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