3. Learning Outcomes
•To identify potential hazards that associated
with chemical processes
•To identify measurement that helps to
control and prevent hazardous deviation
•To introduce basic principles and procedure
to develop Hazard and Operability Study
(HAZOP)
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4. Introduction
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• Legal and moral obligation to safeguard the health
and welfare of its employees and the general public.
• Safety is good business as good management
practices are needed to ensure safe operation and
this will ensure efficient operation.
• Loss prevention an insurance term
LOSS
•Financial loss
- damaged equipment, plant, claim
- Lost production and sales
•Reputation loss
•Human assets loss
5. Introduction
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Safety and loss prevention
in process design
Identification
and assessment
of hazards
Control of the hazards:
eg: containment of flammable and
toxic materials
Control of the process:
Prevention of hazardous deviations in
process variables( pressure, temperature,
flow) by provision of automatic control
systems, interlocks, alarms, trips etc)
Limitation of the loss: The
damage and injury caused if an incident
occurs: pressure relief, plant layout,
provision of fire fighting equipment
6. Introduction
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Intrinsic Safety Extrinsic Safety
which safe operation is
inherent in the nature of the
process, a process which
causes no danger, or negligible
danger under all foreseeable
circumstances.
?
7. Introduction
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Extrinsic Safety
• the safety has to be engineered in
• chemical manufacturing processes are generally inherently unsafe
and dangerous situations can develop if the process conditions
deviate from the design values
• The safe operation of such processes depends on the design and
provision of engineered safety devices and on good operating
practices
• Provision in the design of control systems, alarms, trips, pressure
relief devices, automatic shut-down systems, duplication of key
equipment services, fire fighting equipment, sprinkler system, blast
walls etc
• Examples, PSH, PSHH, LSH, LSHH, PSV, PCV, LCV
9. Hazard – toxicity
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• The potential health hazard to an individual by a material
used in any chemical or process plant is a function of the
inherent toxicity of the material and the frequency and
duration of exposure.
• Have to distinguish between the short term effects (acute)
and long term effects (chronic) of the material
• Highly toxic material that causes immediate injury such as
chlorine or phosgene would be classified as a safety
hazard
• A material whose effect is apparent only after long
exposure at low concentrations, eg vinyl chloride, would
be classified as industrial health and hygiene hazards
10. Hazard – toxicity
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• Both the permissible limits and the precautions to be taken to
ensure that such limits are not exceeded are quite different
for these two classes of toxic materials.
• Inherent toxicity is measured by tests on animals. It is
expressed as the lethal dose at which 50% of the animals do
not survive and this is known as LD50 value.
Test on rats
11. Hazard – toxicity
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• The permissible exposure limit (PEL) of concentration for the
long term exposure of humans to toxic materials is set by the
threshold limit value (TLV)- upper permissible concentration
limit that is safe to humans with exposure of 8hrs/day, 5
days/week over a period of many years
• Most significant source of workplace exposure to toxicity is
inhalation. Hence an understanding of the sources of
contaminants to which workers are exposed is important for
the recognition, evaluation and control of occupational health
hazards
12. Hazard – toxicity
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• Recommended TLVs are published by OSHA (Occupational
Safety and Health Administration)
• Handbook prepared by Sax and Lewis provides a
comprehensive source of data as well as guidance on the
interpretation and use of the data
• Prepare a Material Supply Data Sheet (MSDS) for the
materials used
13. Hazard – toxicity
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• Preventative aspects of the use of hazardous substances are:
- substitution: processes using less hazardous material
- containment: sound design of equipment and piping, avoid
leaks
- ventilation: use open structures or adequate ventilation
- disposal: effective vent stacks
- emergency equipment: escape routes, rescue equipment,
respirators, safety showers and eye baths
14. Hazard – flammability
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• Flammable – materials that will burn
• Hazards caused by flammable material depends on:
• Flash point of material- a measure of the ease of ignition
of liquid. It is the lowest temperature at which the
material will ignite from an open flame.
• Autoignition temperature-temperature at which a
substance will ignite spontaneously in air, without any
external source of ignition. Indication of the maximum
temperature to which a material can be heated in air
15. Hazard – flammability
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• Flammability limits- the lowest and highest concentration in
air, at normal pressure and temperature, at which a flame
will propagate through the mixture. Range of concentration
over which the material will burn in air if ignited.
16. Hazard – flammability
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• For a fire to occur, there must be a fuel, an oxidiser and an
ignition source. In addition, the combustion must be self
sustaining.
• Air is the oxidiser, a minimum concentration of fuel is
necessary for the flame to be ignited. Minimum
concentration required depends on the temperature of the
mixture and to a lesser extent on the pressure, greatest
interest is focused on the ignition conditions necessary at
ambient temperature.
17. Hazard – flammability
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• Flame traps
- fitted in the vent lines of equipment that contains
flammable material
- prevent the propagation of flame through the vents
- providing a heat sink, usually expanded metal grids or
plates to dissipate the heat of the flame
- installed in the plant ditches to prevent the spread of flame
18. Hazard – explosion
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• Explosion is the sudden, catastrophic release of energy,
causing a pressure wave. An explosion can occur without
fire, such as the failure through over-pressure of a steam
boiler or an air receiver.
• Different types of explosions:
1. Confined vapour cloud explosions(CVCE)
2. Unconfined vapour cloud explosions(UCVCE)
3. Boiling liquid expanding vapour explosions (BLEVE)
4. Dust explosions
19. Hazard – source of ignition
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• Electrical equipment
Source of ignition
- sparkling of electrical equipment
- electrically operated instruments, controllers and computer system
Hazardous area – explosive gas-air mixtures are present
Zone 0: explosive gas-air mixtures present continuously for long periods;
intrinsically safe equipment to be used
Zone 1: explosive gas-air mixtures likely to occur in normal operation; intrinsically
safe equipment, or flame proof enclosures to be used
Zone 3: explosive gas-air mixtures not likely to occur during normal operation, but
could occur for short periods; intrinsically safe equipment, or total
enclosure or non-sparkling apparatus to be used.
Hence the standards have to consulted for full specification before selecting
equipment for the designated zones
20. Hazard – source of ignition
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• Static electricity – movement of any non-conducting material, powder,
liquid or gas can generate static electricity, producing sparks. Have to
ensure that all piping is properly earthed (grounded) and that electrical
continuity is maintained around flanges.
- Escaping steam, other vapors or gases can generate a static charge.
Gases escaping from a ruptured vessel can self-ignite
• Process flames – flames from process furnaces and incinerators are
obvious sources of ignition and these equipment have to be located well
away from plant containing flammable materials
• Miscellaneous source
- control entry of obvious source of ignition, matches, cigarettes
- used of portable electrical equipment, welding, spark producing tools and
movement of petrol-driven vehicles
- exhaust gases from diesel engines
21. Hazard – ionizing radiation
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• Radiation emitted by radioactive materials is harmful to living
matter.
• Small quantities of radioactive isotopes are used in the
process industry for various purposes e.g in level and
density measuring instruments and for the non-destructive
testing of equipment..
• Use of radioactive isotopes covered by government
legislation
22. Hazard – pressure
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• Over-pressure (pressure exceeding the system design
pressure) is one of the most serious hazards in chemical
plant operation. Failure of a vessel or the associated piping
can result in a sequence of events that can culminate in a
disaster.
• Pressure vessels are fitted with pressure relief devices set at
the design pressure so that potential over-pressure is
relieved in a controlled manner.
•
23. Hazard – pressure
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• Three different types of relief devices commonly used are:
-Directly actuated valves-spring loaded valves that open at a predetermined
pressure and which close after the pressure has been relieved. The system
pressure provides motive power to operate the valve
-Indirectly actuated valves- pneumatically or electrically operated valves
which are activated by pressure sensing instruments
-Bursting discs- thin discs of material that are designed to fail at a
predetermined pressure, giving a full-bore opening for flow
• Relief valves are normally used to regulate minor excursions of pressure
and bursting discs as safety devices to relieve major over-pressure.
• Relief valves and bursting discs are proprietary items and vendors are
consulted prior selection
• API 520 and 521 for sizing of relief valves and overpressure
25. Hazard – pressure
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• Vent piping
-Relief systems have to ensure that flammable or toxic gases are vented
to a safe location
-Vent to a safe height to ensure the gases are dispersed without creating a
hazard.
-For toxic materials necessary to provide a scrubber. HP Flare Scrubber,
LP Flare Scrubber etc
•-Rate of venting will be determined by the design of the complete venting
system, the relief device and the associated piping. Maximum venting rate
will be limited by the critical sonic velocity.
•-Vessels also have to be protected against external fire so the relief valve
has to be sized for fire etc
27. Hazard – pressure
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• Under-pressure (vacuum)
- Unless designed to withstand external pressure, a vessel has to be
protected against the hazard of under-pressure, as well as over-
pressure
- Under-pressure normally means vacuum on the inside with
atmospheric pressure on the outside. A slight drop in pressure below
atmospheric pressure will result in the tank collapsing
- Not uncommon for a storage tank to be sucked in (collapsed) by the
suction pulled by the discharge pump, due to the tank vents having
become blocked.
- Vacuum breakers should be fitted in such cases
29. Hazard – temperature deviation
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• Excessively high temps, over & above the design temperature can
cause structural failure & initiate a disaster
• High temperatures can result from loss of control of reactors and
heaters, from open fires
• Protection against high temperatures to be provided for processes
where high temperatures are a hazard
- High Temp alarms & interlocks to shutdown reactor feeds or heating
systems
-Provision of emergency cooling systems
-Structural design of eqpt to withstand worst possible temp excursion
-Selection of intrinsically safe heating systems for hazardous material
30. Hazard – temperature deviation
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•Steam and other vapour heating systems are intrinsically safe as the
temperature cannot exceed the saturation temperature at the supply
pressure
•Electrical heating systems are particularly hazardous
• Fire protection
- To protect against structural failure water-deluge systems are usually
installed to keep vessels and structural steelwork cool in a fire.
- Lower sections of structural steel columns are often lagged with
concrete or other suitable material
31. Hazard – noise
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• Excessive noise is a hazard to health and safety. Long
exposure to high noise levels can cause permanent
damage to hearing. At lower levels, noise is a distraction
and causes fatigue
• Sound is measured in decibels
• Permanent damage to hearing
can be caused at sound levels
above 90db
32. Hazard – noise
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• Normal practice to provide ear protection in areas where
noise is above 80db
• Noise pollution from factories
causes complaints from residents
• Compressors, fans, burners and
steam relief valves cause
excessive noise-consider in
plant layout
34. Dow fire and explosion index
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• Hazard classification guide developed by DOW Chemical
Company gives a method of evaluating the potential risk
from a process and assessing the potential loss
• A numerical “Fire & Explosion Index” is calculated.
• The larger the value of the F&EI, the more hazardous the
process.
35. Dow fire and explosion index
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• The potential hazard of a new plant can be calculated
after the P&IDs and equipment layout diagrams have
been prepared.
• Indicate alternative, less hazardous process routes
should be considered.
36. Safety study
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• HAZID - Hazard Identification
• HAZOP - Hazard & Operability Study
• FERA - Fire & Explosion Risk Analysis
• QRA - Quantitative Risk Assessment
• SGIA - Smoke & Gas Ingress Analysis
• EERA - Escape, Evacuation & Rescue Analysis
• ESSA - Emergency System Survival Analysis
• TRIA - Temporary Refuge Impairment Analysis
• HFE - Human Factor Engineering
• HRA - Health Risk Assessment
• ERP - Emergency Risk Assessment
37. Hazard and operability study (hazop)
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• HAZOP - systematic technique for identifying all plant or
equipment hazards and operability problems. When
applied to a process design or an operating plant, it
indicates potential hazards that may arise from deviations
from the intended design conditions.
• In this technique, each segment (pipeline, piece of
equipment, instrument etc) is carefully examined and all
possible deviations are identified. This is accomplished
by fully defining the intent of each segment and then
applying the guide words to each segment.
• Seven guide words for carrying out HAZOP
38. More –
quantitative
increase
eg: higher
temperature
Hazard and operability study (hazop)
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7 guide
words
No or not – no
part of the intent
is achieved and
nothing else
occurs
eg: no flow
Less –
quantitative
decrease
eg: lower
pressure
As well as –
Something in addition
to the design intention
eg: impurities, side
reaction
Reverse –
Reverse of or
opposite to the
design intention
eg: reverse
flow
Part of –
Something missing, only
part of the intention is
realized, such as change in
the composition of a stream
or a missing component
Other than–
No part of the intention is
achieved, something
completely different occurs.
Covers all conceivable
situations other than intended
such as start up, shutdown,
maintenance
39. Hazard and operability study (hazop)
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• These guide words are applied to flow, temperature,
pressure, liquid level, composition and any other
variables affecting the process.
• Normally carried out by Specialists eg HAZOP Leader
and HAZOP Scribe. Include all the main disciplines but
mostly Process Engineers, Instrument and Piping
• Require Flow Sheets (PFDs, UFDs), P&IDs, Plant
Layout
• HAZOP Study Report-Close out the findings
40. Hazard analysis
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• HAZOP identifies the hazards but gives no indication
on the likelihood of an incident occurring or the loss
suffered. The sequence of events that leads to a
hazardous incident can be shown as a fault tree (logic
tree)
• Fault tree analysis (FTA)- means of analysing
hazardous events after they have been identified. Used
to estimate the likelihood of an accident by breaking it
down into its contributing sequences. Use of the fault
tree provides a graphical representation between
certain possible events and undesired consequences.
• Sequence of events forms pathways on the fault tree
with AND and OR gates.
42. Safety audits
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• Safety audits carried out to verify the adequacy of
safety equipment and safety rules.
• Safety equipment includes eqpt for fire protection,
personnel protection and on-site emergency
responses
• Explicit safety rules for new process will be identified
• Safety Checklist for identifying process hazards
• Critical assessment of the checklist will identify the
major hazards in the proposed new facility or existing
facility
• This should be done in the early stages of the design
43. Environmental protection
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• All individuals and companies have a duty to care for
the neighbours and to the environment
• Agencies to monitor this include:
-OSHA: Occupational Safety and Health Administration
-NIOSH: National Institute for Occupational Safety and
Health
-EPA: Environment Protection Agency
• EIA-Environment Impact Assessment gives a full
disclosure statement of project parameters that have a
+ve environmental impact, -ve environmental impact or
no impact at all