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SRP, Sekolah Menengah Derma, Kangar, Perlis 1981SPM, Sekolah Menengah Derma, Kangar, Perlis 1983

B.Sc. Chemical Engineering, University of Tulsa, Oklahoma, USA 1988M.Sc. (Eng.) Process Safety and Loss Prevention, University of Sheffield,

United Kingdom (UK) 1993PhD Environmental Risk Assessment, University of Sheffield, UK 1997

Mohamad Wijayanuddin bin Ali, PhDAssociate Professor, Universiti Teknologi Malaysia

8 December 1966, Perlis

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Video Show

• Let us view a short video presentation

• What have you learned from this video show?

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• To ensure safe process and operation throughout the life of a plant.

• By identifying all potential hazards or incident scenarios and minimizing all risks using loss prevention techniques such as follows :

- technological advances using better design/control

- inherent safety concept in design

- hazard identification methods

Notes

Any potential hazards need to be identified as early as possible so that action can be taken to correct or mitigate the situation.

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Inherent safety is to select a process or equipment which is by nature a safer process by applying keywords such as substitution, intensification and attenuation when developing a flow sheet.

• Substitution - avoid using hazardous material, but instead, use a safer one.

For example, replace flammable refrigerants and heat transfer media by non flammable one or processes which use hazardous raw materials by process which do not.

• Intensification - reduce inventories of hazardous materials in process and storage.

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Before Flixborough accident (1974) it was normal to design plant with huge inventory, and to add on protective equipment such as trips, alarms, and fire protection to control hazards.

Nowadays, inventory reduction can be applied to distillation, reaction, heat transfer and other unit operation.

But, by building many small plants instead of one big one, it increase the number of joints, pumps etc.

Challenge to engineers is to design small plants which can give output as large one.

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• Attenuation - use of hazardous materials under the least hazardous conditions. For example :

- liquefied gases can be stored as refrigerated instead of under pressure.

- An explosive powders are better in slurries forms rather than dry to avoid dust explosion.

- Runaway reactants should be in diluted form, so that it reduces the chance of runaway reactions.

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Methods of inherent safety

• Choice of Process

• Reactor Design

• Distillation Column Design

• Storage installation, for example avoiding

storage by plant relocation, reduction of amount

of materials in storage or storage in a safer

form.

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Choice of process

• Choose process which is less hazardous - this includes intermediate produces, reagent, compatibility of materials, catalysts and also solvents used, for example production of ketone-aldehyde (KA) at Flixborough.

It is an intermediate for nylon production.

Before accident, KA produced by air oxidation of cyclo hexane.

After accident and plant rebuilt, alternative route of process by hydrogenation of phenol was chosen.

This is vapor phase process and less hazardous than oxidation of KA.

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Reactor Design

• Reactors are usually large because reactions are slow and conversion is often low.

• To improve mixing try reduce reaction volume (intensification)

• Speed up the reaction by using a proper catalyst.

• Selection a proper type of reactor.

• For example, reaction of KA mixture was carried out in reactor fitted with external cooler and pump as well as stirrer.

Instead, substitute with internally cooled plug flow reactor.

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Distillation Column Design

• Distillation column usually held up large inventory of boiling liquid.

• So, try to reduce inventory through :

- minimize size of column, use many small column instead of one big one.

- use special design which can reduce inventories and also residence time.

• For example, ICI Higee Distillation column - distillation takes place in rotating packed drum.

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Storage Installation

• Avoid storage by plant relocation - if possible, relocating producing and consuming plant near each other so that to avoid storing and transporting hazardous materials.

• Reduce amount of materials in storage - by making the plant 5% or 10% larger than required. Extra capacity is used to cover delay in arrival of raw material.

• Storage in safer form - for example, some dyestuffs can be supplied as pastes instead of powders to avoid dust explosion. Liquid NH3 stored refrigerated at atmospheric pressure instead of stored under pressure at atmospheric temperature.

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• Complex processes require more advance safety technology.

• Since 1950 significant technological advances and safety analysis techniques have been made in chemical process safety :

- hydrodynamic models for two-phase flow through a vessel relief

- dispersion models representing spread of toxic vapor through a plant after a release

- hazard identification or quantification technique

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• Hazard - A chemical or physical situation with potential to cause harm, injury or damage to either human, property or the environment or some combination of these.

• Risk - A likelihood of hazard occurring in term of frequency (number of accident over a period of time) or in term of probability.

• Safety or loss prevention - An appropriate technology or a method used to identify potential hazard and try to eliminate (or at least reduce) those conditions before led to accident.

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• Hazard Analysis - Involves the identification of undesired events that lead to the materialization of a hazard, the analysis of the mechanisms by which these undesired events could occur. It also involves the estimation of extent, magnitude and likelihood of harmful effects. Many techniques can be used to evaluate process hazards that may involve.

• Quantitative Risk Assessment (QRA) - Quantitative evaluation of the likelihood of undesired events and the likelihood of harm or damage being caused together with value judgements made concerning the significance of the result.

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• Hazard Survey/Hazard Inventory - Identifies all stocks of hazardous material or energy with details of conditions of storage and information on nature of hazard i.e toxic flammable etc (conceptual design stage).

• Hazard indices - Checklist method of hazard identification which provides a comparative ranking of the degree of hazard posed by a particular design conditions, i.e the Mond Index and the Dow Fire and Explosion Index (detailed design stage).

• Hazard and Operability Study (HAZOP) - A formal systematic method of identifying hazards and operability problems by used of guide words (detailed design stage).

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• Failure Mode and Effects Analysis (FMEA) - Hazard identification method where all known failure modes of components or features of a system are considered in turn and undesired outcomes noted. If the chances of failures and the seriousness of the consequences are ranked to identify the most critical features it becomes Failure modes, Effects and Criticality Analysis (FMECA) (detailed design stage).

• Fault Tree Analysis - A method for representing the logical combination of various system states which can lead to a particular hazardous outcome, usually quantified as a form of QRA. (detailed design stage).

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• Event Tree Analysis - A method of illustrating and quantifying the intermediate and final outcomes of a given initiating event, another form of QRA (detailed design stage).

• Safety Audit - A critical examination of all or part of a plant with relevance of safety. Normally refers to a check of hardware and procedures after the plant has been in operation for some time.

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• Ingredients of successful safety and health program :

- safety knowledge

- safety experience (process and procedure)

- technical competence

- safety management support (audit/monitor losses or near misses and make appropriate adjustment)

- commitment (from company/management)

• Good safety and health program identifies and eliminates existing hazards. Excellent one has management system to prevent existence of hazards.

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• Commitment of management for successful safety programs is to have a written safety and health program.

• Safety and health policy is an important element. Policy ensures that :

- All employees must follow safety and health program.

- This program is designed to encourage all employees to promote the safety of their fellow employees and customers.

- To accomplish safety and health goals, all members of management are responsible and accountable for implementing the policy, and to insure it is followed.

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Fundamental principles

• Engineers shall uphold and advance the integrity, honor and dignity of engineering profession by :

- using knowledge & skill for enhancement of human welfare.

- honest and impartial and serving with fidelity to public, employers, clients.

- striving to increase competence and prestige of engineering profession.

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Fundamental canons (for engineers)

• Shall hold paramount safety, health and welfare of public in performance of their professional duties.

• Shall perform services only in areas of their competence.

• Shall issue public statements only in an objective and truthful manner.

• Shall act in professional matters for each employer or client as faithful agents or trustees, shall avoid conflicts of interest.

• Shall build their professional reputations on merits of their services.

• Shall act in such manner as to uphold and enhance the honor, integrity and dignity of engineering profession.

• Shall continue their professional development throughout their careers and shall provide opportunities for professional development of those engineers under their supervision.

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• Accident and loss statistics are used to measure the effectiveness of safety programs.

• Among statistical methods used to characterize accident and loss performance :

- OSHA (Occupational Safety and Health Administration, USA) incidence rate

- Fatal accident rate (FAR)

- Fatality rate or deaths per person per year

• These methods report number of accidents and/or fatalities for fixed number of workers during specified period.

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OSHA incidence rate :

• based on cases per 100 worker years.

1 worker year = 50 work weeks/yr x 40 hrs/weeks = 2000 hrs

• based on 200,000 hrs worker exposure to hazard

• two types of calculation (1) based on injuries and illness (2) based on lost workdays

OSHA (1) = number of injuries & illness x 200,000 / total hrs work by all employees during period covered

OSHA (2) = number of lost workdays x 200,000 / total hrs work by all employees during period covered

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Some OSHA definition :

• Occupational illness. Any injury such as cut, fracture, sprain, amputation etc as a result from work accident or from exposure involving single incident in the work environment.

• Lost workdays. Days which employee normally work but could not because of occupational injury or illness. This day does not include the day of injury.

• Fatality Accident Rates (FAR) is used by British chemical industries. Data is widely available in literature.

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• FAR. Fatalities based on 1000 employees working their lifetime. Employees assumed working total 50 years (108 working hrs).

• FAR = number fatalities x 108 / total working hrs by all employees during period covered

• Fatality rate = number of fatalities per year / total number of people in applicable population

• FAR can be converted to fatality rate (or vice versa) if number of exposed hours is known.

• OSHA incidence rate cannot be converted to FAR or fatality rate because it contains both injury & fatality information.

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Example• Given FAR =2. If employee works 8 hr shift 300 days per

year, compute fatality rateFatality rate = 8 hrs/day x 300 days/year x 2 deaths/108 hrs = 4.8 x 10-5 death per person per year

• More rock climbers are killed travelling by car than are killed during rock climbing. Is this statement supported by statistics?From data, travelling by car, FAR=57, rock climbing, FAR = 4000.Rock climbing produces more fatalities per exposed hrs but spend more time(exposed hrs) travelling by car. Think about this...

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Industry OSHA IncidenceRate

FAR(deaths/ 108hrs)

Chemical 0.49 4.0

Vehicle 1.08 1.3

Steel 1.54 8

Coal Mining 2.22 40

Construction 3.88 67

Agricultural 4.53 100

Activity FAR (deaths/ 108hrs) Fatality Rate

Travelling by car 57 17 x 10-5

Rock climbing 4000 4 x 10-5

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• Risk cannot be eliminated entirely.

• Every chemical process has a certain amount of risk associated with it.

• At some point in the design stage someone needs to decide if the risks are “tolerable".

• Each country has it owns tolerability criteria.

• One tolerability criteria in the UK is "as low as reasonable practicable" (ALARP) concept formalized in 1974 by United Kingdom Health and Safety at Work Act.

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• Individual risk (IR) is the frequency at which a given individual may be expected to sustain a given level of harm from specified hazard.

• Occupational risk is a risk that may happen at the work place. Can be described in term of FAR. It has been suggested that IR ~ 2.2 x 10-5 FAR.

• Societal risk is frequencies with which specified numbers of people in a given population sustain a specified level of harm from specified hazards.

• It is common to plot the frequency of events resulting in a specified consequence magnitude being exceeded (F) versus the number of fatalities (N), known as FN curves.

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• This framework is represented as a three-tier system as shown in figure. It consists of several elements :

(1) Upper-bound on individual (and possibly, societal) risk levels, beyond which risks unacceptable.

(2) Lower-bound on individual (and possibly, societal) risk levels, below which risks are deemed not to warrant regulatory concern.

(3) intermediate region between (1) and (2) above, where further individual and societal risk reductions are required to achieve a level deemed "as low as reasonably practicable (ALARP)".

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INTOLERABLELEVEL(Risk cannot bejustified on anyground)

THE ALARPREGION

(Risk is undertakenonly if benefit is desired)

BROADLYACCEPTABLEREGION

(No need fordetailed workingto demonstrateALARP)

TOLERABLE only if riskreductionis impraticableor if its cost is grosslydisproportionate to theimprovement gained

TOLERABLE if cost ofreduction would exceedthe improvement gained

NEGLIGIBLE RISK

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• General public normally do not understand concept of acceptable risk. This the basis for rejecting some development project.

• From one survey, 28% say chemicals do more good than harm, 29% say more harm than good, 38% say same amount of good and harm.

• Some naturalists suggest eliminating chemical plant hazards by “returning to nature” i.e. to eliminate synthetic fibers produced by chemicals and use natural fibers such as cotton.

• So what? FAR for agriculture is higher.

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• Accidents have direct, indirect and root causes– Direct cause – attribute to equipment failure

or unsafe operating conditions– Indirect cause – not as readily apparent and

can generally be tied to some human failure– Root cause – result of poor management

safety policies, procedures or decisions

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Three significant disasters :

• Flixborough, England 1974Failure of temporary bypass pipe replacing reactor no 5 (from 6 reactors) released 30 tons of cyclohexane, form vapor clouds, killing 28 people, injured 36.

• Bhopal, India 1984Contaminated methyl isocynate (MIC) caused runaway reaction.

Vapor released through pressure relief system but scrubber and flare system not working. 25 tons of MIC vapor released.

Toxic cloud spread nearby town killing 2500 civilian, injured more than 20,000. No plant workers were injured or killed. No plant equipment was damaged.

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• Seveso, Italy 1976

Reactor out of control, produced more side product, TCDD (dioxin - more than originally designed for).

Vapor TCDD released to atmosphere through relief system and heavy rain washed into soil.

TCDD is toxic to man and other species can contaminate drinking water.

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Major Disaster

• Piper Alpha, North Sea…?– Go and search the story……