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 CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 48, 2016 

A publication of 

 
The Italian Association 

of Chemical Engineering 
Online at www.aidic.it/cet 

Guest Editors: Eddy de Rademaeker, Peter Schmelzer
Copyright © 2016, AIDIC Servizi S.r.l., 
ISBN 978-88-95608-39-6; ISSN 2283-9216 

Ignition Frequency in Process Risk Analysis at Solvay 

Simon M. Egan 

Process and Transport Safety Department, Solvay, 20 rue Marcel Sembat, 69190 Saint-Fons, France. 
simon-mark.egan@solvay.com 
 

Roughly half of the scenarios identified in Process Risk Analysis are fires and explosions of gases, vapors or 
combustible dusts. These involve ignition sources including electrical equipment, hot surfaces, friction, 
incandescent substances, gas compression, ionizing radiation, static electricity etc.  To judge whether the risk 
of a given scenario is acceptable or not, it is necessary to estimate the Severity of the consequences of an 
unwanted event for man and the environment, and the Probability of occurrence of the scenario. 
This paper presents the semi-quantitative method used within Solvay to estimate the frequency of all types of 
ignition sources in such scenarios and to judge whether the risk level is acceptable or not. 

1. Introduction 

1.1 The Solvay method 
Solvay has developed a method to identify risk scenarios and to evaluate the Severity, Probability and Risk 
associated with each one.  If the risk level in any scenario applying to a project is unacceptable, then it has to 
be resolved before the project can be started up.  In the case of an existing installation, the problem is flagged 
up at corporate level and has to be resolved within a one year period.  During that time temporary measures 
are taken to reduce the risk level.  The Solvay method is described elsewhere in this issue (Egan, 2016). 

1.2 The problem 
In order to judge the acceptability of risk in a scenario involving fires and gas or dust explosions, we need to 
take account of the frequency of ignition sources.  The way we do that is described in this paper. 

2. Describing the scenario 

2.1 Method 
1. We describe the scenario in chronological order, noting on separate lines, each of the Necessary and 
Sufficient causes to arrive at the unwanted event. 
2. When the phenomenon occurs outside a vessel, we define the extent of the flammable cloud.  In the case 
of a minor leak (rupture of gasket, pump seal, etc.) we refer to in house or external guidelines for sizing 
classified explosive zones. In the case of a major leak (rupture of pipework, etc.) we estimate the flow rate and 
the distance corresponding to the LEL using commercially available software. 
3. We identify the cause (or causes) of the presence of oxidant and rate its frequency. 
4. We identify the cause (or causes) of the presence of fuel within explosive limits and rate its frequency. 
5. We identify ignition sources capable of igniting the mixture in question in the explosive zone defined above. 
6. We check that the various causes are mutually independent. 
7. We assess the effects of explosion (delayed ignition scenario) and fire (immediate ignition scenario).  If the 
consequences are different then we study the scenarios separately. 
8. We assess the consequences of the scenario (s) for humans and the environment. 
9. We rate the Severity of the scenario (s). 
10. We take account of the means of prevention already in place (or already foreseen in a project). 
11. We estimate the residual risk level from the Solvay Severity-Probability-Risk matrix. 
  

                               
 
 

 

 
   

                                                  
DOI: 10.3303/CET1648049

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Please cite this article as: Egan S., 2016, Ignition frequency in process risk analysis at solvay, Chemical Engineering Transactions, 48, 289-
294  DOI:10.3303/CET1648049  

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2.2 Example 
The following scenario was identified on a steel storage tank for toluene operating at atmospheric pressure 
under nitrogen: 

• Cause 1: Plant nitrogen network failure or nitrogen reducing valve failure. During successive 
operations, the nitrogen atmosphere is replaced by air. 

• Cause 2: Presence of toluene between LEL and UEL and thus formation of a toluene / air mixture 
within explosive limits. 

• Cause 3: Ignition source due to electrical equipment fault or an grounding fault on a fixed part or an 
electrostatic brush discharge. 

• Explosion of a gaseous mixture reaching 10 bar gauge whereas the design pressure is 1 bar gauge. 
• Rupture of storage tank (5 m3). 
• Effects: blast wave and projection of fragments: 
• 140 mbar (1% lethality) at 25 m, 
• The number of people within a 25 m radius is less than 10. 
• Human impacts: irreversible severe injury of one or more operators (H) 
• Environmental impacts: internal damage (L) 

3. Rating the frequency of ignition sources 

3.1 General principle for rating ignition  
There are extreme situations in the chemical industry. For example, in a mixing tank of electrically insulating 
viscous silicone, electrostatic discharges are continuously encountered. Silicone gets into the stirrer 
mechanism, such that the stirrer shaft and blades become insulated from the ground. The movement of the 
blades against the silicone causes them to become electrostatically charged. The electrical potential of the 
blades increases until it is sufficient to cause a so-called "spark" discharge between the stirrer and the walls.  
These discharges occur continuously and can be heard from outside the vessel. On the other hand, tens of 
thousands of drums of flammable solvents, such as toluene, can be found worldwide. The vapor space of a 
toluene drum is explosive, in that the oxygen concentration is 21% and the toluene concentration is between 
explosive limits, so long as the ambient temperature is between 4°C and 38°C. However, spontaneous 
explosion of drums of toluene, while they are sealed and in good condition, is not observed. 
 
The frequency of ignition sources in the diverse and varied situations encountered in industry thus covers a 
very wide range. Between the two extremes, there is a ratio which must exceed 10

6. The rating of the 
frequency of ignition sources is a major difficulty, particularly as it is decisive for assessing the level of risk and 
thus for accident prevention.  Rating the frequency of an ignition source should be based on objective criteria. 

3.2 General principle for rating ignition sources 
The frequency of ignition in each situation encountered is rated as follows. 

Table 1:  General principle for rating ignition sources 

Description Example ATEX equivalent Frequency 
class in Solvay 
method 

Ignitions 
per 
incident 

A situation which is 
clearly a continuous 
ignition source 

Silicone mixing tank, 
Electric transformer 

 Continuous 100 

A situation which is likely 
to give rise to an ignition 
source 

Wearing insulating 
shoes 

 Very Frequent ≈10-1 

A somewhat substandard 
practice for handling 
flammable materials 

Wearing dissipative 
shoes which are not 
checked regularly 

Category 3 
equipment (suitable 
for zone 2 or 22) 

Frequent ≈10-2 

Good industrial practice 
for handling flammable 
materials 

Wearing dissipative 
shoes which are 
checked regularly 

Category 2 
equipment (suitable 
for zone 1 or 21) 

Possible ≈10-3 

Effective exclusion of 
ignition sources 

Carburettor of petrol 
car engine 

Category 1 
equipment (suitable 
for zone 0 or 20) 

Improbable ≈10-5 

  

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3.3 Gases of groups IIA, IIB and dusts 
For gases of groups IIA (e.g. methane) and IIB (e.g. ethylene) and for combustible dusts, we use the 
frequency ratings "Continuous", "Very Frequent", "Frequent" and "Possible" according to the judgment of 
the working group.  We do not use the rating “Improbable” except in some exceptional cases where we can 
justify it. 

3.4 Gases of group IIC 
For gases of group IIC (hydrogen, acetylene etc.), as these are very easy to ignite, we only use the frequency 
ratings Continuous" and "Very Frequent".  That is to say, in a risk scenario, even if everything has been 
done in terms of equipment and working practices to prevent ignition, we assume that ignition would still be 
“Very Frequent” on our scale. 

3.5 Frequency of various ignition sources 
In each scenario, examine the possible occurrence of ignition sources capable of causing the ignition of the 
mixture in question, using the tables given in the annex section to this paper.  We select the highest frequency 
applicable to the scenario in question, according to the general approach described below: 

1. Is there a continuous ignition source? If yes: frequency class: "Continuous". 
2. Otherwise, is there a Very Frequent ignition source? If yes, class: Very Frequent "VF", 
3. Otherwise, is there a Frequent ignition source? If yes, class: Frequent "F", 
4. Otherwise, does the scenario correspond to good industrial practice? If yes, frequency Possible "P". 
5. Otherwise, if the scenario is not in line with good industrial practice, there is necessarily an ignition 

source with a frequency greater than Possible "P". 

4. Examples 

4.1 Storage tank for a flammable liquid 
Let us go back to our first example, a scenario of gas phase explosion inside a storage tank for a flammable 
solvent, such as toluene, which is normally under nitrogen and where good industrial practice has been 
followed to prevent ignition.  The first cause, the presence of oxidant, is linked to the failure of the plant 
nitrogen network or failure of the control loop for the nitrogen inlet valve.  We would rate this as “Frequent”, 
meaning between once every ten years and once every year.  We rate the second cause, the presence of fuel 
within explosive limits, as Continuous in this case.  If we are applying good industrial practice to prevent 
ignition sources inside the tank, we can rate the frequency of ignition as “Possible”.  The explosion of the 
storage tank is estimated to lead to an overpressure level of 140 mbar, corresponding to 1 % lethal effects, at 
a distance of 25 m.  The number of people inside this distance is less than 10, so the Severity is rated “High”.  
The combination of one “Frequent” cause and one “Possible” cause leads to a probability level of 3 (once in 
ten thousand years).  From the Solvay risk matrix this corresponds to a level 3 or “acceptable” risk.  Indeed 
this type of situation is generally considered to be acceptable in the industry. 

4.2 Storage tank for a heavy petroleum fraction 
A heavy petroleum fraction is stored in a carbon steel tank under nitrogen.  Now this type of material is often 
of a rather variable composition and usually contains small amounts of dissolved light hydrocarbons (butane, 
etc.) and also hydrogen sulfide.  Over a period of years a reaction occurs between hydrogen sulfide in the gas 
phase and rust on the surface of the carbon steel, forming iron sulfide in a finely divided form.  An obvious risk 
scenario is the loss of the nitrogen atmosphere, caused by the failure of the plant nitrogen network or failure of 
a control loop for the nitrogen inlet valve.  When material is pumped out of the tank, air is drawn in.  A 
flammable atmosphere is likely to be formed, because of the presence of light hydrocarbons.  And when the 
finely divided iron sulfide is exposed to air it becomes incandescent and causes ignition of the gaseous 
mixture inside the tank.  For the sake of simplicity we will assume that the tank is of the same size as in the 
first example and that the Severity of the explosion is “High”.  Only one cause is required: failure of the plant 
nitrogen network or failure of a control loop for the nitrogen inlet valve, which is rated as “Frequent”.  This 
corresponds to a Probability of 1-2 (about once every ten years).  The Severity-Probability-Risk matrix 
indicates a Risk level of 1 i.e. Unacceptable.  Indeed it is considered bad industrial practice to store heavy 
petroleum fractions under nitrogen in carbon steel tanks. 

4.3 Storage tank for recovered sulfuric acid 
Recovered sulfuric acid, derived from refinery operations, is stored in a tank at atmospheric pressure.  It gives 
off light hydrocarbons, which are diluted with nitrogen and fed to a VOC treatment unit, containing a hot 
ceramic bed.  The level of oxygen is monitored by an in-line analyzer.  The nitrogen flow is controlled to give 
an oxygen level of 3 % (the Limiting Oxygen Concentration is estimated at 8 %).  An obvious scenario is the 
failure of the analyzer, such that the nitrogen inlet valve closes.  The gas mixture sent to the VOC treatment 

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unit is within explosive limits and ignites on the hot ceramic bed in the VOC treatment unit.  The flame blows 
back to the storage tank which then explodes.  The tank is much larger than in the previous examples, so the 
distance to 140 mbar (1 % lethal effects) is further.  However, as it is situated on a tank farm, the number of 
people in this zone is still below 10 and the Severity is still rated “High”.  We will first assume that there are no 
safeguards. Only one cause is required, the failure of a control loop, which is rated “Frequent”, leading to 
Probability 1-2 and Risk 1 (unacceptable).  Indeed this situation would not be accepted in the industry.  The 
minimum requirement would be one safeguard consisting of a flame arrestor (this stops the flame going back, 
at least for a short period) plus an independent oxygen analyzer with a Safety Instrumented Function of SIL 1 
which closes a block valve on the line (this acts more slowly, but once the block valve is closed it stops the 
scenario definitively).  If this safeguard is installed, maintained and tested regularly it reduces the probability 
by a factor of 10 to level 2 (once every hundred years) and the Risk level to 2 (intermediate). 

5. Conclusions 

Solvay has developed a method for quoting the frequency of a wide range of ignition sources on a semi-
quantitative scale.  In this paper we have applied it to typical industrial situations and have demonstrated that 
the conclusions are in line with generally accepted ideas about good industrial practice for prevention of 
explosions and fires. 

Reference 

Egan S.M., 2016, Process Risk Analysis within Solvay, Chemical Engineering Transactions, Vol. 48.(on 
Press) 

Annex: tables of ignition source frequency 

The tables of ignition frequency are presented below in the following order: 
• tables 2 and 3: Continuous ignition sources respectively inside and outside of vessels, 
• tables 4 and 5: Very Frequent ignition sources respectively inside and outside of vessels, 
• tables 6 and 7: Frequent ignition sources respectively inside and outside of vessels. 

Table 2:  Continuous ignition sources found inside vessels 

Description Example Remarks 
Naked flame In line burner of off 

gases 
If a flammable gas mixture is sent to an in-line 
burner then flash-back is expected 

Hot surface 
T > AIT 

VOC treatment bed Hot ceramic bed of a Volatile Organic Compound 
(VOC) heat treatment unit 

Incandescent solids Activated Raney nickel 
Ferrous sulphide 

Raney nickel or finely divided FeS becomes 
incandescent when placed in contact with air. 

Brush discharge 
E ≤ 3 mJ - only 
incendive to gases 

Filling of vessel with 
liquid 

Spray filling  
Oil containing dispersed water 
Transfer rate > 10 m/s 

Idem. Electrostatic filter The electric field in this type of filter is expected to 
cause electrostatic brush discharges 

Idem. Filling of vessel with 
powder 

Powder having resistivity > 109 Ω.m. 
Transfer rate > 10 m/s. 

Cone discharge 
E = 10 to 100 mJ 

Filling of vessel with 
insulating powder 

Powder having resistivity > 1012 Ω.m. 
Transfer rate > 10 m/s. 

Table 3:  Continuous ignition sources found outside vessels 

Description Example Remarks 
Friction spark Vessel rupture Group IIC gases (H2, C2H2, CS2, etc.) 
Naked flame Flare stack  
Hot surface 
T > AIT 

Ordinary vehicle If the explosive zone created in the event of 
leakage is capable of exceeding the plant limits, 
the ignition source should be considered to be 
continuous. 

Electric arc 
 

Non-classified 
electrical equipment 

Group IIC gases (H2, C2H2, CS2, etc.) 

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Table 4:  Very frequent ignition sources found inside vessels 

Description Example Remarks 
Friction spark Ordinary shovel to 

unload solid from a 
filter 

Incident of ignition of cyclohexane vapours when 
discharging a filter by hand. 

Hot surface: 
AIT > T > 0.9 AIT 

Heating surface above 
the liquid level 

Hot oil jacket or high pressure steam coil 

Activated charcoal bed 
(absorber, VOC 
treatment, etc.). 

Activated charcoal 
beds are very 
frequently the site of 
exothermal 
decomposition or 
oxidation reactions. 

Activated charcoal bed (absorber, VOC treatment, 
etc.). 

Spark discharges 
E = 0.5 CV2 

Conductive item, with 
no reliable grounding 
or bonding, used with 
insulating or dissipative 
solid or liquid 

A loose metal grid in the charging chute above a 
hopper for an insulating or dissipative powder 
Loose float on a level gauge in a stock tank for an 
insulating or dissipative liquid 

Corona discharge 
E = 0.03 mJ 

Sharp conductive item 
in contact with 
insulating or dissipating 
solid or liquid 

Corona discharges can never be ruled out in an 
industrial facility. 

Brush discharge 
E ≤ 3 mJ - only 
incendive to gases 

Filling of a vessel with 
a liquid 

Spray filling 
Liquid having resistivity > 108 Ω.m 
Transfer rate > limit IEC 60079-32-1. 

Idem. Filling of a vessel with 
a powder 

Powder having resistivity > 109 Ω.m. 
Transfer rate 1 to 9 m/s. 

Propagating brush 
discharge 
E ≤ 1000 mJ 

Emptying of insulating 
powder from type A 
super sack 

Powder having resistivity > 109 Ω.m 

Idem. Crystallisation in 
enamelled steel vessel. 

Crystallisation of a solid from an insulating solvent 
(hexane) in a glass lined steel vessel. 

Cone discharge 
E = 10 to 100 mJ 

Filling of a vessel with 
a powder 

Powder having resistivity > 1012 Ω.m. 
Transfer rate > 1 m/s. 

Table 5:  Very frequent ignition sources found outside vessels 

Description Example Remarks 
Friction spark Sudden vessel rupture Group IIA or IIB gases 

Vessel designed for pressure 
Hot surface: 
AIT > T > 0.9 AIT 

Ordinary vehicle on 
unrestricted roads 
inside plant. 

In some cases it could be appropriate to classify 
the frequency as “Continuous”. 

Electric arc 
Group IIA & IIB gases, 
dusts 

Non-classified electric 
equipment 

Whenever an ordinary switch is operated, an 
electric arc is produced inside it. 

Electric arc 
Group IIC gases 

ATEX category 1, 2 or 
3 electric equipment 

Group IIC gases are very easy to ignite 

Spark discharge 
E ≤ 1000 mJ 

Conductive item, with 
no reliable grounding 
or bonding 

Metal valve on rubber transfer line. 

Spark discharge 
E = 10 mJ 

Non-grounded operator Operator wearing insulating shoes or walking on a 
floor with an insulating surface (for example acid-
proof) or with overshoes in a “white room”. 

Brush discharge 
E ≤ 3 mJ - only 
incendive to gases 

Emptying of insulating 
powder from type A or 
B super sack 

The discharge occurs between the outer surface of 
the super sack and a nearby metal object. 

Propagating brush 
discharge 
E ≤ 1000 mJ 

Emptying of insulating 
powder from type A 
super sack 

Powder of resistivity > 109 Ω.m 

 
  

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Table 6:  Frequent ignition sources found inside vessels 

Description Example Remarks 
Friction spark Stirrer with 

circumferential rate of 
> 10 m/s 

Explosion of dust encountered on powder mixing 
tank. 

Hot surface: 
0.9 AIT > T > 0.8 AIT 

Heating surface above 
the liquid level 

Hot oil jacket or high pressure steam coil 

Electromagnetic radiation Category 3 (zone 2) 
invasive probe 

Category 3 according to ATEX standards 

Spark discharges 
E = 0.5 CV2 

Conductive item, with 
no reliable grounding 
or bonding, used with a 
conductive solid or 
liquid 

A loose metal grid in the charging chute above a 
hopper for a conductive powder 
Loose float on a level gauge in a stock tank for an 
conductive liquid 

Idem. Earthing fault on 
moving part 

Dust filter with automatic filter cake removal, using 
compressed air. The filter cloth is supported by 
metal frames which are mobile and have to be 
electrically bonded to the filter casing. 

Brush discharge 
E ≤ 3 mJ - only 
incendive to gases 

Filling of a vessel with 
a liquid 

Dip pipe 
Liquid having resistivity > 108 Ω.m 
Transfer rate > limit IEC 60079-32-1 

Idem. Filling of a vessel with 
a powder 

Powder having resistivity > 109 Ω.m. 
Transfer rate ≤ 1 m/s. 

Idem. Idem. Powder having resistivity between 108 and 109 Ω.m 
and transfer rate of 1 to 9 m/s. 

Idem. Idem. Powder having resistivity < 108 Ω.m 
Transfer rate > 10 m/s 

Propagating brush 
discharge 
E ≤ 1000 mJ 

Filling of type A super 
sack 

Powder having resistivity > 109 Ω.m 

Cone discharge 
E = 10 to 100 mJ 

Filling of a vessel with 
a powder 

Powder having resistivity > 1012 Ω.m. 
Transfer rate ≤ 1 m/s. 

Idem. Idem. Powder having resistivity between 109 and 1012 
Ω.m and transfer rate > 1 m/s. 

 

Table 7:  Frequent ignition sources found outside vessels 

Description Example Remarks 
Friction spark 
Group IIA or IIB gases 

Ductile vessel rupture Atmospheric storage tank 

Hot surface: 
0.9 AIT > T > 0.8 AIT 

Steam line or hot wall 
of furnace 

 

Electric arc Category 3 ATEX 
electric equipment 

Group IIA or IIB gases; dusts 
Category 3 corresponds to zone 2. 

Spark discharge 
E ≤ 1000 mJ 

Filling or emptying of a 
metal drum, grounded 
by a mobile line 
attached by the 
operator 

The frequency of forgetting to attach a grounding 
line to the drum may be taken to be between 1/100 
and 1/1000 times the frequency of the operation, 
which corresponds to Frequent on the Solvay 
scale in most cases. 

Brush discharge 
E ≤ 3 mJ - only 
incendive to gases 

Operator Operator wearing clothing with < 65 % cotton or 
removing clothing in explosive zone. 

Propagating brush 
discharge 
E ≤ 1000 mJ 

Filling of type A super 
sack 

Powder having resistivity > 109 Ω.m 

 

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