The safe design and operation of - portal.unimap.edu.myportal.unimap.edu.my/portal/page/portal30/Lecturer Notes... · The safe design and operation of facilities is of paramount importance

Chemical Engineering Design

The safe design and operation of facilities is of paramount

importance to every company that is involved in the

manufacture of fuels, chemicals and pharmaceuticals


Processes must meet acceptable safety and environmental performance standards because:

It is required by law

The costs (human, social, economic) of non-compliance can be catastrophic

Negligent attitudes are reflected in insurance premiums, stock prices

Moral and ethical obligations


Factory and Machinery Act (FMA) 1967

◦ 11 regulations of FMA 1967:

1. STEAM BOILER AND UPV 1970

2. ELECTRIC PASSENGER AND GOODS LIFT 1970

3. FENCING OF MACHINERY AND SAFETY 1970

4. SAFETY, HEALTH AND WELFARE 1970

5. CERTIFICATES OF COMPETENCY—EXAMINATIONS 1970

6. NOTIFICATION, CERTIFICATE OF FITNESS AND INSPECTION 1970

7. LEAD 1984

8. ASBESTOS PROCESS 1986

9. BUILDING OPERATIONS AND WORKS OF ENGINEERING CONSTRUCTION (SAFETY) 1986

10. NOISE EXPOSURE 1989

11. MINERAL DUST 1989


The Occupational Safety and Health Act (OSHA) 1994◦ Employers must provide a place of employment free from recognized hazards

to safety and health, such as exposure to toxic chemicals, excessive noise levels, mechanical dangers, heat or cold stress, or unsanitary conditions.

◦ Seven regulations of OSHA 1994:

1. Employers Safety and Health General Policy Statement (Exception) Regulation 1995

2. Control of Industry Major Hazards (CIMAH) Regulations 1996

3. Safety and Health Committee Regulations 1996

4. Classification, Packaging and Labeling of Hazardous Chemicals (CPL) Regulations 1997

5. Safety and Health Officer Regulations 1997

6. Use and Standards of Exposure of Chemicals Hazardous to Health (USECHH) Regulations 2000

7. Notification of Accident, Dangerous Occurrence, Occupational Poisoning and Occupational Disease (NADOPOD) Regulation 2004


The Occupational Safety and Health Act (OSHA) 1994

For all industries

If >5 Employees-Safety & Health Policy

≥40 Employees (S30) -Safety & Health Policy + Safety &

Health Committee

For high risk industries (i.e. construction, ship building, gas etc.)

>100 Employees (Order 1997) -Safety & Health Policy + Safety & Health Committee + a Certified Safety & Health Officer

For low risk industries (other than the above mentioned industries)

>500 Employees (Order 1997) -Safety & Health Policy + Safety & Health Committee + a Certified Safety & Health Officer


i. Physical Hazards e.g. height, noise, vibration, force, lighting etc.

ii. Chemical Hazards e.g. gas, liquid, vapor, fumes, mist, dust etc.

iii. Biological Hazards e.g. microbes, animals, arthropod, toxin plants

iv. Electrical Hazards e.g. current, voltage, sparks

v. Radiation Hazards e.g UV light, lasers

vi. Psychological Hazards e.g. workplace space, organization culture, space.


To design a safe process or product we need to understand and mitigate the associated hazards

Materials hazards◦ Toxicity◦ Flammability◦ Incompatibility (corrosivity and reactivity)

Process hazards◦ Overpressure◦ Explosions ◦ Loss of containment◦ Noise



Almost every chemical is toxic if you get enough of it

Source of exposure – inhalation

Chemical plants tend to have large enough amounts to cause serious concern for workers and local residents

Process design needs to consider◦ Elimination or substitution of the most hazardous compounds

◦ Prevention of releases

◦ Containment

◦ Disposal (via effective collection or vent systems)

◦ Ventilation

◦ Emergency procedures


Acute Effects Symptoms develop rapidly (e.g. burns to skin after direct contact)

Normally the result of short-term exposures

Chronic Effects Symptoms develop over a long period of time (e.g. cancer)

Often but not always the result of long-term exposure

Chronic conditions usually persist or recur frequently

LD50

Lethal dose at which 50% of test animals are killed

Usually expressed in mg/kg body mass

Indicates acute effects only

Threshold Limit Value (TLV) or Permissible Exposure Limit (PEL) Concentration that it is believed the average worker can safely be exposed

to for 40 hr work week

Recommended PEL values are published by OSHA

Recommended TLV values are provided by the American Conference of Government Industrial Hygienists (ACGIH)


Compound PEL (ppm) LD50 (mg/kg)

Carbon monoxide 50 1807

Carbon disulfide 20 3188

Chlorine 1 239

Chlorine dioxide 0.1 292

Chloroform 50 1188

Cyclohexane 300

Dioxane 100 4200

Ethylbenzene 100 3500

Formic acid 5 1100

Furfural 5 260

Hydrogen chloride 5 4701

Hydrogen cyanide 10 3.7

Isopropyl alcohol 400 5045

Toluene 100 5000

Xylene 100 4300

• Examples:

Source: OSHA

Ethanol LD50 = 3450 (oral, mouse) 7060 (oral, rat) 1440 (intravenous, rat)


A fire requires three things:◦ A sufficient amount of fuel◦ A sufficient amount of oxidant◦ A source of ignition (but not always - see autoignition)

Possible ignition sources include◦ Electrical equipment such as motors, actuators Usually specified as flame-proof or non-sparking when fuels

are present

◦ Open flames from furnaces, incinerators & flare stacks◦ Static electricity From any flow, hence pipes, vessels & flanges are always

grounded

◦ Miscellaneous sources Matches, lighters & mobile phones are usually banned


Flash point The lowest temperature at which the material will ignite

from an open flame Function of vapor pressure and flammability limits

Autoignition temperature Temperature at which the substance ignites in air

spontaneously Indicates maximum temperature the material can be

heated to in air, e.g., in drying Flammability limits

Highest and lowest concentrations in air at normal temperature and pressure (ntp) at which a flame will propagate through the mixture

Vary widely for different materials Data can be found in Materials Safety Data Sheets (MSDS) or

safety handbooks


Flame arrestors (flame traps) are specified on vent lines of equipment that contains flammable materials to prevent a flame from propagating back from the vent

Various proprietary designs are available

Basic principle:◦ Remove the heat source (high temperature)

◦ Provide high metal surface area to act as a sink for heat and free radicals

Enardo detonation

flame arrestors

Source: Enardo LLC

www.Enardo.com

http://www.enardo.com/


Mixtures of incompatible materials may undergo violent reaction (exothermic, temperature runaway)◦ Acids and bases

◦ Acids and metals

◦ Fuels and oxidants

◦ Free radical initiators and epoxides, peroxides, unsaturates, …

Incompatibility with materials of construction can lead to loss of containment◦ Corrosion of vessels, internals, instruments

◦ Softening of gaskets, seals, linings

Materials incompatibility is one of the major sources of incidents


• Material Safety Data Sheets (MSDSs)

must be provided to employees and

customers by law in the U.S.A. (OSHA

Hazard Communication Standard 29 CFR Part

1910.1200)

• MSDS contains the information needed

to begin analyzing materials and

process hazards

• Most MSDSs contain a disclaimer

stating that the user should also make

their own evaluation of compatibility and

fitness for use


Always collect MSDS of all components used in the process at as early a stage as possible

Sources: manufacturers, manufacturer’s web sites, libraries, etc.

Because of disclaimers, it is worth checking > 1 source

Good starting points are http://www.msdssearch.com/or http://www.siri.org/msds

Use MSDS information to improve intrinsic safety of process

Eliminate incompatible mixtures

Substitute less hazardous chemicals when possible (e.g. toluene instead of benzene as solvent)

Ensure that design meets regulatory requirements

Vapor recovery

Other emissions

http://www.msdssearch.com/

http://www.siri.org/msds


Substitution – use something less toxic and hazardous

Containment Sound design of plant and equipment

For example, use welded joints instead of flanges

Prevention of releases By design of equipment and disposal systems

Ventilation Use open plant structure or engineered ventilation system

Disposal Effective vent stacks and scrubbers

Collection and treatment of run-off water and liquid from relief systems

Provision of emergency equipment



Occurs when mass, moles or energy accumulate in a contained volume (or space with restricted outflow)

Rate of accumulation determines the pressure rise

Process controls may not be able to respond quickly enough

If pressure is not relieved by pressure safety valve then outcomes could include◦ Vessel rupture◦ Explosion◦ Other loss of containment


A fire requires a flammable mixture and an ignition source

Fires in chemical plants can quickly lead to damage to control systems and equipment, causing overpressure, loss of containment and explosions

Fire protection guidelines are given in several standards◦ NFPA 30, API RP 2001

Legal requirements for fire protection are set by Uniform Building By-Laws 1984


Can you think of possible sources of ignition on a chemical plant?◦ Sparking of electrical equipment Motors, actuators, lighting, electric heaters, …

◦ Process flames Furnaces, flare stacks, incinerators These should always be sited well away from plant, usually

upwind◦ Static electricity See API RP 2003 and NFPA 77

◦ Lightning◦ Vehicles (engines, electrical systems and exhausts)◦ Portable electrical devices◦ Welding and cutting equipment◦ Miscellaneous sources (matches, lighters, etc. are usually

banned)


The use of electrical equipment in chemical plants is regulated by law (OSHA) and by industry design codes National Electrical Code NFPA 70 NFPA standards 496, 497, API RP 500, 505

NFPA 70 defines classified areas in which flammable materials may be present at high enough concentrations to be ignitable Specific precautions must be taken depending on the

classification Equipment must be designed and installed in accordance with

code


Codes should be consulted before selecting equipment for use in classified areas

Codes also govern electrical maintenance work (NFPA 70B). Companies usually have strict ―Lock-out, tag-out‖ procedures to prevent electric shock accidents


An explosion is the sudden, catastrophic release of energy causing a pressure wave (blast wave)

Explosions can be caused by ignition of a flammable mixture◦ Liquid

◦ Vapor

◦ Solid (e.g., finely dispersed dust)

Explosions can also be caused by release of thermal energy◦ Boiler rupture

◦ BLEVE (boiling liquid expanding vapor explosion)


Deflagration◦ Combustion zone propagates at (subsonic) flame speed, usually < 30

m/s◦ Pressure wave generated usually < 10 bar◦ Principal heating mechanism is combustion

Detonation◦ Combustion zone propagates at supersonic velocity, 2000 – 3000 m/s◦ Pressure wave up to 20 bar◦ Principal heating mechanism is shock compression◦ Usually requires confinement or a high-intensity source◦ Deflagration can turn into detonation when propagating along a pipe

Expansion factor◦ Measure of the increase in volume resulting from combustion◦ E = (molar density of reagents)/(molar density of products)◦ Maximum value of E is for adiabatic combustion

Flame speed◦ The rate of propogation of a flame front through a flammable mixture,

with respect to a fixed observer


Upper Lower

Hydrogen H2 4.0 75 54 22.1 2318 6.9 400

Methane CH4 5.0 15 10 2.8 2148 7.5 601

Ethane C2H6 3.0 12.4 6.3 3.4 2168 7.7 515

Propane C3H8 2.1 9.5 4.5 3.3 2198 7.9 450

n-Butane C4H10 1.8 8.4 3.5 3.3 2168 7.9 405

Pentane C5H12 1.4 7.8 2.9 3.4 2232 8.1 260

Hexane C6H14 1.2 7.4 2.5 3.4 2221 8.1 225

Heptane C7H16 1.1 6.7 2.3 3.4 2196 8.1 215

Acetylene C2H2 2.5 80 9.3 14.8 2598 8.7 305

Ethylene C2H4 2.7 36 7.4 6.5 2248 7.8 490

Propylene C3H6 2.4 10.3 5 3.7 2208 7.8 460

Butylene C4H8 1.7 9.7 3.9 3.8 2203 7.9 385

Benzene C6H6 1.3 7.9 3.3 5 2287 8.1 560

Cyclohexane C6H12 1.3 8.0 2.7 4.2 2232 8.1 245

Autoignition

temperature

(ºC)

FormulaFlammability Limits (vol%)

Fuel

Vol% gas at

max flame

speed

Adiabatic

flame Temp

(K)

Expansion

factor

Maximum

flame speed

(m/s)

Dugdale, D. An introduction to Fire Dynamics, Wiley, New York, 1985


Design to prevent explosions from happening Prevent formation of explosive mixtures

whenever possible◦ Operate outside flammability envelope

Consider confined explosion as a pressure relief scenario and ensure that PRV is sized to allow adequate relief load to prevent detonation

Use flame suppressors to prevent deflagration from propagating into detonation


The primary means of protecting the public from toxic chemicals is containment by the plant itself

Loss of containment can occur due to:

◦ Pressure relief

◦ Operator error (e.g. leaving a sample point open)

◦ Poor maintenance procedures

Failure to drain and purge properly

Failure to put everything back together properly

◦ Leaks from degraded equipment

Corrosion

Damaged seals, gaskets

These are mostly operational issues, but design may need to provide for secondary containment if the potential impact of a release is high


Chemical plants can be very noisy, especially compressors, turbines, motors and solids handling

Chronic effects include permanent damage to hearing

Sound is measured in decibels, defined by:

(Note: log scale)

Ear protection should be required in areas where noise > 80 dB

Permanent damage can be caused by noise > 85 dB

dB

102

PapressuresoundRMSlog20levelSound

510

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The safe design and operation of - portal.unimap.edu.myportal.unimap.edu.my/portal/page/portal30/Lecturer Notes... · The safe design and operation of facilities is of paramount importance