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Explosion Hazards in Maize Starch Plant and Safety Measures
Dr Manju Mittal Sr Principal Scientist, Fire Research Laboratory CSIR - CBRI, Roorkee
This paper presents fire and explosion risk associated with maize starch plant and covers brief introduction of a typical maize starch plant, case history of an explosion accident in this plant, anticipated causes and consequences, remedial measures and experimental explosion data of maize starch dust required to design explosion safety measures.

Dust explosions in industries may lead to fire, secondary explosions and even to detonations. Recognising the importance of safety in industries, studies related to explosion have been taken up as a specific activity at CBRI-CSIR, Roorkee. Most explosion-prone plants/units are being selected for explosion risk analysis. Maize starch plant has been identified as one such industry and is the subject of present investigation.

Maize starch is used in several food, pharmaceutical and textile industries. There exists inherent fire and explosion hazard in maize starch plant and other industries using this product. Explosion hazard associated with this plant has been assessed based on analysis of a case history of dust fire/explosion in a maize starch plant, identification of causes that may lead to fire/explosion, explosion characteristics of the materials handled under conditions prevailing in the plant. Safety measures have been outlined. Design of explosion safety measures requires evaluation of thermo kinetic parameters-deflagration index (Kst), maximum explosion pressure (Pmax) and other explosion parameters - Minimum Ignition Temperature of dust cloud (MIT), Minimum Ignition Energy (MIE), Minimum Explosible Concentration (MEC). These parameters were determined experimentally for specific sizes of maize starch and compared with literature data.

Brief Description of Maize Starch Plant
A typical maize starch plant manufactures a wide range of products- starches, dextrins, dextrose, corn syrup, maltadextrins, sorbitol, activated carbon, etc. Processing steps are - cleaning of maize, steeping, germ separation, grinding, fibre screening, gluten separation, processing of wet starch to produce edible starches, industrial starches and dextrins by starch drying and producing dextrose monohydrate, dextrose anhydrous, liquid glucose, altodextrins and sorbitol by processing remaining wet starch.

Starch plant has dry starch and dextrins sections. After separating various constituents of starch, part of wet starch (~35 per cent moisture) is dried in flash drier consisting of air filter, steam heated air heater, drying ducts, flash drier, cyclones and scrubber. Dried starch (12 per cent moisture) collected in bins at bottom of cyclones is conveyed onto first floor.

A part of this starch is sieved (~100 mesh) and conveyed to ground floor blenders and packed and rest is conveyed to dextrins section for processing into dextrins using steam or hot oil. Part of dextrins is humidified and sieved upto 200 mesh on first floor and conveyed to ground floor blenders and packed. Dry starch and dextrins sections involve - production, packing and storing of packed material.

Accident- Case History
Explosion occurred in a plant as described above in Gujarat at around 3 am. Resulting fire affected the starch plant. At about 3:40 am. Municipal Fire Brigade received message of fire. Fire tenders, tankers and ambulances were sent to plant. Firefighting team started their operations at 3:45 am with water, controlled fire at 6 am and extinguished it fully at 11 am. All wooden equipment in starch drying section, and materials in godowns (dry starch and dextrins) were destroyed. 19 people died and 10 injured. There was damage to aluminum equipment, electric motors, cables, electrical fitting and other equipment. The accident resulted in damage of asbestos sheets of roofs of dry starch and dextrins sections and starch/ dextrins godowns, main entrance of dextrins section, corrugated sheet on boundary wall, wall between dextrins and starch sections, bending/twisting of rolling steel shutter on door towards godown in dextrins/starch godowns. Steel flooring on first floor of dextrins section was found bulging upwards. Hopper/blending hoods (wooden) of dextrins/ starch, wooden staircase near starch packing and wooden sifters on first floor were totally burnt. Filters on suction side of starch section blower, aluminum conveyor below the bins of cyclones, conveyor of wet starch to feed box of drier were damaged and melted. Air-heater was detached from the suctionhood of blower. Top side aluminum sheets of scrubber were bent and melted. Paint on steel structures was affected.

Anticipated Causes and Consequences - Analysis
Damage indicated that explosion originated in dextrins section followed by fire spreading in dextrins and starch sections. Main combustible materials involved are starch, HDPE bags and wood equipment. Starch drier blower sucked fire from dextrins section. Explosion disturbed stored starch and dextrins and accumulated dust on surfaces of structures/equipment forming clouds which ignited and exploded leading to secondary explosion, propagation of flame all over and fire spread resulting in damage to plant, machinery, building, asbestos roofs, stored finished products and personnel. Small fires would have damaged vertical wooden elevators leading to dispersion of more dust. This caused primary minor explosion with little thrust causing disturbances from pressure waves and forming more dust clouds resulting into a major explosion followed by fire.

It was difficult to establish the actual source of ignition as there was massive damage due to widespread fire. The probable ignition sources are:

• Heat and small flame generated by smouldering lumps of dextrins discharged from roasters on the floor.
• Lighted match, beedi/cigarette thrown on accumulated dust/ stored material/HDPE woven sacks.
• Electrostatic discharges because of improper earthing.
• Sparks from electrical equipment, short circuiting/overheating of drives due to overload.
• Spark due to mechanical friction and impacts on rotary equipment.

Safety Measures Proposed
• Segregation of dry starch and dextrins sections and production, packing and storage in these to prevent fire/explosion spread.
• Adequate design of handling/packing of dry starches/dextrins to avoid dust and use of dust extraction systems to control release and deposition of dust in building to minimise explosion/fire hazard.

• Provision of mechanical explosion arresting devices such as rotary valves and chokes between different sections to limit explosion and adequate explosion venting and explosion suppression systems designed as per standards using explosion parameters of maize starch in buildings/units.
• Avoiding wooden equipment/structures.
• Laying cabling for power and lighting as per standard practice and earthing equipment properly to prevent build-up of static electricity.
• Good housekeeping and maintenance to minimise secondary explosion hazard and monitoring dust concentration level at various locations so that its value may not exceed MEC.
• Maintaining the temperature of hot surfaces much lower than the MIT of maize starch dust preferably lower than the temperature at which maize starch begins to smoulder (ie, 160°C).

Experimental Studies on Maize Starch Dust at CSIR-CBRI Design of explosion safety measures requires information on explosion parameters of dusts under the conditions prevailing in a plant. Various explosion parameters required for designing fire and explosion safety measures were experimentally determined for maize starch dust.

The MIT tests were conducted in the Godbert-Greenwald furnace; MEC, MIE, Pmax and Kst were measured in the experimental set-ups established at CSIR-CBRI according to international standards [3,4]. Maize starch dust samples of sizes 147, 74 and 38 Ým were prepared by screen analysis. Nominal size of particles was considered to be that of screen opening they passed through, to be retained by next smaller mesh.



Summary of resultant experimental values of various explosion parameters for the three samples is given in Table 1. Experimental values of variation of maximum explosion pressure Pmax and sizenormalised rate of pressure-rise (dP/dt) max.V1/3 (ie, Kst, V being volume of test vessel) are presented in Figure 1. Maximum values of Kst and Pmax are attained at 1000 g/m3 dust concentration. Literature data on explosion parameters of maize starch dust are summarised in Table 2 and Kst values are presented in Figure 2.



Conclusion
The explosion hazard associated with the maize starch plant has been examined and remedial measures to prevent the same have been proposed. It is hoped that the information and data presented in this paper will encourage all maize starch industries to review the safety of their plant.