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Safety in Pneumatic Transport of Powders
Dr Manju Mittal, A A Ansari, Rakesh Kumar, Sushil Kumar. Hazards in pneumatic systems for conveying combustible powders arise due to several reasons such as from probable ignition sources like impingement, friction, static electricity, etc. The article elaborates on powder hazards encountered during pneumatic conveying systems and proposes various safety measures to prevent and mitigate hazard.

Pneumatic conveying systems are used for safe transport of powdered materials in industries especially for in-plant applications.
Fires and explosions in pneumatic systems for conveying combustible powders result due to failure to define operating hazards associated with these properly. Once size of system, fluidising medium and material properties are finalised, designing the system is usually a straightforward application of engineering principles without considering probable sources leading to material decomposition or ignition in the system and explosibility of material under conveying stream conditions. Safety of such a system is assured if design engineer is in a position to analyse presence of potential fire/explosion hazard and risk in operating the system and incorporate modifications required to optimise safety, costs and productivity. Application of System Engineering Approach (SEA) for hazard analysis serves this purpose.

Pneumatic Conveying System for Powders- Description Pneumatic systems convey bulk materials of almost any type and particles comprising of pellets (>2 mm diameter typically above 3 mm), granules (diameter between 0.42 and 2 mm) and dusts (<0.42 mm as low as 1 Ým), and bulk densities of 16 to 3200 kg/ m3. A typical pneumatic conveying system consists of a pickup section - source of compressed air, feed device, transfer duct or pipe, powder air separator (cyclone, cartridge filter or bag house separator) and a receiver (storage silo). Two types of pneumatic conveying used for powder transfer are low pressure/dilute phase (Figure 1) or high pressure/dense phase (Figure 2). Low pressure dilute phase systems employ high system velocities (10 to 25 m/s), low solids concentration (<1 per cent by vol.), lower powder/gas mass ratio (3-9), and low pressure drops per unit length of transport line (typically <5 mbar/m), lower pressures 1-15 psig positive and 25-380 mmHg negative. High pressure/dense phase systems employ low system velocities (0.025 to 10 m/s), high solid concentration (>30 per cent by vol.), powder/gas mass ratio in the range 10-60, high pressure drop per unit length of pipe (typically >20 mbar/m), and higher pressures 10-50 psig positive and 240-600 mm Hg negative.

Analysis of Fire and Explosion Hazards Hazards associated with a pneumatic conveying system for powders can be analysed using System Engineering Approach (SEA) involving engineering analysis for establishing process environment; sensitivity analysis for determining reactivity of processed materials to processing environment; risk evaluation to analyse losses to be expected from system failure, fires and explosions; and optimisation for process safety. Usual criteria of fluidisation velocity, conveying velocity and production rate are considered in designing pneumatic conveying system for powders neglecting the fire and explosion risk. Sources of ignition leading to fire and explosion in pipelines and downstream equipment identified by Engineering Analysis are: • Impingement (in case of pellets)
• Friction
• Static electricity
• Smouldering/flaming nest
• Heated par ticles, sparks or glowing embers

Fire and explosion risk in pneumatic conveying of powders varies according to product, product concentration, airflow rates, and presence of dust cloud and dust layers resulting by particle attrition in case of pellets. At some point in a pneumatic conveying system e.g. at discharge into the receiving vessel or time e.g. during start-up or shut down in conveying cycle, whether dilute or dense phase, the material will be dispersed as explosible suspension leading to fire and explosion on availability of an ignition source of sufficient intensity. Probable sources of ignition in dust handling operations are picked-up by pneumatic conveying systems and transported along with powders leading to fire and explosions in these systems or interconnected units under favourable conditions. Flames or burning materials generated in other areas of processing plant may be transported and ignition initiation in one part of system may propagate as fire or explosion throughout the plant. In pneumatic conveying systems, dust concentration exceeds upper explosion limit under normal operating conditions. During start-up, shutdown or filling operations dust concentration range may be within explosive limits (50 and 1000 g/ m3).

Sensitivity Analysis for Various Probable Ignition Sources
In pneumatic conveying system, sensitivity analysis is used for evaluating chances of fire and explosion of dust-air mixture in presence of ignition sources identified by Engineering Analysis as discussed below.

Impingement
Impingement as a source of ignition requires sensitivity analysis. Equipment used for this purpose consists of a system in which pellets are conveyed at certain velocity (35-40 m/s) and impinged against targets placed at different angles expected to be present in various pneumatic conveying systems. Dust cloud is dispersed around impinging pellets to examine possibility of explosion of dust cloud. Pellet size, free fall velocity and conveying velocity are studied. Pellet size leading to explosions in presence of dust cloud and threshold ignition velocity defined as maximum velocity by which a pellet could be impinged without decomposition taking place are determined.

Friction
Friction as a source of ignition is caused by presence of material/tramp metal trapped in conveying equipment, or flow-control valves in pneumatic conveying systems. Engineering analysis of valves is conducted to determine maximum frictional pressure and solid velocity delivered by valve in metal-to-metal environment. Sensitivity analysis gives safe values of maximum friction and velocity conditions that would yield no ignition. Comparison of actual values for a valve with safe limits decides acceptability of a particular valve for a system.

Static Electricity
Static electricity is electrification of materials through physical contact and separation and static spark is an impulsive discharge of electricity across a gap between two points not in contact. Discharge of electrical energy occurs when field strength exceeds a critical level i.e. 5220 kV/m for 0.6 mm electrode gap, 3150 kV/m for 10 mm gap, 2650 kV/m for 100 mm gap and 2240 kV/m for larger gaps between parallel electrodes in air at atmospheric pressure. Fire and explosion hazard in pneumatic conveying system due to static electricity can be analysed following steps given in Figure 3 and involve: identification of areas where electrostatic discharge could ignite a dust cloud; systematic collection and evaluation of information and data on parameters given in Table 1, flow velocity, mass flow rate/ density, particle size, temperature and humidity in system, plant size and configuration, turbulence due to bends or constrictions, composition of duct/ pipe walls, conditions liable to lead to electrostatic ignition of flammable gas/dust mixture around spark; and identification of ways to prevent hazard occurrence.
Sensitivity of dust clouds to ignition and fire/explosion risk depend on their Minimum Ignition Energies (MIE). For ignition of a flammable mixture electrostatic discharge energy should exceed MIE of material determined using an electric spark as ignition source by standard test methods. Hartmann Apparatus is used for this purpose in CSIR-CBRI. Any object or surface that can accumulate electrical charge acts as capacitor and energy (E) lost in a spark in Joules is given by:
E = 0.5 CV2 (1)
Where,
C Capacitance of the object (F)
V Potential to which object is raised (V)
C and V are measured by appropriate means. Typical values of MIE for various materials are: hydrogen- 0.02 mJ, hydrocarbon vapours- 0.2-2.0 mJ, fine flammable dusts - 1-50 mJ, and coarse flammable dusts - 40-1000 mJ. Published experimental MIE are used for electrostatic hazard analysis. Human operators employed to drum/ sample products are charged to the extent of presenting a hazard as body moves relative to clothing or insulating floors or when it comes into contact with rotating machinery. Capacitance of human body is around 200 pF and that of machinery is 100-1000 pF. Engineering analysis gives man/machinery has potential of a maximum 10,000 Volts and electrostatic discharge value is 10 mJ. These energies may be compared with MIE values for dusts handled. Several categories of static discharges can be identified for describing ignition hazards. Most familiar spark discharge has an almost unlimited effective energy range. Maximum approximate effective energies for others are: corona ~0.1 mJ, brush~1-10 mJ, transitional brush ~10-100 mJ, propagating brush ~100-1000 mJ and bulking brush or cone ~20 mJ. Non-conducting particles moving along walls of conveying lines may become charged and dust clouds may lead to discharge to a less-charged object in powder-air suspension, powder heap in receivers due to carryover of charge, isolated conductive components or settled material. Charged clouds of dusts settling upon insulated surfaces can cause appreciable accumulative ion of static charge. Discharge between a space charged cloud and settled material or between a cloud and grounded metal object may easily contain energy needed for ignition. Charged powder may leak from joints into the atmosphere and electrostatic sparking can occur resulting in an explosion. Spark discharges from ungrounded conductive objects, surfaces and personnel are responsible for majority of industrial fires and explosions. Corona discharges are not hazardous for dusts. Electrostatic brush discharges occur from non-conductive surfaces such as plastics and are unlikely to ignite dust clouds. Transition and propagating brush discharges can be generated during pneumatic transfer of powders (e.g. warm acrylonitrile butadiene styrene) to a storage silo as a thin non-porous layer of high resistively and dielectric strength may deposit in metal pipe over the years. Energy released in such propagating brush discharges can be high enough to ignite explosible dust-air mixtures. If extremely high electrostatic charging occurs and thin layers of insulating powders are present on silo walls, high-energy discharge (upto 1000 mJ) within silo heap to wall can occur. Cone discharges occur across surface of bulked powder in storage silos, containers and hoppers or deep within powder heap for charged powder of resistivity 1010 ohms.
Coehn's law is used to determine appearance and polarity of electrostatic charges in a contact/separation environment such as pneumatic transfer systems. Magnitude of the charge is a function of difference in permittivity of duct/pipe material and powder. Electrostatic charging rate (coulomb/s) in pneumatic conveying pipe for an insulating system with no charge drain off is a product of powder flow rate (kg/s) and characteristic charge generation for powder (coulomb/kg) measured in lab. Charge-to-mass ratio of powders increases with increased conveying velocity in pipes due to increase in charge transfer efficiency with velocity of impact and separation and decreases with increased mass flow density as number of particles-wall collision per unit mass is increased. Typically charge/kg in air conveying pipe is ~ 7 times greater than the value based on testing and mass charge density for pneumatic conveying is 102-106 nC/kg.

Powders are conductive e.g. metals (conductivity >104 pS/m), semi-conductive e.g. many organic powders, such as flour (conductivity 102-104 pS/m) or nonconductive e.g. certain organic powders, many synthetic polymers and some minerals, such as quartz (conductivity <102 pS/m). Charge accumulated on non-conductive powders stays for long time while on conductive or semi-conductive powders it drains off fast. In case of non-conducting powders, layers accumulated on piping, receivers and silos prevent drain off of charge through grounded hardware. If voltage fields of sufficient intensity are generated electrostatic breakdown occurs, which may ignite dust air suspension leading to fire/explosion. Highest voltage fields usually occur at receivers and silos where bulk powder accumulates. Typical source voltage is taken as 1-10 kV when assessing static hazards due to semi conductive or non-conductive powders. Information on design details of system e.g., size of gap and configuration of material/component, helps in predicted capacitance and voltage stored. Voltage fields on accumulated powder surface and in vapour phase transport are calculated using:

E = Q/Ae0 (2)
E = Q1FR/3e0 (3)
Where,
A Surface area of dust (m2)
e0 Absolute permittivity of space= 8.86x10-
12 (coulomb /Vm)
F Dust/air flow rate ratio (kg/m3)
Q Charge (coulomb)
Q1 Charge per unit mass of dust (coulomb/kg)
R Radius of vessel (m)

Electrostatic charge drain off property is determined by test or analysis. Information and data from laboratory scale electrostatic charging tests have been found suitable to be correlated to field conditions. True igniting ability of an electrostatic spark depends not only on total energy but also on inductance, resistance and capacitance of the discharge paths. Only an accurate analysis incorporating sophisticated instruments can determine electrostatic hazard.

Smouldering/Flaming Nests
Fire or explosion may be initiated by smouldering/ flaming nests of powdered material conveyed through a pneumatic conveying system. A smouldering or flaming layer acts either directly as an ignition source for a dust cloud or by means of agglomerations or nests of burning material that break away from deposits and ignite a dust cloud in another part of plant. Ignition probability of a dust cloud by a hot nest depends on whether nest burns only by smouldering or produces either flame or incandescent particles. Experimental investigations indicate that smouldering nests of dusts entering a pneumatic conveying line are poor ignition sources for most dust clouds, failing to ignite dusts even when there is a large difference between nest temperature and Minimum Ignition Temperature (MIT) of dust cloud as these nests are extinguished or cooled to a temperature range in which risk of ignition in downstream equipment is no longer present. Smouldering nests with temperatures above approximately 700- 800 degrees Celsius are able to ignite dust clouds with MIT 280-370 degrees Celsius (e.g. sulphur) but did not ignite dust clouds with MIT values above 400 degrees Celsius. Flaming nests are able to ignite clouds of dusts up to MIT 600-675 degrees Celsius.

Heated Particles, Sparks and Glowing Embers
Heated particles created during grinding or drying are carried into pneumatic conveying systems and fanned to glow by high gas velocity. Sparks or glowing embers are produced by dull tool, processing machines itself, damaged fan bearing, an overheated motor or electric spares, or defective parts in product lines, or from foreign bodies in the conveying material. These potential ignition sources are then picked up by pneumatic system and transported along with highly combustible dust. A single spark entering filters, silos or dust collectors is sufficient to cause fires or dust explosions.

Risk Evaluation and Optimisation
Risk evaluation is based on estimating process failure which could be as simple as a pump motor malfunction or a localised fire leading to temporary shutdown of plant or it could be as complex and lethal as an explosion, secondary explosion or detonation, which not only destroys immediate equipment but also leads to total destruction of manufacturing complex. To evaluate risk designer should be able to assess severity of failure e.g. possible losses in human life and damage to production facilities; frequency of its occurrence; total cost if this failure occurs; and cost to prevent failure. It is not enough to use only the replacement costs as cost of failure, additional factors such as insurance costs and market position are also considered. Design of a pneumatic transportation system for powders should therefore include following aspects in addition to normal mechanical design procedure and evolve optimum design of system from the angle of safety, cost and quality:
• Ignitability, combustibility and explosiveness of dust cloud or dust layers of transported material.
• Presence of ignition sources identified by engineering analysis.
• Probability of explosion propagation initiated in any part of system to other areas through pipes and ducts.
• Application of an adequate method to avoid fire and explosion.

Possibility of expansive modifications can be minimised employing above concept at an early stage of a new plant or during change in operating procedures by pointing out existence of unsuitable equipment, procedures and reactions. The method avoids imposition of costly and undesirable safety margins resulting from an inability to determine the degree of hazard. Failure to detect a hazard can lead to loss of a manufacturing unit which may jeopardise the position of industry in market.

Course of Dust Explosions in Pneumatic Conveying Systems
Flame propagation during fire or explosion within a pipe of pneumatic conveying system for powders may lead to secondary explosions in interconnected vessels or vice versa. For planning and using safety measures (e.g. designing pipe to contain maximum pressure during explosion, designing explosion relief vents and explosion preventive measures like rapid action valve, suppression barriers to separate equipment from effects of an explosion) flame propagation velocity and pressure-time history in pipes of pneumatic conveying systems are required. These parameters are influenced by dust specific properties, dust concentration and its explosion violence characteristics (maximum explosion pressure pmax(bar) and KSt (bar.m/s) determined in 1 m3 or 20 litre spherical vessel (as per standard procedure), pipe diameters, conveying velocities, location of ignition source, length to diameter ratio, etc. KSt for a dust is equal to product of maximum rate of explosion pressure rise (bar/s) and cube root of the volume of test vessel (m). Experimental investigations for maize starch ( pmax 9.4 bar, KSt 215 bar.m./s), lycopodium (pmax 8.5 bar, KSt 155 bar m/s), and wheat flour (pmax 8.5 bar, KSt 115 bar m/s) in pipes of 100, 150 and 200 mm and conveying pipe length 40 or 48 m, mean air velocity 15-30 m/s indicate that low dust concentration (100-500 g/m3) can cause very violent explosions.
If dust-air mixture is ignited near open pipe end combustion gases can flow out nearly unhindered. For maize starch dust with pipe length to diameter ratio 200, optimum dust concentration, igniter position at open end, conveying velocity 20 m/s, experimental values are: pmax -0.3 bar and flame front velocity ~ 50m/s. Oscillating waves of low pressures and low flame velocities in order of conveying velocity could be observed if length/ diameter ratio exceeds 300.

Safety Measures Proposed
High pressure pneumatic conveying systems use relatively strong piping while low-pressure systems utilise weaker pipe work and usually require additional explosion protection. Measures to prevent fire and explosions in pneumatic conveying systems are summarised below:

• Process conditions could lead to ignition of large pellets of size higher than certain limit resulting in explosion in presence of dust. Pellets of sizes higher than safer one should therefore not be transferred by this method. Minimum conveying velocity is usually higher than threshold ignition velocity.
• A valve designed to overcome friction problem should be used. Metal-to-metal contact should not occur even in case of failure of starting the system.
• Measures adopted to prevent fires and explosions of electrostatic origin are aimed at controlling charge generation and accumulation, minimising spark discharge probability and controlling flammable atmosphere. Electrostatic hazard is reduced by reducing fines (-60 mesh) to prevent dust explosion, controlling velocity of transport (7 m/s for pipes upto 200 mm and >1.5 m/s for higher sizes), changing particle size distribution, reducing voltage fields in system by decreasing dust to air flow ratio, using smaller vessels, ionisation (using conventional AC or DC type ionisers or a feedback control-type ioniser system consisting of ioniser, electrostatic field strength metre and computer control equipment), earthing all conductive parts of process system handling powders such as pipes, grinders, conveyors and hoppers, or bonding all parts of a possible spark gap together to bring them to same voltage, increasing drain off of charge by addition of moisture, avoiding insulating coating on inner surfaces of pipelines, etc. Resistance to ground for all conductive components should be <10 ohms.
• Prevention of body charging must take the form of either eliminating source of charge or providing rapid leakage of charge to earth by installing conductive flooring in process areas and requiring operators to wear conductive shoes for which spark tests are conducted. Operator may be electrically isolated from ground charged to 25,000 V and then energy can be discharged through dust or pellets to ground. Personnel are earthed to prevent capacitive spark discharges from humans if powders of minimum ignition energies (MIE) <100 mJ are handled. Grounding of operators loading powders is required so that their resistance to ground is <108 ohms.
• Electrically conductive powders should be earthed by using earthed conductive equipment without non-conductive coating for preventing capacitive discharges from conductive powders. Powders must be discharged into container or silo via intermediate loading equipment, e.g. cyclone fabricated from conductive material. Alternatively, rotary valves, bag dump hoppers or scroll feeder systems can be employed.
• During pneumatic transport of non-conductive powders in metallic pipes non-porous layers of high resistivity and dielectric strength may be formed that may give rise to propagating brush discharges. Periodic inspection and cleaning of the inside walls of pneumatic conveying systems and silos are essential to prevent the risk of deposition of such layers.
• Pneumat i c s y s tems for convey ing powders must be protected against possible effects of explosion by using p r e s s u r e c o n t a i n m e n t ( r e q u i r i n g KSt value of dust being conveyed to select pipeline material and wall thickness), installing explosion relief vents of adequate area, using inert gas in a closed system, installing rapidacting explosion suppression system (which utilises pressure detectors at key points along conveying routes and on sensing ignition, high rate discharge extinguishers apply explosion suppressant preventing an impending explosion in adjacent equipment), or by installing isolation system (chemical suppression or mechanical fast acting slam valves) on pneumatic conveying lines and interconnected equipment to prevent full deflagration that can vent or quench flame and deflagration pressure.
• If ignition source is a smoldering nest or burning ember traveling through pipe/ duct, properly designed and installed spark detection and extinguishing systems (consisting of infra red spark detectors mounted flush to duct walls, control panel and an automatic extinguishing unit) are effective in preventing this scenario from escalating into a downstream deflagration. Upon sensing a spark, extinguishing device located downstream from detector is triggered, producing water spray to extinguish burning material and activates an audible alarm. System reacts in a fraction of a second, and water spray is of such a short duration that no downtime or cleanup is required. System can be programmed to activate other extinguishing agents, alarms, abort mechanics or to shut down equipment.

Conclusions
Adequate safety measures can be provided for pneumatic conveying systems for powders based on quantitative data needed to examine the probability of fire and explosion hazard and to design safety measures thereagainst. Use of Systems Engineering Approach would help the safety engineers in establishing safer working environment. It is strongly recommended that a specialist in the field should be consulted if there is potential explosion risk in pneumatically conveying of any material.