BP
           CHEMICALS


          GUIDELINES FOR
  PREVENTION OF IGNITION SOURCES
CAUSED BY ELECTROSTATICS AND STRAY
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APPENDIX 6 - S...
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2    INTRODUC...
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4         STRA...
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6.2.4   Sampli...
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alternative so...
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In a tank with...
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7     DESIGN...
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ethers, whose ...
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Appendix 1 - L...
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Appendix 2 - C...
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Issue No: 01 ...
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Appendix...
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Appendix 4 - Rel...
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Appendix 5 - Sp...
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Appendix 6 - Sp...
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Appendix 7 - ...
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Appendix 8 - Da...
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Appendix 9 - Li...
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  1. 1. BP CHEMICALS GUIDELINES FOR PREVENTION OF IGNITION SOURCES CAUSED BY ELECTROSTATICS AND STRAY CURRENTS AT SHIP/SHORE TRANSFER Chemical Tankers and Chemical Barges Issue 1 - September 2002 LSU DOCUMENT NO: 28 LOGISTICS SERVICES UNIT
  2. 2. -2- PREVENTION OF IGNITION SOURCES CAUSED BY ELECTROSTATICS AND STRAY CURRENTS FOREWORD It is the Company's policy that safety of operation must be paramount. The implementation of this policy in the distribution field poses special problems because of the extent to which we are dependent on third parties and the difficulty of supervising distribution operations in the field. We must nevertheless be quite satisfied that our distribution operations are carried out competently and safely, and in accordance with national legislation in force. These Guidelines for Prevention of Ignition Sources Caused by Electrostatics and Stray Currents at Ship/Shore transfer has been prepared to help fulfil this aim. This document is published on the Commercial Department’s Website accessible at http://bpc.bpweb.bp.com/Commercial/Distribution/MasterFrames.htm. This document is not published as a paper document apart from a two master reference documents held within the Commercial Department Sunbury. Therefore any paper documents must be treated as uncontrolled copies. Reference to the website above will always provide the most up-to-date copy. Changes to this document will of course be advised to a wide group of business and site based personnel. These BP guidelines have been issued in co-operation with Vopak Shipping, Vopak Barging, Stolt Barging, GEFO, UCT, Belgian Ministry of Labour and Videotel. Issue No: 01 Uncontrolled if Printed Date of Issue: September 02
  3. 3. -3- PREVENTION OF IGNITION SOURCES CAUSED BY ELECTROSTATICS AND STRAY CURRENTS 1 DISTRIBUTION LIST Copy No : Issue To : 1. Manager, Logistics Services Unit, Sunbury 2. Quality System Control, Logistics Services Unit REVISION DETAILS Rev No : Details of Change Date 0 First issue 08/09/2002 Issue No: 01 Uncontrolled if Printed Date of Issue: September 02
  4. 4. -4- PREVENTION OF IGNITION SOURCES CAUSED BY ELECTROSTATICS AND STRAY CURRENTS Contents 1 DISTRIBUTION LIST.......................................................................................................................................3 2 INTRODUCTION AND SCOPE.......................................................................................................................6 3 HAZARDS AND RISKS.....................................................................................................................................6 4 STRAY CURRENTS..........................................................................................................................................7 5 STATIC ACCUMULATORS............................................................................................................................7 6 OPERATIONS.....................................................................................................................................................7 6.1 General precautions against electrostatic and stray current hazards ...............................7 6.1.1 Prevent charge generation.........................................................................................7 6.1.2 Prevent charge accumulation.....................................................................................8 6.1.3 Avoid incendive spark discharge..............................................................................8 6.1.4 Avoid flammable atmospheres..................................................................................8 6.2 Loading procedures .........................................................................................................8 6.2.1 Loading under air .....................................................................................................8 6.2.2 Loading under inert gas/nitrogen..............................................................................9 6.2.3 Filters in the shoreline...............................................................................................9 6.2.4 Sampling /gauging/dipping.....................................................................................10 6.2.5 Clothing...................................................................................................................10 6.3 Vapour treatment............................................................................................................11 6.4 Inerting and related hazards...........................................................................................11 6.5 Grounding/Earthing vs. Insulating flanges.....................................................................11 6.6 Radio transmittance........................................................................................................12 6.7 Cleaning.........................................................................................................................12 6.8 Preparation of operations - communication...................................................................13 7 DESIGN AND CONSTRUCTION..................................................................................................................14 7.1 Pumps.............................................................................................................................14 7.2 Filters..............................................................................................................................14 7.3 Electrical considerations................................................................................................15 8 TRAINING.........................................................................................................................................................15 Appendices APPENDIX 1 - LIST OF STATIC ACCUMULATORS..................................................................................16 APPENDIX 2 - CAUSES OF ELECTROSTATIC DISCHARGES................................................................17 APPENDIX 3 - MAXIMUM LOADING RATES.............................................................................................19 APPENDIX 4 - RELAXATION CHAMBERS..................................................................................................20 APPENDIX 5 - SPECIFIC REQUIREMENTS SEAGOING TANK SHIPS.................................................21 Issue No: 01 Uncontrolled if Printed Date of Issue: September 02
  5. 5. -5- PREVENTION OF IGNITION SOURCES CAUSED BY ELECTROSTATICS AND STRAY CURRENTS APPENDIX 6 - SPECIFIC REQUIREMENTS TANK BARGES..................................................................22 APPENDIX 7 - FORM FOR COMMUNICATING LOADING/UNLOADING ARRANGEMENTS........23 APPENDIX 8 - DATA..........................................................................................................................................24 APPENDIX 9 - LITERATURE...........................................................................................................................25 Issue No: 01 Uncontrolled if Printed Date of Issue: September 02
  6. 6. -6- PREVENTION OF IGNITION SOURCES CAUSED BY ELECTROSTATICS AND STRAY CURRENTS 2 INTRODUCTION AND SCOPE There have been many reports of fires and explosions following the transfer of materials either during loading/unloading, filling or inter-tank transfer. Many of these incidents have been shown to be as a result of sparks from the discharge of static electricity. It is the intention of this Guide to highlight the hazards that may result from the handling of materials that can cause static discharge and present the methods and procedures to minimise these hazards. It is also the intention of this Guide to give some background into the causes of static electricity discharge. This guideline applies to petrochemicals only but parts of it may be applied to mineral oils. Information contained within this Guide is not intended to replace any legal requirements which may be applicable in particular circumstances. It is recommended however, that the guidance outlined in this document is followed, provided that it does not conflict with any specific legal obligations. 3 HAZARDS AND RISKS In any system where there is fuel and oxygen in the flammable range and a source of ignition energy is present, a fire will take place. The continuation of a fire then depends on the whether the conditions for burning are maintained. The consequences of a fire depend much on the circumstances surrounding the fire including whether, for instance, the initial fire results in an explosion or initiates enough energy to produce further fires leading to escalation. The fuel, oxygen and ignition energy are together known as the “Fire Triangle.” To eliminate the possibility of a fire being able to start it is only necessary to eliminate one item from the fire triangle. In order for ignition to take place, sufficient energy must be released by the potential source to reach or exceed a threshold level (i.e. the minimum ignition energy or minimum ignition temperature). The uncontrolled discharge of accumulated static electricity is one such source that may be able to generate sufficient energy. Certain materials have the potential to accumulate static electricity charge. This is usually as a result of physical activities that have taken place with or around the material. Materials that can accumulate static charge can include ungrounded conductive objects, solid plastics and resins, finely divided dusts, petroleum liquids, mists and clouds, suspensions of water in hydrocarbon liquids and high velocity gas jets containing liquid or solid particles. This electrostatic charge will be discharged either to an earthed (grounded) item or between items having differing potentials. To eliminate static discharges as a source of ignition, all the following must be satisfied: • Can charge be generated? • Can charge accumulate? • Can the charge be discharged? • Will the discharge supply enough energy to become a source of ignition? • Will the discharge take place where fuel and oxygen are together in the flammable range? If any of the above can be prevented then the static electricity fire hazard will have been eliminated. Issue No: 01 Uncontrolled if Printed Date of Issue: September 02
  7. 7. -7- PREVENTION OF IGNITION SOURCES CAUSED BY ELECTROSTATICS AND STRAY CURRENTS 4 STRAY CURRENTS The term stray current applies to any electrical current flowing in paths other than those deliberately provided for it. Such other paths include the earth, pipelines, and other metallic objects or structures in contact with the earth. A stray current may be continuous or intermittent, unidirectional or alternating, and is usually distributed among a number of parallel paths in inverse proportion to the paths’ individual resistances. Stray currents can accidentally result from faults in electrical power circuits. They may also be deliberate, as in case of currents applied for cathodic protection of jetties and ships. 5 STATIC ACCUMULATORS For the purpose of this Guide, liquid petrochemicals which have an electrical conductivity of less than 50 pS/ m (2x10E10 Ohm.m) are considered to be capable of generating spark hazards. These fluids are termed Static Accumulators. A list of BP products, which fall under the scope of these Guidelines, is attached in appendix 1. This list may not be complete. All liquid products which are transported in tank ships or barges under the responsibility of BP and which have an electrical conductivity of less than 50 pS/m fall under the scope of these Guidelines. When loading products that do not fall under the scope of this guideline through lines that are shared with other products, and when the lines may contain a significant amount of static accumulator product from a previous loading operation, the loading operation should be handled as a static accumulator until the previous product is cleared from the line. 6 OPERATIONS Electrostatic charges or stray currents may be potential ignition sources. 6.1 General precautions against electrostatic and stray current hazards The most important countermeasure to prevent electrostatic hazard is to bond all metal objects together to prevent potential differences. Certain objects may become charged during tanker operations (e.g. a metal object such as a can floating in a static accumulating liquid). Every effort should be made to ensure that such objects are removed from the tank. This is one example of a “spark promoter.” Under some conditions, a probe (such as a gauging device) in the vapour space can also act as a spark promoter when the rising liquid level approaches the probe. When handling static accumulator cargoes, the product may pick up sufficient charge to constitute a hazard. The charge may arise through one of more different processes: - flow through the pipeline (enhanced when water droplets are suspended in the product) - flow through a micropore filter or fine screen. (a filter with a mesh width of 30 µ or less is considered to be a micropore filter). See also 5.2. - turbulence and splashing Measures to minimise the risk of electrostatic accumulation that can be taken are: 6.1.1 Prevent charge generation - Initial loading pipeline velocity should be below 1m/s until the liquid level reaches 2 times the diameter of the dip tube above the bottom end of the dip pipe. The maximum loading pipeline velocity should be kept below 7m/s. - Avoid the use of filters and bends in the lines as much as possible. Issue No: 01 Uncontrolled if Printed Date of Issue: September 02
  8. 8. -8- PREVENTION OF IGNITION SOURCES CAUSED BY ELECTROSTATICS AND STRAY CURRENTS - No micropore filters in the system unless there is a minimum residence time between product passing the filter and then entering a storage or cargo tank. See 5.2 - Avoid pumping or flowing hydrocarbons containing dispersed water or solids. - Avoid splash filling. 6.1.2 Prevent charge accumulation - Include sufficient fluid residence time downstream of pumps and filters - All metal on seaward side must be electrical continuous to ship - All metal on landward side (lines, pump, tank) must be electrically continuous to the jetty earthing system - Electrical resistance of the complete loading system between tank and loading arm should not exceed 10 ohms - In case of tanks with a non-conductive coating (e.g. epoxy coating), it must be ensured that the liquid is earthed. This can be done by using electrical conductive dip tubes, loading lines, deepwell pumps etc. 6.1.3 Avoid incendive spark discharge - For protection against stray currents, install electrically insulating flanges at the last flange on the loading line as close as possible to the ship’s manifold (liquid and vapour line). Electrically insulating flanges to have a minimum resistance of 1000 ohms. As an alternative, semi conductive hoses can be used. It must be ensured that the insulating flange is not short cut (e.g.; by railing, supporting legs of loading arm etc…). The insulating flange should preferably be installed at the shore side rather than the shipside. - Metal gangways or ladders pose no problem as long as these are not placed or removed during connection or disconnection activities or when a flammable atmosphere may be present. The same applies for steel wire cables - Remove or earth (ground) spark promoters in tank and vessels - Use sufficient waiting period before sampling unless in-line sampling is used (see also 5.2.4) 6.1.4 Avoid flammable atmospheres - Displace air in tank vapour space with nitrogen or inert gas 6.2 Loading procedures 6.2.1 Loading under air The following procedures must be adhered to when loading under normal air: Maximum initial loading pipeline velocity 1 metre/second until the liquid level reaches 2 times the diameter of the dip tube over the bottom end of the dip pipe. Maximum loading pipeline velocity after dip pipe covered is 7 metre/second. (For maximum loading rates: see appendix 3). When multiple tanks are being loaded, the loading rate may only be increased when diptubes of all these tanks are submerged (2 times the diameter of the dip tube) in the liquid. Loading pipeline velocity limits must be ensured in all parts of the loading system, i.e. for calculation purposes, the smallest line diameter in the transfer system must be used. Issue No: 01 Uncontrolled if Printed Date of Issue: September 02
  9. 9. -9- PREVENTION OF IGNITION SOURCES CAUSED BY ELECTROSTATICS AND STRAY CURRENTS Possible different flow distribution into lines of the various compartments aboard a vessel should be considered in making these calculations The initial maximum loading rate is irrespective of the number of cargo tanks which are being filled. (Note the smallest line diameter is the limiting factor and not the number of tanks.) Loading rates must be monitored at all times by both ship and terminal. If two phases are present (e.g. benzene/water mixture), the loading rate must never exceed 1 metre/ second. Avoid the use of filters and other obstructions as much as possible - Do not install mobile filters. - No micropore filters in the system and, if used, the retention times must be taken into account (see appendix 4). Avoid too many bends in the lines Eliminate splash filling Electrically insulating flanges or semi-conductive hoses with a minimum resistance of 1000 ohms should be installed at the last flange on the loading line and vapour return line. All metal on seaward side of the insulation flange should be electrically continuous to ship. All metal on landward side of the insulation flange (lines, pump, tank) should be electrically continuous to the jetty earthing system. Electrical resistance of the complete loading system between tank and loading arm should be a maximum of 10 ohms In addition to the insulating flange, a bonding cable with explosion proof switch may be fitted between the barge/ship and loading installation. It must be ensured that receiving tanks do not contain unearthed objects which can float. These can get charged by charge induction and form an isolated conductor which can cause sparks with the tank wall or other material in the tank. The presence of a non-conductive coating in tanks will impede the relaxation of static charges. The more earthed metal parts present in the tanks, the faster the electric charges will flow away (e.g. via dip tube). If lines must be cleared after loading, this may only be done with nitrogen (do not use compressed air.) 6.2.2 Loading under inert gas/nitrogen Inerting the tanks prior to loading and maintaining an inert atmosphere in the tanks during loading will eliminate any risk of fire/explosion of the cargo should an electrostatic discharge occur. The maximum oxygen concentration in the cargo tanks must be 8 % before starting to load. It must be ensured that the oxygen level remains below 8 % throughout loading. If loading is done under inert gas, loading rate restrictions regarding the risk for static electricity do not apply. 6.2.3 Filters in the shoreline When filters are installed in the shoreline, special precautions must be taken. See 5.2. Issue No: 01 Uncontrolled if Printed Date of Issue: September 02
  10. 10. -10- PREVENTION OF IGNITION SOURCES CAUSED BY ELECTROSTATICS AND STRAY CURRENTS 6.2.4 Sampling /gauging/dipping Whenever possible, in-line sampling systems should be used (e.g. DOPAK system). Sampling/gauging/dipping devices should be either completely conductive or completely non- conductive. Conductive sampling and gauging devices (sampling cage, dip weight) should not be used with a non-conductive lowering device (synthetic fibre rope, rod). In principle, if non-conducting equipment with no metallic parts is used, there are no restrictions. In practice, however, it is difficult to ensure that such equipment remains non conductive, because the accumulation of dirt or other surface contamination may render the equipment surface conductive. A dip tape or sampling cage can be charged by induction if suspended via a non-conductor (e.g. nylon rope). A non-conductive rope can also be charged by rapidly slipping through gloved hands for appreciable distances. By this method an insulated person can also become charged. Man made fibres (e.g. nylon) should not be used as this leads to the generation of static discharges. Conductive sampling and gauging devices should therefore be used with a conductive lowering device (e.g. tape, cable). Conductive sampling and gauging devices should be properly bonded to the tank (by means of bonding cable or by maintaining continuous metal-to-metal contact between the lowering device and the tank hatch. By touching the sample can and/or the lowering device to the metal of the tank, it can be ensured that the tank wall and sampling can/dip rod are at the same potential. Normally the tank wall will have the same potential as the contents. However, after loading time is needed for the charge from the bulk of the liquid to conduct through the liquid to the walls. There is therefore the chance that a sample can at the wall potential would contact liquid at a different potential leading to a spark. For this reason sampling should not take place immediately after the tank has been filled. The minimum waiting time is 30 minutes. Clean dry natural fibre rope may be non-conductive when used in low humidity conditions. When used, natural fibre ropes should maintain contact with the tank hatch. Sampling and dipping operations through sounding pipes are permissible at any time because it is not possible for any significant charge to accumulate on the surface of the liquid within a correctly designed and installed sounding pipe. A sounding pipe is defined as a conductive pipe which extends the full depth of the tank and which is effectively bonded. When inerted tanks are sampled/dipped the precautions as indicated above need not be taken if it can be ensured that the tank remains under inert atmosphere during this operation (closed sampling system) 6.2.5 Clothing Because of its significant capacity to ground, the human body can store an amount of energy in excess of the ignition energy for common hydrocarbons. Body potential of 10-50kV can be attained by individuals. When handling flammable liquids one has to take into account that clothing might be a source of ignition. In zone 0 and 1 areas the following precautions must be taken: - Avoid synthetic clothing - Do not change clothing in zone 0 and 1 areas Issue No: 01 Uncontrolled if Printed Date of Issue: September 02
  11. 11. -11- PREVENTION OF IGNITION SOURCES CAUSED BY ELECTROSTATICS AND STRAY CURRENTS - Wear antistatic shoes (maximum 10E8 Ohms resistance). However, the conductivity of the floor (deck) is important. Rubber mats are non conductive and will render the anti static shoes ineffective and therefore should not be used. 6.3 Vapour treatment It is recommended that vapours are not blown off direct to the atmosphere but that these are: - routed back to the shore tank (vapour return) or - routed to a vapour treatment unit (VTU). Vapour treatment units can be: - scrubbers - active carbon beds - condensers - vapour combustion units It must be ensured that there is adequate protection against flame propagation. Some VTU’s may not be fully explosion proof and are designed to work only with inert gas. In those cases, the vapours must be inerted first. Isolated conductive sections should be avoided in a vapour recovery system. All conductive parts of the vapour connection to the ship should be in electrical contact with the cargo tank. Liquid cascading from one compartment to another as a result of overfilling through the common overhead system can create electrostatic and other hazards. If the barge/ship is isolated from the shore with insulating flanges in the liquid line, an insulating flange must also be fitted in the vapour return line, close to the manifold. 6.4 Inerting and related hazards In order to avoid risks for electrostatic discharges, loading and unloading operations can be done under inert gas. In most cases, nitrogen is used. When working under inert gas conditions it must be ensured that the oxygen level remains below 8 % so that the vapour phase remains out of the explosive range. Inerting by means of nitrogen is recommended. Inerting will pose the additional hazard of asphyxiation. If tanks are inerted, it must be ensured that tank entry without air supplied respirators is forbidden. Even inspection of the tanks from the tank hatches must be forbidden. Therefore, proper labelling of tank hatches is required. Tank entry procedures must be in place to ensure that tanks can be entered safely. 6.5 Grounding/Earthing vs. Insulating flanges Large currents can flow in electrically conducting pipe work and flexible hose systems between ship and shore. The sources can be cathodic protection of the jetty or the hull of the ship or stray currents from galvanic potential differences between ship and shore. An all-metal loading or discharge arm provides a low resistance connection between ship and shore and there is a very real danger of an incendive arc when the ensuing large current is suddenly interrupted during the connecting or disconnecting of the arm at the tanker manifold. Similar arcs can occur with flexible hose strings containing metallic connections between the flanges of each length of hose. It is therefore the recommended practice to insert an insulating flange within the length of the loading arms and at the connection of flexible hose strings to the shore pipeline system. An Issue No: 01 Uncontrolled if Printed Date of Issue: September 02
  12. 12. -12- PREVENTION OF IGNITION SOURCES CAUSED BY ELECTROSTATICS AND STRAY CURRENTS alternative solution with flexible hose strings is to include in each string one length only of insulating or semi conductive hose. The insertion of such a resistance completely blocks the stray current flow of through the loading arm of the hose string. At the same time the whole system must remain earthed either to the ship or to the shore. The insulating flange or piece of hose must have a minimum resistance of 1000 ohms. This resistance must be regularly checked. As an alternative, an antistatic hose can be used which conducts static electricity but not large currents. It must be ensured the ship and shore are not electrically connected/disconnected at places where a flammable atmosphere may be present (e.g. placing/ removing aluminium gangway) : See also 5.1.3. The use of ship/shore bonding cables is not recommended. However in some locations, a bonding cable to the ship/barge may still be required. In that case, it is recommended to install this cable in addition to the insulating flange. The bonding cable must have a maximum resistance to earth of 10 ohms. The bonding cable must be provided with an explosion-proof switch, on the jetty, of a type suitable for use in a Zone 1 hazardous area. The bonding cable must be inspected to see that it is mechanically and electrically sound. It is important to ensure that the switch is always in the ‘off’ position before connecting or disconnecting the cable. Only when the cable is properly fixed and in good contact with the ship/barge should the switch be put in the ‘on’ position. The cable should be attached before the cargo hoses are connected and removed only after the hoses have been disconnected. The connection point of the cable should be well clear of the manifold area. 6.6 Radio transmittance During medium and high frequency radio transmissions (300 kHz – 30 MHz) significant high energy is radiated which can, at distances extending up to 500 m from the transmitting antennae, induce an electrical potential in unearthed ‘receivers’ (derricks, rigging, mast stays, etc) capable of producing an incendive discharge. Therefore: - all stays, derricks and fittings should be earthed. Bearings of booms should be treated with graphite grease to maintain electrical continuity. - transmissions are not allowed during periods when there is likely to be a flammable gas in the region of the transmitting antennae (only for seagoing vessels) Low energy transmissions, such satellite and VHF communications, do not produce the same sources of ignition. 6.7 Cleaning Safe working conditions when cleaning cargo tanks of chemical tank ships/barges can be enhanced when using the following criteria: - Individual washing nozzle capacity should not exceed 17,5 m3/hr - The nozzle diameter should not exceed 12 mm - The nozzle and other equipment introduced in the tank must be bonded to the tank or equipment - Hoses used for cleaning must be tested for electrical continuity and the resistance should not exceed 6 ohms per metre length. - No high pressure jet cleaning (can cause the built up of static charges). Steam cleaning of tanks should be avoided when the tanks contain a flammable atmosphere or may develop a flammable atmosphere during operations since steam can introduce a charge. In addition, condensation of the steam will draw in air, possibly creating an explosive atmosphere. Issue No: 01 Uncontrolled if Printed Date of Issue: September 02
  13. 13. -13- PREVENTION OF IGNITION SOURCES CAUSED BY ELECTROSTATICS AND STRAY CURRENTS In a tank with non-conductive lining, charged condensate from steam can accumulate and will become an conductive ‘object’ in the tank. When water or water-based solutions are used for cleaning with jets, sprays or fog nozzles, the potential exists for static charge generation as a result of mist formation and also from water settling through hydrocarbon product (water droplets are small conductors in a non-conductive environment). Water washings using sprays should only be conducted in an inerted or non-flammable atmosphere. When solvents have to be used for cleaning, the tank should also be inerted. If this is not possible, the following measures must be taken: - use a conductive solvent (conductivity > 50 pS/m) - use a high-flash point material (greater than the operating temperatures) - cleaning system must be conductive and bonded to the tank - use a nozzle that does not create a fine mist or aerosol Do not displace lines with water into a tank containing a static accumulator 6.8 Preparation of operations - communication Immediately after docking, a conference should be held between the jetty operator and the ship’s senior officer to agree on procedures concerning cargo, ballast, bunkering transfer operations and emergency situations. For this purpose a document should be used, an example is attached in appendix 7. The vessel must be informed that the cargo to be loaded is a static accumulator and that therefore special precautions must be taken according to these guidelines. The vessel should be informed when shore tank swing takes place during discharge. The terminal emergency response instructions and the local state and port cargo transfer regulations must also be given to the vessel’s senior officer. Vessel and terminal personnel must be able to communicate effectively in a common language. In addition, before commencing to load or unload, the ship/shore safety checklist must be completed. For seagoing ships this is the IMO checklist, and for barges the ADNR checklist (or equivalent if ADNR does not apply). The jetty operator and the vessel must have the authority to stop operations when safety or pollution violations occur. Portable radios must be available for communication between ship and shore. Issue No: 01 Uncontrolled if Printed Date of Issue: September 02
  14. 14. -14- PREVENTION OF IGNITION SOURCES CAUSED BY ELECTROSTATICS AND STRAY CURRENTS 7 DESIGN AND CONSTRUCTION Chemical tank ships and barges must be constructed according to the requirements in the applicable regulations. In addition, the following minimum requirements should be met: 7.1 Pumps Pumps should be constructed from either cast steel or stainless steel. Plastic pumps or pumps with plastic parts should not be used for cargo transfer. If used in emergency situations, it must be recognized that these pumps may generate static electricity. Since the velocity is of utmost importance when loading ships/barges, pumps with adjustable speed are recommended. The loading velocity must be monitored during the loading operations Mobile pumps (e.g. spill pump) used on board must be properly bonded to the ship/barge steel structure before using. 7.2 Filters Charge generation greatly increases if a filter is placed in a piping system. A filter can produce 10 to 200 times more charge than is produced in the same system without filtration. In some cases, a wire screen can also enhance charge generation. There is no danger from this excessive charge as long as the liquid is contained in the pipe; the absence of air will not allow any flammable mixture to ignite. Furthermore, the high charge developed by the filter tends to decrease as the liquid continues down the pipe. If after filtration, the liquid is discharged from the pipe into a tank where the possibility of a flammable liquid exists, specific precautions must be taken: Filters must be properly earthed. It must be ensured that the filter is slowly filled and emptied. During normal operations, the filter must remain full. When the pore or screen size of the filter is larger than 300 microns ,it is unlikely that hazardous levels of electrostatic charge will be generated in the filter/screen. As the filter pore size decreases, charge generation also increases and may approach a hazardous level. When the pore or screen size is less than 150 microns, a hazardous charge level is likely to be generated. In this situation, a residence time of at least 30 seconds should be provided between the filter/screen and point of discharge. If the pressure drop over the filter becomes excessive, (e.g. when filter is blocking up) the filter must be cleaned because otherwise the charge generation will become too high. This is particularly important when using large pore screens (i.e. >150 microns) because the screen might not have been perceived to be a charge generator during design of the system. For liquids with a low conductivity (<2 pS/m) the residence time between the filter and the receiving vessel must be at least 3 times the relaxation time of the liquid or 100 seconds whichever is smaller. (see also appendix 4). Also, if micropore filters are used (mesh width of 30µ or less) the residence time must be greater than 100 seconds. The maximum residence time need not exceed 100 seconds when all safety precautions as indicated in chapter 10.2 (loading under air) are adhered to. The residence time can also be obtained by using a relaxation chamber or by reducing the flow rate. A relaxation chamber should operate full of liquid to avoid the possibility of sparks occurring in a flammable atmosphere (see appendix 4). The following chart may be used to estimate the residence time requirements for liquids having a dielectric constant of around 2. (The dielectric constant of most hydrocarbons is around 2. Organic liquids, such as Issue No: 01 Uncontrolled if Printed Date of Issue: September 02
  15. 15. -15- PREVENTION OF IGNITION SOURCES CAUSED BY ELECTROSTATICS AND STRAY CURRENTS ethers, whose molecules contain elements other than carbon and hydrogen may have different dielectric constants and should be checked.) See also appendix 4 retention times 60 50 retention time secs 40 30 20 10 0 100 80 50 36 17 15 5 1 conductivity pS/m If it is not possible to meet the requirements of minimum residence time, a minimum waiting time of 30 minutes must be adhered to before cargo tanks are opened for sampling, ullaging, dipping etc. 7.3 Electrical considerations Pumps, filters electric motors and all parts of the system must be effectively earthed to prevent the accumulation of static electrical charges. The resistance to earth must be checked, and should not greater than 10 ohms. 8 TRAINING All personnel involved in the handling of static accumulators must be made aware of the risks associated in handling these cargoes. Therefore training must be given. These guidelines should be used as a basis for this training programme. The training programme must comprise the principles which are outlined in this guideline. Issue No: 01 Uncontrolled if Printed Date of Issue: September 02
  16. 16. -16- PREVENTION OF IGNITION SOURCES CAUSED BY ELECTROSTATICS AND STRAY CURRENTS Appendix 1 - List of Static Accumulators BP product name Electrical conductivity Electrical resistivity pS/m at 25°C M ohms.cm at 25°C Hexane <1 1x108 Cyclo hexane <1 1.43x107 Xylene 0.1 1x105 Toluene 1 1x104 Benzene 4 Styrene 10 1x103 Heptane 10 1x103 Ethylbenzene 30 Ethyl acetate 1x105 0.11 Note: electrical conductivity varies with temperature, composition and purity. This list is not limited. All products with an electrical conductivity of less than 50pS/m are considered static accumulators Issue No: 01 Uncontrolled if Printed Date of Issue: September 02
  17. 17. -17- PREVENTION OF IGNITION SOURCES CAUSED BY ELECTROSTATICS AND STRAY CURRENTS Appendix 2 - Causes of Electrostatic Discharges Whenever two dissimilar materials come into contact charge separation occurs at the interface. At the interface, a charge of one sign moves from material A to B so that materials A and B become respectively negatively and positively charged. Whilst the materials stay in contact and immobile relatively to one another, the charges are extremely close together. The voltage difference between the charges of opposite sign is then very small and no hazard exists. When the charges are separated, a large voltage difference develops between them. In addition, a voltage distribution is set up throughout the neighbouring space and this is known as an electrostatic field. Charges which have been separated attempt to recombine and to neutralize each other. This is know as charge relaxation. If one or both of the separated materials carrying charge is a very poor conductor, recombination is impeded and the material retains or accumulates the charge upon it. The period of time for which the charge is retained is characterised by the relaxation time of the material, which is related to its conductivity: the lower the conductivity, the greater is the relaxation time. If a material has a comparatively high conductivity, the recombination of charges is very rapid and consequently little or no static electricity accumulates on the material. Such a highly conducting material can only retain or accumulate charge if it is insulated by means of a poor conductor and the rate of loss of charge is then dependent upon the relaxation time of this lesser conducting material. Electrostatic breakdown between any two points, giving rise to a discharge, is dependent upon the strength of the electrostatic field in the space between the two points. A field strength of 3000 kilovolts per metre is sufficient to cause breakdown of air or petroleum gases. Conductors (conductivity > 50 pico Siemens/metre): Conductors are not capable of holding a charge. If they are insulated, all the charge available will instantaneously be released into a discharge when an opportunity occurs. Non-conductors (conductivity < 50 pico Siemens/metre) Non-conductors have such a low conductivity that, once they have received a charge, they retain it for a very long period. Under normal conditions, gases are highly insulating. Charged mists are formed during the ejection of wet steam from a nozzle while using tank washing machines. Although the liquid, for example water, may have a very high conductivity, the relaxation on the droplets is hindered by the insulating properties of the surrounding gas. Spark discharge Spark discharges occur between conductive objects that are at different voltages. Usually, one of the objects is not adequately grounded. An example would be a metal can floating on a static accumulator and the side of the tank. Under certain circumstances, discharges with sufficient energy to ignite hydrocarbon gas/air mixtures can occur from unearthed conducting objects already within, or introduced into, a tank filled with charged mist. Avoiding ungrounded conductive objects through design, maintenance and operating practices can prevent this type of spark. Brush discharge Brush discharges can occur between a grounded conductive object and a charged low conductive material. An example would be a spark between the bottom of a filling arm and the surface of the product during splash filling. Brush discharges can be eliminated by avoiding the charge built-up on the product through adequate residence times, flow restrictions etc. Issue No: 01 Uncontrolled if Printed Date of Issue: September 02
  18. 18. -18- PREVENTION OF IGNITION SOURCES CAUSED BY ELECTROSTATICS AND STRAY CURRENTS Issue No: 01 Uncontrolled if Printed Date of Issue: September 02
  19. 19. -19- PREVENTION OF IGNITION SOURCES CAUSED BY ELECTROSTATICS AND STRAY CURRENTS Appendix 3 - Maximum Loading Rates PIPELINE LOADING RATES AT STARTING UP (1m / sec) Loading rates in m³/hr Number of tanks open - velocity in m³/hr Diameter 1 2 3 4 5 6 7 8 6” 65 130 200 260 325 390 450 520 8” 120 240 350 460 580 700 820 10” 180 360 540 720 910 12” 260 520 780 Issue No: 01 Uncontrolled if Printed Date of Issue: September 02
  20. 20. -20- PREVENTION OF IGNITION SOURCES CAUSED BY ELECTROSTATICS AND STRAY CURRENTS Appendix 4 - Relaxation Chambers Relaxation chamber Flow velocity V Vessel Length I I = retention time = 3 × T V where T is the relaxation time E × E0 ×10 E12 T= Lcond where E is the relative liquid dielectric constant (permittivity) E0 is the free space dielectric constant (8.85x10e-12) Lcond is the liquid conductivity in pS/m retention times 60 50 retention time secs 40 30 20 10 0 100 80 50 36 17 15 5 1 conductivity pS/m Issue No: 01 Uncontrolled if Printed Date of Issue: September 02
  21. 21. -21- PREVENTION OF IGNITION SOURCES CAUSED BY ELECTROSTATICS AND STRAY CURRENTS Appendix 5 - Specific Requirements Seagoing Tank Ships See ICS Chemical tanker Safety Guide and ISGOTT Issue No: 01 Uncontrolled if Printed Date of Issue: September 02
  22. 22. -22- PREVENTION OF IGNITION SOURCES CAUSED BY ELECTROSTATICS AND STRAY CURRENTS Appendix 6 - Specific Requirements Tank Barges See ADNR and USA regulations for transport of dangerous goods in barges Issue No: 01 Uncontrolled if Printed Date of Issue: September 02
  23. 23. -23- PREVENTION OF IGNITION SOURCES CAUSED BY ELECTROSTATICS AND STRAY CURRENTS Appendix 7 - Form For Communicating Loading/Unloading Arrangements This form is complementary to the current ADNR and VOW checklists checklists and will be incorporated in the ADNR revision 2005 Jetty:……………………… Ships name:……………….. Ships type………….. Order of Product Carg Ton M³ Max Loading Fillin Stop Shor o Unloading rate g e Unl./Loa Actu Shore/ tanks (m/s) degr d al Tank ee % ship Be Mid En gin dle d A B C Diameter smallest line on board:……………….mm/inch Diameter smallest line ashore:…………………mm/inch Initial loading rate (taking into account smallest diameter of loading lines) will be: 1 m/sec:……… m³/hr Maximum loading rate (taking into account smallest diameter of loading lines) will be: 7m/sec:…..…..m³/hr Loading rate can be increased when all dip tubes in the cargo tanks are submerged in the liquid. Ships crew will inform the shore personnel when this has been done.(note: does not apply when cargo tanks are inerted) Amount of forewarning for changing flow rates:………………….. Nitrogen purging before loading ? Yes/No to ……..%O2 Is nitrogen supplied during unloading ? Yes/No Method for handling vapours:……………………………………………………………. Maximum pump pressure :……………………………bar Has the ship/shore or ship/ship checklist been completed ? yes/no Is a micropore filter installed in the shoreline ? yes/no Date ………………….Time……………… Signed for ship:…………………………Signed for Shore……………………. Signature……………………………… Signature……………………………. Issue No: 01 Uncontrolled if Printed Date of Issue: September 02
  24. 24. -24- PREVENTION OF IGNITION SOURCES CAUSED BY ELECTROSTATICS AND STRAY CURRENTS Appendix 8 - Data Electrical conductivity in pS/ m Static accumulator <50 Electrical Resistance in Ohms Conductive hose (conducts electrostatic charges and large currents) <10E3 Anti static/semi-conductive hose (conducts electrostatic charges but no 10E3-10E6 large currents). Insulating hose (conducts no electrostatic charges and no currents) >10E6 Non conductive line >10E6 Non-conductive coating >10E9 Insulating flange >10E3 Average min field strength to generate spark 300 Volt Micropore filter: Mesh width<= 30µ Issue No: 01 Uncontrolled if Printed Date of Issue: September 02
  25. 25. -25- PREVENTION OF IGNITION SOURCES CAUSED BY ELECTROSTATICS AND STRAY CURRENTS Appendix 9 - Literature - Handboek Explosiebeveiliging Kluwer - ISGOTT - TST notiz 0154/2000:Schutzmassnahmen in Verladeeinrichtungen im Ölhafen Köln-Godorf-D Shell - CDIT Technical Questionnaire - The generation of static electricity during tank washing of chemical tankers:DR J. Bond-BP Chemicals - API standard static electricity guideline (doc 2003) - Ignition Risks during ship/shore transfers (ISMA report SE 00.02.05) - Chemiekaarten edition 2002 Issue No: 01 Uncontrolled if Printed Date of Issue: September 02

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