Dust - Xxiii Paper 10
Dust - Xxiii Paper 10
Dust - Xxiii Paper 10
NEW ENGINEERING GUIDELINE (VDI 2263 PART 7) ON DUST FIRES AND EXPLOSIONS
PROTECTION IN SPRAY DRYING AND INTEGRATED EQUIPMENT
Norbert Jaeger
Syngenta Crop Protection Munchwilen AG., Breitenloh 5, CH 4333 Munchwilen, Switzerland
The new engineering guideline was published in 2011 and describes the state of the art in regards to
fire and explosion protection measures in spray dryers where, during specified normal use, combus-
tible dust/air mixtures, vapor/air-mixtures or hybrid mixtures occur or maybe formed.
The paper will outline the contents of this new engineering guideline in detail. Special focus will
be given to the area of Fire and Explosion risks associated with spray drying operations. The rel-
evant safety characteristics, necessary for a risk assessment, the zoning and consideration of poten-
tial ignition sources will be explained in detail. The final part of paper describes equipment specific
protective measures to afford protection against fire and explosion hazards.
64
SYMPOSIUM SERIES NO. 158 Hazards XXIII # 2012 IChemE
Figure 1. Spray dryer using a vertical airflow, with cone-bottom drying chamber
The less compact a deposited combustible dust is, the . Evaluate the risk.
more violent the combustion reaction will occur after . Consider the measures for reducing the risk.
ignition. Most dusts will combust fairly slow when depos-
The analysis must cover all parts/installation of the
ited. Glowing and smoldering combustion are characteristic.
spray dryer as well as protective systems and components.
In spray dryers, the airflow will accelerate combustion up to
The risk assessment shall take into account:
flaming. Raised dust/air mixtures will combust at such an
extreme rate that an explosion is most likely to occur. . spray dryers, the protective systems and the components
A fire in a spray dryer can be the immediate conse- proper
quence of an explosion, although the main causes are . interaction between the spray dryers, the protective
other ignition sources. On the other hand, an explosion systems and the components as well as the substances
can also occur during a fire due to fire gases or because of handled
dust being raised. . the operations taking place in the spray dryers
In contrast with the EU Directives and the pertinent . interactions between the individual processes in the
harmonized European standards, this VDI guideline various parts of the spray dryers, the protective
addresses not only the manufacturers but the users as well. systems and components
The fire and explosion safety of an installation is also deter- . environment of the spray dryers and potential inter-
mined by risk factors at the installation site and, particularly, actions with neighboring processes.
by the properties of the product(s) processed in each case.
During the planning phase the manufacturer and user must
agree on the design of the safety equipment. The user
shall then ensure, later on, that only such products are pro- 3.1 GUIDANCE FOR MANUFACTURER/USERS
cessed in the context of specified normal use as per the It is the obligation of the manufacturer to conduct a risk
design of the spray dryer. assessment to identify all hazards associated with their
A risk assessment of each individual case has to be spray dryer. They shall design and construct the spray
carried out and should contain the following elements: dryer taking into account the results of their analysis.
Safety characteristics will help to identify the hazards The manufacturer shall carry out the risk assessment
by indicating whether the substances are combustible and on the basis of the intended use also to be specified by them-
how susceptible to ignition they are. selves, i.e. that use for which they declare the spray dryer to
be suited. They shall furthermore check whether the entire
. Determine whether potentially explosive atmospheres spray dryer or only individual systems of the installation,
must be expected to form and determine their likelihood such as the drying system, or individual equipment, such
of occurrence and expected volume. as fans, fall within the scope of the EU Directive 94/9/
. Determine the presence of any ignition sources capable EC/15/. If so, their equipment shall fulfill the basic require-
of igniting combustible substances or potentially explo- ments as well as the supplementary requirements applicable
sive atmospheres, including the likelihood that these to equipment of the categories 1, 2 or 3, depending on the
ignition sources will be present. frequency and likelihood of occurrence of the potentially
. Determine the potential effects of a fire or explosion. explosive atmosphere.
Figure 2. Spray dryer using a vertical or rotating airflow, cone-bottom drying chamber with extract-air outlet in ceiling of drying
chamber
65
SYMPOSIUM SERIES NO. 158 Hazards XXIII # 2012 IChemE
On the basis of this information, the user shall then The following safety characteristics should be known
conduct their own installation-specific risk assessment for drying safety:
taking into account the same elements as mentioned
above. They have to document the result in an explosion Assessing combustion characteristics
protection document. . determination of the combustion class at 208C and at an
The user of a spray dryers is responsible for determin- elevated temperature, usually 1008C
ing which areas of the spray dryer involve explosion
hazards, taking into account the design data supplied by Assessing thermal stability
the manufacturer of the installation, the safety character- . exothermal-decomposition test in an open cup (accord-
istics of the products to be processed and the operating ing to Lutolf ), including combustibility test of the
conditions. decomposition gases and measurement of the volume
The user is also responsible for zoning the environ- of decomposition gases
ment of the spray dryer. To this end, the manufacturer . test for self-ignition in the airflow (according to Grewer)
shall make available data of the potential emissions of the
spray dryer. In addition to these potential emissions of the The temperature range over which these tests are
spray dryer, the user shall also consider other emission carried out depends on the selected drying technique, the
sources that may lead to the occurrence of potentially explo- type of dryer and the users requested drying conditions.
sive atmospheres in the area to be assessed.
Assessing explosion hazard
. minimum ignition temperature of a dust cloud
. minimum ignition energy
3.2 FIRE HAZARDS
. lower explosion limit
Spray dryers processing combustible products as specified
in DIN EN 1127-1/16/ must be assumed to create a fire
hazard as a matter of principle. The fire hazard depends Additionally, where explosion protection by design is
on the quantity of product and its combustion character- required
istics. Fires can occur as a consequence of an explosion . test of the maximum explosion overpressure
or, as is the general rule, independently of explosions. . test of the maximum pressure rise over time
66
SYMPOSIUM SERIES NO. 158 Hazards XXIII # 2012 IChemE
Hot surfaces Glowing nest, friction of Minimum ignition temperature of a Layer thickness, size and shape
rotating parts dust cloud, minimum ignition of the hot surface, grain size
temperature of a dust layer,
combustion class
Flames and hot gases Excessive air inlet temperature, Minimum ignition temperature of a Solvent content, combustible
(including hot direct heating dust cloud, self-ignition smoldering gases
particles) temperature
Mechanically Disk rupture Minimum ignition energy, minimum Temperature, solvent content,
generated sparks ignition temperature of a dust cloud combustible smoldering
Static Electricity Hazardous electrostatic charges Minimum ignition energy gases, humidity, grain size
in components isolating
products
Exothermal reaction Excessive air inlet temperature, Minimum ignition temperature of a Temperature, layer thickness,
including self- deposits on surfaces dust layer, combustion class, size and shape of dust
ignition decomposition temperature accumulation, grain size
concentration. Dust whirled up from deposits may give rise . the volume of product supplied to the installation via the
to explosive dust/air mixtures, posing serious hazards if fines recycling system and the additives feed system
effective ignition sources are present. These hazards . the lower explosion limit (LEL) of the dust, taking into
depend on the frequency and duration of explosive atmos- account the influence of the air temperature on the LEL
phere. Depending on the likelihood of occurrence and the . the distribution of the product in the air streams within
duration of explosive atmosphere, areas are classified into the spray dryer
zones 20, 21 and 22. Appropriate explosion-protection . the air routing within the drying chamber (diverting the
measures shall be selected according to this classification. extract air towards the ceiling of the drying chamber
In spray dryers, explosive dust/air mixtures can occur increases the dust concentration in the drying chamber)
in the drying system, the extract-air system, the internal . the shape of the drying chamber (in the conical part of
powder handling system, the final-drying/cooling system, the drying chamber, the dust concentration will increase
the fines recycling system and the additives feed system. if the extract air is discharged upstream of the cone
Where combustible liquids are used, explosive gas/
air mixtures can also occur in the feed system. Moreover, Zoning of the spray dryer environment must take into
the possibility of hybrid mixtures occurring in the other account the location of the installation. Provided that the
systems cannot be excluded in this case. The presence of spray dryer operating in negative-pressure mode is suffi-
an explosive dust/air mixture in a spray dryer depends on ciently dust-tight, the area around the installation would
not have to be considered at risk of explosion unless other
. the volume of product and the volume flow rate of the air influencing factors at or near the installation site call for
in which the product is dried the classification into a zone. As a rule, increased hygiene
Limiting the concentration Lower Explosion Limit As a rule, the protective measures limiting the
concentration cannot be realized in dust-
carrying parts of a spray dryer
Inerting Limiting oxygen concentration Tightness of installation, temperature, solvent
vapours, combustible decomposition gases
Reducing the release of combustible Lower Explosion Limit Tightness of installation, positive or negative-
substances into the environment of pressure mode
the installation
Avoiding dust deposits inside and Lower Explosion Limit Optimisation of air routing, preventive measures
outside the installation such as air brooms, beaters, etc.; tightness of
installation, cleaning activities
67
SYMPOSIUM SERIES NO. 158 Hazards XXIII # 2012 IChemE
requirements apply in the food-processing industry. Thanks conditions. If appropriate safety measures are not
to the mandatory cleaning management in this case, no implemented then there is a serious risk of damage/destruc-
hazardous dust deposits are to be expected here. tion of equipment and potential for injury or fatalities to per-
The following example exclusively deals with hazar- sonnel in the vicinity of the operation. To assess the
dous explosive atmospheres consisting of dust/air mixtures. likelihood and the severity of an explosion/fire a risk assess-
Where combustible solvents are used, a separate assessment ment has to be done. Based on the likelihood of occurrence
will have to be conducted to clarify whether hybrid mixtures determined and the estimation of possible effects of an
can occur in the installation. It shows a spray dryer using a explosion/fire measures must be defined and applied to
vertical airflow (Figure 3). reduce the risk to a tolerable level.
The manufacturer/user should plan explosion protec-
tion measures as summarized below to reduce the risk of an
3.6 CONSIDERATION OF IGNITION SOURCES explosion/fire to a tolerable risk:
As part of the risk assessment process, all potential ignition
sources must be analyzed with regard to the ignition sensi- . Explosions
W Avoidance of explosible atmospheres
tivity of the explosive atmosphere and the likelihood of
W Avoidance of effective ignition sources
occurrence.
W Limiting the effects of explosions through explosion
The analysis should not just cover the normal oper-
ation cases but should also take process related deviations protection by design (construction explosion protec-
into consideration. tion measures)
The risk assessment should include equipment . Fires
W Avoidance of effective ignition sources
inherent ignition sources and those which are introduced
W Limiting the effects of fires
or acting from outside as well as ignition sources arising
from the dust. The following table (Table 4) outlines the Table 5 will summarize typical examples of measures
different type of ignition sources linked to a spray dryer. applicable for a spray dryer.
Figure 3. Spray dryer using a vertical airflow, with cone-bottom drying chamber
68
SYMPOSIUM SERIES NO. 158 Hazards XXIII # 2012 IChemE
within a written work instruction. In defining fire protective damage shows, that due to operational requirements,
measures it must be ensured that not just the normal oper- reliable fire prevention cannot always be ensured. Damage
ation but also start-up/shut-down phases, maintenance and control measures within the spray dryer are therefore
standstill phases, are included into the decision process. required. It is crucial to detect the start of a fire as soon as
In the event of a fire the following actions should be possible to ensure that within a sufficient time period
carried out: counter measures are activated to fight the fire. Options
for fire detection within spray dryers are:
. Always shut off the heat supply to the spray dryer
. In case of combustible dusts, the fire extinguishing
agents shall be applied in such a manner as to prevent . Maximum Temperature detectors
dusts from being raises (explosion hazard) . Differential detectors
. Shut off the product feed . Temperature measurements
. Shut-off the supply and extract air-fans . Radiation detectors
. Gas monitoring
The prevention of fire is an essential part of preven- . Smoke-gas density meters
tive fire protection in spray dryers. Experience with . Aspiration fire detectors
69
SYMPOSIUM SERIES NO. 158 Hazards XXIII # 2012 IChemE
70