mkm0023's version from 2015-11-30 09:02

Ch 6

Question Answer
What are the elements for combustion?fuel, an oxidizer, and an ignition source
What is fire?the rapid exothermic oxidation of an ignited fuel
When will fire not occur?fuel is not present or is not present in sufficient quantities

an oxidizer is not present or is not present in sufficient quantities

the ignition source is not energetic enough to initiate the fire
Flash point- meaning?the lowest temperature at which it gives off enough vapor to form an ignitable mixture with air

Vapor will only burn briefly because not enough vapor is present to maintain combustion
LFLLower Flammability Limit - The mixture will not burn when the composition is lower than this limit

the mixture is too lean for combustion
UFLUpper Flammability Limit - The mixture is not combustible when the composition is too rich, or above this limit
When is a mixture flammable?A mixture is only flammable when the composition is between the LFL and UFL
Flammability of mixtures containing oxygen and those containing air - are specific mixtures flammable or not?Combustion in pure oxygen also exhibits a lower oxygen limit (LOL) and an upper oxygen limit (UOL), just like the LFL and UFL in air

For most hydrocarbons, the LOL is close to the LFL

A good estimate of the UOL can be found using the UFL
what does the flammability diagram mean?Visual representation of the flammability of a gas or vapor

Each apex represents either 100% fuel, oxygen, or nitrogen

LOC (Limiting Oxygen Concentrations)

Question Answer
What does LOC mean?below the limiting oxygen concentration (LOC) the reaction cannot generate enough energy to heat the entire mixture of gases to the extent required for the self-propagation of the flame

reducing he oxygen concentration regardless of the concentration of the fuel is the basis for the inerting procedure
How is the LOC estimated?the LOC can be estimated using the LFL and UFL
Why are there different LOC’s when adding N2 and when adding CO2 to fuel/air mixtures?adding N2 or CO2 lowers the overall volume, reducing the concentration of oxygen
How much energy do we need to ignite most gases in mJ?Most hydrocarbon gases have MIE’s of about 0.25 mJ


Question Answer
What is the speed of the reaction front for detonation?The reaction front and shock front move at the same speed and at a speed greater than the speed of sound
What is the speed of the shock front for detonation?The reaction front drives the shock front immediately before it
What is the pressure profile for detonation?The pressure profile is a sharp and narrow peak


Question Answer
What is the speed of the reaction front in deflagration?The reaction front moves at a speed less than the speed of sound
What is the speed of the shock front in deflagration?The shock front moves away from the reaction front at the speed of sound
What is the pressure profile in deflagration?The pressure front is characteristically wide (many milliseconds in duration), flat (without an abrupt shock front), and with a maximum pressure much lower than the maximum pressure for a detonation (typically 1 to 2 atm)

The pressure profile is much wider than that of a detonation and looks more like a mound than a spike


Question Answer
What has been the most frequent ignition source in major fires?Electrical sources and wiring of motors
What is the auto-oxidation temperature?The temperature at which liquids begin to oxidize (liquids with relatively low volatility are particularly susceptible to this problem)

Spontaneous combustion is often a result of auto-oxidation

Deflagration index for gases and dusts

Question Answer
What does Deflagration index for gases and dusts mean?Function of maximum pressure increase and volume

Indicates robustness of explosion

Depend on concentration, mixing, vessel shape, ignition energy
What are the characteristics of dust mixtures that are explosive?the particles must be below a certain minimum size, typically less than 400 microns

the particle loading must be between certain limits

the dust loading must be reasonably uniform


Question Answer
What is the blast wave pressure vs. time?A blast wave is composed of the pressure wave and subsequent wind

The blast wave causes most of the damage

Most damage during shock duration

Strong winds accompany the shock duration
What is meant by TNT equivalence?The estimated overpressure of an explosion is compared to the overpressure resulting from a TNT explosion of a known mass an equal distance from the explosive origin
What does TNT equivalence measure for a compound in a process vessel or a vapor cloud?It is a simple method for equating a known energy of a combustible fuel to an equivalent mass of TNT

The overpressure curve for TNT tends to over-predict the overpressure near the vapor cloud explosion (VCE) and to under-predict the distance away from the VCE
What is the overpressure needed to damage the most sensitive parts of houses?The overpressure needed to damage the most sensitive parts of houses (the windows) ranges from 0.15 to 0.3 psig
What is the overpressure needed to distort steal frames of clad buildings?The overpressure needed to distort steel frames of clad buildings is above 1.3 psig
What is the range of blast fragments and missile damage?Maximum range of 6,000 ft

Ch 7

Procedures for avoiding the flammability zone

Question Answer
Inertingthe process of adding an inert gas to a combustible mixture to reduce the concentration of oxygen below the LOC: usually nitrogen or carbon dioxide, sometimes steam
Characteristics of Inertingvacuum purging is the most common inerting procedure for vessels; not used for large storage vessels because they are not usually designed for vacuums

vessels can be pressure-purged by adding inert gases under pressure

sweep-through purging process adds purge gas into a vessel at one opening and withdraws the mixed gas from the vessel to the atmosphere or scrubber from another opening
Taking a Vessel out of serviceefficient procedure: to inert with nitrogen until a point is reached, then air can be introduced

more efficient: first pump air into the vessel until a point is reached on the air stoichiometric line above the UFL, followed by pumping nitrogen into the vessel, followed by more air
Placing a vessel into servicethe vessel begins with air

followed by nitrogen until a point is reached

then fuel is pumped in until a point is reached


Question Answer
What is considered dangerous static electricity?For industrial operations where flammable vapors may be present, any charge accumulation exceeding 0.1 mJ is considered dangerous
What is the most effective and reliable method for preventing ignition?Inerting

Ways to discharge

Question Answer
spark a discharge between two metallic objects; this is an energetic spark that can ignite a flammable dust or gas
propagating brusha discharge from a grounded conductor when it approaches a charged insulator that is backed by a conductor; these discharges are energetic and can ignite flammable gases and dust
conical pilea form of brush-type discharge that occurs at the conical surface of a pie of powder; these are relatively intense discharges that can ignite flammable gases and dusts
brusha discharge between a relatively sharp-pointed conductor and either another conductor or a charged insulated surface; this discharge is less intense and is unlikely to ignite dusts, however it can ignite flammable gases
lightning-likedischarges from a cloud in the air over the powder; do not occur in small-volume vessels; no evidence that lightning-like discharges have resulted in industrial deflagrations
coronasimilar to a brush discharge; has sufficient energy to ignite only the most sensitive gases

Methods for preventing the buildup of charges on a product

Question Answer
Method 1 for preventing the buildup of charges on a productprevent charges from accumulating to dangerous levels by reducing the rate of charge generation and increasing the rate of charge relaxation; usually used when handling liquids
Method 2 for preventing the buildup of charges on a productprevent charges from accumulating to dangerous levels by designing the system to include charge reduction by means of low-energy discharges; usually used when handling powders
Method 3 for preventing the buildup of charges on a productwhen dangerous discharges cannot be eliminated, then prevent the possibility of an ignition by maintaining oxidant levels below the combustible levels or by maintaining fuel levels below the LFL or above the UFL


Question Answer
Bonding and GroundingBonding and grounding reduces the voltage of an entire system to ground level or zero voltage

The voltage difference between two conductive materials is reduced to zero by bonding the two materials

The voltage between sets of bonded materials is reduced to zero by bonding each set to ground
What is a dip pipe (dip leg)?An extended line that reduces the electrical charge that accumulates when a liquid is allowed to free fall


Question Answer
What is an XP-motor?an XP motor is one that has special explosion-proof housing
How does XP motor work?the housings are not designed to prevent flammable vapors and gases from entering but are designed to withstand an internal explosion and prevent the combustion from spreading beyond the inside of the enclosure
Can flammable gases enter an XP motor?Yes
What makes the XP motor able to withstand many small explosions?the enclosure is usually a heavy cast walled box with the strength needed to withstand explosive pressures
What keeps the hot vapors from the mini-explosions from causing a fire outside the motor?if there is a mini-explosion within the motor, the housing is able to withstand/contain it and prevent it from causing a fire or secondary explosion

the explosion-proof design includes the use of conduit with special sealed connections around all junction boxes


Question Answer
What is meant by an explosion proof area?flammable materials (particularly vapors) might be present at certain times
What is meant by a non-explosion proof area?flammable materials are not present, even under abnormal conditions

open flames, heated elements, and other sources of ignition may be present

Ch 8

Question Answer
How can we prevent a runaway reaction in both batch and continuous reactors? (list 4)Provide adequate heat removal systems with back-up coolant systems

Understand the reaction chemistry/kinetics before deigning the system

Properly size relief valves/system

Find a way to stop the reaction once it’s begun – a chemical that could be added or a change in pressure or temperature that could be made

Controlling reactive hazards

Question Answer
Inherent method (list 4)Use a reaction pathway that uses less hazardous chemicals

Use a reaction pathway that is less energetic, slower, or easier to control

Use smaller inventories of reactive chemicals both in the process and in storage; reduce pipe length and size to reduce inventory

Design equipment and/or procedures to prevent an incident in the event of a human error
Passive method (list 4)Ensure that incompatible chemicals are always separated

Provide adequate separation distance between storage vessels, reactors, and other process equipment using reactive chemicals

Provide passive engineering controls, such as dikes and containment, to control reactive chemical spills

Provide passive fire protection for chemical reactors, storage vessels, and process equipment; includes insulation of reactors and storage vessels, and thermal coating of all mechanical supports
Active method (list 4)Screen all chemical for reactive chemical hazards

Provide properly designed control systems to control reactive chemicals in the process

Provide properly designed heat transfer equipment to remove energy released by reactive chemistry

Use quench, stop, or dump systems to quickly stop out-of-control reactive chemistry
Procedural method (list 4)Document chemical reactivity risks and management decisions

Communicate and train on chemical reactivity hazards

Manage process changes that may involve reactive chemicals

Investigate chemical reactivity incidents