Safety at Work 6 E Part 12 pdf

Health and Safety Executive, Health and safety guidance series publication HSG 38 Lighting at work, HSE Books, Sudbury 1998 7.. Health and Safety Executive, Legal series publication L23

Applied ergonomics 635 3.9.3.5 Humidity

The degree of moisture in the air needs to be controlled within certainlimits Excessive levels of moisture (high humidity) can seriouslyinterfere with the body’s ability to sweat and can cause considerablediscomfort Where the production process requires high humidity, such

as in papermaking, exposure times should be kept to a minimum A dryatmosphere (low humidity) can cause dryness of the throat and de-hydration Normal comfort levels of humidity lie between 40% and 50%relative humidity but may vary slightly between different types of work.Extended exposures to a relative humidity below 30% can give rise toadverse pulmonary health effects

In considering optimum temperatures and humidity account should betaken of the clothing normally worn, whether personal choice orcompany issue, the physical nature of the work, exposure to sources ofheat (from the process or naturally from sunlight) and the amount ofventilation provided

The measurement of the thermal environment is discussed in section3.6.2

3.9.3.6 Lighting

To be able to carry out any work effectively and accurately proper andappropriate lighting is essential The eye reacts to strong or bright lightsuch that areas of shadow or darkness are not seen in as much detail if atall With all work situations suitable and sufficient lighting that enablesthe eye to see all the facets of the work and the surrounding area isnecessary Recommended levels of illuminance for various locations andtasks are give in section 3.7.6

While ensuring an adequate level of illumination, care must be taken toavoid positioning illuminaires where they can interfere with the clarity ofvision Typical situations to avoid include:

(a) Glare and dazzle from a source of light positioned behind the object

to be viewed effectively prevents the object from being seen This can

occur with low level lighting on access ways (Figure 3.9.15) or high

level lights in areas of lifting operations Similarly, viewing isinterfered with if the emissions from a source of light shine directly onthe eye

(b) Areas of sharp contrast since the eye reacts to the bright areas with theresult that the darker areas will either not be seen or be seen only with

difficulty by straining the eyes (Figure 3.9.16) Deep shadows and

fluctuating levels of light have the same effect

(c) Reflections of a light source on the object being viewed whetherpaper, metal, desk top or monitor screens

(d) Flicker, which is a cyclic variation of light intensity that is morenoticeable at frequencies below 50Hz It is particularly noticeable atthe edge of the visual field and can be distracting, cause fatigue and,

in some cases, epileptic seizures

636 Safety at Work

Figure 3.9.15 Disability glare from a light fitting (Courtesy The Stationery Office)

Figure 3.9.16 Sharp contrast between exterior light and interior shadow (Courtesy

the Stationery Office)

Applied ergonomics 637

(e) Stroboscopic effect occurs when the flicker from fluorescent lampscoincides with the speed of rotating objects making them appearstationary This can be avoided by utilising twin tube fittings wired90° out of phase

3.9.3.6.1 Types of illuminaires

Sources of artificial light split broadly into two types:

(i) point sources such as the tungsten filament lamp where a glassenvelope containing either a vacuum or a filling of halogen, mercury

or sodium vapour at pressure Since the filament is heated to whiteheat to provide the illumination the surrounding glass envelope canget hot Adequate arrangements for cooling are needed and the lampshould not be located near flammable materials Because this type ofilluminaire is a point source of light it is important that it does not:

 create areas of bright light and deep shadows,

 reflect on work surfaces and

 mask information on VDU monitors

The gas used to fill the bulb – mercury, sodium or halogen – creates

a colour bias in the light emitted This must be allowed for inprocesses where colour recognition is important, such as electricalwiring, paint colour matching, etc

(ii) fluorescent strip light fittings in which the light emanates from thefluorescent coating on the inside of the tube Although giving amuch more even spread of light than tungsten lamps they can stillcause reflections on surfaces This effect can be reduced to aminimum by the use of diffusers and louvre fittings Problems thatare met with this type of illuminaire include:

 flicker and

 stroboscopic effect

The positioning of illuminaires is important to ensure they do not createinterference with viewing Where interference does occur, the object beingviewed should be moved or the illuminaire repositioned Advice on thetype and positioning of luminaires is given in a guidance note5

3.9.3.7 Ventilation

The presence of contaminants in the atmosphere is a potential source ofdistraction and annoyance They may be there as a result of fumes or dustleaking from the process or of someone’s personal habits such assmoking With contaminants there is also an associated potential healthrisk (from hazardous fumes and dusts, tobacco smoke, etc.) Legislation7

requires the supply of a sufficient quantity of fresh or purified air It does not

specify quantities but guidance5 suggests a minimum of 5–8 l/s peroccupant (18–29 m3/h) However, this does not allow for the effects of theproduction process nor the type of work so these quantities may need to

be increased Fresh air is that drawn from outside but care needs to betaken to ensure the intake point is clear of exhaust outlets or other sources

10kg Full height

20kg Shoulder height

25kg Elbow height

20kg

Knuckle height Knee height

again no quantity is specified

In many situations, an adequate supply of fresh air can be obtained from

an open window but in the larger open plan offices and workshops someform of forced air ventilation may be required The outlets from ventilationsystems should be arranged so that they do not play on an individual sincethis can be a source of annoyance and also interfere with sweat rates tobecome a health hazard Outlet velocities and directions of flow shouldensure that the air velocity at any one workstation is not so high as to beunpleasant or uncomfortable In general, the more active the work anairflow as high as 0.5 m/s can be tolerated However, for sedentary workthe flow should be less than 0.1 m/s while in jobs requiring deepconcentration even that level of air movement can be distracting

3.9.4 Manual handling

Ideally, if objects have to be moved it should be done mechanically Wherethis is neither technically feasible nor economically viable manualhandling will have to be employed Manual handling is a known and welldocumented source of occupational injury and in spite of publicity andtraining accident attributed to manual handling remains the highest cause

of absences Legislation8sets out the actions to be taken to reduce thehazards with advice on ways to achieve them given in a Code of Practice9.The ability to handle loads varies with the position of the load with respect

to the body and Figure 3.9.17 indicates a suggested range of maximum

Figure 3.9.17 Suggested maximum loads at various distances from the body

Applied ergonomics 639

weights that can be lifted and carried The values given are typical and willneed to be adjusted to suit the physique and ability of the operator It isimportant to remember that it is not only what is picked up but how.For manual handling, work should be arranged so that:

 the load to be lifted is the smallest technically feasible and cally viable;

economi- loads that cannot be broken down to safe weights are handled bymechanical means such as sack barrow, special purpose handlingequipment, lift trucks, etc.;

 the level from which the load is lifted should be as high as possible up

to waist level;

 ideal height for picking up a load is waist level;

 if necessary an intermediate resting platform is provided;

 close body approach is possible to the delivery platform to prevent theneed to lean with the load;

 the final delivery level is not above shoulder height;

 for placing loads at higher than shoulder level a lift truck or suitablestep ladder is used

Where loads have to be carried manually, the floor surface should belevel, smooth and in good condition

Where manual handling has to be carried out from the sitting position,the ability to lift may need to be reduced to as little as 20% of theequivalent load when standing

3.9.5 Repetitive actions

Actions that involve putting repeated loads on particular muscles,especially on the arms and the wrist, can cause a number of muscularconditions variously referred to as repetitive strain injury (RSI), tenosyno-vitis, carpal tunnel syndrome, work-related upper limb disorder, etc.Symptoms exhibited include soreness in the muscles that initiallydisappears when ceasing work but rapidly returns when work isrecommenced If the same work is continued, the condition can becomevery painful and have long lasting effects

On jobs where this condition is a known or suspected risk ments should be made to:

arrange- eliminate the type of work that causes the condition and replace it byalternative work methods;

 restrict the time engaged on the suspect activity;

 rotate jobs during the shift so that operators carry out a number ofdifferent functions using different muscles;

 ensure tools and equipment used on suspect operations are, and aremaintained, in good condition and do not require excessive force fortheir proper use;

 build into the work programme adequate rest periods;

 instruct supervisors and operators in the symptoms and the action to

be taken if they occur, i.e move to alternative work and seek medicaladvice

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3.9.6 Plant design

Layout of plant should ensure that any movements the operators need tomake are direct, free and unimpeded by other parts or equipment.Operator work areas should be clear, clean, well lit with a good floorsurface If the work platform is at a raised level, it should have a safetyrail and be provided with access steps if the height warrants The treads

of steps should be wide enough to accommodate the full length of anormal shoe Steps at an angle greater than 45° should be avoided, but ifspace limitations dictate steeper steps, proper permanent ladders withhand rails should be provided Any step up (or down) should not begreater than 25 cms (10 ins) Steps higher than this can greatly increase thestrain on the knee and hip muscles with consequent increased fatigue.Adequate space should be left around each machine to permit free andeasy movement for operating it and to allow for maintenance activities.Walkways should be identified by suitable lining and not allowed to beused for storage purposes Services, such as air, water, electrical power,necessary for the work being carried out should be conveniently situatedfor the operators’ use Machines in sequential operations should bepositioned to require the minimum amount of handling of product.Wherever possible that handling should be automated or by mechanicalmeans

The emission of noise and fumes by machinery which can affect theoperator and those on adjacent machines should be reduced to aminimum

3.9.7 Controls and indicators

Controls and instruments are the main interface between the operatorand the machine or plant In the design and layout of them:

 The movement of all controls must be consistent with the naturalmovement of the limb operating it

 Movement of a control in a clockwise direction, to the right or towardsthe operator should cause an increase in the machine function – theexception to this is a tap or valve where clockwise movement results in

a decrease in output, i.e the valve is shut

 Coarse adjustment and adjustments that require some force shouldutilise the full arm, leg or hand movement

 Where foot pedals are used, if actuation is by movement of the wholeleg the pedals should be arranged so they can be operated by eitherfoot If movement of the foot only is required it should be by pivoting

on the heel In both cases the arrangement should ensure that theoperator is not required to stand on one leg for long periods

 Quick, precise or fine adjustments that require little physical effortshould be by the fingers

 Hand operated controls should be located at a height between waistand shoulder level and be in clear view

Applied ergonomics 641

 Controls that have to be actuated frequently should be positionedadjacent to or within easy reach of the operator’s hands Other controlsshould be within easy arms reach

 Adjacent hand or finger operated controls such as push buttons, toggleswitches and rotating knobs should be spaced at least 25 mm (1 in)apart to prevent inadvertent operation

 In the layout and shape of control buttons:

 start buttons should be recessed into the control panel, shrouded, orgated to prevent inadvertent operation;

 stop buttons should be positioned adjacent to the start control, standproud above the panel surface and be red in colour;

 emergency stop buttons should be red, of the mushroom headedtype and lock in the open circuit condition when actuated

 The function of all control actuators should be clearly indicated either

by words or symbols

 Where the condition of the control is important and may need to beknown without looking at it, a datum mark such as a small pin ornotch should be made in the mounting panel and a matching pin ornotch made in the control handle The two should line up at eitherneutral or normal operating position so any deviation from it can easily

 The movement of the condition indicator of an instrument should beconsistent with the change in condition, i.e increase in the conditionshows as a clockwise movement or, in linear gauges, to the right orupwards

 Instruments that measure associated parameters should be positionedtogether and arranged so that the pointers or condition indicators alllie in the same orientation for normal operation allowing any deviantreading to be seen easily

 Where controls have to be actuated over periods of time with littlebody movement, seating should be provided for the operator and thepositioning of the controls and instruments arranged accordingly.Controls that are operated by the feet fall into two categories, those inwhich the whole leg is moved giving only a very coarse degree of controland those using the foot only, when a fine degree of control can beachieved In the former case, movement of the foot is from the hipallowing only a basic ON/OFF type of control without any intermediatepositioning, such are used for the initiation of a press stroke or the footpedals of an organ

Where a fine degree of control over the operating range is necessarythis can be achieved by pivoting the heel of the foot on the floor orsuitable rest An even finer degree of control, such as the accelerator pedal

in a car, can be achieved by providing a support at the outer side of thefoot about which the foot can pivot If a foot control is operated from a

642 Safety at Work

standing position, the arrangement should ensure that part of the bodyweight can be taken by the operating foot, or if this is not possible, thecontrol should allow operation by alternate feet to prevent the excessivestrain imposed when one leg takes the full body weight

3.9.8 Noise and vibrations

In all walks of life sound is a necessity, for communication, for warningand leisure enjoyment (music and the theatre) Unfortunately there arediffering views about what sound is useful and what is an adequateamount of sound Any unwanted sound is regarded as noise and as suchshould be eliminated or reduced to the lowest level possible In generalsound that interferes with people’s enjoyment of their private lives andpursuits becomes a nuisance and has been legislated against11 But excesssound can also interfere with concentration at work and become apotential hazard as well as reducing the operating performance of those

subject to it Examples of typical noise levels are shown in Figure 3.5.2.

Noise in an area is likely to be a hazard if it is necessary, when standing

1 metre apart, to have to shout to carry on a conversation Where thereappears to be a noise problem, sound level readings should be taken toestablish the extent of the problem

The presence of noise has long been recognised as one of the factorsthat reduces the quality of working life While the human brain can ‘tuneout’ consistent and/or irrelevant noises it can only do this up to a point

As noise levels rise so they become more insistent and invasive Similarlyunexpected changes in even quite low levels of noise can stimulate asubconscious response and, in some cases, break completely the currenttrain of thought The problems of noise from machinery and advice on themeasures to combat it are well documented in HSE publications12, 13and

it is not proposed to iterate them here

Vibrations on the other hand, where there is a finite movement of theplant, equipment or a pulsing of the air, are much more invasive and caninterfere with certain body organs ultimately causing ill health

Measures that can be taken to reduce the distracting effects of noiseinclude:

 elimination of sources of noise;

 if that is not possible then:

 enclose the source of noise in a sound proof room but ensureadequate cooling and ventilation is provided;

 provide sound havens or soundproof operating rooms ensuringthere is adequate ventilation;

 use sound absorbing screens and barriers;

 separate work areas from noise sources;

 position potential noise sources away from work areas – thefrequency hum from a transformer can be very invasive;

 directing the outlet ducts from ventilating systems, dust extractionsystems, etc away from affected areas This can include private housebedrooms where fan exhausts can become a nuisance and subject toabatement orders;

be a health hazard since they can induce sympathetic vibrations in certainhuman organs resulting in damage to that organ.

The transmission of mechanical vibrations can be reduced by:

 mounting the equipment on anti-vibration mounts;

 providing flexible connections between the vibrating plant and otherequipment

Air vibrations can be reduced by:

 changing the speed of the fan or blower;

 installing diffusers;

 changing the flow resistance of the air circuit;

 ensuring the intake to the fan is not obstructed

3.9.9 Stress

Stress has many causes including an inability to do what ought to be done

or failure to meet the targets set The cause may be within the individual

or it may be imposed from outside Internally caused stress can only beresolved by the individual himself but imposed stress causes can bereduced or eliminated by following ergonomic principles The build up ofstress in an individual will make him less efficient in his work and mayeven make him a safety hazard To optimise an individual’s performancethe stress suffered should be reduced to a minimum

Typical stress situations, with possible ways to resolve them, include:

 working at a machine led rate which is either faster or slower than the

individual’s natural work rate Wherever possible suitable adjustments should be made to the machine speed;

644 Safety at Work

 being required to undertake work which is either well below or well

above his inherent ability This may require a re-assessment of the operator and moving to other more appropriate work;

 being given inadequate or excessively complex instructions about his

job Instructions should be realistic and comprehensive and in terms and language that the operator can understand;

 being prevented from working at his own natural rate Some means should be provided to adjust the demanded rate of work;

 having to do a job in a less efficient manner than he knows it can be

done Listen to the operator’s suggestions and act on them or explain why not;

 being uncertain of his position in the organisation and not knowing

who his bosses are Provide training in the role and position within the company covering areas of responsibility, extent of authority, subordinates and superiors, etc.;

 having to wait for materials or data Improve planning and expediting;

 being unable to understand and follow work methods Further training and the provision of back-up information;

 working in software in which he has not been properly trained and

without back-up Ensure adequate training and provide competent back-up

to resolve queries;

 at loggerheads with his supervisor A personal matter to be resolved by the individuals or by separating them;

 being pressurised by his peer group;

 under a threat of redundancy without having any details Ensure kept informed of the latest position;

 family affairs;

 frustration with lack of progress on agreed action affecting his work

and working conditions Initiate suitable action or explain why it has not been possible;

 lack of recognition for ideas put forward Improve human relations in the company;

 irritating noises Investigate and eliminate;

 boredom from repetitive uninteresting work Re-assess ability and move

to more demanding work.

3.9.10 Display screen equipment (DSE)

The ergonomic aspects of the use of DSEs has been well documented inthe HSE’s guidance publication14 particularly those aspects concernedwith the physical comfort of the users and operators such as:

 chair with adjustments for seat height and back rest;

 suitable foot rest;

 adequate leg room below work table;

 adjustable screen both rotating and tilting;

 document holder to reduce amount of eye movement;

 limit on time of continuous operation;

 training in the use of the software with back-up immediately available

in case of queries;

Applied ergonomics 645

 screen should have adjustments to ensure stable picture, enable change

of polarity of characters and control over contrast;

 work surface to be large enough to accommodate keyboard, allpapers/documents and any peripherals such as the mouse, printers,disc, imager, etc

DSEs can make the atmosphere very dry and cause discomfort Sources ofmoisture, such as house plants, should be installed to improve thehumidity

3.9.11 Signs and signals

Signs and signals are a vital means of passing information where verbalcontact is not possible or reasonable The signs, generally, in the form ofposters, warnings, etc., are passive while signals, usually by hand or light,are dynamic It is important that those who need to read signs know theircorrect meaning In the passing of operational information by handsignals, such as in the use of cranes, it is important that both the signallerand the receiver (crane driver, etc.) use the same codes15and that both arefully conversant with the full range of hand signals Familiar hand signshave different meanings in different countries and care must be exercisedwhen selecting hand signals to ensure they are not in common usage inworkers’ mother countries where they may have a totally different, andsometimes insulting, meaning

With the number of migrating workers travelling to work in countriesforeign to them, meeting obligations to provide information presentsdifficulties of language This can largely be overcome by the use ofpictograms Standards16, incorporating the requirements of a directive,specify a range of pictogram safety signs with the aim of their beingunderstood regardless of the language of the viewer The standard signsare intended to be stand-alone but text may be added where necessary toprovide additional information

The positioning of signs is important They must be placed where theywill be clearly visible by those at whom they are aimed Emergency signs,such as fire exits, fire points, etc., should be clearly visible from all places

to which employees, visitors and others may have recourse as a normalpart of their activities The height at which signs are located should beconsidered A fire emergency exit sign at chest height is of no use if, in anemergency, the rush of people from the area completely cover it.Emergency safety signs should be positioned above head height so theyare clearly visible from all parts of the area they serve Conversely, signsplaced at high levels are often overlooked because the general trend is tolook downwards rather than upwards Signs should be mounted within

a sight line of 20° above horizontal when viewed from all the areasserved

Care must be exercised when selecting audible signals to ensure, first,that they do not add to the general noise to the extent of raising it abovethe accepted safe levels Second, they must be clearly distinguishablefrom all other audible signals and from other normal sounds in the area

646 Safety at Work

Where audible warnings are used, such as fire alarms, reversing vehicles,etc., the signal must be audible to all those likely to be in a position of riskfrom the danger warned against Audible warnings should not be usedwith such frequency that they become a part of the general backgroundnoise, also that their use does not become an irritant to others workingnearby Where audible warnings are employed all those in areas covered

by the warning should be familiar with the sound and with the action to

be taken

Public places such as cinemas, theatres, stores, supermarkets, etc.,present a particular problem in an emergency Staff should be trained inadvising the public what to do should an alarm be sounded The use ofbroadcast verbal warnings or safety instructions should be avoided sincethe message may be inaudible in some areas, can be misunderstood andgive rise to confusion

3.9.12 Coda

The application of ergonomic principles to work activities can make lifesafer and more pleasant for employees Many of the ergonomictechniques are being incorporated into regulatory requirements and intostandards but there are still many techniques that the employer can adoptthat will further improve not only the safety and quality of working lifebut productivity

References

1 British Standards Institution, BS EN 614–1 Safety of Machinery – Ergonomic design

principles – Part 1: Terminology and general principles, BSI, London (2000)

2 Kroemer, K.H.E and Grandjean, E., Fitting the Task to the Human, 5th edn, Taylor &

Francis, London (1999)

3 British Standards Institution, BS IEC 60529 Degrees of protection provided by enclosures (IP

code), BSI, London (1991)

4 British Standards Institution, BS IEC 60204 Safety of machinery – Electrical equipment of

machines – Part 1: General requirements, Clause 10.2, Push buttons, BSI, London (1997)

5 Health and Safety Executive, Legal series publication L24 Workplace health, safety and

welfare Workplace (Health, Safety and Welfare) Regulations 1992, Approved Code of Practice and Guidance HSE Books, Sudbury (1992)

See also health and safety guidance series publication HSG 202 General ventilation in the

workplace, HSE Books, Sudbury (2000)

6 Health and Safety Executive, Health and safety guidance series publication HSG 38

Lighting at work, HSE Books, Sudbury (1998)

7 Workplace (Health, Safety and Welfare) Regulations 1992, Regulation 6, Ventilation, The

Stationery Office, London (1992)

8 Manual Handling Operations Regulations 1992, The Stationery Office, London (1992)

9 Health and Safety Executive, Legal series publication L23 Manual Handling, Manual

Handling Operations Regulations 1992, Guidance on the Regulations, HSE Books, Sudbury

(1992)

See also Health and safety guidance series publication HSG 115, Manual handling

solutions you can handle, HSE Books, Sudbury (1994)

10 British Standards Institution, BS IEC 61310, Safety of machinery – Indication, marking and

actuation – Part 1: Requirements for visual, auditory and tactile signals, BSI, London

(1995)

Applied ergonomics 647

11 Environmental Protection Act 1990, The Stationery Office, London (1990)

12 Health and Safety Executive Legal series publication L108 Guidance on the Noise at Work

Regulations 1989, HSE Books, Sudbury (1998)

13 Health and Safety Executive, Health and safety guidance series publication L138 Sound

solutions, techniques to reduce noise at work, HSE Books, Sudbury (1995)

14 Health and Safety Executive, Legal series publication L26 Display screen equipment work

– Health and Safety (Display Screen Equipment) Regulations 1992, Guidance on the Regulations, HSE Books, Sudbury (1992)

15 British Standards Institution, BS 7121 Code of Practice for the safe use of cranes, BSI,

London

16 British Standards Institution, BS 5378 Safety signs and colours and BS 5499 Fire safety signs,

notices and graphic symbols, BSI, London

Suggested reading

Kroemer, K.H.E and Grandjean, E., Fitting the Task to the Human, 5th edn, Taylor & Francis,

London (1999)

Bridger, R.S., Introduction to Ergonomics, McGraw Hill, Singapore (1995)

Pheasant, S., Ergonomics, Work and Health, Macmillan Press, London (1991)

Helander, M., A Guide to the Ergonomics of Manufacturing, Taylor & Francis, London (1995) Chartered Institution of Building Services Engineers, Code for Interior Lighting, CIBSE,

London (1994)

McKeown, C and Twiss, M., Workplace Ergonomics: a Practical Guide, IOSH Publishing

Services Ltd., Leicester (2001)

PART IV

Workplace safety

Chapter 4.1 Science in engineering safety (J R Ridley) 651

Chapter 4.2 Fire precautions (Ray Chalklen) 671

Chapter 4.3 Safe use of machinery (J R Ridley) 727

Chapter 4.4 Electricity (E G Hooper and revised by

Chris Buck) 769

Chapter 4.5 Statutory examination of plant and equipment

(J McMullen and updated by J E Caddick) 793Chapter 4.6 Safety on construction sites (R Hudson) 819

Chapter 4.7 Managing chemicals safely (John Adamson) 850

Much of the work undertaken by safety advisers requires an standing of technical industrial processes Even in a single factory unitthe safety adviser may be called upon to advise on avoiding the hazardsfrom a chemical reaction, guarding particular types of machinery, thestandards of safe working to be expected of a building contractor, theprecautions to be taken to prevent fire and the fire fighting equipmentthat should be provided, etc

under-To carry out his duties effectively, the safety adviser should have anunderstanding of basic physics and chemistry and of the current safetytechniques for reducing the risks associated with the more commonly metindustrial processes This Part considers some of these processes and thebasic sciences from which they stem

it Properties may change with use, temperature, operating atmosphere,contamination by surrounding chemicals and for many other reasons It

is necessary to know the properties of the materials and how and whythey have been used so that an assessment can be made of whether likelychanges in the properties may give rise to hazards

These properties stem from the chemical and physical characteristics ofthe different materials and substances used and their behaviour undercertain conditions can determine the safety or otherwise of a process oroperation This chapter looks at some of the characteristics and properties

of materials in common use, their application, circumstances of use andpossible causes of hazards

4.1.2 Structure of matter

Everything that we use in our work and daily life is made up of chemicalsubstances, by themselves or in combination of one sort or another Eachsubstance consists of elements which are the smallest part of matter thatcan exist by itself In its free state, an element comprises one or moreatoms When atoms combine together they form molecules of the element

or, if different atoms combine, of compounds The ratio in which atomscombine is determined by their combining power or valency

Atoms are made up of three particles:

 protons which have a unit mass and carry a positive charge,

 neutrons which have a unit mass but carry no charge, and

 electrons which have negligible mass (i.e 1/2000 proton) but carry anegative charge

651

652 Safety at Work

The core or nucleus of the atom consists of protons and neutrons with

electrons travelling in orbits around the nucleus (Figure 4.1.1) Elements

normally have no overall charge since the number of protons is matched

by an equal number of electrons However, it is possible to upset thisbalance by removing either a proton or an electron resulting in the atomcarrying a charge when it is said to be ionised

In chemistry, atoms are given ‘atomic numbers’ which equal thenumber of protons or electrons in the atom They are also given ‘massnumbers’ which equal the sum of the number of protons plus neutrons.The mass number is always equal to or greater than 2  the atomicnumber except in the case of hydrogen Some elements can occur inconditions where they have the same atomic number, and hence thesame name, but different mass numbers; they are then known asisotopes and are generally referred to as nuclides Very large heavyatoms, such as uranium, can be unstable and easily break down toproduce smaller atoms with the production of particles or energy Theseatoms are radioactive and provide the source of energy in nuclearreactors

Approximately 100 different atoms have been identified and each hasbeen given a name and a coded symbol which usually is the first one ortwo letters of its name: carbon – C, lithium – Li, titanium – Ti, etc.Exceptions in this coding system arise because when it was evolved in theearly 1800s some chemicals were still known by their Latin names, such

as copper (cuprum – Cu) and tin (stannum – Sn)

When atoms join together their molecular formulae are written asgroups of atomic symbols to indicate the number of those atoms present

to form a stable molecule

Molecular formulae

NaOH sodium hydroxide (caustic soda)

Figure 4.1.1 Atomic structures

Science in engineering safety 653

A chemical reaction occurs when the atoms in molecules rearrangeeither by decomposing into smaller molecules or by joining with otheratoms to form different molecules; in both cases the atoms reorganisethemselves to form different structures Endothermic reactions requirethe input of heat to make them happen whereas exothermic reactionsoccur with the evolution of heat

2Na + 2H2O = 2NaOH + H2+ heat

2H2+ O2 = 2H2O + heat

These chemical equations show in chemical shorthand, using thechemical codes, the rearrangement of atoms which occurs in thesereactions, a balance of the number of atoms being maintained during thereaction The molecular mass of molecules can be obtained by addingtogether the mass numbers of the constituent atoms

Compounds which contain atoms of elements other than carbon, butincluding carbon dioxide (CO2), carbon monoxide (CO) and thecarbonates (e.g calcium carbonate CaCO3) are called inorganic chemicals.All other compounds which contain carbon atoms are known as organicchemicals

Carbon is an unusual element; not only is it able to form simplecompounds where one or two carbon atoms are joined to atoms of otherelements, but carbon atoms can link together to form chains or rings ofatoms Almost all other atoms can be joined into these chains and rings tocreate millions of different organic compounds, from the comparativelysimple ones consisting of one carbon with one other type of atom to thehighly complex molecules with hundreds of linked carbon atoms joinedwith other different atoms Organic chemicals include most of thesolvents, plastics, drugs, explosives, pesticides and many other industrialchemical substances

654 Safety at Work

tion of an electric potential across a metal allows the electrons to undergo

a directional flow between the atoms making metals good electrical

conductors Table 4.1.1 lists the properties of some typical metals and

other elements

4.1.3.2 Inorganic compounds

In some compounds one or more of the bonds joining the atoms are theresult of an unequal sharing of electrons between the two atoms and theseproduce ionic compounds which are crystalline solids, usually with ahigh melting point They are often soluble in water giving a solutionwhich conducts electricity

Many other compounds have bonds based on an equal sharing ofelectrons and so do not ionise These compounds can be solids havinglow melting points, or liquids or gases Usually they are not soluble inwater unless they react with it There are also many more compoundswith types of bond intermediate between the two described and which

exhibit properties that relate to both types Table 4.1.2 lists some of the

properties of a selection of inorganic compounds

With the exception of sulphur, stannic chloride and potassium chloride,

all the elements and compounds listed in Tables 4.1.1 and 4.1.2 present

hazards to health

Table 4.1.1 Properties of typical elements

Element Symbol Properties

Reactive metals

Aluminium Al m.p 660°C, good conductor, surface oxide

formation resists attack by air or waterBarium Ba m.p 850°C, soft, spontaneously flammable in air,

reacts with waterLithium Li m.p 186°C, soft, burns vigorously in air, reacts

with waterLess reactive metals

Cobalt Co m.p 1490°C, hard, not attacked by air or waterIron Fe m.p 1525°C, burns in oxygen, reacts slowly with

waterMercury Hg liquid, slowly attacked by oxygen, no reaction with

waterSilver Ag m.p 961°C, ductile, not attacked by oxygen or

waterNon-metals

Bromine Br dark red liquid, b.p 59°C, very reactive, not

flammablePhosphorus P red form, m.p 600°C or white form, m.p 43°C,

burns readily to P2O5, insoluble in waterSulphur S yellow or white, m.p 115°C, burns to SO2,

insoluble in water

Science in engineering safety 655

4.1.3.3 Organic compounds

As most organic compounds contain a relatively large percentage ofcarbon and hydrogen atoms they are flammable and many are toxic Allliving matter is constructed of complex interdependent organic chemicalsand it is because organic compounds interfere with the normalfunctioning of living matter that they constitute fundamental health andhygiene hazards

Although there are very many organic compounds they can begrouped into a small number of classes according to their reactive

properties These broad groups are listed in Table 4.1.3 which gives

examples of compounds in each group

4.1.3.4 Acids and bases

Acids are compounds which dissolve in water to give hydrated hydrogen

water, and the terms strong and concentrated should not be confused.

Acids are corrosive in that they react with both metals and with bodyproteins Acids are dangerous not just because of their acidity but theycan be oxidising agents (HNO , HClO ), violently reactive with water

Table 4.1.2 Properties of a selection of inorganic compounds

Compound Formula Properties

Ammonia NH3 gas, b.p –33°C, dissolves readily in water giving a

basic solutionCarbon monoxide CO gas, b.p –190°C, odourless, almost insoluble in

waterHydrogen chloride HCl gas, b.p –85°C, dissolves readily in water giving

hydrochloric acidHydrogen sulphide H2S gas, b.p –61°C, strong odour, burns in air

Hydrogen peroxide H2O2 liquid, decomposes violently on heating, powerful

oxidising agentStannic chloride SnCl4 liquid, b.p 114°C, fumes, reacts rapidly with waterSulphuric acid H2SO4 liquid, decomposes at 290°C giving SO3, strong

acid, reacts violently with waterAluminium silicate Al2Si2O7 solid, infusible, unreactive, clay silicate

Phosphoric acid H3PO4 solid, m.p 39°C or syrupy liquid, strong acidPotassium chloride KCl solid, m.p 770°C, very soluble in water, unreactiveSodium hydroxide NaOH solid, m.p 318°C, deliquescent, strong alkali

656 Safety at Work

(H2SO4) and many are toxic Phenol (C6H5OH) is one of the mostdangerous acidic organic compounds

Bases are of two types, solid alkalis such as metal hydroxides which

dissolve in water to give hydroxide ions, and gases and liquids such asammonia and the amines, which liberate hydroxide ions on reaction withwater:

NaOH + H2O = Na++ OH–+ H2O

NH3+ H2O = NH+

4 + OH–Some of the bases are toxic, many react exothermally with water and allare highly corrosive or caustic towards proteins Alkalis spilled on theskin penetrate much more rapidly than acids and should be leached outwith copious water and not sealed in by attempting neutralisation.The reaction between an acid and a base is a vigorous, exothermicneutralisation forming a salt The strength of acids and bases can bemeasured in terms of hydrogen ion concentration by the use of eithermeters or test papers, and it is expressed as a pH value on a scale from 0(acid) to 14 (base) Pure water has a neutral pH of 7

Table 4.1.3 Examples of the main groups of organic compounds

Trichloroethane CH3CCl3 solvent

Glycerol C3H5(OH)3 glycerineCarbonyl

compounds

Formaldehyde(methanal)

Benzaldehyde C6H5CHO manfacturing

Ethers Ethyl ether C2H5OC2H5 anaesthetic

Aniline C6H5NH2 manufacturing

Phthalic acid C6H4(CO2H)2 manufacturingEsters Ethyl acetate CH3CO2C2H3 solvent

Science in engineering safety 657 4.1.3.5 Air and water

Air and water deserve to be considered separately since they are everpresent and are necessary for the operation of many processes andresponsible for the degradation of many materials

Air is a physical mixture of gases containing approximately 78%

nitrogen, 21% oxygen and 1% argon These proportions do not varygreatly anywhere on the earth but there can be additional gases as a result

of the local environment: carbon dioxide and pollutants near industrialtowns, sulphur fumes near volcanoes, water vapour and salts near the seaetc

Air can be liquefied and its constituent gases distilled off; liquidnitrogen (b.p –196°C) has many uses as an inert coolant, liquid oxygen(b.p –183°C) is used industrially in gas-burning equipment and inhospitals, and argon (b.p –185.7°C) is used as an inert gas in certainwelding processes Liquid oxygen is highly hazardous as all combustiblematerials will burn with extreme intensity or even explode in its presence.Combustion is a simple exothermic reaction in which the air provides theoxygen needed for oxidation If the concentration of oxygen is increasedthe reaction will accelerate This effect was experienced in the fire onHMS Glasgow1

Water is a compound of hydrogen and oxygen that will not oxidise

further and is the most common fire extinguishant However, cautionmust be exercised in its use on chemical fires since a number of oxidesand metals react energetically with it, in some cases forming hazardousdaughter products and in others producing heat and hydrogen whichfurther exacerbate the fire

4.1.4 Physical properties

All matter, whether solid, liquid or gas, exhibits properties that followpatterns that have been determined experimentally and are wellestablished and proven This section looks at some of the factors thatinfluence the state of matter in its various forms

4.1.4.1 Temperature

Temperature is a measure of the hotness of matter determined in relation

to fixed hotness points of melting ice and boiling water Two scales areuniversally accepted, the Celsius (or Centigrade) scale which is based on

a scale of 100 divisions and the Fahrenheit scale of 180 divisions betweenthese two hotness points Because Fahrenheit had recorded temperatureslower than that of melting ice he gave that hotness point a value of 32degrees Converting from one scale to the other:

(°F – 32)  5/9 = °C

(°C  9/5) + 32 = °F

658 Safety at Work

Man has long been intrigued by the theory of an absolute minimumtemperature This has never been reached but has been determined asbeing –273°C The Kelvin or absolute temperature scale uses this as itszero, O K; thus on the absolute scale ice melts at +273 K

Devices for measuring temperature include the common mercury inglass thermometer, thermocouples, electrical resistance and opticaltechniques

4.1.4.2 Pressure

Pressure is the measure of force exerted by a fluid (i.e air, water, oil etc.)

on an area and is recorded as newtons per square metre (N/m2) Withsolids the term stress is used instead of pressure Datum pressure isnormally taken as that existing at the earth’s surface and is shown as zero

by pressure gauges which indicate ‘gauge pressure’ (i.e the pressureabove atmospheric) However, at the earth’s surface the weight of the air

of the atmosphere exerts a pressure of 1 N/m2or 1 bar Beyond the earth’satmosphere there is no pressure and this is taken as the base for themeasurement of pressure in absolute terms Thus:

gauge pressure = absolute pressure –1 N/m2

or absolute pressure = gauge pressure +1 N/m2

The pressure at the top of a mercury barometer, where the force due to theweight of the atmospheric air outside the tube is balanced by the forceexerted by the weight of the column of mercury inside, is normally taken

as zero (0 N/m2or absolute vacuum), although scientifically there is asmall vapour pressure from the mercury

Pressure can be measured by means of manometers which show thepressure in terms of the different levels of a liquid in a U-tube, bymechanical pressure gauges which record the differential effect ofpressure forces on the inside and outside surfaces of a coiled tube or of adiaphragm, and electronic devices which measure the change of electricalcharacteristic of an element with pressure

4.1.4.3 Volume

Volume is the space taken up by the substance With solids which retaintheir shape, their volume can be measured with comparative ease Liquidvolume can be measured from the size of the containing vessel and theliquid level Gases, on the other hand, will fill any space into which theyare introduced, so to obtain a measure of their volume they must berestrained within a sealed container

Each type of material reacts to changes of temperature and, to a lesserextent with solids and liquids, to changes of pressure, by increases ordecreases in their volume and this fact can be made use of, or has to beallowed for, in many industrial processes and plant

Science in engineering safety 659 4.1.4.4 Changes of state of matter

At ordinary temperatures, matter exists as solid, liquid or gas but manysubstances change their state as temperatures change – for example, icemelts to form water at 0°C and then changes into steam at 100°C Thestages at which these changes of state occur are also influenced by thepressure under which they occur

4.1.4.4.1 Gases

In gases the binding forces between the individual molecules are smallcompared with their kinetic energy so they tend to move freely in thespace in which they exist When heated, i.e additional kinetic energy isgiven to them, they move much more rapidly and if restrained in a fixedvolume impinge more energetically on the walls of the containing vessel,

a condition that is measured as an increase in pressure The relationshipbetween temperature, pressure and volume of gases is defined by thegeneral Gas Law:

PV

T (initial) =

PV

T (final)where P = absolute pressure, V = volume and T = absolute

temperature

Thus in a reaction vessel which has a fixed volume, if the temperature

is increased, so the pressure will increase If the reaction is exothermicand the temperature increase is not controlled there is a risk that thepressure in the vessel could rise above the safe operating level withconsequent risk of vessel failure, a situation that may be met in chemicalprocesses that use autoclaves and reactor vessels

This general law applies with variation when gases are compressed inthat the temperature of the gas rises In air compressors where there islikely to be oil present the temperature of the compressed air must bekept below a certain level to prevent ignition of the contained oil.Conversely, when the pressure of a gas is decreased, the temperaturedrops, a condition that can be seen with bottles of LPG where a frost rimeforms and where in cold weather there is a danger of the temperature ofthe gas dropping so low that the control valve freezes up

Some gases can be compressed at normal temperature until theybecome liquids (e.g carbon dioxide, chlorine, etc.), and can conveniently

be stored in that state, while others, called permanent gases, cannot beliquefied in this way but are stored either as compressed gases (e.g.hydrogen, air etc.) or under pressure in an absorbent substance (e.g.acetylene)

Air, carbon dioxide and a number of other gases which dissolve inwater become more soluble as the pressure increases or the temperaturedecreases Increase in temperature or decrease in pressure causes thedissolved gases to come out of solution, e.g tonic water or fizzylemonade This is why hydraulic systems need venting A similar

= 1.19) being heavier than air tend to sink to the floor and will fill thelower part of pits and storage vessels making entry hazardous Similarly,the vapours of most flammable liquids are heavier than air and tend tocollect in low places in the floor and will flow like water to the lowestpoint creating fire hazards possibly remote from the site of the leakage.4.1.4.4.2 Liquids

In liquids, the kinetic energy of the molecules is sufficient to allow them

to slide over each other but not sufficient for them to move at random inspace since they are subject to the binding force of cohesion and remain

a coherent mass with a definite volume Thus the substance will flow totake up the shape of the container but cannot be compressed Whenheated the kinetic energy of the molecules increases and the liquidexpands until boiling point is reached when the cohesive forces can nolonger hold adjacent molecules together, the liquid evaporates andbehaves as a gas If the temperature falls, i.e the kinetic energy of thevapour is reduced, the vapour will revert to its liquid state by

Table 4.1.4 Properties of some flammable liquids and gases

Substance Sp gr.

of liquid

Sp gr.

of vapour

Boiling point,

OES long- term ppm

OES = occupational exposure standard.

MEL = maximum exposure limit.

* = asphyxiant, requires monitoring for oxygen content of atmosphere.

Science in engineering safety 661

condensing However, at temperatures well below the boiling point, somesurface molecules gain sufficient energy to escape from the surface Thismeans that the liquid is constantly losing some of its surface molecules byvaporisation and the extent to which this is occurring is measured as

vapour pressure (Table 4.1.4) Vapour pressure will increase as

tem-perature rises

In passing from the liquid to the vapour or gaseous state a considerableadditional input of energy is required without raising the temperature

This energy is the ‘latent heat of vaporisation’ (H v in Table 4.1.5) of the

liquid and can be quite substantial This characteristic is made use of infire fighting where a fine spray of water absorbs the heat of the flames tobecome steam and in so doing reduces the gas temperature to below thatrequired to maintain combustion

As liquids are heated they expand volumetrically and this effect has to

be taken into account in the storage of liquids in closed vessels It isnormal to leave an air space above the liquid, the ullage, to allow for thisexpansion, and to fit pressure relief valves to release any excess pressuregenerated An example of what can happen if these precautions are nottaken was seen at Los Alfaques Camping Site at San Carlos de la Rapita

in Spain in July 1978 when a road tanker carrying 23.5 tonnes ofpropylene exploded killing 215 people The circumstances of the incidentwere that the unlagged tanker was filled completely with propyleneduring the cool of the early morning, leaving no ullage The tank was notfitted with any form of pressure relief As the tanker was driven in theheat of the sun, the tank and its contents heated up to a temperaturewhere the vapour pressure of the contents exceeded atmosphericpressure and the propylene (an incompressible liquid), expanding at agreater rate than the tank, caused the tank to rupture as it was passing aholiday camp at 14.30 hours creating a BLEVE (boiling liquid expandingvapour explosion) that caused devastation over an area of fivehectares

The incompressibility of liquids is utilised in many ways in industryparticularly in hydraulic power transmission Two common applications

Table 4.1.5 Heats of vaporisation and fusion

662 Safety at Work

are as power sources for machines and vehicles and in hydrauliccylinders Normally the hydraulic medium is an oil and hazards can arisewhere leaks occur either as oil pools on a floor or from high-pressure lines

as a fine mist which is highly flammable

4.1.4.4.3 Solids

In a solid at room temperature the molecules are tightly packed together,have little kinetic energy and vibrate relative to each other A solid hasform and a rigid surface as a result of the strength of the cohesive forcesbetween the molecules As a solid is heated the vibrations of themolecules increase and the solid expands Eventually a temperature isreached where the kinetic energy allows the molecules to slide over oneanother and the solid becomes a liquid

Solids, and particularly metals, have a number of characteristics thatare of great value in almost everything to do with modern standards ofliving They have stability of form over a wide range of temperatureswhich enables them to be formed and machined into shapes, ameasurable strength over a wide range of stresses and temperatures, andpermanent physical characteristics such as rates of expansion andcontraction with temperature

The change of state from solid to liquid requires the input of a

considerable quantity of heat – the latent heat of fusion (H f in Table 4.1.5)

– without raising its temperature This can easily be seen with melting icewhere the temperature of the water remains at 0°C until all the ice hasmelted At room temperatures some solids, such as solid carbon dioxide,

do not pass through the liquid stage but change directly from a solid to

a gas, a process known as ‘sublimation’, the liquid state only occurring atlow temperature and/or high pressure Organic substances such as woodalso have no liquid stage but decompose on heating, rather than melting,reducing their very large molecules into smaller ones some of which aregiven off as vapours

As solids are heated they expand at rates determined by their

‘coefficients of expansion’ that vary with the different substances Theheat needed to raise the temperature of different solids varies and can bedetermined from its ‘specific heat’ Problems can arise where dissimilarmetals are welded together that have different specific heats and differentcoefficients of expansion Allowance has to be made for differentialexpansions where different metals are moving in contact and havedifferent coefficients of expansion, such as in bearings However, use ismade of these different expansion rates in bimetal strips for temperaturemeasurements and in thermostats

Physical properties of some solids are shown in Table 4.1.6.

Solids, and particularly metals, have great strength and an ability towithstand high levels of stress and strain By alloying metals they can bemade to exhibit particular characteristics to suit particular applications.Characteristics of metals can vary from very ductile lead and goldthrough high-strength high-tensile steels to brittle cast iron, each havingits particular use One exception is the metal mercury which is a liquid atroom temperature

Science in engineering safety 663

4.1.5 Energy and work

There are two types of energy, kinetic and potential Kinetic energy isinvolved in movement and may be available to do work; it can be present

in a number of forms such as heat, light, sound, electricity andmechanical movement Potential energy is stored energy and usuallyrequires an agent to release it, as, for instance, the potential energy of aloose brick lying on a scaffold which needs to be kicked off to producekinetic energy

Heat is a form of energy where the degree of hotness of a material isrelated to the rate of movement of the atoms and molecules which make

it up Heat is transferred from one material to another by conduction,convection or radiation Conduction occurs within and between materialsthat are in contact through the physical agitation of molecules by moreenergetically vibrating neighbours Some materials are better heatconductors than others depending on the ease with which molecules can

be made to vibrate Convection is the conveyance of heat in gases andliquids by the heated fluid rising and heating any surface with which itcomes into contact Initial spread of fire within a building is most likely

to be by convection and a method of control is by venting the hot gases

to outside the building so preventing the lateral spread of fire Radiation

of heat from a heat source occurs as infrared emissions which passthrough the atmosphere by wave propagation

If a force is exerted on an object, no energy is expended until the bodymoves when the force is converted into kinetic energy which is equal tothe work done on the body by the force Mechanical work is donewhenever a body moves when a force is applied to it

Work done (joules) = force (newtons)  distance moved (metres)

In all mechanical devices or machines the work done is never as great

as the energy expended, as some of the energy is lost in overcoming

Table 4.1.6 Physical properties of some solids

kg/m 3

Coefficient of linear expansion

m/mK

Specific heat kJ/kgK

of the vessel and is exerted at right angles to the containing surfaces Withgases the pressure is virtually the same throughout the containing vesselbut with a liquid the pressure varies according to the depth, i.e theweight of liquid above the point of measurement Pressure can beconverted into kinetic energy through the movement of a piston in apneumatic or hydraulic cylinder.

2 Rate of change of momentum is proportional to the force applied andtakes place in the direction in which the force acts

3 To every action there is always an equal and contrary reaction

If a force is applied to a body resting on a plane, initially the body willnot move because of the friction between itself and the plane The force isresisted by an equal and opposite force due to friction Once the appliedforce exceeds the friction force the body will move in the direction of theapplied force However, if the same force continues to act on the body itsspeed will accelerate because sliding friction is less than limiting (i.e.static) friction Hence:

Before movement the force F = W L

where L is the coefficient of static or limiting friction, and W is the

reaction between the body and the plane (i.e its weight = mass gravity)

After movement the excess force F e = F – W s

where is the coefficient of sliding friction