The Port Talbot Steelworks (Power Generation Enhancement) Order. MAH High Level Assessment of Major Accident Hazards

Transcription

1 Planning Act 2008 The Infrastructure Planning (Applications: Prescribed Forms and Procedure) Regulations 2009 The Port Talbot Steelworks (Power Generation Enhancement) Order MAH High Level Assessment of Major Accident Hazards PINS Reference Document No. Author EN MAH1.01 Pinsent Masons Revision Date Description 0 April 2015 Submission Version

2 The Port Talbot Steelworks (Power Generation Enhancement) Order High Level Assessment of Major Accident Hazards MAH1.01 Contents 1. Introduction Executive Summary Review of Major Hazards Major Loss of Containment of Fuel Gas Conclusions Appendices: Appendix 1 - Process Risk Assessment Process Appendix 2 - Calculations Appendix 3 - DNV-GL Corus Technical Note Appendix 4 - Port Talbot Steelworks Safety Report 2013 April

3 The Port Talbot Steelworks (Power Generation Enhancement) Order High Level Assessment of Major Accident Hazards MAH Introduction 1.1 This report has been written in support of an application by Tata Steel UK Limited ("Tata") under the Planning Act 2008 for a development consent order ("DCO") to construct, operate and maintain a process gas-fired power generating station ("the Proposed Development") within the Port Talbot Steelworks at Port Talbot, South Wales ("the Application"). 1.2 The report contains a high-level assessment of the extent and severity of known major hazards at the Proposed Development with the potential to impact on the existing steelworks installation and the local population. 1.3 As a result of the presence of 'hazardous substances' 1 at installations within the Port Talbot steelworks site, the site is a Control of Major Accident Hazards (COMAH) Tier 1 site. Tata is therefore obliged to hold and maintain a 'safety report' for the site under the terms of the Control of Major Accident Hazards Regulations 1999 ("the COMAH Regulations"). 1.4 Although the Proposed Development would not itself be a COMAH site for the purposes of the COMAH Regulations, it would be situated within the formal 'consultation zone' of the steelworks which is a regulated 'establishment' 2. When new facts arise or modifications take place within a regulated establishment, the safety report for that site may need to be reviewed under the terms of regulation 8 of the COMAH Regulations. 1.5 As no additional qualifying inventories of COMAH substances will be present at the Proposed Development, Tata currently considers that the current 2013 COMAH Safety Report for the steelworks site will not need updating. Instead, the presence of the Proposed Development will be reflected in the 2018 Port Talbot COMAH Safety Report. Final decisions in relation to the COMAH Safety Report will be taken once more information becomes available on the design of the Proposed Development. This approach has been verbally agreed with Tata's Principal HSE Inspector, Mr Alan Strawbridge. 1.6 Tata has adopted a six stage risk assessment process to review health and safety risks within the consultation zone of the Port Talbot Steelworks COMAH site. The stages in this process are set out in detail in the 2013 Port Talbot Safety Report, an extract of which outlining these phases is enclosed at Appendix 1 to this report. 1 As defined in regulation 2 of the Control of Major Accident Hazards Regulations As defined in regulation 2 of the COMAH Regulations. April

4 The Port Talbot Steelworks (Power Generation Enhancement) Order High Level Assessment of Major Accident Hazards MAH Tata's Process Safety Department (part of the Health, Safety and Environmental Department) has carried out a high-level, initial assessment of the potential for increased risk to health and safety at the Port Talbot Steelworks as a result of the Proposed Development. This initial review forms part of the existing six-stage framework. The Proposed Development has passed the first (HS1) stage of this package of risk assessments and the findings of that initial assessment are set out in this report, the "HS1 Stage 'High Level' Risk Assessment". 1.8 The Proposed Development will in due course be assessed at the HS2 stage, when more detailed information is available about the detailed design. The HS2 stage will result in a more granular and detailed risk assessment. Both HS1 and HS2 stages are in the Inherently Safe Design phase where actions and recommendations focus on eliminating, reducing, replacing or simplifying the threats and sources of hazards. 1.9 As the design progresses, a more detailed set of Process Hazard Review and Risk Assessments (HS3) will be carried out to ensure that the risks are controlled to As Low As Reasonably Practicable (ALARP). Where Major Accident Hazards are identified, Layer of Protection Analysis (LOPA) studies will be carried out to ensure that the causative mechanisms are credible, that the safeguards claimed are documented, any improvements recommended from the risk assessments captured and an ALARP demonstration made As stated above, it has been agreed with Tata's Principal HSE Inspector that the current 2013 COMAH Safety Report will not need updating at present. Tata will, however, continue to complete the six-stage assessment process in respect of the Proposed Development and will keep the HSE informed of any change in assumptions, risks or proposed approach. April

5 The Port Talbot Steelworks (Power Generation Enhancement) Order High Level Assessment of Major Accident Hazards MAH Executive Summary 2.1 This assessment identifies that the only possible, known major hazard from the Proposed Development is the potential for major loss of containment fuel gas from inside the power generation plant building due to a guillotine of gas pipework and a subsequent escape of fuel gas from the power generation plant building. Such an escape would have the potential to impact both the wider Port Talbot Steelworks site and the local community, most especially those in Tiabach. 2.2 This assessment concludes that such an escape is a barely credible threat and, in the unlikely event that it did occur, a number of controls would be in place to limit the duration of any such release and therefore the risk to the public. The assessment further finds that, even if the control measures failed to prevent or mitigate a catastrophic release of fuel gas, the Proposed Development poses no additional Major Accident Hazard to on-site or off-site populations. In other words, the presence of the Proposed Development would not increase the risk from the rest of the steelworks site. 2.3 The information contained in this report is intended to demonstrate the suitability of the site for the Proposed Development and show that Tata has adequately considered the impact of known hazards from the Proposed Development to the public. Potential risks will continue to be considered as the design of the Proposed Development is progressed to ensure that any such risks are adequately managed and controlled through the design, site safety procedures and through Tata's ongoing obligations under the COMAH Regulations. April

6 The Port Talbot Steelworks (Power Generation Enhancement) Order High Level Assessment of Major Accident Hazards MAH Review of Major Hazards 3.1 An assessment of the existing risks on the Port Talbot is given in the Port Talbot 2013 COMAH Safety Report', which is appended to this report. 3.2 The Proposed Development comprises only a very small addition to the existing inventory of hazardous substances on the Port Talbot Site. The only hazardous substance affected by the Proposed Development is the blast furnace gas (BFG), the volume of which will increase by only around 100m³ (of a current total volume of around 100,000m³). In addition, although the existing pipe work carrying the BFG will be extended slightly to accommodate the Proposed Development, the location of the hazardous substances within the site is not considered to change in any significant way, and in particular the BFG will not be brought significantly closer to the boundary of the site within these new pipes. The risk of known major hazards across the Port Talbot COMAH site is therefore not considered to increase to the extent that the current safety report needs to be reviewed. This conclusion has been discussed with Tata's HSE Principal Inspector and will be kept under continual review as the assessment progresses through Tata's 6 stage process. 3.3 Tata has concluded that the only new or different major hazard that could potentially arise from the Proposed Development is the possibility for major loss of containment of fuel gas from inside the power generation plant building of the Proposed Development and a subsequent escape of gas from that building. The potential consequences of such an incident are described in section 4 below. April

7 The Port Talbot Steelworks (Power Generation Enhancement) Order High Level Assessment of Major Accident Hazards MAH Major Loss of Containment of Fuel Gas 4.1 Major loss of containment of fuel gases (one of which is highly toxic) from inside the proposed power generation plant building and a subsequent escape of gas from that building would have the potential to impact both the wider Port Talbot Steelworks site and the local community, most especially those in Tiabach. 4.2 In order to capture the likelihood and potential severity of such a major loss of containment of fuel gas from the Proposed Development, Tata has reviewed early plans for the design of the Proposed Development and undertaken dispersion modelling. The results of this modelling are set out in Appendix 2 of this report. 4.3 Blast-furnace gas ("BFG") is the primary fuel gas for the Proposed Development. BFG has a high ignition temperature and low calorific value and therefore does not ignite easily. BFG also contains some toxic gases which are harmful to human health, including carbon monoxide. Natural gas, which the proposed development may use as back up fuel, would ignite more easily but is not as toxic as BFG. 4.4 Although the likelihood of either fuel gas igniting (in an uncontrolled way) is very low, should it occur, the Proposed Development will not impact on the risk profile of the wider Port Talbot. This is because: the nature of the hot fired plant means there would be many sources of ignition local to any release, such that fires or delayed ignitions are likely to occur soon after release and inside the power generation building only; and as the Proposed Development is some considerable distance from all of the main installations and buildings located on the wider Port Talbot site, such an ignition with the power generation building would not increase risks to other parts of the site. 4.5 The nature of natural gas means that, were it to escape, it is likely the natural gas would ignite quickly and close to the point of release. In contrast, BFG would not ignite easily upon its release and is therefore more likely to escape from the boundary of the Proposed Development. This means that the Major Accident scenarios impacting beyond the boundary of the Proposed Development are limited to a toxic gas release of BFG from inside the power generation plant building of the Proposed Development. This would only occur due to a guillotine or damage to one or both of the 72 diameter BFG fuel gas pipelines. 4.6 If the fuel pipes carrying BFG were to be damaged outside the buildings housing the Proposed Development, the leaked gas would be contained within the existing ducts carrying the piping and would travel at. The risk posed by damage to external pipes carrying BFG is something which is already covered by the existing Safety Report and, as the length of additional pipework and ducting April

8 The Port Talbot Steelworks (Power Generation Enhancement) Order High Level Assessment of Major Accident Hazards MAH1.01 required for the Proposed Development is minimal (and does not take the BFG piping closer to the site boundary), this is a risk which is not increased by the introduction of the Proposed Development. 4.7 The possibility of fuel pipes carrying BFG being damaged inside buildings of the same size and location as those which will be required to house the Proposed Development is however not a scenario which is directly covered in the existing Safety Report. Tata has therefore considered this in more detail (including undertaking dispersion modelling) as it represents the only Major Accident scenario that could impact beyond the boundary of the Proposed Development. 4.8 It should be noted that it is highly unlikely that fuel gas pipes would be damaged because there are no credible causative mechanisms. Even if the pipe carrying the fuel gases was somehow to be guillotined or damaged, it is considered even more unlikely that such an escape of gas would go unnoticed for a significant period of time because there will be a number of controls in place to limit the duration of such a release. Such controls will include toxic and flammable gas detection systems and alarms both inside and outside the perimeter of the building. 4.9 These controls will be built into the final design of the Proposed Development and will be incorporated into the operation of the Proposed Development via the successful contractor under Tata Steel Design requirements. Dispersion Modelling Methodology 4.10 In the very unlikely event that fuel gas pipes were to be damaged inside the buildings of the Proposed Development, the effect of such a release on local populations would depend on the duration of the release, how it would be contained and the extent to which the gas disperses. To measure the possible effects of such an event, Tata has undertaken dispersion modelling. The results of this modelling are presented in Appendix Whilst the chemistry of the fuel gases is well documented, data about the final design which is needed to enable precision modelling is not available at this stage. Assumptions have therefore been made regarding the location and size of building vents to allow the dispersion modelling to be carried out The modelling has assumed that the building is a pressure vessel that has been pressurised to 50mbg by a massive Loss of Containment (LoC) of un-ignited BFG which is vented to atmosphere from vertically orientated roof vents at 35m elevation. These assumptions are based on the size of volume relief vents elsewhere on the steelworks site and are considered reasonable based on the information presently available. Should these assumptions change as the design progresses, Tata will revisit this modelling as part of the continuing six-stage assessment process it is committed to in order to comply with its COMAH obligations for the steelworks site. April

9 The Port Talbot Steelworks (Power Generation Enhancement) Order High Level Assessment of Major Accident Hazards MAH Tata has completed this initial modelling internally using Det Norke Veritas (DNV- GL) PHAST 7.0 modelling software and has also used previous similar modelling from both DNV-GL and Hazard Research & Risk Consultants Ltd (HazRes) to lend validation to the models Further modelling will be carried out as the design is finalised, to feed into more detailed assessments of the risk of the Proposed Development as part of the sixstage process described above Although Tata Steel have a DNV-GL PHAST software licence and have competence in its use for dispersion and explosion modelling, it is recognised that, at a later stage, it will be appropriate for the models to either be conducted or validated by an independent and competent third party. Such a requirement and the outcome of a further analysis will be discussed with the Health and Safety Executive when appropriate. Dispersion Modelling Estimate of Severity 4.16 Although the models are based on the best data available (including assumptions frequently made and the historical evidence), it must be emphasised that the likelihood of these events occurring is considered negligible, not only in scale but in causative mechanism Nevertheless, should such an event take place, the modelling demonstrates that the most significant potential consequence would be fatalities associated with the toxicity of the BFG, as BFG contains a large proportion of carbon monoxide The dispersion modelling shows that any highly toxic gas plumes emanating from the power generation building of the Proposed Development would rise higher than the 35 metre roof line of the buildings on site and would rise to in excess of 50 metres above as they crossed the perimeter road. By the time any such plumes were to reach neighbouring properties, the plumes would be over 100 metres high and therefore highly unlikely to cause a risk to health If the wind was blowing towards the wider Port Talbot site there is the potential for any escaped plumes (especially plumes of lower concentrations) to reach any person working at the higher heights on the blast furnaces (such work only ever being intermittent and temporary). This event is however far less likely to occur than the impact of a loss of containment of BFG from one blast furnace area to another, which is a risk already discussed in that installation s risk assessments and which would invoke an established plant evacuation procedure As the design of the Proposed Development is finalised, measures in alignment with best practice and applicable guidance and standards to prevent, control and mitigate these scenarios will be implemented These best practice measures will be determined as the design progresses, more accurate design data becomes available and modelling is carried out. The April

10 The Port Talbot Steelworks (Power Generation Enhancement) Order High Level Assessment of Major Accident Hazards MAH1.01 measures will be secured via further risk assessments as the design progresses in accordance with Tata Steel Safety Management System and auditable implementation at the HS4 stage of the Tata Steel six stage project Risk Assessment process. April

11 The Port Talbot Steelworks (Power Generation Enhancement) Order High Level Assessment of Major Accident Hazards MAH Conclusions 5.1 The Proposed Development at the Port Talbot steelworks site would pose no additional Major Accident Hazard to on-site or off-site populations even if the control measures fail to prevent or mitigate a catastrophic release of BFG due to guillotine of the toxic 72 BFG pipework within the power generation plant within the Proposed Development. April

12 The Port Talbot Steelworks (Power Generation Enhancement) Order MAH High Level Assessment of Major Accident Hazards Appendices

13 The Port Talbot Steelworks (Power Generation Enhancement) Order MAH High Level Assessment of Major Accident Hazards Appendix 1: Process Risk Assessment Process

14 Appendix 1: Six Stage Project Risk Assessment Process Reference Page Revision Date Prepared by PT Power Plant Development 1 Issue 1 18/08/2011 Malcolm Warren

15 WHY Process Safety Flowchart OUTCOMES Introduction to Project PROJECT OVERVIEW Produce brief overview describing project Defines ownership and authorisation of Project Key Stages MOC UMBRELLA DOCUMENT: Authoriser, Approver Captures brief project description Determine an outline of proposals together with a cost indication Identify high level/ low cost impacting changes required to satisfy safety, environmental and asset issues Scope Definition Studies and re-design to remove hazards INHERENT SAFETY STAGE HAZARD IDENTIFICATION TENDER (Based On User Requirement Specification) STRATEGIC HAZARDS IDENTIFIED: TEAM Event Carried Out On Scope Definition Studies, Planning Permission Manning, Maintainability Infrastructure, Service/utilities User Requirement Specification developed Review design proposals from selected contractors CONTRACTOR SELECTED Engineering progressed to an Accurate Design Representation and Functional Design Speciification suitable for HAZOP etc. Hazards require risk reduction by Safeguards or re-design Re-Design to remove hazards or include additional Safeguards FUNCTIONAL SAFETY STAGE PROCESS HAZARDS IDENTIFIED: Team Event Inputs include any existing PHR/LOPA/ HAZOP s Define Hazards and Safeguards PROPOSED DESIGN HAZOP ALARP Demonstration and determination of residual risk Carry out SIL/LOPA studies for any SISs All Safeguards defined to Contractor Confirm Final Design Layer Of Protection Analysis (LOPA) and CBA for ALARP Demonstration SAFEGUARD REGISTER FINAL DESIGN (including FDS) fixed for Construction RISKS ASSESSED; HAZAN, QRA Safety Functions Allocated to Safeguards and CBA carried out. SAFEGUARD DEFINITION DOCUMENT: Procedures Safety Instrumented Systems (SIS), Non Return Valves (NRV), Pressure Relief Valves (PRV), DESIGN COMPLETION: risks controlled to ALARP ALARP demonstration cases in progress Manage the design change MOC RECORD ALL CHANGES RECORDED: Risk assessments carried out and recorded for changes. Manage/monitor project tasks MOC TASKLIST PROGRESS PROJECT TASKLIST: attach into MOC record Manage/monitor project phases PROGRESS MOC RECORD THROUGH TO COMPLETION Approval Authorisation Installation & Commissioning Completion Reference Page Revision Date Rev.6 (10/1/13) Prepared by PT Power Plant Development 2 Issue 1 18/08/2011 Malcolm Warren

16 The Port Talbot Steelworks (Power Generation Enhancement) Order High Level Assessment of Major Accident Hazards Appendix 2: Calculations

17 The Port Talbot Steelworks (Power Generation Enhancement) Order High Level Assessment of Major Accident Hazards MAH Calculated Extent PHAST Several projects which included works to the gas infrastructure have been carried out by Tata Steel at Port Talbot Steelworks in recent years (for example BOS Gas Recovery, BF4 Re-Build, New Perimeter Road and the New COG Holder Projects) and, as a result, a great deal of independent dispersion modelling has been carried out for major Losses of Containment. The BOS Gas recovery project in particular engaged DNV-GL to carry out modelling of release of both BOS Gas and BFG from the volume relief vents in the new BOS Gas Holder. To capitalise further on this existing work the BFG release model was used to calibrate and lend validation to the internal model of BFG LoC from roof vents in the new power plant, this calibration can be reviewed by comparing the 5D weather conditions 12800ppm CO plume shown in Graph 2 with Graph 1 from Figure of the DNV-GL Corus Technical Note, see Appendix 3 which shows a reasonable correlation. Graph 1 from of the DNV-GL Corus Technical Note NB: (yellow is 55% CarboxyHemaglobin COHb which broadly corresponds to ppm CO)

18 The Port Talbot Steelworks (Power Generation Enhancement) Order High Level Assessment of Major Accident Hazards MAH1.01 Graph 2 Tata Steel Model Calibration Using the model configuration and settings used to generate Graph 2 the release point and area was changed to 35 metres elevation, the Power Plant roof level, and run for 1 Metre diameter and 2 Metre diameter (total area equivalent of 5 x 1 m dia) vertically orientated roof vents. Graph 3 and Graph 4 show that the plume does not touch the ground in both 2F weather conditions and 5D weather conditions in both cases. Graph 3: 1 metre diameter roof vent

19 The Port Talbot Steelworks (Power Generation Enhancement) Order High Level Assessment of Major Accident Hazards MAH1.01 Graph 4: 5 x 1 metre diameter roof vents

20 The Port Talbot Steelworks (Power Generation Enhancement) Order High Level Assessment of Major Accident Hazards Appendix 3: DNV-GL Corus Technical Note

21 DET NORSKE VERITAS PHAST Modelling of BOS and Blast Furnace Gas Releases Corus Strip Products UK Report no/dnv ref no: Rev 3, December 2008

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23 DET NORSKE VERITAS PHAST Modelling of BOS Gas Releases Corus Strip Products UK MANAGING RISK Table of Content Page 1 INTRODUCTION APPROACH Release Scenarios Hazard Levels Assumptions RESULTS Release Modelling Consequences DISCUSSION APPENDIX I: DERIVATION OF CO CONCENTRATIONS OF INTEREST APPENDIX II: WORST-CASE DISPERSION TO CO CONCENTRATIONS OF INTEREST II.1 Gas Holder II.2 Distribution System II.3 Collection System APPENDIX III: WORST-CASE DISPERSION TO CO CONCENTRATIONS OF INTEREST FOR THE NEW MODELLED CASES III.1 Gas Holder III.2 Distribution system Date : <01/12/2088>

24 DET NORSKE VERITAS PHAST Modelling of BOS Gas Releases Corus Strip Products UK MANAGING RISK Date : <01/12/2088>

25 DET NORSKE VERITAS PHAST Modelling of BOS Gas Releases Corus Strip Products UK MANAGING RISK 1 INTRODUCTION Corus Strip Products plan to build a gasholder for BOS Gas and associated piping at their Port Talbot works in South Wales. BOS gas contains 50% to 60% carbon monoxide depending on temperature, besides hydrogen, nitrogen, carbon dioxide and water vapour. It is therefore both toxic and flammable. Corus have identified potential worst case scenarios for releases of BOS gas from the gas holder, associated piping (which is of large diameter) and vents. They have contracted DNV to model the toxic and flammable consequences of these releases using DNV s commercial software PHAST and to present these in terms of distances to specified hazard levels. Corus hold licences for PHAST and are therefore familiar with the software. Section 2 of this Technical Note summarises the approach DNV have adopted in discussion with Corus, including the scenarios specified, hazard levels modelled and assumptions made. Section 3 first of all sets out the results obtained from the modelling of release rate, which determines the duration of most of the release scenarios and in turn the toxic concentrations of concern. This section then presents in tabular form the worst case hazard distances for each scenario and effect level. The results are briefly discussed in Section 4. 2 APPROACH This section summarises the release scenarios and hazard levels modelled, as well as summarising any assumptions made. 2.1 Release Scenarios The plant items that were considered for the purposes of this analysis are: Gas Holder, which includes the Gas Holder, Inlet Stub, Outlet stub and a series of vents. Distribution System, which mainly includes piping. Collection System, which includes piping and the Cooler Outlet. Corus have indicated that a series of release scenarios from each of the above plant items are to be considered as potential releases. These are summarised in Table 2.1. Only two (2) BOS gas compositions were modelled, at temperatures of 40 and 70 C, saturated, and at pressures slightly over atmospheric. In addition, some new cases have been requested to be modelled. These are summarized in Table 2.2. Three (3) BOS gas compositions were modelled, at temperatures of 20 C, 40 C and 70 C, saturated, and at pressures slightly over atmospheric. In addition, release of Blast Furnace Gas (BFG) from the Gas Holder vents has been taken into account. Date : <01/12/2088> Page 1

26 DET NORSKE VERITAS PHAST Modelling of BOS Gas Releases Corus Strip Products UK MANAGING RISK Table 2.1: Release Scenarios Considered Plant Item Component Vessel type / Pipe and diameter Failure mode Potential release Nm 3 Substance Temperature ( C) Pressure (bara) Release height (m) Gas holder Gas holder Inlet stub Outlet stub Tank Pipe 2300 mm dia Pipe 2300 mm dia Pipe 2300 mm dia Pipe 2300 mm dia Pipe 1370 mm dia Pipe 1370 mm dia Pipe 1370 mm dia Pipe 1370 mm dia Hole 500 mm dia Hole 500 mm dia Guillotine fracture Hole 500 mm dia Guillotine fracture Hole 500 mm dia Guillotine fracture Hole 500 mm dia Guillotine fracture BOS gas BOS gas BOS gas BOS gas BOS gas BOS gas BOS gas BOS gas BOS gas Vent Pipe 650 mm dia Open end Continuous BOS gas Pipe 650 mm dia Open end Continuous BOS gas Vents Pipe 650 mm dia 12 Open end Continuous BOS gas Date : <01/12/2088> Page 2

27 DET NORSKE VERITAS PHAST Modelling of BOS Gas Releases Corus Strip Products UK MANAGING RISK Plant Item Component Vessel type / Pipe and diameter Failure mode Potential release Nm 3 Substance Temperature ( C) Pressure (bara) Release height (m) Pipe 650 mm dia 12 Open end Continuous BOS gas Distribution System Collection system Pipe Pipe Cooler outlet Pipe 1370 mm dia Pipe 1370 mm dia Pipe 1370 mm dia Pipe 1370 mm dia Pipe 2300 mm dia Pipe 2300 mm dia Pipe 2300 mm dia Pipe 2300 mm dia Pipe 2300 mm dia Pipe 2300 mm dia Pipe 2300 mm dia Pipe 2300 mm dia Hole 500 mm dia Guillotine fracture Hole 500 mm dia Guillotine fracture Hole 500 mm dia Guillotine fracture Hole 500 mm dia Guillotine fracture Hole 500 mm dia Guillotine fracture Hole 500 mm dia Guillotine fracture Continuous BOS gas Continuous BOS gas Continuous BOS gas Continuous BOS gas BOS gas BOS gas BOS gas BOS gas BOS gas BOS gas BOS gas BOS gas Date : <01/12/2088> Page 3

28 DET NORSKE VERITAS PHAST Modelling of BOS Gas Releases Corus Strip Products UK MANAGING RISK Table 2.2: Additional Scenarios Considered Plant Item Component Vessel type and diameter Failure mode Potential release Nm 3 Substance Temperature ( C) Pressure (bara) Release height Gas Holder Gas Holder (shell) Inlet Stub Outlet stub Tank Pipe 2300 mm dia Pipe 1200 mm dia Hole 500 mm dia Hole 500 mm dia Hole 500 mm dia Hole 500 mm dia Hole 500 mm dia BOS gas BOS gas BOS gas BOS gas BOS gas Vent Pipe 650 mm dia Open end 30 min Furnace Gas Vents Pipe 650 mm dia x 12 Open end 30 min Furnace Gas Date : <01/12/2088> Page 4

29 DET NORSKE VERITAS PHAST Modelling of BOS Gas Releases Corus Strip Products UK MANAGING RISK Plant Item Component Vessel type and diameter Failure mode Potential release Nm 3 Substance Temperature ( C) Pressure (bara) Release height Distribution system Pipe Pipe 1200 mm dia Hole 500 mm dia 116 tonnes 35 min BOS gas BOS gas BOS gas BOS gas BOS gas BOS gas Date : <01/12/2088> Page 5

30 DET NORSKE VERITAS PHAST Modelling of BOS Gas Releases Corus Strip Products UK MANAGING RISK 2.2 Hazard Levels The BOS gas and BFG compositions modelled are summarised in Table 2.3. Table 2.3: BOS and BFG Gas Compositions Material BOS Gas at 20 C saturated BOS Gas at 40 C saturated BOS Gas at 70 C saturated BFG at 20 C Mol % Mol % Mol % Mol % CO CO H N Water Vapour The following hazard levels were of interest, as specified by Corus, for each of the release scenarios considered: Distances to three (3) CO concentrations, as determined by the CFK equation (refer to Port Talbot Establishment Safety Report) for 25%, 35% and 55% of carboxyhaemoglobin (COHb) in the blood respectively, based on the exposure time (which is assumed to be equal to the release duration, as discussed in Section 2.3). Maximum downwind distances to the Lower Flammable Limit (LFL) and ½ LFL. Distances to thermal radiation levels (from jet fires) of 37.5, 12.5 and 4 kw/m 2. Distances to overpressure levels of 5, 3 and 1 psi. Various CO concentrations derived using the CFK equation for various exposure times, corresponding to 25%, 35% and 55% of COHb in the blood are provided in Appendix I. 2.3 Assumptions This section summarises the key assumptions made for modelling purposes as follows: Two (2) weather categories were modelled, namely D5 and F2, as per UK HSE s guidelines, and agreed with Corus. The PHAST software default atmospheric parameters were used, i.e. a temperature of 10 C, 70% relative humidity and a surface roughness parameter of 0.1, as agreed with Corus. The release orientation was assumed to be horizontal for leaks from piping, and vertical from vents. The guillotine fracture release cases were modelled with both horizontal and downwards, impinging on the ground, release orientations. It is noted here that for a downwards release orientation the default elevation is 0 m (i.e. ). All guillotine fracture releases were modelled as leaks of the equivalent diameter. The vent releases were modelled as open end pipes, i.e. the pipe diameter was used as the equivalent leak hole diameter. Date : <01/12/2088> Page 6

31 DET NORSKE VERITAS PHAST Modelling of BOS Gas Releases Corus Strip Products UK MANAGING RISK A release duration of 1.5 hours was assumed, as agreed with Corus, for all releases described in Table 2.1 as Continuous, using the discharge rate as predicted by the PHAST software. For releases with a defined, limited inventory, the discharge rate was assumed to be constant throughout the release duration, which was determined by the available inventory. For the original scenarios (as set out in Table 2.1), the maximum release duration was assumed, as for the Continuous release cases, to be 1.5 hours. For the additional Gas Holder cases (Table 2.2), the release duration is calculated as the potential release quantity divided by the discharge rate, with no limit. All distances to toxic concentrations and radiation levels of interest are reported at a height of 1.5 m. For the toxic effects, the averaging time was assumed equal to the exposure time, i.e. the release duration (see Section 2.2 for CO concentrations of interest). The maximum downwind distances to LFL and ½ LFL are reported at the cloud centreline height. The explosion modelling was conducted with the PHAST software, using the TNT explosion model (an unconfined vapour cloud explosion model) with the software default values. 3 RESULTS This section summarises the results for all the release scenarios considered, in terms of the release and consequence modelling respectively. 3.1 Release Modelling The release modelling results are summarised in terms of the initial release rate, which was used to determine the release duration for each scenario. For the original scenarios only the maximum release duration was set to 1.5 hours, as discussed in Section 2.3. The release durations were used to determine the CO concentrations of interest for 25%, 35% and 55% of COHb in the blood using the data in Appendix I. The results are summarised in Table 3.1 (original scenarios) and Table 3.2 (new cases). 3.2 Consequences The consequence results for the hazard levels of interest are summarised in Table 3.3. The results are presented for the hazards levels set out in Section 2.2. Side views of the dispersion, showing the hazard levels of interest, are presented in Appendix II. New cases are summarised in Table 3.4 and Appendix III. The values given are for the worst-case outcomes for the different weather categories and release orientations considered (as indicated in the brackets): see footnote to the table. A brief discussion of the results is provided in Section 4. Date : <01/12/2088> Page 7

32 DET NORSKE VERITAS PHAST Modelling of BOS Gas Releases Corus Strip Products UK MANAGING RISK (this page left blank) Date : <01/12/2088> Page 8

33 DET NORSKE VERITAS PHAST Modelling of BOS Gas Releases orus Strip Products UK MANAGING RISK Table 3.1: Summary of Release Modelling Parameters and Results Plant Item Component Vessel type and diameter Failure mode Potential Release (Nm 3 ) Potential Release (kg) Substance Release Orientation Release Rate (kg/s) Duration of Interest (mins) CO Concentrations of Interest (ppm) for % COHb Tank Hole 500 mm dia BOS gas (40) Horizontal Gas holder Horizontal Horizontal Horizontal Pipe 2300 mm dia Hole 500 mm dia BOS gas (70) Horizontal Pipe 2300 mm dia Guillotine fracture BOS gas (70) Horizontal/Down- Impinging on the Inlet stub ground Pipe 2300 mm dia Hole 500 mm dia BOS gas (40) Horizontal Pipe 2300 mm dia Guillotine fracture BOS gas (40) Horizontal/Down- Impinging on the Gas holder ground Pipe 1370 mm dia Hole 500 mm dia BOS gas (70) Horizontal Pipe 1370 mm dia Guillotine fracture BOS gas (70) Horizontal/Down- Impinging on the Outlet stub ground Pipe 1370 mm dia Hole 500 mm dia BOS gas (40) Horizontal Pipe 1370 mm dia Guillotine fracture BOS gas (40) Horizontal/Down- Impinging on the ground Vent Pipe 650 mm dia Open end Continuous Continuous BOS gas (40) Vertical Pipe 650 mm dia Open end Continuous Continuous BOS gas (70) Vertical Vents Pipe 650 mm dia 12 Open end Continuous Continuous BOS gas (40) Vertical Pipe 650 mm dia 12 Open end Continuous Continuous BOS gas (70) Vertical Distribution System Pipe Pipe 1370 mm dia Hole 500 mm dia Continuous Continuous BOS gas (40) Horizontal Pipe 1370 mm dia Guillotine fracture Continuous Continuous BOS gas (40) Horizontal/Down Impinging on the ground Distribution System Pipe Pipe 1370 mm dia Hole 500 mm dia Continuous Continuous BOS gas (70) Horizontal Pipe 1370 mm dia Guillotine fracture Continuous Continuous BOS gas (70) Horizontal/Down- Impinging on the ground Date : <01/12/2088> Page 9

34 DET NORSKE VERITAS PHAST Modelling of BOS Gas Releases orus Strip Products UK MANAGING RISK Plant Item Collection system Component Pipe Cooler outlet Vessel type and diameter Failure mode Potential Release (Nm 3 ) Potential Release (kg) Substance Release Orientation Release Rate (kg/s) Duration of Interest (mins) CO Concentrations of Interest (ppm) for % COHb Pipe 2300 mm dia Hole 500 mm dia BOS gas (70) Horizontal Horizontal Pipe 2300 mm dia Guillotine fracture BOS gas (70) Horizontal/Down- Impinging on the ground Horizontal/Down- Impinging on the ground Pipe 2300 mm dia Hole 500 mm dia BOS gas (40) Horizontal Horizontal/Down- Pipe 2300 mm dia Guillotine fracture BOS gas (40) Impinging on the ground Pipe 2300 mm dia Hole 500 mm dia BOS gas (70) Horizontal Horizontal/Down- Pipe 2300 mm dia Guillotine fracture BOS gas (70) Impinging on the ground Pipe 2300 mm dia Hole 500 mm dia BOS gas (40) Horizontal Pipe 2300 mm dia Guillotine fracture BOS gas (40) Horizontal/Down Impinging on the ground Date : <01/12/2088> Page 10

35 DET NORSKE VERITAS PHAST Modelling of BOS Gas Releases orus Strip Products UK MANAGING RISK Table 3.2: Summary of Release Modelling Parameters and Results (New Cases) Plant Item Component Vessel type and diameter Failure mode Potential release (Nm 3 ) Potential Release (kg) Substance Release Orientation Release Rate (kg/s) Duration of Interest (mins) CO Concentrations of Interest (ppm) for % COHb Hole 500 mm dia BOS gas (70) Horizontal Gas Holder (shell) Tank Hole 500 mm dia BOS gas (40) Horizontal Gas Holder Hole 500 mm dia BOS gas (20) Horizontal Inlet Stub Pipe 2300 mm dia Hole 500 mm dia Outlet stub Pipe 1200 mm dia Hole 500 mm dia Horizontal BOS gas (70, 40, 20) Horizontal Horizontal Horizontal BOS gas (70, 40, 20) Horizontal Horizontal Vent Pipe 650 mm dia Open end - 30 min Furnace Gas Vertical Vents Pipe 650 mm dia x 12 Open end - 30 min Furnace Gas Vertical BOS gas (70) BOS gas (40) BOS gas (20) Distribution system Pipe Pipe 1200 mm dia Hole 500 mm dia - Horizontal BOS gas (70) min release BOS gas (40) BOS gas (20) Date : <01/12/2088> Page 11

36 DET NORSKE VERITAS PHAST Modelling of BOS Gas Releases orus Strip Products UK MANAGING RISK Table 3.3: Summary of Worst-Case Consequence Results Plant Item Gas holder Distribution System Component Vessel type and diameter Failure mode Gas holder Tank Hole 500 mm dia Inlet stub Outlet stub Vent Vents Pipe Pipe 2300 mm dia Pipe 2300 mm dia Pipe 2300 mm dia Pipe 2300 mm dia Pipe 1370 mm dia Pipe 1370 mm dia Pipe 1370 mm dia Pipe 1370 mm dia Pipe 650 mm dia Pipe 650 mm dia Pipe 650 mm dia 12 Pipe 650 mm dia 12 Pipe 1370 mm dia Pipe 1370 mm dia Pipe 1370 mm dia Pipe 1370 mm dia Hole 500 mm dia Guillotine fracture Hole 500 mm dia Guillotine fracture Hole 500 mm dia Guillotine fracture Hole 500 mm dia Guillotine fracture Open end Open end Open end Open end Hole 500 mm dia Guillotine fracture Hole 500 mm dia Guillotine fracture Substance BOS gas (40) BOS gas (70) BOS gas (70) BOS gas (40) BOS gas (40) BOS gas (70) BOS gas (70) BOS gas (40) BOS gas (40) BOS gas (40) BOS gas (70) BOS gas (40) BOS gas (70) BOS gas (40) BOS gas (40) BOS gas (70) BOS gas (70) Release height (m) Reference Figure Distance (m) to CO Concentrations (m) for % COHb Downwind Distance (m) to Distance (m) to Jet Fire Radiation Level (kw/m 2 ) Distance (m) to Overpressure Level (psi) LFL ½ LFL Figure II (5D) 197 (5D) 245 (5D) 7.0 (2F) 17.5 (2F) (2F) 177 (2F) 200 (2F) 8.0 (2F) 17 (2F) (2F) 161 (2F) 8.5 (2F) 17 (2F) 8.5 (2F) 17.5 (2F) 2 Figure II.2 83 (5D) 95 (5D) 105 (5D) 5 (2F) 10.5 (2F) 34 (2F) 41 (2F) (2F) 40 (2F) (2F) (2F) (2F) 39 (2F) Figure II.3 92 (5D, H) 112 (5D, H) 130 (5D, H) 29 (2F, D) 53 (2F, H) 84 (5D, H) 124 (2F, H) 148 (2F, H) 59 (H) 62 (H) 75 (H) 2 Figure II (5D) 210 (5D) 240 (5D) 8.0 (2F) 15.0 (2F) 34 (2F) 41 (2F) Figure II (5D, D) 603 (5D, D) 738 (5D, D) 89 (5D, D) 160 (5D, D) 108 (2F, H) 130 (2F, H) 156 (2F, H) 174 (D) 182 (D) 215 (D) 2 As Figure II.2 83 (5D) 95 (5D) 105 (5D) 5 (2F) 10.5 (2F) 1 Figure II (5D, D) 400 (5D, D) 500 (5D, D) 30 (5D, D) 51 (5D, D) 2 As Figure II (5D) 210 (5D) 240 (5D) 8.0 (2F) 15.0 (2F) 33 (2F) 39 (2F) (2F, H) 94 (2F, H) 60 (D) 64 (D) 78 (D) 34 (2F) 41 (2F) Figure II (5D, D) 560 (5D, D) 680 (5D, D) 60 (5D, D) 110 (5D, D) 57 (5D, H) 83 (2F, H) 100 (2F, H) 115 (D) 121 (D) 142 (D) 52 Figure II.8 52 Figure II.9 52 Figure II Figure II.11 <1 <3 <1 < Figure II (5D) 540 (5D) 590 (5D) 9.0 (2F) 17 (2F) 41 (2F) 48 (2F) 57 (2F) Figure II (5D, D) 1685 (5D, D) 2020 (5D, D) 94 (5D, D) 187 (5D, D) 101 (2F, H) 117 (2F, H) 140 (2F, H) 199 (D) 205 (D) 232 (D) 2 Figure II (5D) 230 (5D) 260 (5D) 6 (2F) 13.0 (2F) 32 (5D) 45 (2F) 53 (2F) Figure II (5D, H) 420 (5D, H) 475 (5D, H) 52 (5D, D) 90 (5D, D) 94 (2F, H) 110 (2F, H) 130 (2F, H) 93 (D) 98 (D) 116 (D) Date : <01/12/2088> Page 12

37 DET NORSKE VERITAS PHAST Modelling of BOS Gas Releases orus Strip Products UK MANAGING RISK Plant Item Collection system Component Pipe Cooler outlet Vessel type and diameter Failure mode Substance Pipe 2300 mm dia Hole 500 mm dia BOS gas (70) Pipe 2300 mm dia Pipe 2300 mm dia Pipe 2300 mm dia Pipe 2300 mm dia Pipe 2300 mm dia Pipe 2300 mm dia Pipe 2300 mm dia Guillotine fracture Hole 500 mm dia Guillotine fracture Hole 500 mm dia Guillotine fracture Hole 500 mm dia Guillotine fracture Notes: 1. Distances given are for worst-case results (weather category in parentheses). 2. Distances to toxic concentrations of interest and radiation levels given at 1.5 m elevation. 3. Maximum downwind distances to LFL, 1/2 LFL are given at the cloud centreline height. 4. Explosion modelling conducted with PHAST, using the TNT explosion model. BOS gas (70) BOS gas (40) BOS gas (40) BOS gas (70) BOS gas (70) BOS gas (40) BOS gas (40) Release height (m) 2 Reference Figure As Figure II.2 8 Figure II.16 Distance (m) to CO Concentrations (m) for % COHb Downwind Distance (m) to Distance (m) to Jet Fire Radiation Level (kw/m 2 ) Distance (m) to Overpressure Level (psi) LFL ½ LFL (5D) 95 (5D) 105 (5D) 5 (2F) 11 (2F) 5.0 (2F) 12.0 (2F) 33 (2F) 39 (2F) (2F) As Figure II.3 92 (5D, H) 112 (5D, H) 130 (5D, H) 29 (2F, D) 53 (2F, H) 84 (5D, H) 124 (2F, H) 148 (2F, H) 59 (H) 62 (H) 75 (H) 8 Figure II (5D, D) 75 (5D, H) 100 (5D, H) 29 (2F, D) 47 (2F, H) 83 (5D, H) 123 (2F, H) 147 (2F, H) 49 (H) 51 (H/D) 72 (D) 8 Figure II (2F) 152 (2F) 170 (2F) 9.0 (2F) 17 (2F) 8 As Figure II.5 18 Figure II (2F) (5D, D) 603 (5D, D) 738 (5D, D) 89 (5D, D) 160 (5D, D) 108 (2F, H) 129 (2F, H) 156 (2F, H) 174 (D) 182 (D) 215 (D) 6.0 (2F) 13.0 (2F) Figure II (5D, D) 61 (5D, D) 70 (5D, D) 29 (2F, D) 48 (2F, H) 82 (5D, H) 115 (2F, H) 142 (2F, H) 49 (H) 52 (H) 72 (D) 18 Figure II As Figure II.5 9 (2F) 18 (2F) 18 (2F) (5D, D) 603 (5D, D) 738 (5D, D) 89 (5D, D) 160 (5D, D) 105 (2F, H) 122 (2F, H) 152 (2F, H) 174 (D) 182 (D) 215 (D) Date : <01/12/2088> Page 13

38 DET NORSKE VERITAS PHAST Modelling of BOS Gas Releases orus Strip Products UK MANAGING RISK Table 3.4: Summary of Worst-Case Consequence Results (New Cases) Plant Item Component Vessel type and diameter Failure mode Substance Temperature Release height (m) Reference Figure Distance to CO Concentrations (m) for % COHb Downwind Distance to (m) Distance (m) to Jet Fire Radiation Level (kw/m 2 ) Distance [m] Overpressure (psi) Gas Holder Gas Holder (shell) Inlet Stub Outlet stub Vent Tank Pipe 2300 mm dia Pipe 1200 mm dia Pipe 650 mm dia Hole 500 mm dia Hole 500 mm dia Hole 500 mm dia Hole 500 mm dia Hole 500 mm dia Open end BOS gas 70ºC BOS gas 40ºC BOS gas 20ºC BOS gas BOS gas Furnace Gas 70ºC 1 40ºC 1 20ºC LFL 1/2 LFL Figure III (5D) 133 (5D) 172 (5D) 4.81 (2F) (2F) 32.1 (2F) 38.3 (2F) Figure III.2 93 (5D) 121 (5D) 167 (5D) 4.99 (2F) (2F) (2F) 37.3 (2F) Figure III (5D) 5.07 (2F) (2F) 32.3 (2F) Figure III (2F) (2F) 1 Figure III (5D) 296 (5D) 359 (5D) 6.87 (2F) (2F) 5 Figure III (2F) 242 (5D) 306 (5D) 8.01 (2F) (2F) 10 Figure III (2F) 182 (2F) 229 (5D) 8.23 (2F) (2F) 15 Figure III (2F) (2F) 1 Figure III (2F) 1602 (2F) 2073 (2F) 7.98 (2F) (2F) 5 Figure III (2F) 1284 (2F) 1705 (2F) 8.89 (2F) (2F) 10 Figure III (2F) 884 (2F) 1241 (2F) 9.11 (2F) 18.3 (2F) 15 Figure III (2F) 531 (2F) 832 (2F) 9.25 (2F) 18.7 (2F) As Figure III.1 As Figure III.5 As Figure III (5D) 133 (5D) 172 (5D) 4.81 (2F) (2F) 229 (5D) 296 (5D) 359 (5D) 6.87 (2F) (2F) 1114 (2F) 1602 (2F) 2073 (2F) 7.98 (2F) (2F) 70ºC 2.2 Figure III (5D) 137(5D) 175 (5D) 4.9 (2F) (2F) 40ºC 2.2 Figure III (5D) 292 (5D) 356(5D) 7.75 (2F) (2F) 20ºC 2.2 Figure III (2F) 1522 (2F) 1988 (2F) 8.62 (2F) (2F) 20ºC 52 Figure III.16 <1 < (2F) 40.6 (2F) (2F) 39.7 (2F) (2F) (2F) (5D) 53.4 (5D) (5D) (5D) (2F) 38.3 (2F) (2F) 40.6 (2F) (5D) 53.4 (5D) (2F) 38.3 (2F) (2F) 40.6 (2F) (5D) 53.3 (5D) Vents Pipe 650 mm dia x 12 Open end Furnace Gas 20ºC 52 Figure III.17 <1 2.5 (5D) Date : <01/12/2088> Page 14

39 DET NORSKE VERITAS PHAST Modelling of BOS Gas Releases orus Strip Products UK MANAGING RISK Plant Item Component Vessel type and diameter Failure mode Substance Temperature Release height (m) Reference Figure Distance to CO Concentrations (m) for % COHb Downwind Distance to (m) Distance (m) to Jet Fire Radiation Level (kw/m 2 ) Distance [m] Overpressure (psi) BOS gas 70ºC LFL 1/2 LFL Figure III (5D) 241 (5D) 275 (5D) 6.02 (2F) (2F) 41.6 (2F) 52.3 (2F) Figure III (5D) 238 (5D) 279 (5D) 6.04 (2F) (2F) 16 Figure III (5D) 6.1 (2F) (2F) 50.7 (2F) (2F) Figure III (2F) 629 (2F) 712 (2F) 8.89 (2F) (2F) 40.8 (2F) 46.2 (2F) 57.4 (2F) Distribution system Pipe Pipe 1200 mm dia Hole 500 mm dia BOS gas 40ºC BOS gas 20ºC BOS gas 70ºC 10 Figure III (2F) 613 (2F) 698 (2F) 8.92 (2F) (2F) 16 Figure III (2F) 537 (2F) 629 (2F) 8.97 (2F) (2F) 8 Figure III (2F) 1483 (2F) 1680 (2F) 9.72 (2F) (2F) 10 Figure III (2F) 1481 (2F) 1682 (2F) 9.75 (2F) (2F) 16 Figure III (2F) 293 (5D) 1632 (2F) 9.8 (2F) (2F) 8 Figure III (5D) 173 (5D) 210 (5D) 6.02 (2F) (2F) 10 Figure III Figure III (5D) 177 (5D) 6.04 (2F) (2F) 6.1 (2F) (2F) 44.1 (2F) 56.1 (2F) (5D) 52.3 (2F) (5D) 64.1 (5D) (5D) (5D) (2F) 52.3 (2F) (2F) (2F) Figure III (2F) 443 (2F) 544 (2F) 8.89 (2F) (2F) 40.8 (2F) 46.2 (2F) 57.4 (2F) BOS gas 40ºC BOS gas 20ºC 10 Figure III (2F) 414 (2F) 521 (2F) 8.92 (2F) (2F) 16 Figure III (2F) 415 (2F) 8.97 (2F) (2F) 8 Figure III (2F) 1150 (2F) 1341 (2F) 9.72 (2F) (2F) 10 Figure III (2F) 1135 (2F) 1334 (2F) 9.75 (2F) (2F) 16 Figure III (2F) 880 (2F) 1225 (2F) 9.8 (2F) (2F) 44.1 (2F) 56.1 (2F) (5D) 52.3 (2F) (5D) 64.1 (5D) (5D) (5D) Notes: 1. Distances given are for worst-case results (weather category in parentheses). 2.Distances to toxic concentrations of interest and radiation levels given at 1.5 m elevation. 3. Maximum downwind distances to LFL, 1/2 LFL are given at the cloud centreline height. 4. Explosion modelling conducted with PHAST, using the TNT explosion model. Date : <01/12/2088> Page 15