Introduction
Not our words … but those of Trevor Kletz; renowned Guru in the eld of process safety. What
did he mean? Well, he was talking about ‘bungalows’ (single story buildings) and how they relate to
INHERENT SAFETY – the very rst topic which should spring to mind in any hazard or risk assessment i.e.
how can we eliminate the hazard in the rst place?
Later in the third section of this book, we look at the classication of hazardous areas and cite an incident in Mexico involving a water immiscible solvent (n-Hexane) which entered a town drainage system. Had the manufacturing process used a water-miscible solvent, many of those killed in the violent explosion would be alive today.
EU Areas and equipment in which combustible gases and/or vapours may form and in which
airborne clouds may be produced, fall within the scope of the Dangerous Substances and Explosive
Atmosphere Regulations 2002 Statutory Instrument No. 2776. A more detailed account of the principles involved is discussed in International Standard IEC 61241-19. The Regulation (Section 6.4 Risk
Reduction) states the following measures, in order of priority, are those specied for risk control:
- Reduction of the quantity of dangerous substances to a minimum
- Avoidance or minimising the release of a dangerous substance
- Control of the release of a dangerous substance at source
- Prevention of the formation of an explosive atmosphere, including the application of appropriate ventilation
- Ensuring that any release of a dangerous substance which may give rise to risk is suitably collected, safely contained, removed to a safe place, or otherwise rendered safe, as appropriate
- Avoidance of ignition sources including electrostatic discharges and adverse conditions which could cause dangerous substances to give rise to harmful physical eects
- Segregation of incompatible dangerous substances
Dilution
Simple steps, for example, could help eliminate the hazard – like adding water to an alcohol or changing the process operation e.g. method of addition. Pure iso-Propyl Alcohol (IPA) will form ammable atmospheres in air under ambient conditions because it has a low ash point value i.e. 12 °C.
Hence, hazardous areas will arise during handling and inside process vessels under normal ambient
temperature conditions. But could an IPA-water mixture be used in place of the pure solvent?
Dilution with water results in an increase in the closed-cup ash point value. This arises because
the rate of evaporation is suppressed. The graph shows empirical data from which a ‘line-of-best-t’
relationship has been determined:
y = 106.7 x-0.4674
where:
y = ash point (°C), and
x = IPA concentration (% v/v)
Ref: http://www.nugentec.com/FAQ/Fr…
With a 5K safety margin, if the temperature of an
IPA/H2O solution can be restricted to normal room
temperature (say 25 ºC) or below, hazardous
vapour-air mixtures can be avoided by controlling the alcohol content to 15 % IPA by volume or less (equivalent Flash Point = 30 ºC). Clearly, lower concentrations provide an even greater margin of safety.
Change in Procedure
When pouring liquids with moderate ash points (e.g. Flavours) in to a heated batch, to preclude the
formation of hazardous areas within the vessel, some liquids (Flavours) have to be restricted,
dependent on their ash point. To eliminate this, one option is to dilute the avours with some of the
batch liquor in a separate area (i.e. within a ventilated cubicle or fume cupboard), prior to the
addition of the (then) diluted mixture. Occasionally, to achieve the same goal, the batch
temperature can be lowered, although this is not always a tenable solution. Increased ventilation
oers a third alternative, whereby extraction (LEV) is provided immediately above the point of addition
i.e. rather than at (say) ceiling level.
Ventilation
With increased ventilation, the extent of the hazardous area will be reduced. Suitable ventilation
rates can also avoid persistence of the explosive atmosphere, thus inuencing the type (and/or
extent) of a zone. However, key points need to be considered in the use of ventilation:-
- Eectiveness should be controlled and monitored
- Extract discharge point requires consideration
- Air should be drawn from a non-hazardous area
- Release conditions must be dened Need to consider changes in gas densities (with temperature)
- Need to consider ow of heavier-than-air gases
- Need to consider local obstacles/impediments to air movement
Ventilation is often categorised as follows:
High ventilation (VH) – can reduce the concentration at source virtually instantaneously, resulting in a
concentration below the LEL. A zone of small (or even negligible) extent results.
Medium ventilation (VM) – can control concentration, resulting in a stable situation where
the concentration beyond the zone boundary is below the LEL whilst the release is in progress and
where the hazardous area does not persist unduly, after the release stops.
Low ventilation (VL) – cannot control the concentration whilst release is in progress or
prevent undue persistence of hazardous area after release has stopped.
Hazardous Area Classication
The process of area classication involves the identication of all ammable materials, the
identication and grading of all releases of ammable material, the assessment of the level of
ventilation and/or housekeeping and the determination of the resulting types and extents of
the zones. In turn, the designation of zones enables the correct equipment, practices and procedures to be applied to protect the health and safety of the workers concerned with the facility.
It is important to note that area classication only deals with reasonably foreseeable events and does
not consider highly improbable (‘catastrophic’) events. EN 60079-10 section 1.1(d) denes
‘catastrophic’ failures as ‘beyond the concept of abnormality dealt with in the standard’ and lists ‘the
rupture of a process vessel or pipeline and events that are not predictable’ as examples.
Thus, a ‘catastrophic’ failure may cause an explosive atmosphere to be present in an area dened by
area classication as ‘non-hazardous’ and such situations are subject to a risk assessment by the
operator under other legislation.
Quick-x ‘Gaer Tape’, often used for a temporary repair, is not a sound engineering solution.
Moreover, in many cases, it becomes a permanent xture! Certainly not an example of catastrophic
failure.
Warehousing is not immune to risk either! A recent audit found several contraventions of HSE
Guidelines:
- Storage of ammable liquids within the connes of the building
- Shared storage of oxidising materials and ammable liquids in the building
- No provision of natural or forced (mechanical) ventilation (battery charging)
- Processing (mixing) operations undertaken within a warehousing environment
- Limited segregation of operations
- An opportunity for the release of gaseous oxidants within the building
Simple procedures can help hugely; such as protecting containers against banging or other
physical damage when storing, transferring or using them and not using wooden pallets or other
combustible pallets for storing containers of oxidizing materials and of course, ensure containers
are suitably labelled.
In 2004, ICL was ned £400,000 over a factory explosion at Stockline Plastics in Maryhill, Glasgow.
The blast killed nine workers and injured 40 others and was Scotland’s worst industrial disaster since
the Piper Alpha oil rig explosion and re in 1988.
The blast was caused by a build-up of liquid petroleum gas that had leaked from pipes. The pipes dated back to 1969 and were so badly corroded that escaped gas was ignited when a
builder icked a switch in the factory. The High Court in Scotland was told that the
pipework in question would only have cost £405 to replace and that one risk assessment undertaken
was carried out by a college student doing vacation work.
Characterising Your Material Hazards
Industries producing and/or handling materials which may form hazardous (ammable)
atmospheres must comply with the ATEX Directives. In the UK, this requires compliance with the
Dangerous Substances & Explosive Atmospheres Regulations (DSEAR). Similar NFPA Codes apply In
the USA.
You cannot dene the necessary systematic approach to protecting people and plant, without a
knowledge of the potential hazards in your work place – are ammable atmospheres present within
your plant or processing areas?
The first step is to check your suppliers’ MSDS’s, your own records and the open literature for
relevant test data.
Gestis is an EU funded database of combustion and explosion characteristics of more than 6000 dust samples from virtually all sectors of industry to help establish a basis of safety for the safe handling of combustible dusts. Gestis data is generic and may be indicative only e.g. it may not cover specicfic formulations or compounds, in which case, testing will be required.
How do you know a material is dangerous unless you test it?
Group A/B (Vertical Tube) Test: Essentially, the dust under test is dispersed in air at ambient
temperatures, past a source of ignition and observations of flame propagation made by the
operative. The photograph shows the test apparatus with the tube removed for clarity. This
classification test is a qualitative assessment of the ability of a dust to take part in an explosion:
- Group A is a dust, which is able to ignite and propagate ame.
- Group B is a dust, which does not propagate ame.
As in all explosion testing, the sample selected must be representative of the material from the plant at risk, typically the nest and driest material found. Further testing is required for Group B samples if
handled at elevated temperatures >110 degree C. Although this test may be superseded by the 20-litre
sphere, it does give a very good visual feel for how the dust reacts to an ignition source at ambient
temperature.
Once you have established that the dust is Group A Explosible, the next step is to examine the specic
ignition and explosion parameters.
Dust Concentration
Lower Explosion Level (LEL for gases and vapours) or Minimum Explosible Concentration (MEC for
powders) is typically of the order 45g/m3 and 30-60 g/m3 respectively. The Upper Explosion Level (UEL) not as clearly dened for powders, but usually is 1000g/m3.
Do you leave footprints where you walk? If so, there may be a secondary explosion risk.
Consider a room 10 m x 10 m x 5 m high. What thickness of deposits will form a ammable
(explosible) atmosphere throughout the room?
A thickness of just 1 mm can equate to 75 kg of powder (depending on density) and if this was
dispersed in the 500 m3 volume, the resulting concentration is 75,000 / 500 = 150 g/m3 i.e. well
within the ammable range.
How might this be dispersed, you may ask? Duringcleaning operations using air-jetting!
Particle Size
This is a very important factor as particles greater than 500micron are unlikely to cause dust explosion
hazards.This is why most tests are carried out below 100 micron or the standardised 63 micron. This may be a futile action due to attrition, as transportation of granular material may create a nes fraction.
Moisture Eects
Explosion violence falls at higher moisture contents:
- 0-5% has little eect
- 5-10% decreases sensitivity
- 25% particles unlikely to stay in suspension
Minimum Ignition Temperature: MIT Cloud
The Minimum Ignition Temperature of a dust suspension is the lowest temperature at which it will
ignite spontaneously and propagate ame. This MIT value is particularly relevant to problems
involving relatively large heated areas of plant e.g. surfaces of dryers, mills, electrical equipment, etc.
For example, you need to ensure that any escape of dust (as a cloud) does not spontaneously ignite on
nearby heated surfaces e.g. electrical motors, insect-o-cuters, etc. Ancillary equipment such as this is often ‘missed’ (overlooked) when shutting down plant for cleaning and maintenance purposes.
Minimum Ignition Temperature: MIT (LIT) 5 mm Layer
The test determines the minimum temperature of a prescribed hot surface, which will result in the
decomposition and/or ignition of a layer of powder of specied thickness. The test is particularly
relevant to industrial equipment with hot surfaces, on which dust deposits may form.
For example, you need to ensure that any escape of dust (which forms a layer) does not result in
smouldering. Equipment must be kept clean with surface layers not exceeding 5 mm. Thicker layers
will invalidate the T rating of equipment and may cause ignition.
Minimum Ignition Energy: The Dust Cloud
Minimum Ignition Energy of a dust suspension is the lowest energy at which ignition and ame
propagation away from the spark kernel occurs – the value being particularly relevant to identifying
potential electrostatic ignition hazards. In principle, the test is similar to the Group A/B Test.
Two circuits are employed – capacitive (for electrostatic ignition) and inductive (for friction
spark ignition).
Essentially, the dierence between them lies in the duration of the spark i.e. increasing inductance will lengthen the duration of the discharge making it more incendive. Consequently, the MIE value with this circuit will be lower than that with a capacitated circuit e.g. 10 mJ compared to 30 mJ. Typically, MIEcap values range from 3 – 1000 mJ for dusts and 0.017 – 0.4 mJ for gases and vapours.
Minimum Oxygen for Combustion / Limiting Oxygen Concentration (MOC / LOC):
Essentially, a ammable atmosphere (be it gas, vapour or airborne dust) cannot exist below this
level of depleted oxygen. The data is needed when basing safety on the use of inert gas, in certain
milling or solvent handling operations, for example. The test is conducted in a 20 Litre Explosion Sphere.
Typical MOC values lie in the range 8 – 15 % v/v (for powders) and 5 – 10 % v/v for gases and vapours. It
is important to note that the MOC value depends on the type of material and the type of inert gas.
The following schematic shows the ammable envelope for Methane.
Explosion Characteristics: Kst & Pmax
Although the methods above are relevant for prevention of flame and explosions, to confirm the
quantitative assessment of a design for explosion protection you will need to conduct an actual
controlled dust explosion in either the 20 litre or 1m3 test apparatus.
The dust sample is homogenously dispersed via deector plates (typically) and ignited by a 10kJ
source.
The development of the explosion pressure with time is monitored and the maximum peak pressure
Pmax and maximum rate of pressure rise is obtained after a testing over a range of dust concentrations.
The peak value of the maximum rate of pressure rise (dp/dt) max is used to calculate a dust specic
explosibility characteristic called Kst or Kmax = dp/dt max. V1/3.
As shown, dust explosion pressures can reach the same as gases/vapours and some metal dusts can
be as dangerous as Hydrogen.
Thermal Instability Testing
The Diusion Cell, Aerated Cell and Layer Tests are used to assess the thermal stability of a material in
bulk or layer form e.g. inside Dryers, etc. The cells are located in fan assisted ovens which can
be run isothermally or in ramped mode. In addition, large scale basket tests may be required to assess
the eect of scale.
A separate Combustibility Test (comprising a train of powder ≈ 200mm long, ≈ 30mm wide and ≈ 20mm
high) is used to assess the type and rate of flame/smoulder propagation along a layer of material e.g. inside ductwork, etc.
It can also be used as an indicator of UN. Div. 4.1 ‘Flammable Solid’ material for transport purposes.
Hazardous Area Classification
The dangers of siting electrical apparatus in areas where explosive mixtures of gases and air could
occur were first recognised in the mining industry early in the 20th Century.
The chemical and petrochemical industries recognised that, unlike coalmining, the occurrence
of ammable atmospheres was due to mechanical and process failures or deliberate situations created
by man.
This newspaper article covers a widespread nHexane vapour explosion through a drainage
system, although reporters referred to is as a ‘gas’ explosion.
Hazardous Zones
A fairly simple set of rules was developed dividing areas where ammable atmospheres could occur
into 3 areas of risk, based upon frequency and persistence.
This approach was taken up nationally and appeared in a British Standard Code in 1959.
Today hazardous area classication (HAC) covering natural gas installations, solvent handling, oil
pumping, etc. is a statutory requirement throughout EU Member States and under the Dangerous
Substances & Explosive Atmosphere Regulations 2002 (in the UK), in particular.
The zone denitions are as follows:-
zone 0
A place in which an explosive atmosphere ( from gas, vapour, mist or spray) is present
continuously or for long periods or frequently – e.g. solvent storage tank operating continuously above
the ash point of the liquid, etc.
zone 1
A place in which an explosive atmosphere (from gas, vapour, mist or spray) is likely to occur in normal
operation occasionally – e.g. sampling point where the liquid is above its ash point temperature, etc.
A place in which an explosive atmosphere (from gas, vapour, mist or spray) is likely to occur in normal
operation occasionally – e.g. sampling point where the liquid is above its ash point temperature, etc.
zone 2
A place in which an explosive atmosphere (from gas, vapour, mist or spray) is not likely to occur in
normal operation but, if it does occur, will persist for a short period only – e.g. leakage from gas pipework anges, ttings, etc.
Key parameters to be considered include liquid ash point, gas or vapour density, the leak or
emission rate, the prevailing level of ventilation (natural or forced), pressure, temperature, LEL,
height of release, etc.
Oddly, historically, the HAC job was often given to electrical or instrument personnel when it was (and
still is) a matter for process engineering!
A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present
continuously or for long periods or frequently – e.g. inside cyclones, hoppers, containers, pipework, etc.
A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to
occur in normal operation occasionally – e.g. vicinity of powder lling/discharge points, weigh stations,
sampling points, etc. and where dust layers occur and are likely in normal operation to give rise to an
explosive concentration.
zone 22
A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is not
likely to occur in normal operation but, if it does occur, will persist for a short period only – e.g.
outlets from cyclones, clean-side of dust lters, etc.
Gas Installations
Historically, the following probability values have been used for guidance (with gases/vapours) and
these are commonly applied to solids handling also.
| Continuous source | (> 1000 hrs.yr-1) yields a Zone 0 / 20 |
| Primary source | (10 – 1000 hrs.yr-1) yields a Zone 1 / 21 |
| Secondary source | (<10 hrs.yr-1) yields a Zone 2 / 22 |
The relevant code of practice for Gas Installations with working pressures up to 2.0 bar.g is
IGEM/UP/16 Comm. 1756
Essentially, the use of eective and appropriate ventilation is the key to precluding hazardous areas
in Boiler Houses, battery charging areas, etc. other than Zone 2 (NE) i.e. of negligible extent.
Flammable vapour atmospheres will arise when liquids are handled or processed above their ash
point temperature.
A 5K safety margin is normally applied for pure materials (solvents) when using ash point
temperatures to dene whether or not ammable vapour-air mixtures will be formed. A larger 15K
safety factor is recommended, however, for liquid/liquid or liquid/solid mixtures. For spillages,
the open cup ash point (rather than close cup) value is more appropriate.
The above discussion relates vapour pressure to temperature and in essence, assumes ‘saturated
vapour-liquid equilibrium’. Care is needed with operations which produce high shear forces on a
fluid, resulting in ‘non-equilibrium’ conditions e.g. high speed mixing, splash loading, etc.
Similarly, ammable atmospheres can arise through mist or spray formation (by mechanical means or
condensation). Fine mists can form ammable mixtures well below (may be 200K or more) the
ash point of the liquid. The potential hazard arises from leakage of uids under pressure, from
compressions joints, oil seals, etc. The use of angeguards can eliminate the hazard, however.
Dust
In the case of dusts, most sources of release in normal operation should be small, and should be
controlled at source, since the increasing concern to reduce occupational health risks from breathing in
airborne material should have reduced the number of areas where dust is released regularly into the
general atmosphere.
With regard to the extent of a hazardous area (Zone), this very much depends on how much dust
is released and whether it would be released as a heap that settled quickly on the oor or as a large
cloud. For instance, a sack that tore as someone picked it up might release (say) 5-10 kg, but most
would settle very quickly. However, a Flexible BigBag (FIBC) where the tie became loose after lifting
might release 500 kg (or more) at high level. Also, a pneumatic transfer line might release a very large
amount as a cloud.
Unlike releases involving gases and vapours, there is no simple method available of calculating how far
from the source a release of dust of a given quantity with a known pressure behind it will form a dense
cloud.
However, it is clear that if the release could produce a dense cloud (say larger than a couple of metres
from the source), a means of minimising quantity of dust that could be released and its spread should
be sought. Often, the provision of simple plastic or fabric curtains are considered to be helpful.
Predictable Hazardous Areas
Equipment which is opened up for cleaning, maintenance, etc. may give rise to explosible dust
clouds externally if material held-up on internal ledges, back-sides of doors, etc. is dislodged.
To cover this, a ‘generic’ Zone 22 area of 1.0 m extent (to solid oor level) is often proposed.
However, since the formation of such hazardous areas is predictable, it is not necessary to provide
certied equipment within the hazardous area so formed if the work is done under a STRICT permit
system i.e. a safeguard whereby uncertied (unsuitable) equipment within the vicinity has been
de-energised and isolated. The hazardous area still remains, however.
Dust Extraction
Dust extraction is important to limit fugitive emissions and it should be routinely monitored to
ensure satisfactory performance. In addition, the level of ventilation should ensure that the dust-air
concentration does not routinely exceed 25 % of the lower explosive limit LEL or minimum explosible
concentration MEC).
The ‘capture velocity’ is also important and this depends on the level of air movement in the region
of interest.
| Release type | Capture velocity |
| Still air / low momentum | 0.5 – 1.0 m.s-1 |
| Moving air / particles | 1.0 – 2.5 m.s-1 |
| Turbulent / high momentum | 2.5 – 10 m.s-1 |
Predictable Hazardous Areas
Equipment which is opened up for cleaning, maintenance, etc. may give rise to explosible dust
clouds externally if material held-up on internal ledges, back-sides of doors, etc. is dislodged.
To cover this, a ‘generic’ Zone 22 area of 1.0 m extent (to solid oor level) is often proposed.
However, since the formation of such hazardous areas is predictable, it is not necessary to provide
certied equipment within the hazardous area so formed if the work is done under a STRICT permit
system i.e. a safeguard whereby uncertied (unsuitable) equipment within the vicinity has been
de-energised and isolated. The hazardous area still remains, however.
Dust Extraction
Dust extraction is important to limit fugitive emissions and it should be routinely monitored to
ensure satisfactory performance. In addition, the level of ventilation should ensure that the dust-air
concentration does not routinely exceed 25 % of the lower explosive limit LEL or minimum explosible
concentration MEC).
The ‘capture velocity’ is also important and this depends on the level of air movement in the region
of interest.
| Release type | Capture velocity |
| Still air / low momentum | 0.5 – 1.0 m.s-1 |
| Moving air / particles | 1.0 – 2.5 m.s-1 |
| Turbulent / high momentum | 2.5 – 10 m.s-1 |
Dusts, which are not removed by mechanical extraction ventilation, settle out, at a rate depending
on properties such as particle size, into layers or accumulations and account has to be taken of the
fact that dilute or small continuous sources of release (in time) may produce a potentially
hazardous dust layer.
Principally, three risks are presented by dust layers:
Risk 1: A primary explosion within a building may raise dust layers into clouds, and cause secondary
explosions more damaging than the primary event. Dust layers should always be controlled to reduce
this risk.
Risk 2: Dust layers may be ignited by the heat flux from equipment on which the layer rests. The risk is
of re, rather than explosion, and this may be a slow process.
Risk 3: A dust layer may be raised into a cloud, ignite on a hot surface and cause an explosion. In
practice, dust cloud ignition temperatures are often much higher than layer ignition temperatures. For
example, Paper nes have a layer ignition temperature of 210 – 275 °C, but a cloud ignition temperature of > 450°C.
These risks depend on the properties of the dust and the thickness of layers, which is inuenced by
the nature of the housekeeping. The likelihood of a layer catching re should be controlled by the
correct selection of equipment and eective housekeeping.
Signage
Arrangements must be made for the display of ‘EX’ signage in appropriate locations. Additional information (wording) can be incorporated to highlight the specic hazard i.e. Explosive
Gas/Vapour/Mist Hazard or Explosible Dust Hazard
Potential Sources of Ignition
EU ‘ATmosphere EXplosif’ (ATEX) Directives require manufacturers to provide safe (certied) equipment
for use in hazardous areas under the Equipment Directive previously referred to as ATEX 95 now 114.
Similarly, users of equipment are required to assess both the likelihood of forming a hazardous area and the risks from ignition (identication and control) under the Use Directive previously referred to as
ATEX 137 now 153.
Standard BS EN 1127-1:2011
‘Explosive Atmospheres – Explosion Prevention and Protection Part 1. Basic concepts and methodology’ distinguishes between 13 types of ignition source:
| Common | Less Common |
| Mechanically generated sparks | Stray currents & cathodic protection |
| Hot surfaces (e.g.binding friction) | Electromagnetic elds (9 – 300 GHz) |
| Flames and hot gases | Electromagnetic radiation (3 x 1011 to 3 x 1015 Hz or wavelength range from 1000 µm to 0.1 µm (optical spectrum) |
| Electrical apparatus | Ionising radiation |
| Static electricity | Ultrasonics |
| Lightning | Adiabatic compression & shock waves |
| Chemical reactions (inc. spont. ignition) |
This section covers some of the more common sources of ignition. Static electricity and chemical
reaction / thermal instability will be covered in the next section.
| Equipment Category | Explosive Atmosphere | Control of ‘eective’ ignition sources |
| Cat. 1G / D | Zone 0 / 20 | No sources of ignition in normal operation, during expected malfunctions AND during rare malfunctions |
| Cat. 2G / D | Zone 1 / 21 | No sources of ignition in normal operation AND during expected malfunctions |
| Cat. 3G / D | Zone 2 / 22 | No sources of ignition in normal operation |
The table above shows the general philosophy behind matching the required level of ignition
control to the likelihood of forming a ammable mixture.
In practice, some of the above can be avoided, for example, by management procedures or by
specication of special (i.e. suitable) equipment.
However, many potential sources of ignition are present by virtue of the plant hardware, materials in
use or mode of operation and as such, are inherent to the specic process and in need of control.
Mechanical Sparks
Ignition risks can arise when two hardened materials come into contact with force resulting
glancing impact and friction sparks.
With certain materials, the impact energy can initiate an exothermic reaction (i.e. the Thermite
reaction associated with aluminium, titanium, magnesium, zirconium and their alloys), resulting in
highly incendive sparks of ‘white hot’ appearance. Thermite friction sparks (particularly with rusty steel
and aluminium) are known to be extremely energetic and capable of igniting many ammable
gases, vapours, mists and dusts. In contrast, sparks which are formed between, say, steel on steel are
much lower in temperature and energy.
Sometimes, pneumatic separators are used in the feed supply to remove ‘debris’, upstream from a
Mill, for example; a good option where metal detection may not be 100 % reliable.
In reality, the ignition risk from any form of spark depends on the sensitivity of the fuel-air mixture.
More stringent requirements are needed for Group IIB and IIC gases/vapours, because of their greater
sensitivity, compared to Group IIA materials. Research has been undertaken to evaluate the
ignition hazard with respect to airborne dusts and this, together with similar work in Germany, has led
to a better understanding of friction spark ignition capability e.g. the eect of the level of thrust at the
point of impact.
In many instances, steel-on-steel friction and grinding sparks do not give cause for concern, due
to the relatively slow rotational speeds of equipment i.e. if contact occurs at all, it is likely to
result in increased surface temperatures rather than multiple sparks.
Therefore, potential sources of ignition can be eliminated by maintaining low circumferential
(contact) velocities i.e. below 1 m.s-1.
Hot Surfaces
In accordance with BS EN 1127-1:1998 ‘Explosive Atmospheres – Explosion Prevention and
Protection’, for gases, vapours and mists, the maximum surface temperature must not exceed:-
80% of the AIT value in C (even in the case of rare malfunctions) for Category 1 equipment
(i.e. Zone 0 areas)
80% of the AIT value in C (during normal operation and in the case of rare malfunctions) for Category 2 equipment (i.e. Zone 1 areas)
100% of the AIT value in C (during normal operation) for Category 3 equipment (i.e. Zone 2 areas)
Hot surfaces can arise in many situations, both intentionally (e.g. drying operations) or spuriously
(e.g. binding friction from Screw Conveyors and similar mechanised equipment, in this case the
1m/s does not apply. Hot particles from exhausts need to be eliminated in zoned areas, also.
Control of drying temperatures is of particular importance. To eliminate hot particles from direct
red systems, it is necessary to:-
- Clean / ltered air if re-circulated
- Clean burners regularly
- Fit 3mm mesh on air inlet to restrict large
- (glowing) particles
The Dryer inlet and outlet temperatures must be restricted also. Typical values for an explosion protected Dryer are:-
50K below MIT cloud value 10-20K below Air-Over-Layer exotherm onset temp
Flames & Hot Gases
The most obvious risk of ame arises from unauthorised ‘hot-work’ and clearly procedures
should be in place to ensure that this cannot arise – a near-miss was witnessed when an operator was
sweeping dust from one side of a large sieve screen whilst welding work was being undertaken on the
opposite side.
Hot work is a well-known cause of dust explosions and this activity needs special consideration.
Instances are given in the open literature where (unknowingly) smouldering material has arisen
inside the plant item, which then resulted in ignition on start-up.
In bulk storage areas, burning embers from the engine of trucks should be precluded by means of a
certied ame arrester. Provided the necessary maintenance and inspection regimes, associated
with this device (and any other heated surfaces e.g. engine block, manifold, etc.) are adhered to, this
form of ignition hazard can be eliminated.
Electrical Equipment
Electrical apparatus used within the hazardous areas should be certied for the Gas Group (gases,
vapours and mists/sprays) and the Temperature Class (‘T’ rating).
The maximum surface temperature of equipment (‘T’ rating) is the highest temperature attained in
service under the most adverse operating conditions (but within the recognised tolerance) by
any part or surface of the equipment which would be able to produce an ignition of the surrounding
potentially explosive atmosphere.
The ‘T’ rating of equipment intended for use in ammable atmospheres and based on a max.
ambient temperature of 40 °C, is listed as follows:-
| Temperature Class | Max. Surface Temperature (C) |
| T1 | 450 |
| T2 | 300 |
| T3 | 200 |
| T4 | 135 |
| T5 | 100 |
| T6 | 85 |
The criteria for selection of equipment is :-
- less than 2/3rds MITcloud
- AND
- more than 75K below MIT5mm layer
Using ‘typical’ wood nes as an example :-
MITcloud : 470 * 2/3 = 313 ºC
MIT5mmlayer : 260 – 75 = 185 ºC
Therefore, the required Rating is T4 – T6 (for max. 5mm thickness)
It is important to use the correct Ingress Protection (IP) rating also i.e. typically IP5X or IP6X for dusts) –
the rst and second digits refer to restricting dust and water ingress, respectively.
In general terms, if electrical equipment needs cleaning more often than daily to keep dust
deposits to negligible thickness and it is not dust tight (IP6X), it should be replaced or relocated.
Overheating of dust layers on electrical equipment (e.g. motors) leading to a re is usually a slow
process. Dust layers should be removed regularly to control this problem and in any case, should not
exceed 5 mm thickness.
Lightning
If lightning strikes a ammable atmosphere, ignition will always occur. Moreover, there is also a
possibility of ignition due to the high temperature reached by lightning conductors.
Large currents ow from where the lightning strikes and these currents can produce sparks in the
vicinity of the point of impact.
Even in the absence of lightning strikes, thunderstorms can cause high induced voltages in
equipment, protective systems and components.
In practice, the probability of a lightning strike depends on the specic location of the site and a
specialist should be consulted to ascertain whether additional precautionary measures are required.
As a general precaution against external sources of ignition, all vents handling ammable gases and
vapours should be tted with a suitable ame-trap!
Static Ignition &Thermal Instability
Static electricity is part of everyday life and is generated when materials come in to contact with
each other and then separate i.e. electrostatic charging occurs at the interface. The phenomenon
is known as a ‘contact electrication’ and this gives rise to a ‘streaming’ or charging current.
Potential electrostatic charging hazards are shown above and include uid ow in pipework, drum charging operations, walking across a oor, pouring powders, lm transport rollers, spraying, etc.
The electric eld produced can give rise to dierent types of discharge, with diering energy levels. The
highest charging currents arise from the use of electrically insulating materials (e.g. PTFE lined
equipment, Poly-Tubs, plastic sheeting and insulating (low conductivity) liquids such as Toluene,
n-Hexane and many others.
Operatives isolated from earth via their footwear or the ooring can become charged unknowingly,
simply by standing within an electrostatic eld.
For example, at rest, humans are likely to be charge neutral i.e. no signicant excess positive or negative
charge.
However, if they stand next to a highly charged surface (e.g. negatively charged FIBC), the charges
on their body will ‘polarise’ i.e. the residual negative charges will be repelled and the positive charges
attracted.
Whilst in this position, if the person touches an earthed item, the free (negative) charges will ow to
earth and as he or she walks away, they will be left with net positive charge, which is a potential source
of ignition.
Charge generation per se is not the issue; the problem is the accumulation of charge because it
can create a source of ignition.
The most basic precaution against electrostatic ignition is earthing – the structure of the plant and
associated vessels/equipment are a potential source of electrostatic charge accumulation and therefore, must be earthed to avoid this risk.
All metal items and ttings (e.g. funnels, metal spirals in exible LEV trunking, etc.) should be in
good contact with each other and with earth. For example, Blo-line / Morris couplings have an integral
earthing strip (as shown LH photo) to ensure electrical continuity through the coupling and along
the pipework.
Electrical continuity checks should be made before equipment is brought into use and following any
maintenance work. Frequently, however, earthing failures are evident during audits e.g. unattached /
ungrounded metal reinforcing helices in flexible trunking, grilles in loading chutes, etc.
The requirements for resistance levels are essentially those normally used by electrical
engineers i.e. LESS THAN 10 to earth. In practice, higher resistances may be used (up to 106 Ω) but
only where the reason for the high resistance is known and controlled.
Types of Discharge
Spark: Occurs between two conductors (the plasma channel is over the entire gap)
Corona: Point discharge with no plasma channel
Brush: Occurs between a charged non-conducting surface and a (rounded) earthed conductor or electrode
Propagating Brush: Polarised insulating sheets of very high charge density
Cone: Occurs from highly charged, high resistivity granular materials
People
People who are insulated from earth (by their footwear or the ooring) can easily acquire and
retain an electrostatic charge. If the MIE of the ammable atmosphere is lower than 30 mJ,
consideration needs to be given to grounding personnel i.e. use of ‘charge dissipative’ footwear
and flooring.
In addition, clothing should be as close fitting and should not be removed or unfastened within the
hazardous area.
Insulating Plastics
Insulating plastics can present a risk of ignition in zoned areas, also. ‘Brush discharges’ with energies
up to 4 mJ can occur from charged insulating surfaces and this is well in excess of the minimum
required for ignition of low ash point solvents (e.g. Acetone, Toluene, Methanol, etc.) and sensitive
airborne powders.
Thus, the exposed surface areas of insulating surfaces need to be restricted, as shown below.
Restrictions on Surface Area (cm2) – Sheets
| Gas Type | Group IIA | Group IIB | Group IIC |
| 0 | 50 | 25 | 4 |
| 1 | 100 | 100 | 20 |
| 2 | No Limit | No Limit | No Limit |
For example, high resistivity plastics exceeding 100 sq.cm in area cannot be used with IIA or IIB solvents
in Zone 1 areas, unless it can be shown that charge generation (or incendive discharges) will not arise
even in the case of likely malfunctions. Clearly, this excludes all but the smallest of plastic items.
Thus, an electrostatic hazard assessment requires a systematic study of where (and the levels of) charge that can be generated, whether it can accumulate, the type and energy of the discharge and the ignition sensitivity of the ammable atmosphere.
Thermal Instability
General guidance for risk control is specied in DSEAR Section 6.4 (Risk Reduction) and one of the
key elements is ‘Avoid Adverse Conditions’. Essentially, in heated equipment such as Dryers, Hot
Boxes, etc.), this relates to the identication and control of areas where hot dry material can collect
(Dryer inlets/outlets, Mechanical Conveyors, Mills, etc.).
To preclude highly energetic ‘propagating brush discharges’, special types of FIBC are required. Type
A bags (which have no dissipative properties and high breakdown strengths) should NOT be used
with explosible powders.
A – no special requirements
B – breakdown FIBC wall < 4 kV
C – resistance to earth from any location < 100 M.Ω
D – interwoven conductive threads not connected together
In most cases, small scale tests such as DTA or DSC? will not be sucient; Diusion Cells, Aerated Cell or
Air Over Layer tests are preferred since the availability of air during self-heating can have a
profound eect. The key features are the onset temperature of self heating Tonset, the points at
which the exotherm becomes marked Tmarked (> 5 K/min) and rapid Trapid (> 50 K/min).
Thermal stability test results can indicate signicant self-heating hazards e.g. smouldering nests inside
bulked product, smoulders on ledges inside Dryers, etc. Some materials exhibit relatively low exotherm
onset temperatures from about 130 ºC and high peak temperature of > 700 ºC, even on the small
scale.
However, these tests are still for screening purposes because they don’t allow the eect of scale to be
determined.
Where exothermic activity near the process operating temperature is likely, further ‘Basket
Tests’ will be required to allow safe operating temperatures to be quantied. The tests are run
isothermally in wire baskets of dierent size.
The eect of scale is predicted by plotting scale (volume/surface area) against the reciprocal selfheat temperature TC (K). An example is given as follows:-
Log (V/SA) ≈ [2189.4 / Tc(K)] – 6.758
The self-heat onset temperature will increase as the surface area of the bulked material increases. For
example, if a material starts to self-heat at 93 ºC as a 1 m3 mass, thermal predictions may show that as a 50 cm thick layer, the onset would be nearer 107 ºC due to greater cooling by the larger surface area.
Fibrous Insulation
Thermal instability can also occur in brous insulation. The potential ignition / re hazard arises
from auto-oxidation of insulation – a gradual exothermic oxidation of combustible material,
accompanied by the generation of heat. Hence, auto-oxidation needs to be controlled whenever
there is a possibility of insulation being contaminated, particularly by oils at elevated
temperatures.
Auto-oxidation and escalation in temperature is exacerbated by the inherent insulation (low thermal
conductivity) properties and high surface area. Hence, it is often prudent to use a closed-cell type of
insulation on hot oil systems, particularly around joints, known leakage points, etc. since this type of
insulation is not prone to oil seepage or wetting.
NOTE: Great care is needed when removing contaminated lagging as this too can spontaneously
ignite. Oil contaminated lagging should be removed only under a HOT WORK permit.
Prevention
As mentioned in the rst ‘introductory’ section, prevention of the formation of an explosive
atmosphere, including the application of appropriate ventilation, is an acceptable approach
to control of risk.
Prevention techniques can be simple, dilution for example, equally applicable to certain types of
solvent handling operations and evaporative Ovens / Dryers as dust extraction systems.
The question is – can we under all normal and abnormal operations control (i.e. eliminate) one of
the THREE basic components for combustion?
Codes of practice, for Dryers and Ovens in which ammable substances are released, set out basic
design criteria (dilution of the vapour). This is aimed at processes where the ammable VOC
concentration exceeds 3 % of the Lower Explosive Limit.
Essentially, the ‘maximum admissible quantity of ammable substance’ needs to be determined and
controlled against the ‘minimum forced ventilation owrate’ in order to maintain conditions below the
‘maximum admissible concentration’ below the LEL.
This includes any short-term operations. Indeed, a formal HAZOP study, which examines ALL of the
conditions which can lead to the formation of extensive ammable atmospheres inside plant
equipment, should be undertaken to allow the necessary ventilation rates to be determined.
Dilution
Following on from the rst Introductory Article, water miscible solvents can be made ‘safer’ simply
by adding water, to an alcohol for example. If the maximum ambient temperature is (say) 25 ËšC, using
a 5K safety factor, concentrations up to 40 % v/v would be permissible.
Dilution of the liquid is sometimes employed where solvents are used for cleaning purposes, thereby
eliminating the formation of hazardous areas (and the potential for ignition) both inside and outside
equipment.
Ventilation
Ventilation (or rather Extraction) can be employed to dilute airborne dust, in a spray coating operation
for example.
Typically, a primary source of release will exist within the connes of the spray / extraction hood (i.e. ‘concentration gradients’ will exist between the source [spray head] and the ventilated
surroundings, allowing localised ammable atmospheres to occur) but, given the correct level of
ventilation, ammable atmospheres (hazardous areas) should not extend in to the extraction
system.
The necessary extraction rates are determined from the spray rates. For example, at a mass feed rate of 355 g.min-1 per gun at 100 % and 24 gun operation, assuming 50 % overspray, the mass feed rate to the extraction system is 256 kg.hr-1.
As a rule, the maximum concentration within the ductwork should be 25 % MEC (typically between 10
– 100 g.m-3) and in view of the fact that the airow through the extraction ductwork forms part of an
explosion safety system, this should be monitored or (at least) aorded a ow switch to indicate poor
performance i.e. reduced flow.
Ventilation also plays a key ‘preventative’ role in gas- red systems e.g. Boilers, Burners, etc. Ecient and
eective ventilation can render a Boiler House as
Zone 2 (NE) i.e. of negligible extent.
Gas detection has a role to play also, when interlocked to the supply and thereby limiting the
persistence of the hazardous area. The siting of the detector head is critical, however; governed by
neighbouring equipment and local air movement due to thermal gradients and forced or natural
ventilation.
Smoke tests (or childrens’ ‘Bubbles’ – YES, BUBBLES) can be used to check that there is free movement of air around the equipment i.e. to ensure that the proposed ventilation is eective at the point of
interest and the Zone 2 NE (negligible extent) designation can be upheld.
Inerting
Inerting is another preventative technique but here, we are controlling the Oxidant concentration rather than the fuel. The diagram (below) expresses the
ammability characteristics (or ‘envelope’) for methane, depicting lower and upper explosive limits
and the ‘nose’ of the curve at which the MOC occurs. The principle of the ammability envelope
also holds for vapours and airborne dusts i.e. as the oxygen concentration is reduced, the lower and
upper ammability limits converge to a point where ame propagation cannot be sustained.
Inert gas blanketing or purging is commonly used asthe sole basis of safety. Thus, it is imperative to
maintain the reduced oxygen level inside the processing environment whilst the potential source of ignition may be present.
It can be seen that a reduction in oxygen content does not change the lower limit value but markedly
reduces the upper limit. This is because oxygen is in excess at the lower ammable limit.
Generally, a safety factor is applied to the required oxygen level (MOC or LOC [Limiting Oxygen
Concentration]) to take account of changes in environmental conditions and also, to allow for
inaccuracies that may arise when monitoring or sampling the internal atmosphere – a safety factor
of 2.0 – 3.0% below the limiting value is recommended.
Literature information (NFPA 69 Explosion Prevention Systems Appendix C Table C-1 – National
Fire Protection Association) indicates an MOC value of 9.5 % by volume for Toluene, where nitrogen is
the purge gas. Hence, the design basis value would be a maximum of 5 % oxygen by volume under
normal running conditions with shut down at 7.5 % oxygen by volume.
It must be borne in mind that MOC values like ammability limits, are pressure and temperature
dependent. Also, gases other than nitrogen can be used for inerting purposes e.g. halogenated
compounds, water vapour, carbon dioxide, etc. and their eciency is dependent upon their gaseous
specic heat capacity.
Inert gas blanketing/purging is used in many industries as a Basis of Safety e.g. Metal Purication,
Pharmaceuticals, Coal Handling Plant (Mills), etc. Even ‘partial inerting’ has benets. This is where the
concentration of the oxidant lies above the MOC value but below the prevailing atmospheric level.
Flame propagation can still occur but at a reduced rate, depending on the extent of oxygen depletion.
In such cases, the gas/vapour-air or dust-air mixtures will lie within the ammable envelope but
at a point where the explosive limits are closing.
This inuences not only explosion severity but also ignition sensitivity – combustion through the
mixture can still occur but the explosion eects will be reduced. Ignition sensitivity will decrease also. A
reduction in oxygen content can result in marked increases in both hot surface ignition temperature
and minimum ignition energy. As discussed in previous sections, we can base safety on ‘Elimination of Ignition Sources’ IF the basis for the hazardous area classication (Zoning) is sound, ALL equipment within the Zones iscorrectly ATEX certied AND we don’t introduce ‘user’ or process intrinsic ignition sources such as static electricity, smoulders, hot surfaces, etc.
Control of Ignition Sources
We cannot stress this enough – without knowing the ignition sensitivity of the material (be it gas, vapour, mist or dust), we cannot assess the ignition potential with any accuracy.
Likewise, we need to know the extent of the hazardous Zones accurately so we can assess which
operations / equipment can give rise to the ignition sources.
Intrinsic ignition sources are those which exist solely due to the operation, for example:
- Electrostatic sparks (poly-liners, unearthed personnel, etc.)
- Thermite sparks (aluminium scoops, foil, etc.)
- Mechanical friction (screw feeders, milling,
- etc.)
- Pyrophoricity (metal powders, catalysts, etc.)
- Smouldering deposits (dryers, grinders, etc.)
- Auto-ignition (dryers, furnaces, etc.)
But the consequences can be huge if you get it wrong!
Protection
The ‘protection’ concept is based on the assumption that ignition and an explosion (gas, vapour, mist or
dust) may arise and steps need to be taken to mitigate the eects, thereby safeguarding personnel
and (as far as practicable) maintaining the integrity of the plant. The options available are dependent on the plant vessels / layout and the characteristics of the materials. There is a choice of two designs.
An Explosion Pressure Resistant Design (EPRD) doesnot allow for any deformation of the vessel whereas an Explosion Pressure Shock Resistant Design (EPSRD) does – i.e. deformation is acceptable, but not total failure.
Explosion Pressure Relief
Explosion pressure relief is perhaps the most common but essentially there is a ‘loss of containment’ i.e. process material and/or fume will be emitted from the protected vessel during the
explosion. Conversely, explosion suppression and total explosion pressure containment systems do
not give rise to a release of process material.
Venting
Explosion venting is a protective measure preventing unacceptable high explosion pressure
build-up inside vessels / enclosures. Normally explosion venting is applied such that the maximum
reduced explosion pressure (Pred,max) does not exceed the known design pressure of the vessel; the
lower the vent opening pressure (Pstat) and the larger the vent area, the lower the reduced
explosion pressure.
Moreover, ALL parts of the enclosure, including valves, access ports, ductwork, etc. exposed to the
explosion pressure, must be taken into account when estimating the design pressure of the vessel –
this is to ensure that the relief of the explosion pressure is accomplished in a controlled manner.
Venting does not prevent an explosion, it limits the explosion pressure. Hence, ame and pressure
eects outside the enclosure and ying debris must be anticipated and accounted for. To preclude this,
ameless venting devices (depicted below) may be used – this form of venting is particularly useful for
plant sited in (or close to) the middle of the work area; otherwise, long vent ducts would be required
to safely vent the explosion outside the connes of the building.
Explosion Suppression
As mentioned above, explosion suppression does provide containment of the ‘reduced explosion
pressure’ (Pred). This is achieved by detection of the incipient explosion i.e. in its early stages. Whilst the combustion is taking place (most of the time quite rapidly over a few milliseconds), once detected,
suppressant is injected in to the growing reball to quench the ame.
The predominant eect is absorption of heat, temperature reduction and stoppage of ame
transmission. Once again, ALL parts / components of the vessel must be taken into account when
estimating the design pressure.
In practice, the quantity of suppressant (number of suppressors) and their location will depend on the
violence of the explosion (Pred and Kst from dust testing), the geometry of the vessel and its design
pressure.
Explosion Pressure Containment
With the exception of some milling operations, as a safety concept, explosion pressure containment is
less common. This is due to the high design strength needed – typically of the order 8 – 10 bar.g
for dust explosion containment. Common gases and hydrocarbons have lower peak explosion pressures of about 6 – 8 bar.g.
It is important to recognise, however, that ANY explosion protection system MUST cover upstream
and downstream interconnections i.e. it is imperative to provide isolation to prevent propagation of burning particles, flame and pressure.
Isolation
Isolation can take the form of an ATEX certied rotary valve or active of passive slam-shut (rapid
action) valve, chemical extinguishing barrier, product choke, etc. as described in prEN 15089
Explosion Isolation Systems.
For complete isolation (e.g. by use of a slam-shut valve), the design pressure must be applied up to
the isolation device. Hence, any ducting or pipework up to this point would need to withstand
the maximum anticipated pressure i.e. Pred or Pmax.
A typical arrangement is shown in the schematic below. The closure time of the rapid action valve, together with the response time of the detection/control system and ame speed, denes
the required minimum distance (L) from the source of the explosion – typically L > 5000 mm.
A vented explosion must discharge to a safe area and often, this requires the use of a vent duct. The
action of venting, in most cases, will be accompanied by ejection of burned and unburned
gases and ames and measures must be taken to ensure that nearby plant and personnel will not be
at risk from the vented reball.
It is important also to note that a vent duct will increase the back-pressure during the relief process
requiring a greater pressure resistance for equipment and vessels.
Flames ejecting from a vent opening will spread in all directions but especially in the main lateral
venting direction due to inertia. Moreover, the ames will represent a thermal radiation hazard. In
certain cases, a deector plate (depicted below) can be used to limit the length of the ejected ame.
In addition, if the vent is situated in the side wall, the recoil force on the enclosure must be considered in the design. Also, as shown in the photograph above, unburned dust can be ejected ahead of the reball during venting increasing external thermal radiation and overpressure effects.
As part of the isolation concept, equipment must be shut down automatically, in the event of an
explosion, to prevent transfer of burning material, etc. With venting, this is normally achieved by
sensors tted to the vent panel. Of course, this should not result in frequent spurious shutdowns,
since some will nd ways of by-passing the problem – by means of wood and scaolding poles to keep
vent doors shut, for example, as shown. The CORRECT course of action would have been to
examine why the explosion doors keep opening! When tting explosion protection, from a process
viewpoint, it is important to think about any repercussions. One example is the use of Rotary
Valves for explosion isolation purposes as this is often contentious due to the likely ‘wear rates’ and
the need to maintain certain tolerances (in particular, the gap between the blades and the casing).
Another example is the use of Flap-Valves in dust laden ductwork. The on-going ‘user obligations’
regarding inspection and maintenance of such items is not always appreciated. For example, periodic inspection checks must be undertaken to ensure that the explosion isolation capability does not deteriorate (e.g. due to corrosion, abrasion, dust built up on the flap, dust build up inside the body of the ap valve). The positioning of explosion vents on Dust Filters is important too, to ensure that the internal filter membranes do not obstruct (compromise) the protection.
Vent panels sited close to membranes can result in an increase in the ‘reduced explosion pressure’
(Pred value) and over-pressurisation of the vessel – the filter bags can be blown out of the vent on
activation.
A further consideration is that of providing automatic re suppression since Filters can be
terminally damaged by secondary thermal stresses due to burning bags or product, following the
explosion.
Management Procedures
Do we need to take precautions, as part of our managerial responsibilities and if so, why and to what extent? Well, there may be a risk of injury or fatality (to Plant Operators or members of the public), there are Statutory Requirements (Legal Obligations) which we must adhere to and there are
consequences when things go wrong:-
- Interruption to the business
- Fines for breach in legislation
- Loss of market share
- Loss of customer condence
- Harm to corporate image
Risk Matrix
We need to ask ourselves, is the risk we have identied, high enough to justify the investment in order to reduce it to a tolerable level. To this end, a commonly used Risk Matrix is shown below.
More detailed forms of the matrix with costs, event frequencies, etc. are available but this shows the general idea. Today, risk assessments are common place and used in all walks of life, often without a proper understanding of the true meaning i.e. risk is the product of probability (of the undesired event) and consequence.
The categorization of a risk as insignicant does not necessarily mean that it will be perceived as such by
those aected; it relates rather to the action required by the decision maker.
Neither does tolerable mean acceptable – it refers to the willingness to live with a risk to secure certain
beniets and in the condience that it is being properly controlled.
To tolerate a risk means that we do not regard it as negligible or something we might ignore, but rather as something we need to keep under review, and reduce still further if, and as, we can (Hazard
Identification and Risk Assessment – Geo Wells, IChemE 1996).
Management Obligations
Management obligations are wide ranging and for that reason, they can be quite daunting. However,
some simple measures can be put in to place which not only help massively reduce risk but also do it
both eectively and economically.
For example, bearing in mind the need to control releases of the ‘hazardous material’, all equipment
must be well maintained and subject to regula inspections, in line with good engineering practice.
The use of improper seals/joints IS NOT acceptable. For example, ‘gaa tape’ should never be used on
leaking joints. Moreover, there is a tendency for this type of temporary x to become permanent.
Likewise, seals (and replacement thereof) need to be robust to reduce the extent of dust egress.
Items of plant and equipment must be sealed effectively i.e. not simply relying on gravity to hold a
door or panel shut. For example, often inspection hatches are not bolted down – although this might
be considered unnecessary for (say) granular materials, it sends the wrong message to operators
and maintenance staff (e.g. there should be a consistent approach).
Such items should be bolted shut and form an effective seal; although this may not lead to the
formation of a hazardous area per se, fugitive emissions from poorly sealed plant will give rise to
housekeeping issues.
Some inspection hatches, which by their nature are opened routinely, should be sealed and aorded a
‘quick-release’ opening mechanism e.g. sprungloaded, whilst other ‘access’ panels should be sealed
and bolted shut.
Air-jetting (on dust deposits) must not be used – this simply moves dust around in to spaces which are
even more inaccessible and it aggravates the problem. Use of dust extraction points (where provided) or vacuuming when opening up equipment should be encouraged – this will help reduce the cleaning frequency.
If extraction is provided for dust control, make sure the ‘capture’ velocities are correct for the materials in use. Also, all too often there is no indication of poor performance i.e. low ow, so t a low-ow (LF) alarm with low-low-ow (LLF) trip. If the extraction fails, you’ll know about it rather than throwing
manpower at what appears to be more and more dust deposits!
From an operational (and safety) viewpoint, Conveyors should comprise one or a combination of
the following – blockage detection, tracking sensors, motor overload protection, rotation sensors, etc.
and ALL metal items should be bonded to earth.
Procedures must be in place which ensure that Hot Work is controlled. Indeed, ALL plant personnel
must be made aware of both the potential ignition/explosion hazards associated with their plant and the means of control. To this end, formal DSEAR / ATEX Training Sessions should be conducted on a regular basis and Attendance Certicates issued.
HAZOP studies should be considered. This is a form of design review – the key features being it is a team study, it concentrates on how well the design will cope with abnormal conditions (rather than how well it will perform in normal operation) and it is systematic and detailed.
A study considers each process stream or pipeline and vessel examining for each possible cause and
consequence of a wide range of process upsets or abnormalities.
It is a very thorough but mainly qualitative approach to ‘Failure Mode and Eect Analysis’. When done
correctly, the technique is extremely powerful, often highlighting many ‘operability’ issues not just
ignition hazards.
Portable equipment is often brought in to work areas for a specic reason and with good intentions
e.g. more heat, more light, etc. However, it is essential that you assess this ‘change’ i.e. follow
your Management of Change Procedures. Assuming you have some of course!
AND IF NOT, WHY NOT?
The heat lamp may have zero consequences, but there again, it might have been brought in to
increase the evaporation rate of solvent wet material – Yes, it happens! As we’ve said, electrical and echanical equipment must be certied for the specic hazardous area AND ‘process specic’ sources such electrostatic ignition e.g. from solvent or powder transfers, etc.
Predictive Maintenance
Good engineering practices and strict maintenance regimes are vital also. Hopefully, the days of ‘if it
ain’t broke don’t x it’ are long gone; and now there is a ‘new kid’ on the block – Predictive Maintenance.
What critical items do you have and what is the best means of identifying the predicted fault?
ATEX Establishing a Basis for Safety
The potential failure of ‘process critical’ or ‘safety critical’ items should be monitored since this will
help identify problems BEFORE they arise – the key benet being that YOU shut down the plant when
it’s convenient to you rather than having an unscheduled breakdown (which is never convenient).
In the example shown above, Roller Bearing defects were identied early, by vibration monitoring.
This ebook has been developed to help you establish a simple basis of safety in your plant
and dispel some of the myths associated with process and safety risk assessments.
We hope you have found the book both informative and helpful. We are here to help you if you require
assistance and we look forward to the opportunity of working with you in the future.
Stay safe!