Chemical & Process Safety Management > Chapter-1 > Topic-Enhancing Safety In Chemical Industry > Subtopic-Introduction to concept: Criteria for siting and layout of chemical plant, Hazardous area classification, Layers of Protection analysis, Instrumentation for safe and efficient operations of plants, safety Integrity level.
Introduction to concept: Criteria for siting and layout of chemical plant, Hazardous area classification, Layers of Protection analysis, Instrumentation for safe and efficient operations of plants, safety Integrity level.
Criteria for siting and layout of chemical plant-
1.PLANT SITING AND LAYOUT-
Rural, Urban with mixed areas, low population density
of high population density.
If hazard is toxic gas release, effect of distance to
reduce gas concentrations.
2. PLANT LAYOUT CONSIDERATIONS-
Segregation of different risks.
Ø Separation
of flameproof and non-flameproof areas as per Factories Act and Tariff Advisory
Committee.
Ø Segregation
of Plants having explosion potentials, keeping costs on utilities low.
Ø Minimization
of vulnerable pipe work.
Ø Containment
of accidents.
Ø Efficient
and safe construction of match factories light roof construction.
Ø Facilitation
of process operations.
Ø Efficient
and safe maintenance - clear distances to facilitate this
Ø Minimization
of personal injuries - minimum number of operators at times working behind
barrier walls, with mirrors to observe processes.
Ø Safe
control room design - entrance to be guarded by blast wall.
Ø Emergency
control facilities - ECC Disaster Plan.
Ø Congregation
points, security problems, firefighting facilities inside factory and in neighborhood,
access plant for emergency Services.
3. FLOW PRINCIPLES-
Ø Process layout
Ø Functional layout
Ø Materials always on
the more in a straight forwards manner
Ø Materials flow to
follow process flow design
Ø Importance of
efficient materials holding quantities, necessity of
Ø large inventories
4. LAYOUT
TECHNIQUES-
Ø Method study for
best layout.
Ø Use of 2
dimensional and 3 dimensional templates.
Ø Algebraic matrix to
determine minimum cost of material handling movements.
5. SITE LAYOUT-
Ø Preliminary layout.
Ø Main layout topography,
weather, environment, transport, power, water and effluent services, legal
constraints.
6. SEGREGATION OF AREAS:
Zones 0, 1 and 2
Ø Class A, B and C as
per Petroleum Act.
Ø Classification of
flammable liquids - as per NFPA.
Ø Safe separation distances
- for bulk storages as per Petroleum Act (underground - above ground), as per
SMPV Rules under Explosives Act depending upon proportion of chemicals stored and
quantities stored - distance between vessels - different types of transformers
Ø Protection screen
walls - no double tier storages.
Ø Vapour travel
barrier walls - blast walls.
Ø Dished ends of vessels not to face each other.
7. SERVICES / UTILITIES
Ø
Boiler,
Thermic Fluid Heater, Compressors, Electric sub-station, pumping stations,
transformers
8. EFFLUENTS DISPOSAL
Ø
Incinerator,
biological treatment, liquid and acid effluents.
Ø
Hazardous
solid wastes disposal – Hazardous Waste Management Rules.
9. TRAFFICE
Ø
Types
of traffic inside work areas. Adequate space for road tankers parking.
Ø
Parking
of employees' and visitors' vehicles.
Ø
Parking
lots - angular, parallel parking.
Ø
Outwards
traffic to merge slowly into the main road traffic.
Ø
Rail
lines inside work areas.
Ø
Entry
into plants form main and side roads.
10. EMERGENCIES
Ø
ECC,
Congregation points
Ø
Emergency
service vehicles parking.
Ø
Ring
main - hydrant system.
11.SECURITY
Ø
Boundary
fence, gate house, watch and word training.
12. PLOT LAYOUT
Ø
General
considerations - gravity flow, process flow principles, gas leaks.
13. HAZARDS
Ø
Earthquakes,
thunderstorms.
Ø
Types
of ventilation - natural, artificial, safety, vacuum exhaust, A/c - heating
toxics, ventilation from fire, health, and comfort points of view.
14. FIRE FIGHTING FACILITIES –
Ø
Types,
mutual aids schemes.
15. EQUIPMENT LAYOUT
Ø
General,
considerations, corrosive materials.
16.CONTROL ROOMS-
Ø
General
considerations, ventilation, inlet of air from nearly plant areas, control
facilities, layout construction.
17.PIPE WORK LAYOUT-
Ø
May be
run as a double layer bed - service lies on top upper and process lies on lower
duct, compatibility of adjacent pipe work, splash guards for acid / alkali
splashes through flanged joints.
Ø
Sample
points at 1 m above floor and not at eye level. Pipe bridges over roads should
be minimum necessary damage by truck forklifts and mobile cranes.
Ø
Discharge
from pressure vessels, relief system (valves and bursting rapture discs) to be
piped away in a closed system. Some scrubbing may also be necessary -
limitations.
18. STORAGE LAYOUTS-
Ø
Flammable
liquids / gases – at atmospheric pressure and under pressure.
Ø
Controls
should not allow flammable liquids or heavy vapor to collect in a depression.
Ø
Krebs,
dyke walls – principles e.g. LPG vessels full of liquids are very heavy, hence
site should have good load bearing characteristics.
Ø
Segregation
of storages from process areas Minimum 15meters.
Ø
Storage
area should be in groups - groupings should be such as to allow common bunding,
common firefighting facilities for each group.
Ø
Access
on all 4 sides of each bund area, if materials are highly hazardous, terminals
should be near the entrance may be at site boundary, provided it does not
affect neighbor’s installations.
Ø
Monitoring
of storage conditions, provision of windsocks
Hazardous area classification
Hazardous Area Classification for Flammable Gases and Vapours Area classification may be carried out by direct analogy with typical installations described in established codes, or by more quantitative methods that require a more detailed knowledge of the plant. The starting point is to identify sources of release of flammable gas or vapour. These may arise from constant activities; from time to time in normal operation; or as the result of some unplanned event. In addition, inside process equipment may be a hazardous area, if both gas/vapour and air are present, though there is no actual release.
Catastrophic failures, such as vessel or line rupture are not considered by an area classification study-A hazard identification process such as a Preliminary Hazard Analysis (PHA) or a Hazard and Operability Study (HAZOP) should consider these abnormal events.
The most used
standard in the UK for determining area extent and classification is BS EN60079
part 10', which has broad applicability. The current version makes clear the
direct link between the amounts of flammable vapor that may be released, the
ventilation at that location, and the zone number. It contains a simplistic
calculation relating the size of zone to a rate of release of gas or vapor,
but it is not helpful for liquid releases, where the rate of vaporization controls the size of the hazardous area.
Other sources of advice, which describe more sophisticated approaches, are the Institute of Petroleum Model Code of Practice (Area Classification Code for Petroleum Installations, 2002), and the Institution of Gas Engineers Safety Recommendations SR25, (2001). The IP code is for use by refinery and petrochemical type operations. The IGC code addresses specifically transmission distribution and storage facilities for natural gas, rather than gas utilisation plant, but some of the information will be relevant to larger scale users.
Zoning
Hazardous areas are
defined in DSCAR as "any place in which an explosive atmosphere may occur
in quantities such as to require special precautions to protect the safety of
workers". In this context, ‘special precautions' is best taken as relating
to the construction, installation, and use of apparatus, as given in BS EN
60079-10'.
Area classification
is a method of analysing and classifying the environment where explosive gas
atmospheres may occur. The main purpose is to facilitate the proper selection
and installation of apparatus to be used safely in that environment, taking
into account the properties of the flammable materials that will be present.
DSEAR specifically extends the original scope of this analysis, to take into
account non-electrical sources of ignition, and mobile equipment that creates
an ignition risk. Hazardous areas are classified into zones based on an
assessment of the frequency of the occurrence and duration of an explosive gas
atmosphere, as follows:
Zone 0: An area in
which an explosive gas atmosphere is present continuously or for long periods:
Zone 1: An area in
which an explosive gas atmosphere is likely to occur in normal operation:
Zone 2: An area in which an explosive gas atmosphere is not likely to occur in normal operation and, if it occurs, will only exist for a short time.
Various sources have tried to place time limits on to these zones, but none have been officially adopted. The most common values used are:
Zone 0: Explosive atmosphere for more than 1000h/yr
Zone 1: Explosive atmosphere for more than 10, but less than 1000 h/yr.
Zone 2: Explosive atmosphere for less than 10h/yr., but still sufficiently likely as to controls over ignition sources.
Where people wish to quantify the zone definitions, these values are the most appropriate, but for the majority of situations a purely qualitative approach is adequate.
When the hazardous areas of a plant have been classified, the remainder will be defined as non-hazardous, sometimes referred to as 'safe areas.
The zone
definitions take no account of the consequences of a release. If this aspect is
important, it may be addressed by upgrading the specification of equipment or
controls over activities allowed within the zone. The alternative of specifying
the extent of zones more conservatively is not generally recommended, as it
leads to more difficulties with equipment selection, and illogicality in
respect of control over health effects from vapors assumed to be present.
Where occupiers choose to define extensive areas as Zone 1, the practical
consequences could usefully be discussed during site inspection.
Layers of Protection analysis (LOPA)
Layer of Protection
Analysis is a simplified form of quantitative risk assessment. In a typical
process plant, various protection layers are in place to lower the frequency of
undesired consequences: the process design (including inherently safer
concepts); the basic process control system; safety instrumented system’s
passive devices (such as dikes and blast walls); active devices (such as relief
valves); human intervention; etc.
The layers of
protections are:
1. Process Control and supervision
2. Preventive Control & monitoring
3. Protective measures & Control
4. Onsite Emergency measures
5. Offsite Emergency measures
LOPA aims to answer the
questions: How many protection layers are needed? How much risk reduction should
each layer provide?
In LOPA, the individual protection layers
proposed or provided are analyzed for their effectiveness. The combined effects
of the protection layers are then compared against risk tolerance criteria.
LOPA is not a hazard identification technique and scenarios for investigation
must be identified by another method. LOPA can be applied at any stage in the
life cycle of a plant design. At the earliest stages, it can be used to compare
alternative concepts to determine which is inherently safer. In detail design
or when modifications are made, LOPA can be used to complement HAZOP and other
forms of Process Hazard Analysis.
If a safety
instrumented function (SIF) is needed, LOPA can be used to determine the
required Safety Integrity Level (SIL). LOPA can be used to identify safety
critical equipment and operator actions and responses that are critical to
safety.
Instrumentation for safe and efficient operations of plants
The hazard in
Instrumentation can be categorized as follows
1) Hazard in online
testing of instruments-
This is mainly because of testing of Instrument in running condition without switching of the energy supply
Safety Precautions: -
a) Plant supervisor
& operator shall be informed about the testing.
b) The Instruments
in Auto mode should be switched over to manual mode before
attending
c) Ensure proper
operation/closing of isolating valves before disconnecting from the
process.
2. Hazard in
off-line testing of Instrument -
a) Pressure Testing of Glass equipment e.g.: - Rotameters.
Safety Precautions-
Ø Only Hydraulic test
to be carried out no pneumatic testing.
Ø Glass parts to be suitably covered with wire mesh to prevent flying glass splinters, which may cause accidents.
b) Testing of gases may result in explosive mixture in the testing apparatus. E.g., Testing of oxygen / hydrogen.
Safety Precautions-
i) Gases analysis
equipment should be used for a particular gas.
ii) Gas analysis equipment should be purged with Nz before changing from one gas to another.
3. Hazards caused
by Instrumentation failure-
Malfunctioning of Instrument can be minimized / eliminated by-
a) Proper &
periodic calibration of instruments to ensure correct measurement of process
variables.
b) Alarm activators
should be periodically checked & maintained to detect any
instrument mal
functioning.
c) Safety
Interlocks should be checked and simulated.
4. Hazard caused by
inadequate/defective instrumentation systems-
Poorly designed Instrumentation may lead major hazard which can be eliminated by
1) Proper assessing
of Hazard Potential and designing Instrument accordingly.
2) Any change in
process operation should be periodically reviewed to ensure safety
norms are not deviated.
Safe Sampling
Gauging & Instrumentation for safe operation-
1) Positioning of
Instruments should be about 1 meter above floor level. Not on eye
level.
2) Accuracy of
electrical direct reading Instruments may be affected by Improper
maintenance, lack
of calibration, poisoning of catalyst due to interference of
Of atmosphere
conditions, change in air flow rate, volume etc, these can be eliminated by proper
maintenance & calibration.
3) Ensure proper
safety sampling techniques.
4) Explosive limits
are physical constraints, which can be modified only within narrow
limits in contrast
to threshold limit values which can be interpreted broadly.
5) A provision of
remote gauge measuring equipment to tall tanks to avoid frequent
walking to the tank
roof top.
6) Gauging of
flammable & combustible liquids.
7) A substantial shield should be provided in front of high-pressure gauges.
Safety in Chemical Laboratory-
1) Proper labelling
of chemicals & reagent bottles storing them in their respective places.
2) Keep volatile,
combustible & flammable materials away from heat source.
3) Poisonous
substance must be kept under lock with proper authorization.
4) Reactions
liberating toxic, poisonous flammable vapours have to be carried out in fume
board by keeping
the exhaust fan, well before starting of reaction.
5) Use rubber bulbs
while pipetting toxic, flammable, acids etc.
6) Using proper PPE
while sampling.
7) To prevent
condensate water flowing into the oil bath a filter slip must be placed
around the neck of
the flask.
8) As per as
maintain quantity of hazardous chemical storage well below 5 litres.
9) Liquids which
have a tendency to form peroxides are often responsible for small
unexpected
explosions. As the peroxides are built due to photo chemicals reactions.
Those forming
peroxide are best be kept in dark bottles. Before distilling these
substances must be
tested for peroxides.
10) While pouring
solvent (flammables) care should be taken for static charge dissipation. Avoid
free fall of solvent.
11) Small electric
ovens are used in laboratory. Hence never place hazardous materials on the bottom
plate which in cases electric heaters. The surface temperature of this plate could
be several degrees higher than the critical temperature which may lead to explosive.
12) While working with glass apparatus sudden changes in the temp. must never beplaced in a desiccator. While evacuating desiccators use of protective shield of wiremesh plastic recommended.
13) There are
energy lines like gas, vacuum, hot water etc. These could be inspected occasionally
for leakages.
SAETY INTEGRITY LEVEL (SIL) ASSESSMENT
In language of instrumentation (to be used in
hazardous process are:), SIL
stands for Safety Integrity Level. There are
four SIL levels with SIL-1 is of
lowest safety integrity and SIL-4 is of the
highest level of safety integrity. Of
the overall Safety Instrumented System (SIS),
Safety Integrity Level is a part.
When we call for or specify an instrumented safeguard, we
typically tend
to assume that the safeguard will work when needed. We are
thus taking for
granted the reliability of the protection device. However,
the efficiency and
sufficiency of protection measures can be judged during
HAZOP processes.
Hence, additional safeguards may be necessary and
recommended for adequate
protection. These additional safeguards are the secondary
levels of protection
suggested when primary devices are assumed to be unable to
provide protection
as desired. Safety Integrity Levels are assigned, as part
of a Safety Instrumented
System, to provide protection or mitigation based on the
potentials of different
hazards. The SIL numbers from 1-4 is assigned to describe
the severity of the
consequences in the event of a potential hazard causing an
emergency in the
process unit.
SIL |
Consequence |
4 |
Catastrophic
Community Impact |
3 |
Employee
and Community Impact |
2 |
Major
Property and Production Impact |
1 |
Minor
Property and Production Impact |
SIL levels have been assigned and guidelines to choose a
particular SIL level
have been issued as per ANSI/ISA S84.01 and the IEC 61508
standard. Basically
the necessity of an assigned SIL level for the SIS for any
process comes after
a successful Process Hazard Analysis where it may be found
that the existing
process controls would not be able to provide adequate
protection or mitigation
of hazards. SIL assessment is a risk-based approach to
identify the required
safety integrity levels (SIL) for safety instrumented
functions (SIFs), the purpose
of which to control or mitigate the hazard under a process
upset condition.
For assignment of a SIL level for
instrumentation required to perform
certain safety instrumented functions,
different methods are used, out of which
some are traditional and few are progressive
and widely used.
Ø Conventional
Fault Tree Analysis (FTA)/Event Tree Analysis (ETA) will provide quantitative
measures
Ø Risk
Graphs is a qualitative method and
Ø Layers
of Protection Analysis (LOPA) is also a qualitative method and
widely used in the process industry
Generally, a combination of the methods is employed. But
quantitative
assessments are normally needed to critically assess the
functions, after
screening process through qualitative methods. A SIL
verification study is then
needed to verify the results of a SIL assessment exercise.
References-
K.U.Mistry
Industrial diaster management and quick response-Book by U. K. Chakrabarty
NSO Notes
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