Hazardous Chemicals Handbook 2 Episode 11 docx

Table 11.5 Approximate half-thickness values for a selection of shield materials and γ-emitters Table 11.6 General control measures for work with radioactive substances Consult experts i

Generally, sampling of waterways should be at the fastest flowing part of the stream/river,usually mid-depth unless the contaminant is less dense than water and could float, or is moredense and could accumulate near the river bed For lakes representative samples should be takennear to the inflow, outflow and other locations If two phases are present both may requiresampling Sample preservation by refrigeration, pH adjustment, elimination of light, filtration,and extraction may be important.

Table 10.34 summarizes the information to be recorded during an investigation (A code ofpractice for the identification of potentially contaminated land and its investigation is given inBritish Standard DD 175/1988.) Consideration should be given to the application of appropriatequality control and quality assurance procedures such as those advocated by ‘good laboratorypractice’ or ISO 17025 to ensure the sampling, the analysis and interpretation/reporting of dataare robust since results will dictate action and may be subject to scrutiny by third parties Duplicatesamples may need to be retained for disclosure to third parties There may be legal requirements

to notify results to relevant authorities

POLLUTION MONITORING STRATEGIES IN INCIDENT INVESTIGATION 389

Radioactive chemicals

The main chemical elements are listed in Chapter 18 Each comprises a nucleus of

positively-charged protons and neutral neutrons orbited by negative electrons The mass number A is given

by

A = Z + N

where Z is the number of protons, or atomic number

N is the number of neutrons.

Atoms with the same value of Z but different values of A are isotopes (Table 11.1) Many isotopes

are stable but others are naturally or artificially radioactive, i.e their atomic nuclei disintegrate,emitting particles or radiation This changes the nuclear structure of the atom and often results inthe production of a different element

Table 11.1 Nuclear composition of selected istopes

Element Symbol Atomic Protons Neutrons Total number Atomic

Natural sources of ionizing radiation include cosmic rays and nucleides such as potassium-40,carbon-14 and isotopes of thorium and uranium which are present in rocks, earth and buildingmaterials Industrial sources of radiation include nuclear reactors, X-ray radiography, electronmicroscopy, X-ray diffractors, thickness gauges, smoke detectors, electron beam welding andcertain processes including chemical analysis, polymer curing, chemical/biological tracing, foodand medical sterilization, and mining The radiation source can be sealed, when the radiation can

be switched off, or unsealed Examples of the former are smoke detectors and electrical devicesfor producing radiation

Hazards

The chemistry, and hence hazards, of ‘hot’, or radioactive, elements parallel those of their ‘cold’isotopes However, the radiation poses additional toxicity hazards A qualitative classification ofselected isotopes in terms of toxicity is given in Table 11.2 The biological effects of ionizingradiation stem mainly from damage to individual cells following ionization of the water content.Oxidizing species, e.g hydrogen peroxide, form together with ions and free radicals, all capable

of chemical attack on important organic moieties within the cells, e.g nucleic acids Biologicaleffects are influenced by the type of radiation, the dose, duration of exposure, exposed organ androute of entry Effects on cells include death, mutation and delayed reproduction Acute adverseeffects of exposure are illustrated in Table 11.3

Table 11.2 Classification of isotopes according to relative radiotoxicity per unit activity

The isotopes in each class are listed in order of increasing atomic number

Very high toxicity Sr-90 + Y-90, *Pb-210 + Bi-210 (Ra D + E), Po-210, At-211, Ra-226 + 55 per cent *daughter

products, Ac-227, *U-233, Pu-239, *Am-241, Cm-242.

High toxicity Ca-45, *Fe-59, Sr-89, Y-91, Ru-106 + *Rh-106, *I-131, *Ba-140 + La-140, Ce-144 + *Pr-144,

Sm-151, *Eu-154, *Tm-170, *Th-234 + *Pa-234, *natural uranium.

Moderate toxicity *Na-22, *Na-24, P-32, S-35, Cl-36, *K-42, *Sc-46, Sc-47, *Sc-48, *V-48, 52, 54,

*Mn-56, Fe-55, *Co-58, *Co-60, Ni-59, *Cu-64, *Zn-65, *Ga-72, *As-74, *As-76, *Br-82, *Rb-86,

*Zr-95 + *Nb-95, *Nb-95, *Mo-99, Tc-98, *Rh-105, Pd-103 + Rh-103, *Ag-105, Ag-111,

Cd-109 + *Ag-Cd-109, *Sn-113, *Te-127, *Te-129, *I-132, Cs-137 + *Ba-137, *La-140, Pr-143,

Pm-147, *Ho-166, *Lu-177, *Ta-182, *W-181, *Re-183, *Ir-190, *Ir-192, Pt-191, *Pt-193,

1 α-Particles (helium nuclei, i.e 2 neutrons plus 2 protons): on emission the original isotopedegrades into an element of two atomic numbers or less, e.g uranium 238 produces thorium 234.Such transformations are usually accompanied by γ-radiation or X-radiation α-Particles have avelocity about one-tenth that of light with a range in air of 3–9 cm Because of their relatively

TYPES OF RADIATION 391

large size and double positive charge they do not penetrate matter very readily and are stopped bypaper, cellophane, aluminium foil and even skin If inhaled or ingested, however, absorption of α-particles within tissues may cause intense local ionization.

2 β-Rays comprise electrons of velocity approaching that of light with a range of severalmetres and an energy of 0–4 MeV β-Particles of <0.07 MeV do not penetrate the epidermiswhereas those >2.5 MeV penetrate 1–2 cm of soft tissue Thus β-emitters pose both an internaland an external radiation hazard: skin burns and malignancies can result Once inside the bodythey are extremely hazardous, though less so than γ-rays About 1 mm of aluminium is needed tostop these particles Most β-emissions are accompanied by γ- or X-radiation and result intransformation into the element of one atomic number higher or lower but with the same atomic mass

3 γ-Radiation is similar to, but shorter in wavelength than, X-rays and is associated with manyα- or β-radiations γ-Radiation does not transform isotopes/elements Like X-rays, γ-rays are verypenetrating; they are capable of penetrating the whole body and thus require heavy shielding, e.g.γ-rays from 60Co penetrate 15 cm steel

4 X-Radiation like γ-radiation is electromagnetic in nature It can be emitted when β-particlesreact with atoms More often it is electrically generated by accelerating electrons in a vacuumtube The latter source can be switched off X-rays are extremely penetrating and are merelyattenuated by distance and shielding

5 Neutron radiation is emitted in fission and generally not spontaneously, although a fewheavy radionucleides, e.g plutonium, undergo spontaneous fission More often it results frombombarding beryllium atoms with an α-emitter Neutron radiation decays into protons and electronswith a half-life of about 12 min and is extremely penetrating

The same type of radiation emitted by different isotopes may differ significantly in energy, e.g.γ-radiation from potassium-42 has about four times the energy of γ-radiation from gold-198.Units of radiation are the becquerel (Bq), the gray (Gy) and the sievert (Sv)

Control measures

The control of ionizing radiation is heavily regulated Expert advice should be sought prior

to introducing sources of radiation onto the premises The general provisos for their controlare that:

• All practices resulting in exposure shall be justified by the advantages produced

• All exposures shall be as low as reasonably practicable

Table 11.3 Effects of acute exposures to X- and γ-radiation

Dose Effects

(Gy)

<1 No clinical effects but small depletions in normal white cells count and in platelets likely within 2 days.

1 About 15% of those exposed show symptoms of loss of appetite, nausea, vomiting, fatigue etc 1 Gy delivered

to whole body or 5 Gy delivered to bone marrow produces leukaemia.

2 Some fatalities occur.

3.5–4 LD50 (see Ch 5), death occurring within 30 days Erythema (reddening of skin) within 3 weeks.

7–10 LD100, death occurring within 10 days.

• The dose received shall not exceed specified limits As with most hygiene standards theselimits vary slightly between nations: local values should be consulted Limits set within the UKare summarized in Table 11.4.

• All regulatory requirements will be followed

• All incidents will be investigated and reported

Table 11.4 UK exposure limits (the Ionizing Radiations Regulations, 1999)

Dose (mSv in any Employee aged 18 or Trainee under 18 All others

Equivalent dose for the

Abdomen of women of child bearing 13***

capacity at work (in any consecutive

period of 3 months)

* Where these limits are impracticable having regard to the nature of the work the employer may apply a dose limit of

100 mSv in any period of 5 consecutive months subject to a maximum effective dose of 50 mSv in any single calendar year, and to prior approval by the Radiation Protection Adviser, the affected employee(s), and the Health and Safety Executive.

** As applied to the dose averaged over an area of 1 cm 2 regardless of area exposed.

*** Once an employer has been informed that an employee is pregnant the equivalent dose to a foetus should not exceed

1 mSv during the remainder of the pregnancy and significant bodily contamination of breast-feeding employees must be prevented.

In the UK, annual doses of 20 mSv, or any case of suspected overexposure, must be investigated,reported and recorded The HSE must also be notified of any spillages of radioactive substancebeyond specified amounts Companies are obliged to monitor exposures and investigate excursionsbeyond action limits, and to maintain records for specified periods Checks on surface contaminationare aimed at avoiding exposure, preventing spread of contaminant, detecting failures in containment

or departures from good practice, and providing data for planning further monitoring programmes.Air monitoring will be required, e.g., when volatiles are handled in quantity, where use ofradioactive isotopes has led to unacceptable workplace contamination, when processing plutonium

or other transuranic elements, when handling unsealed sources in hospitals in therapeutic amounts,and in the use of ‘hot’ cells/reactors and critical facilities Routine monitoring of skin, notably thehands, may be required

Monitoring for both external and internal radiation may be required, e.g when handling largequantities of volatile ‘hot’ chemicals or in the commercial manufacture of radionucleides, innatural and enriched uranium processes, in the processing of plutonium or other transuranicelements, and in uranium milling and refining The nature of biological monitoring is influenced

by the isotope: e.g., faeces, urine and breath monitoring is used for α- and β-emitters and body monitoring for γ-sources

whole-Exposure is minimized by choice of source, by duration of exposure, by distance from source(at 1 m the radiation level is reduced almost 10-fold), and by shielding The greater the mass perunit area of shield material the greater the shielding efficiency Whereas α- and β-particles posefew problems (the former can be absorbed by, e.g., paper and the latter by 1 cm Perspex) γ- andX-rays are not completely absorbed by shield material but attenuated exponentially such thatradiation emerging from the shield is given by:

CONTROL MEASURES 393

D t = D0e–ut

where D0 is the dose without a shield

D t is the dose rate emerging from a shield of thickness t

u is the linear absorption coefficient of shield material.

Half-thickness values (H-TV) i.e thickness to reduce intensity to half the incidence value, formaterials commonly used as shields for selected γ-rays are exemplified by Table 11.5

Table 11.5 Approximate half-thickness values for a selection of shield materials and γ-emitters

Table 11.6 General control measures for work with radioactive substances

Consult experts including competent authorities (in UK the HSE must be given 28 days prior notice of specified work with ionizing radiation).

Conduct a risk assessment to any employee and other persons to identify measures needed to restrict exposure to ionizing radiation and to assess magnitude of risk including identifiable accidents.

Conduct work in designated controlled areas (e.g in UK these are areas in which instantaneous dose rates >7.5 µSv/hour occur, or where employees may exceed 6 mSv annual dose limit, or where air concentration or surface contamination exceeds specified levels).

Provide barriers for identification and display of appropriate warning notices, e.g Trefoil symbol.

Control exposures by engineering techniques, e.g containment, shielding, ventilation (consider need for in-duct filters to remove contamination prior to exhausting to atmosphere), backed up by systems of work and personal protection including approved respirators where necessary.

Use remote handling techniques where necessary.

Appoint a Radiation Protection Adviser: all staff involved with radioactive work should be adequately trained and instructed Limit access to designated areas to classified persons (e.g in UK persons likely to receive doses in excess of 6 mSv per year

or an equivalent dose which exceeds 30% of relevant hygiene standard) Access may need to be limited by trapped keys

or interlocks for high dose rate enclosures.

Prepare as appropriate written rules for work in designated areas and appoint Radiation Protection Supervisors.

Check exposures routinely by personal dosimetry or following accidents (in the UK dosimetry services must be approved) and keep records (e.g in UK for at least 50 years) Notify the relevant employees and authorities as appropriate Monitor background contamination periodically using equipment that has been checked by qualified persons and keep records

of levels (e.g for 2 years).

Investigate accidents which may have led to persons receiving effective doses in excess of 6 mSv or an equivalent dose greater than 30% of any relevant dose limit Investigate and report to the authorities loss of materials from accidental release to atmosphere, spillages, theft The Regulations provide a comprehensive list of notifiable concentrations for each radionuclide isotope.

Provide mandatory medical surveillance for classified workers, e.g medical examinations prior to commencement of radioactive work followed by check-ups annually or when overexposure may have occurred (In the UK the surveillance must be undertaken by appointed doctors and records retained for at least 50 years.)

Maintain high standards of personal hygiene and housekeeping.

Detailed precautions for handling radioactive substances will be dictated by the nature andquantity of isotope and the likely level of exposure Thus for some materials laboratory coats andgloves may be adequate; for others a fully enclosed suit and respirator may be more appropriate.Some general precautions are listed in Table 11.6

Do not eat, drink, smoke, apply cosmetics or use mouth pipettes in controlled areas Dress any wounds prior to entering the area.

Wherever practicable, store radioactive substances in sealed, properly labelled containers Check for leaks periodically (e.g 2-yearly intervals) and maintain records of stocks including sealed sources (e.g for 2 years).

Carry out work over spill trays to contain leakages and use impervious work surfaces.

Decontaminate apparatus and prevent cross-contamination.

Collect waste for treatment or disposal and deal with spillages immediately.

Remove protective clothing in a changing area provided with wash basin, and lockers for clean and dirty clothing.

Table 11.6 Cont’d

CONTROL MEASURES 395

Common matters, e.g ventilation, temperature and lighting; floor, wall and ceiling surfaces;workspace allocation; workstation design and arrangement; floors and traffic routes; safeguardsagainst falls or being struck by a falling object; glazing; doors and gates; travelators and escalators;sanitary and washing facilities; drinking water supply; accommodation for clothing; facilities forchanging, resting and meals; are all covered by the Workplace (Health, Safety and WelfareRegulations) 1992.

Design procedures

To ensure safety consult flowsheets/engineering line diagrams and consider both the materials(raw materials storage, processing, product storage, disposal and transportation) and the processdetails (scale, batch vs continuous, temperature, pressure, materials of construction, monitoring,safety features, e.g fail-safe or ‘second chance’ design) See Table 12.1 Subject the proposals todetailed scrutiny, as in Table 12.2 or using a HAZOP study, fault tree analysis, etc for both theplanned operation and anticipated major deviations from normal operation (Table 12.3)

A HAZOP (Hazard and Operability Study) involves a formal review of process and instrumentationdiagrams by a specialist team using a structured technique, based upon key words These comprise

‘property words’ and ‘guide words’, e.g as in Table 12.4

Where possible plants of intrinsically safe design are preferred, i.e those which have beendesigned to be ‘self-correcting’ rather than those where equipment has been ‘added on’ to controlhazards Some characteristics of intrinsically safe plants are:

• Low inventory (small plant with less inherently hazardous materials on site)

• Substitution of hazardous materials with less dangerous chemicals

• Attenuation of risk by using hazardous materials in the least dangerous form

• Simplification of plant, instrumentation, operating procedures to reduce the chance of humanerror

• Domino effects eliminated so that adverse events are self-terminated and do not initiate newevents

• Incorrect assembly of plant made impossible by equipment design

Table 12.1 Some considerations in reaction process selection and design

Are unstable reactions and side reactions possible, e.g spontaneous combustion or polymerization?

Could poor mixing or inefficient distribution of reactants and heat sources result in undesirable side reactions, hot spots, runaway reactions, fouling, etc?

Can hazards from the reaction be reduced by changing the relative concentration of reactants or other operating conditions? Can side reactions produce toxic or explosive material, or cause dangerous fouling?

Will materials absorb moisture from the air and then swell, adhere to surfaces, form toxic or corrosive liquid or gas, etc?

What is the effect of impurities on chemical reactions and upon process mixture characteristics?

Are materials of construction compatible mutually and with process materials?

Can dangerous materials build up in the process, e.g traces of combustible and non-condensible materials?

What are the effects of catalyst behaviour, e.g aging, poisoning, disintegration, activation, regeneration?

Are inherently hazardous operations involved:

Vaporization and diffusion of flammable/toxic liquids or gases?

Dusting and dispersion of combustible/toxic solids?

Spraying, misting or fogging of flammable/combustible materials or strong oxidizing agents?

Mixing of flammable materials and combustible solids with strong oxidizing agents?

Separation of hazardous chemicals from inerts or diluents?

Increase in temperature and/or pressure of unstable liquids?

• The status of plant should be immediately obvious

• The tolerance should be such that small mistakes do not lead to major problems

• Leaks should be small

Table 12.5 summarizes the application of these principles (after Kletz – see Bibliography) Wherethis approach is not feasible, external features of plant must ensure the minimization of unwantedconsequences

Layout

Factory layout has a significant bearing on safety Relevant considerations include:

• Relative positions of storage and process areas; control room, laboratories and offices – i.e.areas of highest population density; switch-house; materials receipt and despatch areas; effluenttreatment facilities Spacing distances according to standard guidelines

• Need for normal and emergency access (and escape)

• Security, e.g fencing requirement, control of access

• Topography

• Tendency for flooding

• Location of public roads

• Prevailing wind direction

• Zoning of electrical equipment (Table 12.6)

• Positions of neighbouring developments, housing, public roads, controlled water courses, etc.Segregation is practised to allow for housekeeping, construction and maintenance requirementsand to reduce the risk of an accident resulting in a ‘domino effect’, e.g from a fire, explosion ortoxic release For very toxic substances, e.g prussic acid (HCN) or tetraethyl lead, this mayinvolve isolating the entire manufacturing operation in a separate unoccupied building or sealed-off area

LAYOUT 397

Table 12.2 Chemical process hazard identification

Materials and reaction Identify all hazardous process materials, intermediates and wastes

Produce material information sheets for each process material Check the toxicity of process materials, identify short and long term effects for various modes

of entry into the body and different exposure tolerance Identify the relationship between odour and toxicity for all process materials Determine the means for industrial hygiene recognition, evaluation and control Determine relevant physical properties of process materials under all process conditions, check source and reliability of data

Determine the quantities and physical states of material at each stage of production, handling and storage, relate these to the danger and second-degree hazards

Identify any hazard the product might present to transporters and public while in transit Consult process material supplier regarding properties, characteristics, safety in storage, handling and use

Identify all possible chemical reactions, both planned and unplanned Determine the inter-dependence of reaction rate and variables, establish the limiting values to prevent undesirable reactions, excessive heat development etc.

Ensure that unstable chemicals are handled so as to minimize their exposure to heat, pressure, shock and friction

Are the construction materials compatible with each other and with the chemical process materials, under all foreseeable conditions?

Can hazardous materials build-up in the process, e.g traces of combustible and noncondensible materials?

General process Are the scale, type and integration of the process correct, bearing in mind the safety and

specification health hazards?

Identify the major safety hazards and eliminate them, if possible Locate critical areas on the flow diagrams and layout drawings

Is selection of a specific process route, or other design option, more appropriate on safety grounds?

Can the process sequence be changed to improve the safety of the process?

Could less hazardous materials be used?

Are emissions of material necessary?

Are necessary emissions discharged safely and in accordance with good practice and legislation? Can any unit or item be eliminated and does this improve safety, e.g by reducing inventory

or improving reliability?

Is the process design correct?

Are normal conditions described adequately?

Are all relevant parameters controlled?

Are the operations and heat transfer facilities properly designed, instrumented and controlled? Has scale-up of the process been carried out correctly?

Does the process fail safe in respect of heat, pressure, fire and explosion?

Has second chance design been used?

Spacing distances may be reduced in the light of:

• explosion relief, blast-proofing, blast walls, earth banks;

• bunds, dykes;

• steam and water curtains; foam blanketing provisions;

• inter-positioning of sacrificial plant, e.g cooling towers, unpopulated buildings;

• provision of refuges, e.g for toxic release incidents

Adequate distance frequently serves to mitigate the consequences of an accidental release ofchemicals, e.g a flammable liquid spillage or toxic gas escape

Distances are recommended for zoning of electrical equipment, separation of storage frombuildings etc Distances are also proposed (on the basis of experience) to minimize the escalation

Table 12.3 Checklist for major deviations from normal operation

Start-up and shutdown;

What else, apart from normal operation, can happen?

Is suitable provision made for such events?

Can start-up and shutdown of plant, or placing of plant on hot standby, be expedited easily and safely?

Can the plant pressure or the inventory of process materials, or both, be reduced effectively and safely in a major emergency? Are the operating parameter limits which require remedial action, known and measured, e.g temperature, pressure, flow, concentration?

Should plant be shutdown for any deviation beyond the operating limits?

Does this require the installation of alarm, trip, or both, i.e to what externt is manual intervention expected?

Does material change phase from its state in normal operation, during the start-up, and shutdown of plant? Is this acceptable, e.g does it involve expansion or contraction, solidification, etc.?

Can effluent and relief systems cope with large or abnormal discharges, during start-up, shutdown, hot standby, commissioning and fire-fighting?

Are adequate supplies of utilities and miscellaneous chemicals available for all activities, e.g absorbents for spillage control?

Is inert gas immediately available in all locations where it may be required urgently?

Is there a standby supply?

Is any material added during start-up and shutdown, which can create a hazard on contact with process or plant materials?

Is the means of lighting flames, e.g on burners and flares, safe on every occasion?

Table 12.4Key words for use in HAZOP studies

Typical property words

Common Application-dependent

Temperature Flash point

Pressure Vapour pressure

Level Heat transfer

Concentration Separate

Absorb etc.

Guide words for HAZOP studies

NO/NOT The complete negation of these No part of the intentions is achieved but nothing

intentions else happens MORE/LESS Quantitative increase or decrease Refer to quantities and properties such as flowrates

and temperatures as well as activities like ‘heat’,

‘react’

AS WELL AS Qualitative increase All the design and operating intentions are achieved

together with some additional activity PART OF Qualitative decrease Only some of the intentions are achieved: some are

not REVERSE The logical opposite of the intention Mostly applicable to activities, e.g reverse flow,

chemical reaction Can also be applied to substances, e.g ‘poison’ instead of ‘antidote’, ‘D’ instead of ‘L’ optical isomers

OTHER THAN Complete substitution No part of the original intention is achieved;

something quite different happens

LAYOUT 399

Table 12.5 Features of intrinsically safe plants

Examples

Low inventory

Tubular reactor Pot reactor Liquids separation Centrifugal Gravity settling

Substitution

Heat transfer media Non-flammable Flammable

Solvents Non-flammable/low toxicity Flammable/high toxicity

Attenuation

Any material Vapour, superheated liquid Liquid

Moderate temperature High or very low temperature

Simplification

Hazards avoided Hazards controlled by added

equipment Single stream Multistream with many

crossovers Dedicated plant Multipurpose plant Liquid or powder transfer One big plant Many small plants

Domino effects

Open construction Closed buildings Fire breaks No fire breaks

Horizontal cylinder Pointing away from other Pointing at other equipment or

equipment and buildings buildings

Incorrect assembly impossible

Compressor valves Non-interchangeable Interchangeable

Device for adding water to oil Cannot point upstream Can point upstream

Obvious state

Rising spindle valve or ball Non-rising spindle valve valve with fixed handle

Tolerant of maloperation or poor Continuous plant Batch plant

maintenance Spiral-wound gasket Fibre gasket

Bolted arm Quick-release coupling

Self-sealing couplings Standard couplings

Low leak rate Spiral-wound gasket Fibre gasket

Tubular reactor Pot reactor Vapour-phase reactor Liquid-phase reactor

or effects on site of fire, explosion, toxic release or similar incident Selected sources of informationare summarized in Table 12.7.

Storage

Chemicals in packages

The design of any building or outside compound for the storage of chemicals in packages (e.g.drums, cylinders, sacks) will depend upon their hazardous characteristics (pages 228, 248 and272)

For a storage building the considerations include:

• Siting to minimize risk to nearby premises on and off site in a fire

• Access for delivery and transfer of chemicals and for emergency purposes

• Fire-proof construction

Table 12.6 Electrical zoning

Classification of hazard areas according to the probability of a flammable concentration of vapour occurring (to BS 5345 Part 1)

Zone 0 Area in which an explosive gas–air mixture is continuously present, or present for long periods

Zone 1 Area in which an explosive gas–air mixture is likely to occur in normal operation

Zone 2 Area in which an explosive gas–air mixture is not likely to occur in normal operation, and if it occurs will

exist only for a short time

Safe area By implication, an area that is not classified Zone 0, 1 or 2 is deemed to be a non-hazardous or safe area

with respect to BS 5345

Examples of electrical area classification for various operations

Vapour source Extent of classified area Area classification

Vapour space inside Within the vapour space of the tank Zone 0

storage tank

Storage tank outside Vertically from ground level up to 2 m above the Zone 2

buildings tank connections and horizontally within 2 m

from the tank connections or shell Discharge from vent (a) Where liquids or vapours discharged from the Fixed electrical equipment

(b) Within 2 m in all other directions from point of Zone 2 discharge

Tank vehicle loading (a) Vertically from ground level up to 2 m above, Zone 1

and horizontally outwards for 2 m from any point where connections are regularly made or disconnected for product transfer

(b) Vertically and horizontally between 2 m and Zone 2

4 m from the points of connection or disconnection

Pumps and sample Within 2 m in all directions Zone 2

points in the open air

Pump house building Within the building Zone 1

Notes

1 Where any area is classified under more than one factor, the higher classification should prevail.

2 Any bunded area, pit, trench, depression or drain falling within a Zone 1 or Zone 2 area should be treated as being a Zone

1 area throughout.

3 Pump seals should be properly maintained.

STORAGE 401

• Security, i.e control of access by staff and means to prevent entry by vandals or trespassers.

• General ventilation provisions, particularly if volatile toxic or highly flammable liquids, ortoxic or flammable pressurized gases are stored

• Lighting, and emergency lighting, provisions

• Correct selection of heating equipment and zoning of electrical equipment to reduce the chance

of an ignition source arising

• Provision of fire detection and extinguishment equipment

The same factors arise for a dedicated compartment of a building In either case identification andwarning notices need to be provided

Operation of the store (see Chapter 13) then needs to account for:

• A system for checking that all packages entering the store have identifiable labels whichindicate their contents and any hazardous characteristics

• A system for inspecting all packages received, and routinely in-store, for leaks, damage,external corrosion, etc

Table 12.7 Selected sources of spacing distances with hazardous chemicals (see Bibliography)

Preliminary minimum distances

Liquid oxygen A Code of Practice for the Bulk Storage of Liquid Oxygen at

Production Sites (HSE, 1977) Liquefied flammable gases Process Plant Layout page 562 (Mecklenburgh, 1985)

Liquids stored at ambient temperature Process Plant Layout page 564

and pressure

Electrical area classification distances Process Plant Layout pages 568–577

Distances for storage of explosives Explosive and Toxic Hazard Materials page 370 (Meidl, 1970)

Safe Handling Requirements during Explosive, Propellant and Pyrotechnic Manufacture (HSE, SIR 31)

General recommendations for spacing General Recommendations for Spacing in Refineries, Petrochemical

Plants, Gasoline Plants, Terminals, Oil Pump Stations and Offshore Properties (Oil Insurance Association, No 361)

‘Consultation’ distances (in relation to HSE assessments for consultation distances for major hazard

major hazard and other sites, e.g LPG, installations, Chapter 5 in Safety Cases (Lees and Ang, 1989)

chlorine, for land use planning)

Minimum recommended spacing distances

Flammable liquids Storage of Flammable Liquids in Tanks (HSE, HSG 176)

Storage of Flammable Liquids in Containers (HSE, HSG 51) Storage of Highly Flammable Liquids (HSE, CS2)

Highly Flammable Materials on Construction Sites (HSE, HSG 3) Storage of Flammable Liquids in Fixed Tanks, up to 10 000 m 3 Total Capacity (HSE, HSG 50)

Liquefied petroleum gas Storage of LPG at Fixed Installations (HSE, HSG 34)

Storage and Use of LPG at Metered Estates (HSE, CS11) Storage and Use of LPG on Construction Sites (HSE, CS6) Keeping of LPG in Cylinders and Similar Containers (HSE, CS4)

Distances for storage and handling of Storage and Handling of Organic Peroxides (HSE, CS21)

dangerous chemicals Chemical Warehousing Storage of Packaged Dangerous Substances

(HSE, HS(G) 71) Safety Advice for Bulk Chlorine Installations (HSE, HSG 28) Storage and Handling of Industrial Nitrocellulose (HSE, HSG 135) Storage and Handling of Ammonium Nitrate (HSE, CS18)

• A system for the segregation of chemicals, following the principles explained on page 136 and248.

• In-store handling of packages Manual handling should be eliminated or reduced as far aspossible Fork-lift trucks should be regularly maintained, be provided with adequate accessways; driving should be restricted to fully-trained personnel

• Safe stacking practice This will include provision of suitable racking, limitations on stack sizeand height, e.g having regard to the potential severity of a fire

• Specification of a storage capacity and a procedure to avoid overfilling

If outdoor storage is not reasonably practical a specially designed storeroom, preferably in aseparate building, may be used

A summary of recommendations for outdoor drum storage is given in Table 12.8

Table 12.8 Outdoor drum storage recommendations

• If possible, situate area remote from buildings and plant Avoid location beneath pipe bridges or electric cable runs.

• Secure the area to prevent tampering or trespassing.

• Limit permissible fire load to 79 × 10 5 kJ or 250 tons of hydrocarbon.

• Label each drum; affix appropriate warning, e.g ‘Highly Flammable’, ‘Corrosive’.

• Provide suitable absorbent materials to deal with spillages.

• Limit stack heights, e.g 4.5 m for 200 litres drums., 5 m high for drums stored on end or 4 m high on pallets,

4 m high for drums on their sides.

• Segregate highly flammable liquids and mark the stack; classify the area as Zone 2 Place material >15 m from any working building, amenity building or plant and >7.5 m from plant boundary and boundary fence.

• Space combustible material stacks >7.5 m from buildings and 4 m from plant boundary fences.

• Limit areas for any stack of drums to 250 m 2 with generally a maximum length of 18.5 m (A reduced area applies to particularly hazardous chemicals.)

• Restrict number of 180 litre drums to 1500.

• Ground should be impervious, and sloped and drained, with demarcation lines.

• Spillages from this area should be contained in a bund, or similar arrangement.

• Provide >5 m clearance between adjacent stacks with access on three sides for fire-fighting, etc.

• Provide an adequate number of fire hydrants for fixed monitors for drenching the stacks with water Stack drums 2 m from any hydrants and leave clear access.

• Leave space for possible hose runs for a fire in any stack and provide dry powder extinguishers around the area.

LPG in cylinders

The risk with liquefied petroleum gas in cylinders is significantly greater than with a highlyflammable liquid in drums because of the potential for rapid release of heavy flammable gas In

a fire around a cylinder there is a potential BLEVE hazard (see p 178); ignition of a leak from

a valve will cause a jet fire

Therefore detailed recommendations relate to the keeping of liquefied petroleum gas in cylindersand similar containers General recommendations for storage, other than on rooftops, are summarized

in Tables 9.16–9.18

STORAGE 403

Equipment design

The design and instrumentation of individual equipment items are beyond the scope of this text

It is important to note, however, that:

• All work equipment must be constructed or adapted to be suitable for its use, and its selectionshould have regard to the working conditions and risks to health and safety of persons where

it is used and any additional risks it poses (Provision and Use of Work Equipment 1998Reg 2)

• In addition to guarding dangerous machinery (Reg 11) measures are required to prevent oradequately control hazards from work equipment due to:

– any article or substance falling or being ejected;

– rupture or disintegration of parts;

– it catching fire or overheating;

– the unintended discharge or premature discharge of any gas, dust, liquid, vapour or othersubstance; consideration should also be given to fugitive emissions or leaks from valves,flanges, connections, seals, pumps, agitators, etc by, for example, using average emissionfactors for common plant equipment such as those given in Table 12.9 (Such assessmentsmay be important in reducing material loss or in estimating potential employee exposure orenvironmental pollution levels);

– the unintended or premature explosion of it or any substance in it (Reg 12)

• The selection of materials of construction is subject to the considerations of corrosion summarized

on page 54

• Consideration should be given to the safety features listed in Table 7.21

• For any pressure system there are obligatory requirements summarized on page 423

Table 12.9 Average emission factors for plant items

Equipment Service Emission factors (kg/hr)*

Refineries Synthetic organic

*Multiplying the total number of items in a unit by the factor provides an indication of the emission rate.

Piping arrangements

Complex piping systems may be required for the transfer of chemicals, balancing of pressures,venting, drainage, supply of services, etc

Safe operation is generally assisted by simplification of piping and valve arrangements, and bytheir identification, e.g by colour coding or tags Logical arrangement can also serve as a prompt

in identification

Design of piping systems is a special discipline but some guidance is given in Table 12.10

Table 12.10 Design measures to reduce leaks from pipework

• Minimize the number of branches and deadlegs.

• Minimize the number of small drain lines.

• Design small diameter lines to the same codes, and test to the same standards, as main lines; reinforce junctions as necessary.

• Provide flexibility to allow for thermal expansion, or contraction, of pipework and connected equipment.

• Direct discharges from automatic drains to visible locations.

• Provide gaskets compatible with the internal fluid over the full range of temperatures and pressures.

• Provide removable plugs on valve sample points.

• Provide adequate pipe supports.

• Minimize the number of flanges on vacuum lines.

• Consider duplication, locking, temporary blanking of valves on ‘open’ discharge lines.

• Either avoid pipe runs along routes subject to possible mechanical damage, e.g from fork-lift trucks or tankers, or provide shields and barriers.

• Provide walkways, ladders, etc to avoid pipework being clambered over.

• Minimize the use of flexible hoses; if used:

– ensure hose and couplings are to the appropriate standard;

– provide for rapid isolation in any emergency;

– use bolted, not jubilee, clips;

– consider self-sealing couplings;

– provide adequate support and protection whilst in use;

– protect from crushing or contamination when not in use.

Services and utilities

Ensure the adequacy (in terms of quality, quantity and reliability) of services/utilities, e.g steam,process/cooling water, electricity, compressed air, inert gas, fire suppression systems, ventilation.Stand-by or emergency services may be required Some general safety design considerations aresummarized in Table 12.11

Ventilation can be provided as general dilution ventilation or local extraction ventilation

Dilution ventilation (general ventilation)

Open construction is preferable wherever practicable for areas processing hazardous chemicals,

to provide general ventilation and assist in the dispersion of leaking gas or vapour, to maximizeexplosion-venting area and to facilitate fire-fighting Local dilution ventilation is provided toflush the workplace atmosphere with clean air and thus dilute the level of contaminant in ambientair to acceptable levels The siting of the extract fans and the air inlets require careful consideration

to minimize operator exposure (Figure 12.1) The general rules are:

• Site the exhaust fan near to the source of contaminant

• Ensure that fresh air movement is from worker to leak source and not vice versa

• Ensure that air inlet supply is not contaminated with exhaust effluent

• Provide back-up for air inlet where necessary

PIPING ARRANGEMENTS 405

For continuous release of gas or vapour the steady-state dilution ventilation required to reduce theatmospheric pollutant to a level below its hygiene standard is given by

where Q is the ventilation rate (m/min)

SG is the specific gravity of the evaporating liquid

ER is the rate of evaporation (l/hr)

MW is the molecular weight

K is the design factor

HS is the hygiene standard.

In general, dilution ventilation alone is inappropriate for highly-toxic substances, carcinogens,dusts or fumes or for widely fluctuating levels of pollutants Since hygiene standards are oftenrevised (usually downwards), specifications of existing systems may prove inadequate An inherentproblem is that contaminants may have to pass through a worker’s breathing zone before dilution

Local exhaust ventilation

Local exhaust ventilation serves to remove a contaminant near its source of emission into theatmosphere of a workplace A system normally comprises a hood, ducting which conveys exhaustedair and contaminants, a fan, equipment for contaminants collection/removal and a stack fordispersion of decontaminated air Hoods normally comprise an enclosure, a booth, a captor hood

or a receptor hood Those relying on other than complete enclosure should be as close as practicable

to the source of pollution to achieve maximum efficiency

Total enclosure may be in the form of a room with grilles to facilitate air flow; this functions

as a hood and operates under a slight negative pressure with controls located externally Entry isrestricted and usually entails use of comprehensive personal protective equipment Ancillaryrequirements may include air filters/scrubbers, atmospheric monitoring, decontamination proceduresand a permit-to-work system (see page 417)

Partial enclosure allows small openings for charging/removal of apparatus and chemicals The

requisite air velocity to prevent dust or fumes leaking out determines the air extraction rate, e.g

Table 12.11 Safety considerations for plant services design

• The need to duplicate power supplies to equipment.

• The need to duplicate or triplicate equipment.

• Selection between steam, or electricity, or a combination for pumps and compressors.

• Provision of automatic start for spare pumps.

• Provision of voltage protection for key equipment which must be kept on-line or be restarted quickly.

• Order for restarting equipment after a power failure.

• Extent of emergency power supplies for lighting, communication systems, and key items of equipment (e.g cooling facilities, reactor agitators, exhaust ventilation) and instruments/alarms.

• Provision for cooling emergency equipment, e.g diesel generators if cooling water is disrupted.

• Required positioning of control valves (i.e shut, open or ‘as is’) upon power failure.

• Provision of compressed air, inert gas from reservoirs for a limited period.

• Provision for alternative fire water supply.

• Provision of efficient drift eliminators of water cooling towers; consideration of replacement by air cooling systems.

• Thermal insulation to protect personnel from contact with hot or cold surfaces; prevention of water supply disruption by freezing.

• Design of hot and cold water services to avoid water standing undisturbed for long periods Use of covered tanks and cisterns with approved fittings and materials.

• Avoidance of cold water temperatures of 20°C–45°C, storage of hot water at 60°C and circulation at 50°C.

Poor air inlet Fair air inlet Good air inlet

Best air inlet Best air inlet Best exhaust (local)

Best air inlet

Good fan locations

Plenum

Plenum

Poor air inlet Fair air inlet Good air inlet

Poor air inlet Fair air inlet Good air inlet

Poor fan locations

in the range 0.28 to 0.56 m/s A higher velocity is required if there is significant dispersion insidethe enclosure

A booth should be of sufficient size to contain any naturally occurring emissions and so

minimize escape via the open face An air velocity of 0.56 m/s is required over the whole openface; a higher velocity is needed if there is significant air movement within the booth or to copewith convection currents Booths should be deep enough to contain eddies at the rear corners;baffle plates or multiple offtakes may be necessary with shallow booths

No operator should work between the source of pollutant and the rear of the booth (i.e all workshould be handled from the front open face, as illustrated by Figure 12.1)

If enclosure or use of a booth is impracticable, a captor hood is used This is placed some

distance from the source of pollution and the rate of air flow needs to be such as to capturecontaminants at the furthermost point of origin

Typical capture velocities are given in Table 12.12 Since velocity falls off rapidly with distancefrom the face of the hood, as shown in Figure 12.2, any source of dust should be within one hood

Figure 12.1 Dilution ventilation: inlet air may require tempering according to outside conditions

PIPING ARRANGEMENTS 407

100% 50% 30% 15% 7.5%

% of diameter

Table 12.12 Range of capture velocities

Condition of dispersion of contaminant Examples Capture velocity

(m/s) Released with practically no velocity into quiet air Evaporation from tanks 0.25–0.51

Degreasing vats etc.

Released at low velocity into moderately still air Spray booths 0.51–1.02

Intermittent container filling Low-speed conveyor transfers Welding

Plating Pickling Active generation into zone of rapid air motion Spray painting in shallow booths 1.02–2.54

Barrel filling Conveyor loading Crushers Released at high initial velocity into zone of very Grinding 2.54–10.2

rapid air motion Abrasive blasting Tumbling

Figure 12.2 Reduction in air velocity with distance from captor hood (distance given as % of hood diameter)

diameter Efficiency can be significantly improved by the use of flanges and by avoiding abruptchanges in direction of the ducting

A receptor hood receives a contaminant driven into it by the source of generation The flowrate

needs to ensure that the hood is emptied more rapidly than the process fills it and to overcomedraughts No operator should work between the hood and the source of the contaminants