Chemical Process Safety Slides for Final Exam
yellow diamond
reactivity
LOPA is limited to what?
to evaluating a single cause consequence pair as scenario
puff model
transient. sudden discharge of a finite quantity of material. function of position and time
deflagration
a fire or reaction which propagates at below the speed of sound
chronic hazard
a hazard whose effects manifest themselves over a long period of time
leading indicator
a measure preceding or indicating a future event used to drive and measure activities carried out to prevent and control injury. proactive in nature and report what employees are doing on a regular basis
detonation
a reaction where the reaction front travels at greater than the speed of sound. because the reaction happens faster than the speed of sound, the amount of energy that can be scrubbed off by expansion is limited. this causes a much higher pressure and temperature behind the blast wave increasing the blast efficiency
3 types of industrial hygiene control
administrative controls, engineering controls, personal protective equipment
alk
alkali
adiabatic temperature rise
allows one to determine if a loss of cooling control will raise the temperature enough to trigger a secondary reaction
confined space entry
an OSHA requirement. required permit at most refineries, chemical plants, and many other industries
effective dose ED50
dose which produces measureable but reversible effects in 50% of population
dust pentagon
dust explosions can be prevented by removing any side. ignition source, dispersion, oxygen, fuel, confinement
blue diamond
health hazard
response of reaction to temperature
higher T makes all reactions go faster. different reactions respond to temperature changes differently according to arrhenius law
facility siting
identify, evaluate, and manage the potential hazards and risks to onsite and offsite occupied areas from potential fire, explosion and toxic material release events. protect personnel in occupied site buildings from the potential serious effects of releases. protect the public in offsite but nearby buildings from the same
IDLH
immediately dangerous to life and health
inherently safer design strategies
minimize, substitute, moderate, simplify
disadvantages to rupture discs
dont reclose after relief, burst pressure cannot be tested, require periodic replacement, greater sensitivity to mechanical damage, greater sensitivity to temperature
toxic dose TD50
dose which produces irreversible harmful effects in 50% of population
what reduces ground level concentration?
raising height of release point
3 key actions in reducing risk
1. identifying and classifying a risk level 2. identifying ways to decrease the risk 3. quantify the effects of the risk reduction efforts
divisions
I is flammable concentrations are normally present II is flammable concentrations are present only under abnormal situations
classes
I is locations with flammable gases or vapors II is locations with flammable dusts
PSM management
a comprehensive set of policies, procedures, and practices to ensure barriers are in place to mitigate the chance of a release of chemicals
process safety
a discipline that is focused on prevention and preparation of releases of chemicals
grounding
connecting equipment to ground
hazards are relatively ranked into what?
consequence categories based on a few available parameters
cor
corrosive
risk
coupling of the consequence and frequency of a scenario
buckling pins
designed to be a non reclosing pressure relief device similar to a rupture disc
modeling of consequences
dispersion modeling, explosion modeling, and toxic modeling are usually not done at the LOPA stage
conditions for dust explosion to occur
dust must be combustible, must have particle size distribution that allows propagation of flame, atmosphere in which dust is dispersed must contain sufficient oxidant
bonding
electrically connecting two equipment pieces
causes of chatter
excessive inlet pressure drop, excessive built up back pressure, oversized valve, valve handling widely differing rates
red diamond
fire hazard
fire triangle
fires can be prevented by removing any single leg. oxidant, fuel, ignition source
assumptions for gas flow through hole
fluid is compressible, friction can be expressed as discharge coefficient, expansion is adiabatic, change in height is insignificant
assumptions for liquid flow through hole
fluid is non compressible, friction can be expressed as discharge coefficient, pressure is constant, change in liquid height is zero
reaction calorimetry
for scale up the heat flow produced by a given volume of reaction mass cannot exceed the heat removal capacity of the reactor if temperature control is to be maintained
heat transfer
for the reactor to stay isothermal, heat transfer q must equal the heat flow produced by the reaction plus the agitator power
static electricity
formation mechanisms are rubbing, falling, moving, flowing, separation.
qualitative
frequency is often expressed as likelihood. consequence may be expressed as in general terms (major, moderate, minor)
GHS
globally harmonized system. requires chemical manufactures to determine the hazards associated with their chemicals labels be utilized, material safety data sheets be provided
HAZOP
hazard and operability studies
entry route for toxicants
ingestion, inhalation, injection, dermal absorption
LOPA is commonly used in industry
is a risk assessment and mitigation tool, is a semi quantitative tool, is to be used with PHAs. is not a replacement for PHA
lethal dose LD50
kills 50% of populaion
LOPA
layers of protection analysis. a semi quantitative tool used to determine risk
LOC
limiting oxygen concentration. oxygen concentration below which combustion is not possible, with any fuel mixture. lowest concentration of oxygen which will form a flammable mixture
types of release
liquid/gas flow through hole, liquid/gas flow through open pipe, flashing liquid, liquid evaporation, gasket failures
control of hazardous energy
lockout tagout is an OSHA standard but not required. requires isolation of electrical and other hazardous energy sources before working on equipment
LFL
lower flammability limit. below LFL, mixture will not burn
MAWP
maximum allowable working pressure. maximum design pressure at the top of a vessel for a designated temperature
MTSR
maximum temperature (adiabatic) of the synthetic reaction. the temperature the reaction would reach in the absence of cooling
lagging indicator
measure a company's incidents in the form of past accident statistics. tell you how many people got hurt and how badly
HAZOP advantages
meets regulatory requirements, plant operates better, less down time, product quality improved, employees are happier
MEC
minimum explosible concentration. the minimum concentration of an explosible dust cloud that is capable of propagating a deflagration through a uniform mixture of dust and air under the specified conditions
MIE
minimum ignition energies. energy required to ignite a flammable mixture
advantages to conventional spring loaded valve
most reliable type if properly sized and operated. versatile
secondary explosions
most severe and destructive. caused when the powder/dust on the floor and beams is jarred loose and thrown into one large dust cloud and ignited by the fireball from primary reaction
NEP
national emphasis program on combustible dusts
HAZOP guidewords
no, more, less, as well as, part of, reverse, other than, sooner than
propagating explosions
normally starts as a primary explosion in a piece of equipment and propagates to all of the interconnected equipment
what is a confined space?
not designed for human occupancy. large enough to enter and preform work. limited means of entry/exit.
process/reaction hazard testing purpose
obtain data for emergency relief system design, evaluate thermal stability of a material, obtain pt data for process systems, detect and measure gas generation and rate of pressure rise
oxy
oxidizer
factors influencing sensitivity and explosion characteristics of dust clouds
particle size, surface area, chemical composition, moisture content, initial pressure temperature of the gas phase, distribution of particle sizes, degree of dispersion
inherently safer
permanently and inseparably reduces or eliminates process hazards. avoidance of hazards rather than their control
hot work permits cont
plant areas with combustible vapors and dusts control ignition sources. hot work helps prevent fires and explosions. electrical area classifications and explosion proof electrical equipment also eliminate ignition sources. hot work permits control intermittent ignition sources needed for maintenance or other special operations
set pressure
pressure at which the relief device begins to open
overpressure
pressure increase over set pressure during relieving
pressure relief devices
pressure relief valves, rupture disks, buckling pins
combustible dust explosions
primary explosions, propagating explosions, secondary explosions
PSM
process safety management. divided into 14 elements
general classification risk approaches
qualitative, semi-quantitative, quantitative (QRA)
advantages to rupture discs
reduced fugitive emissions, no simmering or leakage prior to bursting, protect against rapid pressure rise caused by heat exchanger tube ruptures, less expensive, less tendency to foul or plug
what do source models provide?
release rate, total amount released, state of material
advantages to pilot operated valve
relieving pressure is not affected by backpressure, can operate at up to 98% of set pressure, less susceptible to chatter
advantages to bellows valve
relieving pressure not affected by back pressure. can handle higher built up back pressure. protects spring from corrosion
hot work permits
required by OSHA PSM. a key is to make sure there are no flammable hydrocarbons in the hot work area or introduced into the area by the people doing the work
isolation procedures
shutdown/shutoff the source of hazardous energy. remove any residual energy (pressure, potential, electrical, depressure) lock electrical breakers in the closed position, remove pneumatic air or electricity that allows automatic valves to operate, lock valves in their closed position, place a tag on each lock signed by the person isolating, try to start any electrically locked equipment
types of fires
smoldering fires, flash fires, spreading fires
acute hazard
something which is hazardous immediately and generally in high concentration
plume model
steady state charge of material. function of position
two types of backpressure
superimposed and built up
disadvantages to pilot operated valve
susceptible to plugging, limited chemical and high temperature use, vapor condensation and liquid accumulation above the piston may cause problems
flash point
temperature above which a liquid produces enough vapor to form an ignitable mixture with air or the lowest temperature where a non propagating fire will occur above a pool of liquid
HAZOP process parameters
temperature, pressure, flow, level, concentration, agitation, reaction, start, stop, power, component
Pmax
the maximum pressure developed in a contained deflagration of an optimum mixture
where is the highest concentration in dispersion
the point of release
backpressure
the pressure downstream of the relief device during the relieving process
accumulation
the pressure increase over the MAWP of the vessel during the relief process
chatter
the rapid alternating opening and closing of a PR valve
TLV
threshold limit value. max concentration which TWA- limit over 8hr STEL-limit in any 15 min C- instantaneous limit
HAZOP disadvantages
time, people and effort, information management challenges
UFL
upper flammability limit. above UFL mixture will not burn
w
use no water
quantitative evaluation of frequencies from fault trees and event trees
usually not done at this LOPA stage
how can a cloud be better dispersed?
wind and less stable atmosphere