PRACTICAL APPLICATION OF STATIC HAZARD ASSESSMENT FOR DSEAR COMPLIANCE Graeme R Eis, Senior Safety Consutant ABB Engineering Services, Daresbury Park, Daresbury, Warrington, Cheshire WA4 4BT, UK KEYWORDS: Static hazard assessment, DSEAR, risk based INTRODUCTION This paper describes the methodoogy and practica experiences of carrying out a Static Hazard Assessment for Chemtura Manufacturing (UK) Ltd at the Trafford Park site, in compiance with the Dangerous Substance and Exposive Atmosphere Reguations 2002 (DSEAR). Companies handing substances capabe of creating exposive atmospheres are required to carry out a forma risk assessment [Ref 1]. This must consider the extent of foreseeabe exposive atmospheres within and externa to the process, and ensure that suitabe equipment is instaed to contro a potentia ignition sources. Within the Process Industry there has been a focus on cassifying hazardous areas caused by eaks of fammabe substances from the process and assessing the eectrica and mechanica ignition sources within these areas. Less attention has been given to potentia exposive atmospheres within equipment and the need to appy measures to contro a ignition sources incuding eectrostatic discharges. Many substances handed in the Process Industry can create exposive atmospheres in air due to fammabe vapours or combustibe dusts. Low conductivity iquids and highy resistive powders are capabe of generating eectrostatic charges during processing resuting in the potentia for a static discharge with sufficient energy to cause ignition. It is thought that fires and exposions caused by static discharges occur weeky in the UK. The atest standard for the contro of static hazards [Ref 2] provides detaied guidance on the contro measures that need to be appied. Many companies are uncear about the eve of risk posed by static hazards and tend to appy a range of generic contro measures without assessing their criticaity or reiabiity. A risk based methodoogy was deveoped for the Static Hazard Assessment described in this paper. The initia step invoved coection of physica property data on the conductivity of iquids and resistivity of powders, with ow hazard substances screened from further assessment. The operations invoving medium and high static risk substances were then reviewed to identify credibe exposive atmospheres within equipment and possibe charging mechanisms. 2008 ABB Engineering Services. Third parties ony have access for imited use and no right to copy any further. Inteectua property rights of IChemE aow them to make this paper avaiabe. ABB are acknowedged as the owner.
A risk grid approach was used to determine the required reiabiity of the anti-static contro measures with a Layer of Protection Anaysis (LOPA) for high risk cases. For events where the anti-static measures were judged critica, a review was carried out to ensure fu compiance with reevant standards, and foow the more onerous design options where appicabe. BACKGROUND The Trafford Park site first started operations as The Geigy Coour Company Limited on Christmas Eve 1939. In the ast 30 years the Site has been owned by Ciba Geigy and then as FMC before being bought by Great Lakes Chemica Corporation in 1999. In Juy 2005 Crompton Corporation merged with Great Lakes to form Chemtura Corporation. A range of speciaity chemicas incuding phosphorus fame retardants and fuids as we as industria water treatment additives are manufactured on the site in a number of batch processing pants. The Chemtura site is reguated by the Heath and Safety Executive (HSE) and Environmenta Agency (EA) under the Contro of Major Accident Hazard (COMAH) reguations as a top tier site. As required by the COMAH reguations, a safety report was submitted to the HSE in 2000 and updated in 2005. A genera risk assessment was used to identify major accident hazards, some of which were reated to exposive atmospheres and static ignition. A number of actions were raised during the COMAH risk assessment to provide further demonstrations that reevant good practice is being foowed to contro static hazards. It was decided to carry out a structured review of static reated hazards covering a operations on site, in compiance with DSEAR and demonstrating reevant good practice. The scope of this assessment needed to incude dust exposion hazards that were excuded from the COMAH risk assessment as dusts are not cassified as dangerous substances under this reguation. This work woud compete the various activities being carried out on the site for compiance with DSEAR, incuding re-assessments of hazardous areas on the site to ensure that suitabe eectrica and mechanica equipment is instaed in zoned areas. REQUIREMENTS OF STANDARDS The Dangerous Substance and Exposive Atmosphere Reguations 2002 (DSEAR) are intended to protect peope from the harmfu physica effects caused by dangerous substances with the potentia to cause fires and exposions, incuding fammabe vapours, combustibe dusts and reactive materias. This incudes the creation of an exposive atmosphere of the substance in air and ignition eading to harmfu effects from therma burns, bast overpressure or oxygen depetion. A risk assessment is required to identify fire and exposion hazards during processing and to agree appropriate contros to eiminate the hazard or if this is not possibe then to reduce the risk as far as reasonaby practicabe. 2
Further guidance is provided on contro and mitigation measures [Ref 3]. Hazardous area cassification shoud be used to identify where exposive atmospheres may form, specifying zones dependent on the frequency and duration of the exposive atmosphere. This must incude areas within processing equipment where the potentia for an exposive atmosphere exists, in addition to externa areas where eaks can ead to exposive atmospheres outside of the process. The risk assessment must identify a possibe ignition sources, assess whether these coud cause a fire or exposion causing harmfu effects, and introduce contro measures to prevent the ignition sources occurring. Ignition sources incude heat energy, eectrica energy (incuding discharges of static eectricity), mechanica energy and chemica energy. Reference is made to the reevant British Standard [Ref 4] for advice on the contro of eectrostatic discharges. This code of practice provides information on the generation of static eectricity in soids, iquids, gases and on persons, with recommendations for suitabe contro measures. STATIC HAZARD ASSESSMENT METHOD A structured methodoogy was deveoped for the assessment to meet the requirements of the standards and buiding on existing risk assessment techniques in use on the site. The assessment invoved a Process Safety consutant from ABB and a team of experienced technica and operations staff from Chemtura. The methodoogy invoved a 7-step process as described beow. Step 1 Data on Dangerous Substances Reevant data was initiay gathered on a fammabe and combustibe substances handed on the site with the potentia to cause an exposive atmosphere in pant equipment or generate a significant eectrostatic charge. This data incuded fash point and eectrica conductivity for iquids, and minimum exposion concentration, minimum ignition energy and voume resistivity for dusts and powders. Where avaiabe for raw materias this data was obtained from pubished sources. For some intermediate materias, such as recovered xyene, tests were carried out by Chemtura on sampes taken from the pant. Step 2 Identify Exposive Atmospheres A activities on the Trafford Park site invoving fammabe iquids and combustibe dusts were assessed to identify the potentia for exposive atmospheres within equipment. This was based on major accident scenarios from the COMAH Safety Report with an additiona assessment of the powder handing operations. The assessment considered the potentia for exposive atmospheres either during norma processing or foreseeabe upset conditions such as faiure of nitrogen banketing systems and/or heating of substances above their fash point due to temperature contro faiure. 3
Step 3 Assess consequences For a credibe exposive atmospheres identified in step 2, the consequences of ignition were assessed. Scenarios were screened from further assessment if the exposive atmosphere is judged to be too sma to cause significant harmfu effects or if the exposion is contained within the process. The atter case invoves consideration of the maximum credibe exposion pressure in comparison with the equipment design and test pressure rating. Scenarios invoving powders were screened from further assessment if the minimum ignition energy for the dust is >10J. Above this imit no measures are required to avoid static hazards [Ref 2], due to there being no credibe operations in the Process Industry that can generate eectrostatic discharges with such high energy eves. This screening was not permitted if other materias that coud form an exposive atmosphere were present in addition to the powder, such as a fammabe vapour. In cases where an exposion coud cause significant harm to peope, the severity of the event was assessed using the foowing word modes based on the Chemtura risk matrix, as used for COMAH risk assessments. Leve 1 Catastrophic mutipe fataities on-site Leve 2 Disastrous fataity or mutipe major injuries on-site Leve 3 Serious major injury or mutipe severe injuries on-site Leve 4 Significant Reportabe accident/lta Leve 5 Minor first aid cases. Step 4 Identify Charging Mechanisms Where the ignition of an exposive atmosphere can cause significant harm, potentia charging mechanisms during processing activities were identified that have the potentia to ead to a static discharge. Consideration was given to norma charging mechanisms for iquids such as fow in pipes, spash fiing into tanks or stirring in vesses. For powders the mechanisms incuded charging from bags to vesses and pneumatic conveying. Consideration was aso given to the potentia for static discharges caused by charged operating personne approaching an exposive atmosphere. Scenarios were screened from further assessment on the basis that the charging mechanisms invove ow static risk iquids with conductivity >1,000 ps/m or ow static risk powders with resistivity <10 6 ohm/m [Ref 2], if processed within we earthed and conductive equipment. It is very unikey for iquids or powders outside these imits to generate significant charges during norma processing activities found in the Process Industry. Step 5 Assess criticaity of Anti-static contro measures A criticaity assessment was carried out for anti-static contro measures to determine those judged to be critica. This used a risk based methodoogy simiar to the method used on the site for Safety Integrity Leve (SIL) determinations for safety instrumented systems. The method considers a number of factors to determine if the anti-static contro 4
measures need to be Standard or Critica, with the atter requiring the highest standard of contro measures. The first stage invoves assess the ikeihood for an exposive atmosphere within equipment taking account of whether an exposive atmosphere is a norma occurrence or whether one or more faiures need to occur before an exposive atmosphere can be formed. From this assessment the foowing hazardous area zones are determined: Zone 0 exposive atmosphere continuousy, for ong periods or frequenty, normay assumed >1,000 hr/yr, probabiity assumed =<1.0 Zone 1 exposive atmosphere in norma operation occasionay, normay assumed 10 1,000 hr/yr, probabiity assumed <0.1 Zone 2 exposive atmosphere not ikey in norma operation and ony for a short period, normay assumed <10 hr/yr, probabiity assumed <0.001 An occupancy factor is seected as F B (100%) for heaviy occupied areas or where the operator causes the exposion to occur, or F A (10%) for areas that are not usuay occupied. An avoidance factor is seected as P B (100%) where avoidance is not ikey or P A (10%) to account for avoidance factors such as exposion reief, exposion suppression or where it is judged unikey for the operator to suffer the stated eve of harm. Using these factors and the consequence eve in step 3, the criticaity of anti-static contro measures is determined from the risk graph in Tabe 1. This risk graph has been caibrated to achieve a risk beow the target of beow 3 10-5 per year for operator fataity. The static ignition frequency used in the caibration was estimated as 3/yr for Standard measures and 0.03/yr for Critica measures. These vaues are considered to be conservative estimates athough there is a ack of pubished sources for ignition frequencies. The risk graph determines anti-static contro measures as at one of the foowing eves; S (Standard) or C (Critica). Step 6 Layer of protection Anaysis For cases where the risk graph gives a Q (Quantify) resut, a more detaied risk assessment technique is required. In this case it invoved a Layer of Protection Anaysis (LOPA) as used by Chemtura for SIL determination for high hazard safety instrumented systems and Tabe 1. Risk graph for determination of criticaity for anti-static measures Consequence Leve 5 4 3 2 1 F A F B P A P B Zone 0 Zone 1 Zone 2 S S S C S S C C S Q C S Q Q C Q Q C Q Q Q 5
for aspects of the COMAH Safety Report. The same approach was used for this study based on an Exce spreadsheet. Step 7 Verification of Anti-static contro measures A anti-static contro measures in pace at Trafford Park for the events in step 4 were reviewed against reevant good practice [Ref 2]. If the contro measures were assessed as Critica, a proportionatey more in-depth review was competed ooking for compiance with the most stringent requirements from the standards. For exampe, where earth cabes need to be attached to mobie equipment, there shoud be an earth proving interock to prevent the charging mechanism. The verification process invoved checking the system design on the P&ID s, discussions with Chemtura staff and a site audit to inspect the equipment systems. This stage of the assessment resuted in a number of recommendations for improvements to the anti-static contro measures. PRACTICAL EXPERIENCE Physica Properties Data Gathering conductivity and resistivity data on the wide range of iquids and powders handed on the site was made difficut by the imited number of sources. This type of information is generay unavaiabe on Materia Safety Data Sheets (MSDS) but can be found for common substances in speciaist pubications [Ref 5]. In some cases it was necessary to take sampes of substances from the process for anaysis, especiay for process intermediates. An exampe was waste xyene that had previousy been assumed as high conductivity and ow risk due to the presence of impurities incuding 2 3% acetone. Anaysis of sampes showed the conductivity for waste xyene to be typicay around 400 ps/m, and therefore requiring further assessment as a medium risk substance. Of nineteen fammabe iquids handed on the site, ten were screened out with high conductivity and therefore ow static risk, with the highest risks posed by an organic peroxide, xyene, diese and thermino heat transfer fuid. A three of the organic powders handed on the site were known to be high resistivity from previous speciaist anaysis. One of these materias was screened from further assessment as its minimum ignition energy was above 500J. Exposive Atmospheres For a fammabe iquids and combustibe powders handed on the site an assessment was carried out on the unit operations where these substances are processed, and a tabe competed with the foowing headings: Unit Operation: Pant, main equipment item and fowsheet reference Exposive atmosphere: Description of the mechanism by which an exposive atmosphere may form in the equipment and an assessment of the hazardous area as zone 0, 1 6
or 2. This made reference to the fash point for vapours, inert banket systems on the equipment, heating systems and any trips or aarms. For powder handing operations it aso considered the potentia for dust couds above the LEL to be formed. In some cases invoving vapours it was concuded that an exposive atmosphere was not credibe and the unit operation was screened from further assessment. Consequence of Ignition: The consequences of ignition of the exposive atmosphere in terms of harmfu effects to peope were assessed and the worst credibe severity rating set as eve 1 to 5. This assessment considered the maximum exposion pressure and the ikeihood for vesse rupture, pus the scae of the credibe exposion effects. Reference was made to consequence assessment of scenarios in the COMAH risk assessment to ensure consistency. Some events were screened from further assessment due to the exposion being contained in the equipment or of such a sma scae that significant harm was not credibe. Charging Mechanism: The credibe mechanisms for static charge generation were identified for each unit operation, such as pumped offoading into a storage tank, powder fow from a big bag during emptying, or charged operators approaching an exposive atmosphere. These mechanisms were screened out as not credibe for substances with ow static risk, i.e. high conductivity iquids or ow resistivity powders processed in we earthed and conductive equipment. Criticaity: For credibe charging mechanisms, the criticaity of the contro measures was assessed as Standard or Critica using the risk graph or LOPA method described earier. This coumn gives the resuts of the assessment and any assumptions made for occupancy and avoidance factors. Anaysis of Resuts Thirty four unit operations were assessed in tota with five screened out as no credibe mechanism for an exposive atmosphere. One event was screened out for minor consequences due to the sma voume of exposive dust/air present. Sixteen of the events were screened out for no charging mechanism, mosty due to the ow static risk substances invoved. Nine events were assessed using the risk graph approach with seven rated Standard and two rated Critica. The remaining three events were rated high risk on the risk graph method and were assessed using LOPA, a resuting in a rating as Standard. Verification of Contro Measures The tweve events with static contro measures rated as Standard or Critica were verified against the requirements of the standards, and a number of improvement recommendations made, with exampes as foows: Provide anti-static footwear for operators and re-instate equipment and procedure for checking operator earth resistance at start of shift. Earth proving system at peroxide drum offoading to be interocked with offoading pump. 7
Provide nitrogen inerting system on peroxide and xyene storage tanks with ow pressure trip of transfer pump. Check that air diaphragm pump cannot deiver at veocity above 7 m/s even with air reguator adjusted in error. Confirm routine inspection and testing of fixed earthing on equipment to achieve earth resistance beow 10 ohm. Confirm that powder is suppied in type C big bags with provision for attaching an earthing camp for interocked earth proving system. Provide anti-static fexibe hoses on dust extraction system to ensure earth continuity of system. Provide earth camp system for soids charge chute to reactor. Provide dip-pipe on head tank fiing ine to prevent spash fiing. Exampe 1: Waste Xyene Road Tanker Loading Waste sovent is top oaded from a storage tank via a pump to a road tanker barre. The fash point of waste xyene is at or above 21 degc, meaning that an exposive atmosphere coud occur reguary in the tanker. It is not feasibe to nitrogen banket the tanker prior to oading. The atmosphere inside the tanker is therefore assessed as zone 0, as it wi reguary contain an exposive xyene/air mixture. An exposion in the tanker woud cause the manid to be bown open (this is oose during tanker oading) causing a fash fire from the opening and possibe missie hazards. This was assessed as ikey to cause a serious injury to a person in the immediate vicinity of the road tanker, therefore consequence eve 4. It was assumed that occupancy in the area is 100% as the tanker driver stays during oading, and the avoidance of a serious injury was judged as 10% based on the position of the driver and the fash fire expected to be directed verticay from the opening. Using the risk graph approach with consequence eve 4, zone 0, occupancy 100% and avoidance 10%, the anti-static contro measures were rated as Critica. Eectrostatic charge coud be generated by fow through the 3 charging ine but the fowrate of 12 m 3 per hour imits the ine veocity to 0.7 m/s. This is we within the imit for singe phase fow from the standard [Ref 2] of 7 m/s, and beow the imit of 1 m/s for 2-phase fow. Charge coud aso be generated by spash fiing of the road tanker from the top entry fi ine as there is no dip-pipe on the tanker. A recommendation was raised for modifications to the fiing ine to prevent spash fiing. As part of the tanker oading procedure the operator connects an earth cip to the tanker to ensure that charge cannot accumuate on the tanker. It is possibe for the cip to be eft off in error. It was recommended that an earth proving unit is provided with an automatic interock to the oading pump to prevent oading if the earth cip is not effectivey connected. Exampe 2: Dust Fiter Exposion A dust fiter handing exposive dust was judged to be capabe of causing a major injury to operators in the area if an exposion occurred. Using the risk graph method the anti-static 8
measures were assessed as eve Q, therefore requiring a more detaied quantified assessment. Using the LOPA method, the frequency of static ignitions capabe of causing an ignition were initiay estimated as 3/yr, based on Standard contro measures. The atmosphere within the fiter was cassified as zone 1 due to the potentia for frequent exposive atmospheres. These figures were considered very conservative as they suggest an exposion woud occur 3 times per year. In practice there have been no exposions in the fiter over severa years of operation. The risk of major injury is then reduced by 0.01 for the exposion reief pane on the fiter, by 0.1 for the ow occupancy in the area, and by 0.1 for the chance of suffering a major injury. The LOPA assessment concudes that the anti-static contro measures shoud be at the Standard eve. CONCLUSIONS This paper has described a Static Hazard Assessment carried out on the Chemtura COMAH top tier site at Trafford Park, in accordance with the requirements of DSEAR. A structured methodoogy was deveoped that coud be appied to other sites that hande a range of fammabe or combustibe materias. The approach was to screen out ow risk scenarios and focus attention on cases where the risk of static ignition causing an exposion is judged to be significant. Cases were screened out when there was no potentia for an exposive atmosphere, no significant harmfu effects from an exposion or a ow risk of eectrostatic discharge due to the high conductivity of the substance. For cases where the static hazard was judged to present a significant risk, a risk graph approach was used to assess the criticaity of the anti-static contro measures. This identified situations where the contro of static is judged to be Critica, and the most stringent measures need to be appied. This structured approach provides a justification for taking extra measures for some situations whist appying generic measures in others. A number of specific improvements were identified during the assessment that provide a targeted reduction in static risks and a demonstration that the risk of static reated major accidents has been reduced to as ow as reasonaby practicabe. The imited number of static hazards assessed as requiring Critica contro measures refects the good design principes that have been appied at the Trafford Park site. The genera design approach has been to avoid exposive atmospheres in equipment by operating beow the materia fash point or using a nitrogen inert atmosphere where this is not possibe. For situations where an exposive atmosphere cannot be prevented, mitigation measures have normay been provided in the form of exposion reief systems to reduce the effects of an exposion. REFERENCES 1. HSE, L138, 2003, Dangerous Substances and Exposive Atmospheres Reguations 2002, Approved Code of Practice and Guidance, HSE Books 9
2. BSI, 2003, Eectrostatics Code of practice for the avoidance of hazards due to static eectricity, PD CLC/TR 50404:2003, British Standards Institute 3. HSE, L136, 2003, Contro and Mitigation Measures, Approved Code of Practice and Guidance, HSE Books 4. BSI, 1991, Code of practice for contro of undesirabe static eectricity Parts 1 & 2, BS5958-1 & BS5958-2:1991, British Standards Institute 5. Britton, L., 1999, Avoiding static ignition hazards in chemica operations, CCPS, American Institute of Chemica Engineers 10