and process scores
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Comparison & Interpretation