Bulk Distributor May/Jun 18 - Page 14

14 B ULK D ISTRIBUTOR Powder Processing Ignition source May/June 2018 James Grimshaw outlines the risk of electrostatic ignition during powder processing operations T he problem of static electricity in hazardous atmospheres is ever present in many sectors of processing industries. This case study investigates the factors behind the ignition source of a static discharge during a powder processing operation. Pneumatic transport systems represent the heart of any granular bulk material handling system throughout many industries today. Being an efficient method of transporting granular material, such systems allow for quick transportation of powders between processes enabling companies to keep up with an ever-growing demand on production. However, such processes are not without their risks. When the product being processed is considered combustible and has an appreciable portion of fine material, the potential for having an explosion increases dramatically. Fine powders with low MIE (minimum ignition energies) will regularly reach the MEC (minimum explosive concentration) along the conveying system and may be at risk of combustion by several sources of ignition. One such ignition source is electrostatic discharge. Electrostatic charge Pneumatic conveying systems have the ability to generate vast quantities of electrostatic charge via the movement of product through the plant equipment. The most common method of electrostatic charging on such process operations is due to tribo-electrification, which is simply the contact and separation of the powder with the walls of the processing equipment, the powder molecules itself or other factors that can cause charging, like surface contaminants. In this incident a process operator working on a pneumatic conveying system heard a crackling noise when powdered material was being transported between the classifier and the loading hopper. During investigation of the noise, the operator came into contact with a section of the duct and received a significant static shock. Although the operator was unharmed, the severity of the incident warranted a full system shutdown to investigate how static charges had been able to accumulate on a particular section of ducting. During the inspection the duct was examined and it was identified that the section of duct was not suitably grounded. When tested it was found the duct had a resistance path back to ground well in excess of 1011Ω, exceeding the recommended resistance of less than 10 Ω for metal plant items in good contact with ground, stated in IEC 60079- 32-1:2013 Explosive atmospheres Part 32-1: Electrostatic hazards, guidance. Further inspection found that the unusually high resistance was a result of a single grounding clip that had not been properly installed after a clean down operation. Consequently, the piping between the two ducts acted as an isolated conductor resulting in the generation and subsequent accumulation of charge. The lack of continuity to ground meant that the charge could not be dissipated, allowing an excessively high voltage potential to develop on the duct which eventually discharged onto the operator. Given the high rate of charge generation and spark discharge by a poorly fitted grounding clip; a review of grounding and bonding of all metallic parts was carried out. The inspection scrutinised the grounding and bonding integrity of all equipment units, all sections of ducts, bags and cages in the bag filters. As a result, many deficiencies were found and swiftly rectified. Only to expected The nature of a powder processing operation means that the generation of static electricity is to be expected in all parts of the system because of the movement of the particles through equipment. Therefore, regular maintenance is required to stop material from clogging up the machinery. Regular disassembly for cleaning and maintenance can result in bonding connections As an example In Fig 1, the system is configured to ground 7 sections of ducting (1–7) and a drum (8). Each channel is individually monitored back to ground to a resistance of less than 10 ohms and interlocked with the control equipment responsible for the flow of product. All monitored channels including the drum (1–8) need to have a path to ground (via bonding straps or grounding clamp) before the system will go permissive allowing the operation and flow of product to commence. Powder processing equipment presents more of a challenge compared to standard applications as there are metal parts that can make up larger assemblies that can be electrically isolated from each other. The risk of removable sections becoming isolated conductors will occur if: 1. Each section does not have a sufficiently low path to ground to safely dissipate charge. 2. The correct reassembly of equipment after cleaning between operations and regular examination of bonding straps between the metal pipework and duct sections by plant personnel is not routinely carried out. Figure 1 Figure 2 being missed or not made correctly when the equipment is reassembled. Vibration and corrosion may also degrade assembly connections so it i 0W&FfRFV7W&RFB'G2FR76V&ǐ&V6R6FVBg&G'VRV'Fw&VBf'GVFVǒF2'F7V"66V&&vW 66R6FVBv2fFVB'FRV6bW&F"bFR6FVBGV7FrBB&VVfVBFRWF6R6VBfR&VVfW'FffW&VB7FF2F66&vRFR&vB6PF&Vv6'W7F&RF7W&Rr琧6V7FbGV7Fr6VBfR&W7VFVB6vf6BvF6FVBWGFrFRƗfW2`VVW2BB76WG2B&6FR7BVffV7FfRvbV7W&r6WWVVBW6VBvFW"&6W76rW&F06B67VVFR7FF2VV7G&6G2F&fFRFVF6FVB7FF2w&VFr6WFFB26&PbF&rFRw&VB6V7FF6VG2B&6b6F7V66WF6VB6&R&RF&WfVBFRfrb&GV7@BW'BW'6VFFVF&B6VB6VB6RG26V7FFw&VBF20W7V6ǒ'FBbFRw&VB6V7FBFFRWVVB2B&VFǒf6&R"6( @V6ǒ66W76&Rf"WRFRw&VFr6Ɨ0VFVBF266V&vB7F26VBfR&VVFVF&WfV@F26FVCB2vǒW6&RFB6&vR67VVF6FVBGV7Fr6V7FB&W7VFVBg&֗76VBw&VFr6ƗVFRFW2GW&p&WfW2W&F2vFWBf6&RVV7G&7FF0vF6FVBWfW"67W'&rvFWBf&R6'W7F&RF7W&R&Vr&W6V@FR7&vvV7V6F66&vR67W'2g&WVVBF66&vW26VBfR&VwV&ǒvPVF6VBF226fVGW&Rb&6W70W&F2FBfR7VffW&VBg&FP66WVV6W2bf&R"W66W6VB'7FF2VV7G&6GFRf'7B6RF7F'B2FFWFW&֖RvVV7G&7FF26&vRv2( W&֗GFVN( F67VVFRFR6V7FbGV7FrF266PVV7G&7FF26&vRB&VVvVBF67VVFR&V6W6RFR72b6FVG&W7VFV@FR6V7F&VrVV7G&6ǒ6FVBg&FPvVW&72bw&VBBFR6V7F&VV6V7FVBFG'VRw&VB6&vRvVB@fR&VV67VVFVBG27W&f6R7FV@W6W72VV7G&7FF26&vW2vVB6ǒfPfVBFV"vFw&VB666&F6RvFGW7G'wVFVƖW2ƖResrBT2csУ3"FR6FVB6V7FF266RFRVwFbGV7Fr6VBfRB6FVGF&VvFfW&fVBw&VBvF&W67F6Rb2 W72दW2w&6r2&WFrvW"BWw6vRV6RFRF266R7GVG2&VfW&V6V@g&F&B'G6W&6RB2BvƖVBFFRW&F2bWw6vP7W7FW'2wwrWw6vR6VWw6v^( 2V'F&FRTDB77FV6fW2FW6R&&V2'V7W&rFB'G2bFPWVVBfR6FVGF&VvFw&VBvF&W67F6R6V7FbW72F2FP&VƖ6RVFW'fVFFW&f&&VwV"&W67F6R6V622WfFVBGVRFFRF&pBFW&6r6&ƗFW2bFRV'F&FRTDBखbFR72b6FVG&WGvVVRbFRGV7Fr6V7F2FFRfW&fVBV'FWfW"67W'2FP77FVvvW&֗76fRBFRW&Fv6V6RF6FVRFR77FVǒW&֗G2FP&GV7BG&6fW"&6W72vVFRw&VB&W67F6RbV6WFƗ6VB6V2W72F22&V6VFVBFRf&W2FW&F7FF&G2f"FR6G&bVFW6&&R7FF2VV7G&6GFR77FV2454W2DUBT4W&ff"W6R&FW2F7W&W2BVWG27W'&VBT2F&V7FfW2fr"FV7G&FW2FRV'F&FRTDB77FVw&VFrBF&rVFR'G2`FR6fWr77FVv66VB&RB&6b6F