Introduction

Chemical and processing units are at risk of fire and explosion due to a variety of reasons including fire hazards, chemical reactivity, and leakage of materials. Catastrophic events, production cessation, and damage to the equipment and organizational investments may occur due to the lack of accurate recognition and assessment of these hazards. The accidents that occurred in the Flixborough chemical complex (England), Pasadena chemical complex (USA), Mexico City LPG Terminal, Feyzin Refinery (France), and Piper Alpha Oil Production Platform can be given as examples of these catastrophic events. Therefore, the mentioned hazards should be identified, assessed, and controlled to ensure the security of the processing industries (1).

Towards this goal, the processes of recognition and risk assessment of fire and explosion hazards have been performed through different methods. The occurrence of various accidents induced by fire and explosion hazards indicates that the classical techniques of risk assessment are not effective enough due to the unspecialized point of view to the risk assessment processes. Thus, accurate and dexterous methods are* required to identify and assess fire and explosion risks from a specialized viewpoint. Dow’s Fire and Explosion Index (F&EI) has been recently developed to identify and investigate fire and explosion hazards. This index (which does not require high levels of specialty and details) calculates the general risks of processing units through a simple and comprehensive method.

This method is based on historical loss data, the energy potential of materials, and the extent to which loss prevention practices are currently applied. Dow uses numerical values of hazard factors associated with different material and process characteristics to determine fire and explosion hazards in a step-by-step objective evaluation. Therefore, incorporation of such a method (especially for recognizing the critical points) is indispensable due to its key advantages including economic identification, saving time and concentration on the fire and explosion control activities in important and critical sections. In addition to the investigation and quantification of total effective parameters in fire and explosion occurrence, Dow’s index can efficiently assess other useful information such as the maximum amount of potential damage, maximum days of production cessation in probable explosion and fire (2, 3). The main aim of this method was not to classify the facilities into safe and unsafe categories; however, a relative ranking of hazards and risks in an organization can be provided (4).

The F&EI was designed by Dow and the American Institute of Chemical Engineers (AIChE) in 1967. This method has since been revised six times. Its last revision (7^th edition) was published by Suardin (5). Etowa et al. developed a computer program based on this method to automate F&EI calculation (6). Different researches have been conducted on the incorporation of this method in different applications. Etowa et al. employed this method to investigate the inherent safety of reservoirs used for storing methyl isocyanate(6). Rigas et al. performed a comprehensive study for the safety analysis of a new production line in a pesticide factory in Northern Italy using Dow’s method(7) In this regard, Suardin (5) and Hendershot(8) can also be studied.

Moreover, Ahmadi et al. conducted a study for the rational ranking of fire and explosion hazards in a petrochemical industry (9). The quantitative determination of fire and explosion risk in a processing unit was also performed by Ahmadi et al. (10).

The main aims of the present study were to identify fire and explosion hazards and assess the induced risk in a processing company. Other goals of the study were to predict the maximum probable damage and determine the maximum number of days of production cessation using Dow’s F&EI, and to present appropriate control guidelines.

Material and Methods

The present qualitative case study was performed on a processing company (in Southern Iran) in 2015 using Dow’s F&EI. Dow’s index was first introduced in 1964 by a chemical materials production company named Dow. It has since then been revised 6 times and the last version was presented in 1994. Dow’s F&EI (as one of the specific and useful methods for risk assessment and evaluation of the damage induced by fire and explosion in processing industries) has provided an appropriate framework for identification and assessment of fire and explosion damages. It has also provided effective ways for controlling of the identified hazards. This method calculates the risks of fire and explosion hazard in processing units in a simple, fast, and comprehensive way. Moreover, it does not require high level of specialty and process details(11, 12).

The implementation process of the present study is defined in the following stages.

Preliminary data collection: In the first stage, the required information were obtained for coordinates of units’ installation and location, equipment, and the main piping routes and cables by investigation of the plot plan. In addition, the required information on the schematic view of the studied system, the flow between the principal system elements, and the basic design, such as quantity and quality of the utilized materials in the processes, were collected by investigating the process flow diagrams (PFDs). The required detailed information on the relationship between the parts, machines, valves, fittings, and other mechanical parts were determined using piping and instrumentation diagrams/drawings (P&IDs) (3).

Processing units: After the preliminary data collection and familiarization with the processes, the studied company was classified into processing units. In this study, the processing unit was defined as an element from the processing equipment that could be investigated as an independent system such as reactor, distillation column, absorption tower, compressor, pump, furnace, and reservoir. Any identified processing unit was then accurately investigated in terms of stock materials, the materials’ potential chemical energy, operational conditions, records of past damages, and its potential ability to stop the production. The effects of the mentioned parameters were investigated on the whole process regarding fire and explosion to select those processing units that had severe adverse effects on the process (2, 3).

Material factor (MF): In this stage, the quantitative and qualitative characteristics of the chemicals were determined, and based on that, the material factors were calculated for each processing unit. Material factor can be defined as the material’s emission intensity and release of potential energy which may be calculated by considering its flammability and
reactivity. The material factor usually ranges from 1 to 40 and can be determined based on standards NFPA-325M and NFPA-49 considering material flammability (NF) and reactivity (NR).

The calculated parameter is the base factor representing the hazard level at surrounding environment’s temperature and pressure conditions. The material factors were revised because the environmental conditions differed from the processing conditions in the studied units. Since, several materials were usually used in each processing unit, the maximum values between the calculated material factors were considered according to the weight percentage of the material(3, 11).

Process general hazards factor: In this stage, the process’s general hazards (which are generally classified into 6 main categories) were identified. Since existence of these hazards in processing units could induce the risk of fire and explosion, a specific penalty value was assigned for each of the identified hazards according to table 1. Clearly, the higher levels of hazards induced received higher penalty. Furthermore, the penalty value was ignored if there was no hazard. Finally, the total values of the assigned penalties were calculated for the identified hazards and process general hazards factor (F₁) was obtained for the processing units by addition of 1 to its value (2, 3).

Table 1: General process hazards and corresponding penalties

Penalty value	Description	General process hazards (F₁or GPHs)
0.30	Mild reactions such as hydrogenation, hydrolysis, isomerization, sulfating, and neutralization	Exothermic chemical processes
0.50	Moderate reactions such as alkylation, esterification, oxidation, polymerization, condensation, and incremental reactions
1.00	Severe reactions within which the control of reaction conditions was difficult and critical such as halogenation
1.25	Sensitive exothermic chemical processes such as nitration
0.20	Endothermic chemical reactions that occurred in the reactor	Endothermic chemical processes
0.40	Endothermic chemical reactions that occurred in the reactors and their energy sources were provided from combustion of solid, liquid, and gas fuels such as lime production (calcination) and materials pyrolysis induced by direct contact with fire	Endothermic chemical processes
0.50	Performance of loading and unloading of grade one flammable liquids and liquid pressurized gases (LPG) in one way in a continuous or discontinuous way	Manual handling, transportation and material warehousing
0.50	Those processes within which the detonative mixes may appear during material addition (induced by contact with air) or other reactivity hazards may be revealed such as centrifuge, discontinuous or interrupted reactions, and modular mixing.
The following penalties have been applied according to the materials in cases in which they were stored in roofed warehouses or outdoor environment (unroofed warehouses).
0.85	Flammable gases or liquids with NF = 3 or 4
0.65	Volatile solids with NF = 3
0.40	Volatile solids with NF = 2
0.65	Flammable liquids with 140 ºF/60 ºC < FP_closed_cup > 100 ºF/37.8 ºC
Note: The penalty value was increased by 0.2 if the mentioned materials were stored in racks lacking rack sprinkler.
0.50	In cases in which the filters or dust collectors were in a closed area	Enclosed/closed or internal processing units
0.30	Processes within which flammable liquids were incorporated in temperatures higher than their flash points
0.45	In cases in which the flammable liquid mass was higher than 10 Mlb
0.60	Processes within which LPG or flammable liquids were incorporated in temperatures higher than their boiling point
0.90	In cases in which the liquid weight was more than 10000 lb (equal to 1000 American gallons)
Note: The penalty values were decreased up to 50% if appropriate ventilation systems were installed for the above mentioned cases.
0.35	In cases of inappropriate accessibility to the processing regions with area of more than 10000 ft²(925 m²)	Accessibility
0.35	In case of inappropriate accessibility to the warehouses with area of more than 25000 ft² (2312 m²)	Accessibility
In processes within which flammable materials were incorporated in flash points of above 140 ºF or for materials that were employed in process condition above their flash point if		Drainage and leakage control
0.50	Barrier walls existed which surrounded the equipment capable of causing fire
0.50	A flat area existed near the processing unit which facilitated the materials leakage development and exposed the region to high risk of fire
0.00	Barrier walls existed that surrounded the processing unit from three sides which guided the leaked materials to a discharge pool or covered drainage canal
0.50	Pools or discharge canals in a processing unit passing telephone and electricity lines specified regions or the safe distances from these lines were not considered in processing unit design

Table 2: Specific process hazards and corresponding penalties

Penalty value				Description					Specific process hazards (F₂ or SPHs)
				-					Toxic materials
0.50				Processes that were performed at absolute pressure of below 500 mmHg or processes that are hazardous due to leakage of air into them					Pressure below the barometric pressure
0.50				- Flammable liquid reservoirs with flammability degree (NF) of 3 or 4 that air could enter into during sudden cooling or removal of its contents - Open gates and vacuum pressure relief systems where no gas emission was observed during their application - Storage of flammable liquids in conditions that were above their flash points (in sealed containers)					Operation at (or near to) the flammable limits
0.00				- A vapor recovery system with corresponding airlocks
0.30				- Processing units or reservoirs in which processing conditions were at or near the flammability limits (just in case a fault was observed in tools and equipment)
Penalty factor		Taylor mesh size			Particle sizes		Processing units which generate dust such as material handling, materials mixing, grinding, and packing (The penalty value is determined according to the particles and mesh size.)		Dust explosion
0.25		60-80			> 175
0.50		80-100			150-175
0.75		100-150			100-150
1.25		150-200			75-100
2.00		> 200			< 75
In processing units which were working above the barometric pressure, the penalty value was determined based on the operational pressure.									Relief pressure
For pressure limit of 1000 psig, the penalty value was determined using the following equation:
Penalty value				Pressure (psig)				For pressure of more than 1000 psig
0.86				1000
0.92				1500
0.96				2000
0.98				2500
1.00				3000-1000
1.50				> 10000
In processes within which the temperature may decrease to lower than transition point because of normal or abnormal operational conditions.									Low temperature
0.30				- Hard steel was used or the operational conditions were at or below the transition temperature (soft a.nd flexible/brittle and fragile)
0.20				- Materials other than steel were used or the operational conditions was at or below the transition temperature
Having the potential heat transfer value (in BTU) the penalty factor was determined using the following equation:				The first category (the liquids and gases existing in the process) which consist of flammable liquids with flash points below 140 ºF (60 ºC) and flammable gases, flammable liquid gases, and flammable liquids with flash points above 140 ºF (60 ºC)					Flammable and unstable materials
The second category (liquids and gases stored in the reservoir) which were located outside the processing region consisted of
				1. Liquid gases
				2. Class I flammable liquids (with flash points below 100 ºF/37.8 ºC)
				3. Class II flammable liquids (100 ºF/37.8 ºC < FP < 140 ºF/60 ºC)
The third category contained volatile solids in the inventory or the dust created in the process. The considered penalty value depends on the amount of materials in the inventory.
				1. Materials with a density above
				1. Materials with a density below
The amount of corrosion was defined as the summation of the internal and external corrosion degrees									Corrosion and wear
0.10				Corrosion below 0.5 in/year (0.127 mm/year)
0.20				Corrosion above 0.5 in/year (0.127 mm/year) and below 1 mm/year
0.2				Corrosion above 1 in/year (0.254 mm/year)
0.75				The risk of cracking existed due to the corrosion stress
0.20				Special coatings were used for prevention of corrosion
0.40				Processing units that incorporate materials with natural penetration capability or abrasive watery solutions which cause various problems in equipment sealing. Processes that used detachable seals					Leakage induced by fittings and sealing washers
1.50				Processing units with optical glass and expansion joints
0.10				Pumps and seals with slight leakage
0.30				Pumps, compressors and flange fittings with continuous leakage
0.10				processes with pressurized and thermal cycles
Processing units which are placed near heaters with flame or themselves have flamed heaters The considered penalty values depend on the distance between the heater and the potential leakage point (in ft) which may be calculated using following equations:									Equipment with flames (burning equipment)
				Processing units with potential emission of materials employed above their flash point Processing units within which flammable dust is incorporated
				Processing units within which the probability of emission of materials above their boiling points exists
0.00				Hot oil heat exchangers were assessed as an independent process. The utilized hot oil is not flammable or is utilized below its flash point					Hot oil heat exchangers
Penalty value				The penalty value for processing units using hot oil heat exchangers or flammable hot oil
Oils used above boiling point	Oils used above flash point		Volume (m²)			Amount of oil (American gallon)
0.25	0.15		< 18.9			< 500
0.45	0.30		18.9-37.9			500-1000
0.75	0.50		37.9-94.6			1000-2500
1.15	0.75		> 94.6			> 2500
The penalty value was determined according to the previous section of the table.				The existence of a flamed hot oil heat exchanger in the processing unit
0.50				Processing units including one of the following equipment: - Compressor with power above 600 hp - Pumps with power above 75 hp - Mixers or rotating pumps with ability of creating exothermic reactions in case some defects occurred - Large rotating equipment with high rotational velocity such as centrifuges with history of significant defects					Rotating equipment

Table 4: Determination of the process control factor

Row	Description	Process control factor (C₁)
a	Existence of emergency electricity	0.98
b	Existence of cooling systems	0.97-0.99
c	Existence of explosion control systems	0.84-0.98
d	Existence of emergency stop systems	0.96-0.99
e	Existence of computer control	0.93-0.99
f	Incorporation of noble gases in the process	0.94-0.96
g	Existence of operational manuals	0.91-0.99
h	Investigation of chemical reactions	0.91-0.98
i	Analysis of process hazards	0.91-0.98

Table 5: Determination of the material separation factor

Row	Description	Materials separation factor (C₂)
a	Remote control valves	0.96-0.98
b	Waste discharge/pressurized discharge	0.96-0.98
c	Drainage	0.91-0.97
d	Automatic locks	0.98

Table 6: Determination of the fire protection and prevention factor

Row	Description	Fire protection and prevention factor (C₃)
a	Existence of leakage identification system	0.94-0.98
b	Existence of steel structures	0.95-0.98
c	Existence of water resources for extinguishing a fire	0.94-0.97
d	Existence of special systems	0.91
e	Existence of sprinklers	0.74-0.97
f	Existence of water curtains	0.97-0.98
g	Existence of fire extinguishing suds	0.92-0.97
h	Existence of handy fire extinguishers	0.93-0.98
i	Existence of shields for cables	0.94-0.98

Table 7: Processing units with their materials and material factors

No.	Processing unit	Material/main materials	Material factor	Replaced material factor
1	Reactor R-A	Hydrogen, butane, ethane, and methane	21	21
2	Reactor R-B	Hydrogen, methane, and heptane	21	21
3	Reactor R-C	Hydrogen, propane, and propylene	21	21
4	Tower DC₁	Benzene, toluene, and xylene	16	21
5	Tower DC₂	Ethylene dichloride	16	21
6	Tower DC₃	Diethyl ether	21	21
7	Tower AC₁	Naphtha	16	21
8	Tower AC₃	Methyl ethyl ketone	16	16
9	Furnace F₃	Diesel fuel	10	16
10	Reservoir ST₁	Naphtha	16	21
11	Reservoir ST₂	Hydrogen	21	21

Table 8: Fire and explosion index and hazards level

	General Process Hazards Factor	Special Process Hazards Factor	Processing Unit Hazards Factor	Dow’s fire and explosion index	Fire and explosion hazards level
Reactor R-A	1.25	6.75	8.43	177.03	Severe
Reactor R-B	2.20	5.50	12.10	254.10	Severe
Reactor R-C	2.10	6.95	14.60	306.60	Severe
Tower DC₁	1.50	4.50	6.75	141.75	Severe
Tower DC₂	1.50	5.50	8.25	173.25	Severe
Tower DC₃	1.50	6.80	10.20	214.20	Severe
Tower AC₁	1.50	7.85	11.77	247.17	Severe
Tower AC₃	1.50	6.75	10.12	161.92	Severe
Furnace F₃	1.50	5.50	8.25	173.25	Severe
Reservoir ST₁	1.50	5.00	7.50	120.00	High/heavy
Reservoir ST₂	1.50	4.20	6.30	132.30	High/heavy

Table 9: Results of risk analysis for the processing units

	Exposure radius	Area of exposure	Investment accumulation rate	Value of area exposed		Damage factor	Maximum probable property damage	Damage control credit factor	Actual maximum probable property damage (million dollars)	Maximum probable days outage	Business interruption loss
	Exposure radius	Area of exposure	Investment accumulation rate	Original value	Replacement value	Damage factor	Maximum probable property damage	Damage control credit factor		Maximum probable days outage	Business interruption loss
Reactor R-C	79	19597	0.00130	25.47	24.01	0.83	19.92	0.58	11.55	49	114
Reactor R-B	65	13266.5	0.00100	13.26	12.50	0.83	10.37	0.60	6.22	34	79
Tower AC₁	63	12463	0.00141	17.57	16.56	0.83	13.74	0.62	8.51	41	96
Tower DC₃	55	9498.5	0.00128	12.15	11.45	0.83	9.50	0.54	5.13	30	70
Reactor R-A	45	6358.5	0.00125	7.94	7.48	0.83	6.30	0.62	3.84	25	58
Tower DC₂	44.5	6218	0.00135	8.39	7.91	0.83	6.56	0.61	4.00	26	61
Furnace F₃	44.5	6218	0.00129	8.02	7.56	0.83	6.27	0.63	3.95	25	58
Tower AC₃	41	5278	0.00135	7.28	8.86	0.68	4.66	0.48	2.23	18	42
Tower DC₁	36	4069	0.00140	5.69	5.36	0.83	4.44	0.49	2.17	18	50
Reservoir ST₂	34	3630	0.00126	4.57	4.30	0.77	3.31	0.59	1.95	17	40
Reservoir ST₁	31	3018	0.00138	4.16	3.92	0.65	2.54	0.51	1.29	13	30

The risk of fire and explosion in processing units was investigated through the use of the calculated fire and explosion index (Table 9). Based on the results, the actual MPPDs for Reactor R-C and Reservoir ST₁were the highest (11.55) and lowest (1.29), respectively. The MPDO was determined for Reactor R-C (49 days). As can be observed in table 9, Furnace F₃ and Tower AC₃had the maximum (0.63) and minimum (0.48) damage control credit factor, respectively. The Actual maximum and minimum probable property were, respectively, obtained for Reactor R-C (11.55 million dollars) and Reservoir ST₁ (1.29 million dollars). Tower AC₁ and Reactor R-B were at the next levels with actual MPPDs of equal to 8.51 and 6.22 million dollars, respectively. Moreover, maximum and minimum BI loss was obtained for Reactor R-C (114) and Reservoir ST₁(30), respectively. Maximum and minimum VAE was obtained for Reactor R-C (25.47) and Reservoir ST₁(4.16), respectively. The maximum and minimum investment accumulation rates (OCD) were obtained for Tower AC₁ (0.00141) and Reactor R-B (0.00100), respectively. The maximum and minimum AOEs were that of Reactor R-C (19597) and Reservoir ST₁(3018), respectively. Furthermore, the maximum and minimum values of ERs were obtained for Reactor R-C (79) and Reservoir ST₂ (34), respectively.

Discussion

The results of the present study indicated that Reactor R-C had the maximum F&EI. In addition, damage to property and production cessation days of Reactor R-B and Tower AC₁ were at the second and third levels, respectively. The F₃ of Reactor R-C was equal to 14.6 which was higher than the other units. Moreover, the F₁ of this unit was 6.95 which was the highest value after Tower AC₁ (F₁ = 7.85). It was concluded that F₃ and F₁ can be important parameters in determining the risks of fire and explosion hazards level. This conclusion is in accordance with the results of researches conducted by Etowa et al. (5) and Suardin et al. (7) who have concluded that material usage reduction results in lower F&EI.

The obtained indices for the studied processing units were higher than the fire and explosion index of the isocyanate storage reservoir at Bupal event (F&EI was equal to 238) which caused about 2000 deaths and poisoning of 10000 individuals (13). This indicates the critical conditions of the mentioned processing units. However, the calculated value was less than the fire and explosion index obtained by Nezameddini et al. at an oil extraction company which was equal to 243.68 (15). The same index was obtained as 161 in the research conducted by Gupta et al. on an ammoniac synthesis reactor (12).

The other processing units were ranked according to their criticality (from high to low) as Tower DC₃, Reactor R-A, Tower DC₂, Furnace F₃, Tower AC₃, Tower DC₁, Reservoir ST₂, and Reservoirs ST₁.

The results of the study indicated that the fire and explosion hazards level were severe at 82% of the studied processing units and heavy/high at the rest of units. This reveals the high level of fire and explosion hazards at the studied company. In the research conducted by Jafari et al. (16), the fire and explosion hazards level was determined as severe for 75% of the refinery Isomax unit. Results of the present study indicated that Reactor R-C had the maximum ER of 79 m, while ER was calculated as 41 m in the research conducted by Gupta et al. (12).

Furthermore, the costs of probable accidents were calculated as 51 million dollars with the induced 296 days of production cessation. The maximum cost of probable accidents was obtained for Reactor R-C as 11.55 million dollars while the same parameter was calculated as 21 million dollars in the research conducted by Gupta et al. on an ammoniac synthesis reactor (12). The estimated value represents the huge economic losses due to the occurrence of such accidents. The obtained value is the most realistic loss which is calculated by considering the existing safety and protective operations and equipment.

In some studies, the F&EI is successfully used as a tool to evaluate the inherent safety of the chemical process and it can provide a more understandable view of the process risks. In order to reduce the F&EI in the studied processing units, reduction of existing hazardous materials and processes operation pressure can be useful. The results of the study by Etowa et al. showed that when the operating pressure and the existing materials are reduced, F&EI is reduced in accordance with the principles of inherent immunity. However, changes in the amount of material in the processes had greater effect on the F&EI compared with pressure changes (6).

F&EI is an important tool for the determination of the risk of industrial processes and over time its weaknesses have been eliminated. For example, Gupta et al. stated in their study that the current methods of F&EI calculation not consider control measures effect on the F&EI value Therefore this makes the industry consider more dangerous. They suggested that the effects of lack of control measures should be included in F&EI calculation (12). In addition, Suardin et al. suggested that F&EI can be used as a measure in the optimization of design by integrating the F&EI in design optimization (5).

Conclusion

Results of the present case study emphasized the existence of several weakness points at the selected unit by determining the fire and explosion hazards of its processing units. According to the identified hazards and parameters effective on their occurrence, it was concluded that the control and reduction of process hazards require incorporation of comprehensive engineering controls. Hence, Dow’s index may be incorporated as an efficient tool by process design engineers in achieving safe and low-hazard chemical processes. Thus, the utilization of this index is suggested at all stages of a chemical company’s life cycle for the prevention of fire and explosion accidents occurrence.

Acknowledgments

The authors are grateful to all managers and staff of the studied processing company for their cooperation in this project.

Conflict of Interest: None declared

References

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* Corresponding author: Omran Ahmadi, Dept. of Sciences of Occupational Health, Tarbiat Modares University, Tehran, Iran.

Email: O.ahmadi@modares.ac.ir

Hazards level assessment (According to the 5-7^th revisions)	Dow’s F&EI index limit
Low/slight	1-50
Limited	51-81
Average	82-107
High/heavy	108-133
Severe	≥ 134