Lecture 11 - Safety, Fire and Explosions

8/10/2019 Lecture 11 - Safety, Fire and Explosions

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CBEE Chemical, Biological and Environmental Engineering

Process Safety Nobody can teach you Safety in only 5 weeks.

Similar to teaching you Art or Math in a month. Very broad fields, not nearly enough time. Take home: Know what you dont know.

Reading assignments are listed in the syllabus

All material in the assigned readings is fair gamefor homework and exams (but place specialemphasis on topics we discussed in class).

Exams will be open-book, open-notes, closed-neighbor, closed-network.


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Process Safety

Safety can be separated into (arbitrary) categories: Occupational safety (hard hats & safety goggles)

Industrial hygiene (chemical exposure & control) Physical Security (police & fire response) Information Security (computers and networks)

Public Relations (neighbors and reputation) Process Safety (pipes, vessels and pumps)


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Into the Danger Zone

Where and how do most accidents occur?

50% of incidents occur during startup, shutdown,

maintenance or abnormal operations 40% could have been prevented by a hazard and

operability study (down to 15% in 1990)

25% in storage/blending areas, flammable vaporsand leaks (liquified petroleum gas, gasoline)

24% of ignitions unexplained (unknown source)

21% caused by auto-ignition or nearby flames

Keep in mind: ACCIDENTS HAPPEN FAST!

Approximate statistics from Kletz What Went Wrong (5th Ed., 2009), Appendix 1


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Recurring Primary Causes

18% runaway reactions / incompatible chemicals

12% due to corrosion, usually unsuitable materials

11% due to abnormally high temperatures (lack ofeffective alarms/trips and inadequate procedures)

10% unexpected result of modifications to process

equipment or operating procedures 9% flammable vapors in confined space (4% in tanks)

8% uncontrolled/unexpcted flows in drains/vents

7% opening pressurized vessels

7% failure of safety instruments / alarms



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Responsibility for Accidents

Many accidents could be prevented by

Better process design and safety features (40%)

Well-documented operating procedures (35%)

Improved training and control board layouts (20%)

Systematic inspection and maintenance of processequipment (53% from mechanical failure)

Piping systems (28%), reactors (25%), tanks (15%)

Surprisingly, pumps, valves and compressors < 5%



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A Note on Blame

After an industrial accident occurs, the typicalresponse from management is to blame the

plant operators on duty at the time: Operator X21 closed the wrong valve, which lead

to overfilling of the tank and the subsequent fire.

So Whos to blame?

The operator? Their supervisor? Upper management?The plant designer or equipment vendor? Government?

Blaming accidents caused by poor institutionalpractices or design faults on an individual is notonly unproductive, but ultimately irrelevant.


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C SE STUDIES

How to have a very bad day

Image source: h ttp://hulldailymail.co.uk (Flixborough distaster, 1974)


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Case Studies

Those who cannot remember the past arecondemned to repeat it.

George Santanya, 1905

Understanding the causes of major incidentshelps us learn to prevent future accidents.

Seven must-know disaster examples

c.f. Trevor Kletzs What Went Wrong (5th ed.) Valley Library Reserves, VR-761


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Flixborough (England, 1974)

Cause: Improper temporary piping

Result: 28 dead, 36+ wounded. Plant destroyed.

The Chemical Engineer(April 2005); Kletz 2009


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Bhopal (India, 1984)

Cause: Runaway reaction vented toxic vapors

Underlying cause: Containment system

(scrubber, flare) for vapors was not operational Result: 3,787 deaths, 558,212 injuries.

Source: Wikimedia


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Seveso (Italy, 1976)

Release of 2kg of highly-toxic dioxin (TCDD)

Cause: Runaway reaction, failure of cooling

No containment of vented toxins (sound familiar?)

Result: 10 sq. miles contaminated, uninhabitable.> 750 cases of chloracne. Now non-disaster.

Toxipedia.com / CDC/NIOSH / CNN.com


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Pasadena (Texas, 1989)

Cause: Failed valve, release and ignition of85,000 pounds of flammable gas

Result: 23 dead, 314 wounded, $715M loss Poor maintenance

Valve control air lineswere switched (off/on)

Days between permit

and maintenance ops Take-off branch notisolated before service

Wikimedia.org


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Cause: Startup procedures not followed.

Contributing: Excess liquid alarm inoperable.

Flammable liquid spilled out of tank & ignited. Result: 15 dead, 170 injured, much damage

Lack of knowledgeable operators on-site

Maintenance trailers located next to hazardousoperations

No supervisingsenior personduring startup

Texas City (Texas, 2005)

http://www.csb.gov/bp-texas-city-investigative-photos/


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Jacksonville (Florida, 2007)

Cause: MCMT side-reaction runaway due tohigh temperature excursion

Result: 4 dead, 32 injured (28 public). Thermal runaway due to inadequate design of

reactor cooling system (no redundancy)

Company was unaware ofhazardous side-reaction

Relief system improperly sizedto handle reaction products

175 previous batches okayCSB.gov / Wikimedia.org


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Port Wentworth (Georgia, 2008)

Cause: Sugar dust collected due to coveredconveyors produced massive dust explosion

First explosion dislodged accumulated sugar dustfrom surfacesmany secondary explosions

Concrete floors, stairwells, brick walls collapsed

Result: 14 dead, 36 injuries

Company was aware of dust explosion hazards

200+ dust explosions cataloguedbetween 1980-2005

100+ dead, 600+ injuries

CSB.gov / Wikimedia.org


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CHEMIC L EXPOSURE LIMITS

How long can you stand that smell?


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Exposure Limits

American Conference of GovernmentalIndustrial Hygienists (ACGIH) defines TLVs

Threshold Limit Values = concentration thatcauses no adverse effect during workers lifetime.

Three types defined:

TLV-TWA (Time-Weighted Average most common)

TLV-STEL (Short-Time Exposure Limit)

TLV-C (Ceiling limit)

Guidelines, not legal limits (OSHA PEL = legal)


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Threshold Limit Values (TLVs)

TLV-TWA (Time-Weighted Average)

Acceptable exposure 8 hours/day, 5 days/week

Most commonly used guideline, conservative

TLV-STEL (Short-Term Exposure Limit) Permissible 15-minute exposure (> TWA)

Not to be exceeded, even if TWA below limits

No more than 4 exposures/day of < 15 minutes,and at least 60 minutes between exposures.


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NFPA Diamond Diagrams

Quick characterization of major hazards of achemical compound (aka NFPA 704 signage)

Widely used for chemical storage and shipping Simple, effective warning of major hazards

Designed for firefighters and

other emergency personnel All containers of hazardous

materials must be clearly

labeled with at least thecommon name, formula, andany known hazards

http://my.firefighternation.com


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NFPA Diamond Diagrams

compliancesigns.com

3

3 3

W

Example: Sodium Cyanoborohydride (NaCNBH3)


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FIRES ND EXPLOSIONS

The science behind

Bremen chemical waste facilit y explosion (9/9/2014). 2014 BBC Media


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Fire Triangle

Fires generally require three components:

FUEL (gas or vapor from hot solid/liquid)

OXIDIZER (often air, but may be solid/liquid) IGNITION (usually hard to eliminate completely)


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Explosions

Fires release chemical energy fairly slowly

Explosions release lots of energy very rapidly,typically within microseconds

Consider an inflated automobile tire

Release the pressure through the nozzle no problem.

Blow out on the road big problem!

Same result and energy release,only the time scale is different


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Definitions

Flammability limits

Lower Flammability Limit (LFL)

Upper Flammability Limit (UFL) Explosions can only occur if mixture is between

LFL and UFL otherwise too lean or too rich

Explosion Rapid expansion of gases thatproduces a shock front

Mechanical sudden release of pressurized gas Deflagaration shock front moves < Mach 1

Detonation shock front moves > Mach 1 (fast)


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Deflagaration vs. Detonation

Deflagaration:

Mythbusters Coffee Creamer Cannon

http://www.youtube.com/watch?v=XWcR5nv1N8I

Explosion:

Mythbusters Concrete Mixer http://www.youtube.com/watch?v=Gxm_qpKh7Jw


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Definitions (continued)

Autoignition Temperature (AIT) Lowest (fixed) temperature at which a mixture will

ignite without external ignition source Ignition Energy

Energy input needed to initiate combustion ( LFL)

Fire Point Lowest temperature (must be > flash point) at which a

liquid will burn continuously once ignited


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Explosions

Explosions occur because of a rapid releaseof energy.

Essentially the immediate environment cantcontain the immediate accumulation of energy sothe energy is dissipated outward through anumber of mechanisms

Pressure Wave Projectiles (aka Shrapnel)

Thermal Radiation

Acoustic Energy

The Damage is caused by the Energy as itDissipates


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Detonation and Deflagration

Figure 6-14 Comparison of detonation and deflagration gas dynamics. The explosion is in itiatedto the


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Blast Damage

An explosion of dust or gas results in a

reaction front proceeded by either a shockwave or pressure front. This pressurewave causes significant damage much

more typically than the thermal radiation oracoustic energy (shrapnel is always badwhen it hits you).


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Blast Damage

The explosion damage depends mainly on the

over pressure (peak pressure reached) 0.03 psig, large glass windows break

0.15 psig, typical glass windows break

0.3 psig, safe distance limit limit of projectiles 1 psig, partial demolition of houses (unlivable)

5 psig, utility poles snap, large equipment damaged

10 psig, probable total destruction of non-explosionproof buildings; heavy equipment destroyed

300 psig, limit at crater lip (nothing remains alive!)


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Overpressure Estimation

Experiments with explosives demonstrate

Figure 6-23

TNT Energy = 1120 cal/g


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Overpressure Estimation

Experiments with explosives demonstrate

TNT Energy = 1120 cal/g


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Equivalent TNT

A simple way to compare the known

energy of a combustible fuel to TNT

TNT Energy = 1120 cal/g = 4686 kJ/kg = 2016 BTU/lb

is the empirical explosion efficiency


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Equivalent TNT

Damage Estimates can be made by:1. Determine the total quantity of flammable

material involved in the explosion2. Estimate the explosion efficiency, and calculate

the total equivalent mass of TNT3. Use the scaling law to estimate the peak over-

pressure.4. Use an appropriate table (like Table 6-9 in the

text) to estimate the damage to commonstructures and process equipment.

The reverse procedure can be used toestimate the quantity of material involvedbased on observed damage.


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Explosive Energies

Chemical Reaction Explosion

Calculated from Heat of Combustion

Mechanical Explosion (Vessel Failure) Dependent on release of energy stored as

pressure. Frequently estimated based on

Isentropic Expansion:

1 1


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Flammability Properties

Concentrationof

Flammable

Vapor

TemperatureFP AIT

Mists(may be flammable)

Liquid VaporPressure

Auto-IgnitionRegion

FLAMMABLEREGION


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Flammability Diagrams

Figure 6-6: Flammability diagram for methane at an initial temperature and pressure of 25C and 1 atm.

Pure N2Pure O2

Pure CH4

2Fuel O productsz

100%

1

z

z


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Flammability Diagrams

Inerting (eliminate flammable vapor in a tank)

Must be done before entry or hot work

More on thistopic later

Figure 6-7


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Flash Point Calculations

Flash point of pure compounds are tabulatedor can be derived from empirical correlations

Satyanarayana and Rao correlation (p. 251):

Tf is flash temperature (K)

a, b, c are constants for a given material

Tb is the boiling point temperature (K)

2 /

2/

/

1

b

b

c T

b

fc T

b c T eT a

e


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Flash Point Calculations

Flash points of mixtures of flammable andinert liquids can be determined

FP is temperature at which vapor pressure of theflammable component in the mixture is the sameas that of the pure component at its flash point

See example 6.1 for details

Multicomponent mixtures are hard to model flash points should be measured directly


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l b l


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Flammability Ranges

The flammable mixture range (LFLUFL)becomes wider with increasing temperature

Empirical correlations are also available based onreaction stoichiometry or heats of combustion. See pg. 257-258 for details

250.75

25c

LFL T LFL TH

25 0.75 25c

UFL T UFL T H

i i i i


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Limiting Oxygen Concentrations

LFL/UFL are based on fuel in air, but air isonly 20% oxygen

Below the Limiting Oxygen Concentration(LOC), the reaction cannot generate enoughheat to propagate a flame

LOCs are tabulated (Table 6-3)

LOCs are dependent upon the inert gas

Depends on heat capacity, etc., of inert gas

A id i ( b i )


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Auto-Oxidation (Spontaneous Combustion)

Slow oxidation and evolution of heat

Can lead to autoignition if heat is not removed

Can be a problem in a number of systems

Oxidizable liquids with low volatility Classic example: Oily rags burst into flame

Evaporation of liquid efficiently sheds heat

Biomass piles (e.g. woodchips) Heat generated in the center by decay cant be

shed, because the rest of the pile insulates it

Adi b i C i


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Adiabatic Compression

If a gas is compressed without heat beingremoved, the temperature will increase

If the temperature of the gas increasesabove the AIT, and the gas mixture isflammable, an explosion will occur!

This was the suspected ignition source for theTexas City refinery explosion in 2005.

1

f

f i

i

PT T

P

I iti S


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Ignition Sources

23% - Electrical wiring faults and motors

18% - Smoking near flammable vapor/materials

10% - Friction from worn bearings, broken parts 14% - Hot surfaces, open flames (torches)

11% - Sparks from welding, cutting, grinding

8% - Overheated/abnormally hot materials/parts

4% - Spontaneous combustion (autoignition)

3% - Static electricity, lightning, etc.

Often unknown (practically impossible toeliminate all ignition sources in a plant!)

BLEVE


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BLEVEs

Boiling-Liquid Expanding-Vapor Explosion Most common source of large, damaging explosions

1. A fire develops adjacent to a tank containing liquid.

2. The fire heats the walls of the tank.3. The tank walls below the liquid level are cooled by

the liquid via heating and evaporation of the liquid this raises the temperature and pressure in the tank.

4. If the flames reach the tank walls or roof wherethere is only vapor and no liquid to remove the heat,the tank metal temperature rises until the tank losesits structural strength.

5. The tank ruptures, explosively vaporizing itscontents (which if flammable may ignite and explodeas a vapor cloud explosion).

BLEVE id


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BLEVE videos

See these and other videos for examples: http://www.youtube.com/watch?v=UM0jtD_OWLU

BLEVE demonstration training video

http://www.youtube.com/watch?v=Xf3WKTwHpIU Multiple BLEVE explosions following train derailment

Documents

Lecture 11 - Safety, Fire and Explosions