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 first young man refused the challenge.

He lived safe and died chaste.


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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 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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Degraded Layers of Protection

Potential Incidents

Layers

ofProtectio

n

Higher Actual Risk

Degraded

Degraded


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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

Documents

Plant Layout Considerations