Process Safety Midterms

Toxic release models

___________ are routinely used to estimate the effects of a release on the plant and community environments.

Neutrally buoyant dispersion models

__________________ models are used to estimate the concentrations downwind of arelease in which the gas is mixed with fresh air to the point that the resulting mixture is neutrally buoyant.

parts per million

Neutrally buoyant dispersion models apply to gases at low concentrations, typically in the _________ range.

Plume models

describes the steady-state concentration of material released from a continuous source.

Plume

_________ is simply the release of continuous puffs.

smoke stack

Plume models were originally developed for dispersion from a ___________

plume

Inan emergency if there is a leak in a large tank then a ______ can develop.

Puff models

describes the temporal concentration of material from a single release of a fixed amount of material.

Puff models

are used when you have essentially an instantaneous release and the cloud is swept downwind.

Puff model

___________ can used to describe a plume.

Initial puff

This is formed by instantaneous release of material

Concentrations (in puff model)

This is the same on all three surface

Puff moves downwind and dissipates by mixing with fresh air

________ moves downwind and dissipates by mixing with ________

Because diffusivity constantly changes due to fluctuations and turbulence.

Why do traditional transport phenomena equations not accurately predict dispersion?

Fluctuations and turbulence in the flow.

What causes diffusivity to constantly change?

They make dispersion harder to predict using traditional equations.

What is the effect of turbulence and fluctuations on dispersion prediction?

Assume that materials spread out in a normal Gaussian-type distribution.

What is the solution to handle the unpredictable changes in diffusivity?

A normal (Gaussian) distribution.

What type of distribution is used to model material spreading in turbulent flow?

Pasquill-Gifford or Gaussian Dispersion Models

As the plume is swept downwind, the concenfration profile spreads out and decreases.

Pasquill-Gifford Models

Gaussian Dispersion Models are also known as ____________

Gaussian Distribution model

_______________________ applies only to neutrally buoyant dispersion of gases in which the turbulent mixing is the dominant feature of the dispersion.

turbulent mixing

Neutrally buoyant dispersion of gases in which the _____________ is the dominant feature of the dispersion.

Gaussian Distribution model

It is typically valid only for a distance of 0.1-10 km from the release point

0.1-10 km

Gaussian Distribution model is typically valid only for a distance of _____ km from the release point

Dense Gas Dispersion: Britter and McQuaid Model

Dense Gas Dispersion Model is also known as __________________________

Dense Gas Dispersion Model

The model is best suited for instantaneous or continuous ground-level releases of dense gases.

Dense Gas Dispersion Model

The release is assumed to occur at ambient temperature and without aerosol or liquid droplet formation.

Dense Gas Dispersion Model

The model requires a specification of the initial cloud volume, the initial plume volume flux, the duration of release, and the initial gas density.

• initial cloud volume

• initial plume volume flux

• duration of release

• initial gas density.

Dense Gas Dispersion Model requires a specification of the:

• Wind speed at a height of 10 m

• distance downwind

• ambient gas density

These are the additional requirements in the Dense Gas Dispersion Model

dense gas

A _________ is defined as any gas whose density is greater than the density of the ambient air through which it is being dispersed.

• gas with a molecular weight greater than that of air

• gas with a low temperature resulting from auto refrigeration

What can dense gases result from?

Release mitigation

____________ is defined as "lessening the risk of a release incident by acting on the source (af the point of release)

• in a preventive way by reducing the likelihood of an event that could generate a hazardous vapor cloud

• in a protective way by reducing the magnitude of the release and/or the exposure of local persons or property

Ways of release mitigation

• Inventory reduction

• Chemical substitution

• Process attenuation

Inherent safety

• Physical integrity of seals and construction

• Process integrity

• Emergency control

• Spill containment

Engineering Design

• Policies and procedures

• Training for vapor release

• Audits & inspections

• Equipment testing

• Routine maintenance

• Management of change

• Security

Management

• Sensors

• Personnel

Early Vapor Detection

• Water sprays

• Steam curtains

• Deliberate ignition

• Foams

Countermeasures

• On-site communications

• Emergency shutdown

• Site evacuation

• Safe havens

• PPE

• Medical treatment

• On-site emergency plans

Emergency Response

• fires

• explosions

• toxic releases

The three most common chemical plant accidents are:

Organic solvents

_____________ are the most common source of fires and explosions in the chemical industry.

• The fire and explosion properties of materials

• The nature of the fire and explosion process

• Procedures to reduce fire and explosion hazards.

To prevent accidents resulting from fires and explosions, engineers must be familiar with:

• Oxygen

• Fuel

• Heat

Elements of the fire triangle

FIRE

_______ is a CHEMICAL REACTION between Fuel, Oxygen and Heat.

Fire

It is a self-sustaining process of rapid oxidation or combustion of a fuel, which produces heat and light.

rate of energy release

The major distinction between fires and explosions is the ______________

• Fires release energy slowly

• Explosions release energy rapidly, typically on the order of microseconds

Differences between fires and explosions

TRUE

TRUE or FALSE
Fires can also result from explosions, and explosions can result from fires.

Combustion or fire

_______________ is a chemical reaction in which a substance combines with an oxidant and releases energy.

sustain the reaction

Part of the energy released during a fire is used to ______________

Ignition

May be caused by a flammable mixture coming in contact with a source of ignition with sufficient energy or the gas reaching a temperature high enough fo cause the gas to autoignite.

Avutoignition temperature (AIT)

A fixed temperature above which adequate energy is available in the environment to provide an ignition source. The temperature at which a product can undergo spontaneous ignition.

flash point

The ___________ of a liquid is the lowest temperature at which it gives off enough vapor to form an ignitable mixture with air.

fire point

The __________ is the lowest temperature at which a vapor above a liquid will continue to burn once ignited; the fire point temperature is higher than the flash point.

Flammability limits

The range of concentrations of gases in air which will support the explosive process bounded by measurable limits

• Upper Explosive Limits (UEL) or Upper Flammable Limit (UFL)

• Lower Explosive Limits (LEL) or Lower Flammable Limit (LFL)

Different flammability limits

LEL: 1.4%

UEL: 7.6%

LEL and UEL of Gasoline vapors

explosion

An ____________ is a rapid expansion of gases resulting in a rapidly moving pressure or shock wave.

• mechanical

• caused by a chemical reaction

An explosion can be:

pressure or shock wave

Explosion damage is caused by the ______________________

Mechanical explosion

An explosion resulting from the sudden failure of a vessel containing high-pressure nonreactive gas.

Deflagration

An explosion in which the reaction front moves at a speed less than the speed of sound in the unreacted medium.

Detonation

An explosion in which the reaction front moves at a speed greater than the speed of sound in the unreacted medium.

Confined explosion

An explosion occurring within a vessel or a building. These are most common and usually result in injury to the building inhabitants and extensive damage.

Unconfined explosions

These occur in the open

Unconfined explosions

This type of explosion is usually the result of a gas spill.

Unconfined explosions

The gas is dispersed and mixed with air until it comes in contact with an ignition source.

Unconfined

Which are rarer, Unconfined/confined explosions?

Unconfined explosions are rarer than confined explosions because the explosive material is frequently diluted below the LFL by wind dispersion.

Boiling-liquid expanding-vapor explosion (BLEVE)

occurs if a vessel that contains a liguid at a temperature above its atmospheric pressure boiling point ruptures.

BLEVE

The subsequent _____ is the explosive vaporization of a large fraction of the vessel

Dust explosion

This explosion results from the rapid combustion of fine solid particles. Many solid materials (including common metals such as iron and aluminum) become flammable when reduced to a fine powder.

Shock wave

An abrupt pressure wave moving through a gas.

strong wind

A shock wave in open air is followed by a __________

mostly adiabatic

The pressure increase in the shock wave is so rapid that the process is _______________

Blast wave

The combined shock wave and strong wind. Pressure wave propagating in air.

Overpressure

The pressure on an object as a result of an impacting shock wave.

Minimum ignition energy (MIE)

is the minimum energy input required to initiate combustion. All flasmmable materials (including dusts) have this

are explosions that occur because of the release of flammable vapor over a large volume and are most commonly deflagrations.

TNT equivalency

_____________ is a simple method for equating a known energy of a combustible fuel to an equivalent mass of TNT.

exploding TNT

The approach is based on the assumption that an exploding fuel mass behaves like _____________ on an equivalent energy basis

1120 cal/g = 4686 kJ1kg = 2016 Btu/lb.

A typical value for the energy of explosion of TNT is:

heat of combustion

The ____________________ for the flammable gas can be used in place of the energy of explosion for the combustible gas.

• thermal heating of the reaction products

• the change in the total number of moles by reaction

The blast wave resulting from a chemical explosion is generated by the rapid expansion of gases at the explosion site. This expansion can be caused by two mechanisms:

thermodynamics

The energy released during a reaction explosion is computed using standard ______________________

work required to expand the gases

The released energy is equal to the ______________________. This expansion work is a form of mechanical energy.

blast wave impact

Unconfined explosions also create missiles by _______________ and subsequent translation of structures.

Missiles

___________ are frequently a means by which an accident propagates throughout a plant facility.

profit (probability unit) analysis

Blast damage effects are estimated using _____________________

Vapor Cloud Explosions (VCEs)

The most dangerous and destructive explosions in the chemical process industries

• Sudden release of a large quantity of flammable vapor

• Dispersion of the vapor throughout the plant site while mixing with air

• Ignition of the resulting vapor cloud

Steps in Vapor Cloud Explosions (VCEs)

Sudden release of a large quantity of flammable vapor

(typically this occurs when a vessel, containing a superheated and pressurized liquid, ruptures)

BLEVE

is a special type of accident that can release large quantities of materials. If the materials are flammable, a VCE might result; if they are toxic, a large area might be subjected to toxic materials.

BLEVE

A ______ occurs when a tank containing a liquid held above its atmospheric pressure boiling point ruptures, resulting in the explosive vaporization of a large fraction of the tank contents.

The liquid may ignite as the tank ruptures.

What can happen if a flammable liquid is involved in a BLEVE caused by fire?

It behaves like rocket fuel, propelling vessel parts over great distances.

How does the boiling and burning liquid behave during a fire-induced BLEVE?

A vapor cloud may form, resulting in a vapor cloud explosion (VCE).

What might happen if a BLEVE is not caused by a fire?

No, only a fraction of the liquid vaporizes.

When a BLEVE occurs, does all the liquid vaporize?

The physical and thermodynamic conditions of the vessel contents.

What determines how much liquid vaporizes during a BLEVE?