Risk assessment for the siting of developments near liquefied ...

I.CHEM.E. SYMPOSIUM SERIES NO. 110

RISK ASSESSMENT FOR THE SITING OF DEVELOPMENTS NEAR LIQUEFIED PETROLEUM GAS INSTALLATIONS

Crossthwaite P J,* Fitzpatrick R D , ' Hurst N W,*

A computer based model has been developed by HSE for quantified risk assessment of above ground LPG installaty ions, initially for vessels up to 300 te. A representative set of events and their associated frequencies has been used to predict individual and societal risk levels. Sensitivity studies indicate that the BLEVE event frequency and, to a lesser extent, fireball mass have most significance on the predicted risk levels. Societal risk is shown to be strongly affected by the location of the population and assumptions regarding its density. The model gives reasonable agreement with another assessment method which has been used in Holland to develop siting criteria._ It is concluded that the model could be used as the basis for a LPG siting and development control policy in the U.K.

INTRODUCTION

Since the accidents at Flixborough (1974) and Seveso (1976) the interest in the control and management of chemical major hazards has increased. In the UK the Advisory Committee on Major Hazards (ACMH) was established and produced 3 reports (1-3) which laid down proposals for legislation and provided technical advice. The ACMH proposed a 3 stage approach to the management of major hazards: identification, prevention of accidents and mitigation of their effects. Identification is achieved by the Notification of Installations Handling Hazardous Substances Regulations 1982 (NIHHS) which require operators to inform the Health and Safety Executive (HSE) if more than a specified minimum quantity of a defined hazardous substance is stored, manufactured, processed or used at an installation. Prevention of accidents takes place o site in response to the general requirements of the Health and Safety at Work etc Act 1974 and under the Control of Industrial Major Accident Hazards Regulations 1984 (CIMAH). Mitigation is dealt with by emergency planning required under the CIMAH regulations and also by the control of siting and development required under UK planning legislation.

There are some 1600 installations subject to the NIHHS regulations in the UK, approximately 500 of which are bulk LPG installations. As a result of Department of Environment Circular 9/84 local planning authorities (which determine all applications for development) are advised to seek the view of HSE on significant developments within a certain distance of notifiable installations. This distance, generally termed the consultation distance is determined by HSE and some distances currently advised for LPG installations

* Health and Safety Executive, Technology Division, Bootle + Health and Safety Executive, Research and Laboratory Services Division,

Sheffield.

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are 150 m, 400 m and 600 m respectively for vessels in size ranges 6-10 te, 41-80 te and 121-300 te. For any new proposed LPG installation with more than 25 te storage capacity, HSE is always consulted by the local authority for its views on the suitability of the proposed location.

LPG is usually stored in above ground, horizontal vessels, and many of these installations are in urban areas ie there is both a considerable existing population within the consultation distance and many proposals for development of land nearby. The type of advice which HSE gives to local planning authorities has been described in both general (4) and more specific terms (5). This advice has been based on a quantitative assessment of the consequences of major releases of LPG coupled with a qualitative consideration of the likelihood of these events. In order to improve the quality of its advice on LPG notifiable installations and to be consistent with HSE's approach on toxic major hazards (6) a quantified risk assessment technique has been developed for such installations. This has recently been described elsewhere (7).

This paper gives further details of the predicted levels of individual risk of fatality and an indication of the significance of various inputs and assumptions by means of a sensitivity analysis. Societal risk levels are also calculated, and finally the results are compared with another risk assessment. It is intended that HSE will shortly implement this assessment method to replace the currently used hazard based method in accordance with HSE's policy of utilising risk quantification in this judgemental process.

THE RISK PROGRAM

The general basis of the risk calculations has been described in (7) . The program was developed so as to:

- cater specifically for LPG storage installations be able to take into account relevant site specific factors

- be simple and inexpensive to apply - provide a number of different but complementary outputs

be modular in form so that it can be easily updated.

An overview of the program indicating the major steps involved is shown in Figure 1. The main inputs required are:

Size(s) of vessel(s) - LPG type (propane or butane) - Cold catastrophic vessel failure rate - Hot catastrophic vessel failure rate (BLEVE)

Limited vessel failure rates (cracks or holes) - Failure rates of associated plant eg vaporisers, pumps and pipework - Probability of ignition of plant/pipework releases and - Existing use of land surrounding the storage installation

The program calculates the levels of thermal radiation dose [(kWm-2)1,33s] and blast overpressure (psi) which could occur at specified locations defined by a cartesian grid around the installation together with associated

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probabilities. These data may then be used to derive probability contours for specified levels of radiation and overpressure ie each contour gives the distance from the source at which a level of radiation or overpressure within a specified range will occur at a particular probability level eg 10-5, 10-6

etc yr-1. In addition the probabilities of these calculated levels of radiation and overpressure can be used with appropriate relationships linking dose with probability of injury (probit equations) to derive individual risk of fatality at the various distances, which may again be used to give contours. If the population around the installation is included in the calculation (by means of the cartesian grid) the results can be expressed as a societal risk. This is a prediction of the frequency, F, at which a specified number of people, N, could be killed and is displayed in graphical form (F/N).

THE BASE CASE

In order to demonstrate the capability of the program and provide a bench mark for sensitivity testing a base case has been used. Inputs and assumptions for this base case are given in Appendix 1. As with any risk assessment program, the values of the outputs depend on the input values selected and the assumptions embodied in the program by way of consequence models etc. For the base case, the inputs have values which are considered to reasonably reflect the relative likelihood of the events eg a cold catastrophic whole vessel failure is somewhat less likely than a vessel BLEVE which in turn is somewhat less likely than a 25 mm equivalent hole leak from plant or pipework. These assumed input values are also considered to be reasonable in absolute terms, and so their use in the base case will indicate the critical assumptions and significant contributors to the predicted risks. The values should not however be regarded as fixed and necessarily applicable at specific LPG installations. Some of the base case inputs and program assumptions are discussed below.

Failure Rates

Cold vessel failure rate The most serious cold failure of the vessel is a catastrophic failure, defined as one which would cause the whole contents of the vessel to be released almost instantaneously. As far as is known this type of failure has never occurred in a pressurised LPG storage vessel, but it is possible. HSE generally assumes that chlorine storage vessels have catastrophic failure rates within the range 2 to 6 x lO-6yr-1, and it is considered that LPG storage vessel failure rates are unlikely to be higher than those for chlorine storage vessels (as they are both usually constructed to similar standards). Experience within the UK for 510,000 LPG vessel years, approximately 95% of which have a capacity of 5 te or less, indicates (for zero failures) a vessel failure rate of less than 1.4 x 10-6yr-1 (50% confidence level) or less than 5.9 x 10-6yr-1 (95% confidence level). For the purposes of the base case, the catastrophic cold vessel failure rate has been assumed to be 2 x 10-6yr-1. Vessels can also fail in a limited way eg due to nozzle failure. The values used for this type of failure are given in Appendix 1.

Hot vessel failure rate (BLEVE) LPG vessels have failed as a result of flame impingement and consequently it is reasonable to assume that the failure rate for a BLEVE is higher than

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that for cold catastrophic failure. A recent detailed study of LPG (8) used fault tree analysis to estimate BLEVE frequencies. Vessels less than approximately 10 te in capacity had estimated frequencies of 1 or 2 x 10-5

yr-1 and a large horizontal vessel at a depot had an estimated frequency of 4 x 10-' yr-1. In the UK there has been one BLEVE of a fixed LPG storage vessel, and this indicates a historical UK BLEVE frequency of less than 3.3 x 10-6yr-1 (50% confidence level)or less than 9.9 x 10-6yr-1 (95% confidence level). For the purposes of the base case it has been assumed that the hot vessel (BLEVE) failure frequency is 1 x 10-5yr-1.

Plant/pipework failure rate These failures are separated into 13 mm, 25 mm and 50 mm equivalent hole sizes, and it is generally considered that the small releases are more likely to occur than larger releases. It is further assumed that the 50 mm hole size is less likely by half an order of magnitude than the 25 mm hole size and similarly for the 25 mm/13 mm holes. The actual values used, which are similar to values which have been used by HSE in other assessments, are given in Appendix 1.

Ignition Probabilities Cold catastrophic vessel failure. Following cold catastrophic failure, it is assumed that 5% of releases ignite immediately. Calculation of delayed ignition probability follows the assumption that an instantaneous 200 te release which drifts (i.e. release without momentum) over industrial land in D5 weather is almost certain to ignite, while for F2 weather the probability of ignition will be slightly less (0.9). This is an attempt to reconcile a larger flammable cloud area in F2 weather with a wider but lower cloud and a likely lower density of ignition sources when F2 weather occurs (generally at night). The delayed ignition probability of a momentum driven vessel release is calculated by using the same assumption, deriving an ignition probability for each grid and applying this to the size of cloud.

Limited Vessel failure Limited vessel failure is assumed to give a continuous release. Immediate ignitions produce negligible direct off-site risk and are neglected (7). Delayed ignition probabilities are derived from a consideration of reported ignitions following accidental releases of LPG, the assumption made above for the ignition of a large drifting cloud and the respective areas of the LFL envelopes for the three release rates.

Plant/pipework failure Plant/pipework failures also give a continuous release. An immediate ignition probability of 0.05 has been assigned to each release. Delayed ignition probability is selected as either high, medium or low depending on the general distribution and density of ignition sources around the installation as determined by a plant inspection. Using similar considerations to the limited vessel releases the delayed ignition probability of the 50 mm equivalent hole release has been assigned values of 0.8 (high), 0.6 (medium) and 0.4 (low). Delayed ignition probabilities of releases from smaller holes have been derived from those of the 50mm release by assuming proportionality to the corresponding LFL plume areas. For the base case medium ignition probabilities were used.

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Flash fire/Vapour cloud explosion ratio The delayed ignition of a significant cloud of flammable mixture may under certain circumstances generate overpressure as well as radiation. If damaging overpressures outside the LFL envelope are produced, combustion has been termed 'vapour cloud explosion' (VCE). If damaging overpressures outside the LFL cloud are not produced, ignition and subsequent combustion has been termed 'flash fire'. The type of combustion which will occur on ignition is dependent on many factors but a consideration of LPG releases from an incident data bank (9) indicated that in circumstances similar to those likely to exist at UK storage sites the flash fire/VCE ratio is approximately 4:1. For the base case it has been assumed that the ratio is 3:1 in D5 weather and 10:1 in F2 weather. However for the specific case of the highly turbulent momentum driven release following cold catastrophic vessel failure a ratio of 1:1 has been used.

BASE CASE PREDICTIONS

Some of the predictions from the risk program for the base case inputs are shown in Tables 1-3 and Figure 2. Table 1 shows values for individual risk of death, taking into account the time a person may be expected to spend indoors and outdoors, at source and at distances of 100 m and 300 m. Values given are in terms of probability per 106 years. Table 2 shows the probability of exceeding certain specified injury criteria (for illustration purposes levels of 800(kW/m2)a•33s for radiation and 2 psi (0.14 bar) for overpressure have been used) at distances up to 400 m from the installation. The predictions show (Table 3 and Fig 2) that at source the major contributor to the risk is a release from plant or pipework. By 50 m, however, the BLEVE fireball is dominant and remains so until about 350 m. Beyond this distance the drifting clouds from whole catastrophic vessel failure are the only measurable contributors (Table 2) but at this range the predicted risk levels are very low (about 5 orders of magnitude less than the predicted risk at source). At source overpressure contributes about one third of the risk, and also at distance (>300m) when drifting cloud events are of similar magnitude to BLEVE events. At intermediate distances, when BLEVE (radiation only) dominates, the overpressure contribution is very small.

The risks at locations which are significant for offsite planning purposes, typically in excess of 50 m are dominated by the BLEVE (more than 80% of the total individual risk is due to BLEVE at distances between 50 and 300 m). Societal risk predictions are shown in Figure 4. The base grid contains relatively few people (approximately 11 people within 200m of the installation) and it can be seen that persons close to the installation are most affected by the BLEVE, and few people are likely to be killed by other events.

BASE CASE SENSITIVITY STUDIES

The sensitivity of the program outputs to certain inputs and assumptions has been tested in order to indicate their significance on predicted levels of individual risk. From the base case it would appear however that BLEVE event frequency and fireball mass are most significant for individual risk, and these together with population density assumptions are significant for societal risk.

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Hot Vessel Failure (BLEVE)

The three inputs for the BLEVE are event frequency, fireball mass and rupture pressure. The latter is primarily intended to enable higher relief valve settings (and the effect this may have on fireball surface emissive power) to be modelled. However it may also be used to incorporate ruptures at pressures below normal relief valve pressure which may result from jet flame impingement on the vessel vapour space. The mass in the vessel at time of rupture, and hence the mass used for the fireball model is dependent on the operating cycle of the vessel at a particular site. The program can accommodate up to 5 different vessel fractions for the fireball each with a specified probability. The sensitivity to these inputs of the individual risk due to BLEVE is indicated in Table 4 and Figure 3.

It can be seen that risk is relatively insensitive to vessel burst pressure, except that at distance (300m) the higher burst pressure gives almost an order of magnitude increase in risk compared to the base case. This difference assumes more significance when considering societal risk rather than individual risk should there be a large number of people within 250-350m of the vessel.

Risk levels outside the fireball radius are sensitive to fireball mass, eg at 250m if it is assumed that the fireball mass is always 160te (80%) there is a doubling of the predicted risk whereas if the fireball mass is assumed to be 80te (40%) there is an order of magnitude reduction. Event frequency is of particular significance, having a direct effect on risk levels (changing event frequency by an order of magnitude produces an order of magnitude change in risk level). This demonstrates the importance of safety features incorporated to prevent (eg mounding) or reduce the probability (eg remotely operated shut off valves) of BLEVE. Because of this dependence of risk on BLEVE event frequency a quantified fault tree technique has been developed by the Safety and Reliability Directorate (SRD) (10) on behalf of HSE to predict it for LPG vessels at any specified installation taking into account site specific factors e.g. presence of water sprays etc.

Several models have been proposed for assessing the size, duration and effects of the BLEVE fireball. The model used in this assessment has been compared with an alternative model (used by TNO) details of which are given in Appendix 2. The predicted individual risk using the two models is shown in Figure 3. Although the fireball models used are similar, because the TNO method assumes that the maximum vessel capacity enters the fireball, 100% fatality is predicted to extend to a greater distance from the source. Beyond the predicted fireball radius TNO assume a discontinuity in the predicted risk. The use of a probit approach in the HSE model for individual risk determination gives a gradual reduction, rather than a discontinuity, outside the fireball radius. Although the two methods give similar risk predictions close to the source and at distance (300m) there is a considerable difference at intermediate distances, eg more than an order of magnitude difference at 200m, ie those distances which are likely to be of importance in terms of offsite development.

Cold Vessel Failure

Catastrophic cold vessel failure is not a significant contributor to risk unless the event frequency were to be about an order of magnitude greater

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T

w

p

Tp

P

b

oT

than BLEVE event frequency. The effects, however, extend further from the source than the effects from any other event.

he significance of two assumptions (a) ignition probability and (b) flash fire/VCE ratio, has been examined. For (a) the ignition probability of the drifting cloud over industrial land was reduced to 0.8 in both D5 and F2 eather conditions. This change causes a reduction in individual risk at source by a factor of about 4 or 5. The reduction factor gradually becomes less until at 250m individual risk predictions are comparable. At greater distances the reduced ignition probability gives higher individual risk redictions than the base case. This effect is caused by clouds travelling further from the source before being ignited.

he sensitivity to the flash fire/VCE ration was tested by assuming an equal robability for flash fire and VCE. This change causes an increase of approximately 10% in individual risk due to cold vessel failure at all distances, being a balance between reduction of radiation risks and increase in overpressure risks.

Plant/Pipework failures

Iant/pipework releases are of significance because of their relatively high frequency. The sensitivity to delayed ignition probability and flash fire/VCE ratio has been tested. Table 5 shows the effect of low and high delayed ignition probabilities.

It can be seen that the value selected for delayed ignition probability is of limited importance, however as the contribution to the total individual risk y these events at distances above 50 m is small because of the short consequence range. However, ignited releases of this type, although not directly producing high levels of risk could cause a vessel BLEVE and it is for this reason that the overall ignition probability can be of importance.

If the probability of a VCE is increased with a corresponding reduction in that for flash fire, so that they are of equal probability the proportion of risk due to overpressure increases. The individual risk at source is increased by about 10% but that at 100m is a factor of 3 higher because verpressure causes fatalities to people indoors and beyond the LFL envelope. his assumption, therefore, may be important at installations where higher release rates could occur.

Location of People In the estimation of the effect on people of overpressure or radiation, their location either inside a building or outside in the open air is important. This assessment assumes that buildings provide a certain degree of protection for people inside in the event of a radiation hazard (see Appendix 2). However, for overpressure it is only people inside buildings who are assumed to be affected. The effect of location on individual risk is shown in Table 6. The base case (90% indoors in D5 weather) is virtually indentical to the case when all people are assumed to be indoors.

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The assumption on location is most significant at source (where pipework events dominate) and at distances beyond about 2 BLEVE fireball radii (where buildings provide significant protection for the assumed 90% indoors). At intermediate distances the levels of risk are similar.

SOCIETAL RISK

In order to demonstrate societal risk an area of housing 200m x 200m was introduced into two locations within the rural section of the base case grid. (See Appendix 3) . Population densities of (i) 15000 people/km2 (ii) 10,000 people/km2 and (iii) 3000 people/km2 were used for the analysis. These are considered to be representative of terraced housing, semi detached housing and detached housing in the U K ( 1 1 ) .

The effect of introducing housing at a distance (Grid 1) is shown in Figure A (curve B for 10,000 people/km 2 and curve C for 15,000 people/km2). It can be seen that at frequencies above 5 x 10-8 yr-1 there is little change from the base case. However, at event frequencies below 5 x 10-8 yr-1, when drifting cloud events affect the housing, the societal risk is charged. The different density assumptions, even for a relatively small area of housing are also apparent (at 10"8 yr"1 the number of people predicted to be fatally injured increases from 250 to 500 with the increase in population density).

Curves D, E and F show societal risk predictions with housing close to the installation (Grid 2) for the three housing densities. There is a two order of magnitude increase in number of people killed at event frequencies between 10"5 and 10"7 yr"1 for the two denser populations compared with the base case (curve A, no housing). Also the curves do not follow a common path in the high F region as, with the housing close to the installation, more people are subject to the high frequency pipework events.

RISK ASSESSMENT COMPARISON

A detailed risk assessment has been carried out on several Dutch LPG installations (8) which has been used as a basis for a LPG siting policy (12). The HSE model was applied to two of these installations - storage at a depot in a horizontal vessel (75te) and storage at a filling station (9te). Major event frequencies and release rates from (8) were input. Minor events were neglected and it was necessary to make certain assumptions for delayed ignition probability (assumed 0.9) and densities of population (assumed as in Appendix 3, with housing at 10,000 people/km2).

Individual risk plots for the installations are shown in Figures 5 and 6. Societal risk predictions for two environments are shown in Figures 7 and 8. The individual risk predictions by the two models show reasonable agreement bearing in mind that minor events with high frequencies have been neglected by HSE, and the inbuilt discontinuities in the TNO assessment. With regard to the latter, although significant differences in risk are predicted at certain distances (e.g. for the filling station, Figure 5, at 100m TNO predicts 3 x 10-7 yr-1 whilst HSE predicts 7 x 10-6 yr-1) the more important conclusion, which is predicted by both models is the sharp fall in risk

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between 80m and 200m (and this is the main basis for the policy adopted by the Dutch for siting purposes around this type of installation).

The societal risk predictions are of the same form for both models but TNO predicts significantly fewer people being affected. This is likely to be as a result of different population density assumptions (actual density used by TNO, general density assumption for comparison purposes by HSE). As discussed previously, the population density assumption is critical for the societal risk predictions as it determines the total number of people (ie the limiting value of N) who might be affected by the hazardous events.

COMMENTS

The risk assessment model developed by HSE for above ground LPG installations gives predictions of individual and societal risk of death, and can also give probabilities of being exposed to any specified range of thermal radiation dose or overpressure. Use of the model to evaluate risks by assuming reasonable values of event frequencies indicates that the BLEVE event is likely to be the most important at distances where some form of planning control is appropriate. BLEVE event frequency is most dominant for individual risk, but mass in the fireball is also significant. Pipework events have a small consequence range and, despite their high frequency, do not contribute significantly to risk at distances in excess of 50m. Cold whole vessel events are of minor importance because of the assumed event frequency and the additional reduction in risk due to wind variability. For societal risk, the critical factors (in addition to BLEVE event frequency) are the population density and its distribution within about 2 fireball radii of the installation. The predictions of the model give reasonable agreement with TNO predictions for specific installations, particularly for the distance range within which there is a significant reduction in levels of individual risk. Consequently it is considered that the model could be used to derive a LPG siting policy in the UK based on quantitative risk assessment.

ACKNOWLEDGEMENTS

The authors wish to acknowldge: the contribution to this work by Mr D Carter and Mr G Clay (Health and Safety Executive) and to thank TNO for permission to reproduce predictions of individual and societal risk (Figures 5-8).

c Crown Copyright 1988

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9

References

1. Advisory Committee on Major Hazards. First Report 1976 HMSO.

2. Advisory Committee on Major Hazards. Second Report 1979 HMSO.

3. Advisory Committee on Major Hazards. Third Report 1984 HMSO.

4. Pape R P. 1984 General Considerations in Producing a Development Control Policy around a Notifiable Installation. Seminar at Loughborough University, September 1984.

5. Crossthwaite P J. 1985 Development Control in the vicinity of certain LPG installations. Gastech. Nice.

6. Pape R P and Nussey C. 1985. I Chem E Symposium Series no 93.

7. Clay G A, Fitzpatrick R D, Hurst N W, Carter D A and Crossthwaite P J. 1987 Risk Assessment for installations where liquefied petroleum gas is stored in bulk vessels above ground. International meeting on Major Hazards in the transport and storage of pressure liquefied gases. Fredericton NB, Canada.

8. LPG A Study. 1983 Ministry of Housing, Physical Planning and the Environment, conducted by TNO, Apeldoorn, May 1983.

. MHIDAS Data Base Service. Safety and Reliability Directorate.Warrington

10. Blything K, Harding A, and O'Donnell K, SRD report. To be published.

11. Petts J I, Withers R M J and Lees F P 1987 Jnl Haz Mat 14 (337)

12. LPG Integral Memorandum (Appendix 5) Netherlands Ministry of Housing, Planning and the Environment, Ministry of Economic Affairs, Ministry of Transport and Public Works.

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APPENDIX 1

Details of Inputs and Assumptions for the Base Case

Inputs

Inputs are specified by the user for each assessment. The following

inputs are the values used in the base case.

Vessel size

LPG type

Catastrophic vessel failure rate (hot) ie BLEVE

Catastrophic vessel failure rate (cold)

Limited vessel failure - 13 mm equivalent hole

in liquid space 25 mm equivalent hole

50 mm equivalent hole

Plant/pipework failure 13 mm equivalent hole

(liquid pipework) 25 mm equivalent hole

50 mm equivalent hole

Ignition probability of plant/pipework release : Medium (see below)

The environment surrounding the vessel was assumed to be rural to the

west and industrial to the east, ie

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Assumptions

Assumptions are built into the logic of the program, and are not normally altered by the user. Many of the assumptions are embodied into the 3 main event trees given below. Numbers on the event tree represent the appropriate probabilities

A, B and C are different positions on the cartesian grid.

Whole Vessel Event Tree

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LIQUID IMMEDIATE STABLE DELAYED FLASHFIRE/ CONSEQUENCE RELEASE IGNITION WEATHER IGNITION VCE

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Other assumptions are:

(i) The probability of ignition of a drifting cloud arising from an

instantaneous release of 200 te of butane passing over industrial

land in D5 weather is and in F2 weather is 0.9.

(ii) The ratio of ignition probabilities for a drifting cloud over urban and

rural areas is 0.8 and 0.04 respectively of the ignition probability of

an industrial area.

(iii) The wind direction is of uniform probability - using 12 directions at

30° intervals

(vi) People are indoors for 90% of the time when D5 weather exists and 99%

of the time during F2 weather.

(v) Thermal radiation probit is:

Y = -14.9 + 2.56 In V

Y = probit function for fatal response

Y = thermal radiation dose (kWm-2)1,33s

If V 2:3000, then Y = 8

(vi) Overpressure probit is:

Y = 1.47 + 1.35 In P

Y = probit function for fatal response

P = overpressure (psi)

If then Y = 8

Details of the consequence models used are given in Reference (7).

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APPENDIX 2

Calculation of Individual Risk from the BLEVE fireball:

The consequence model for the BLEVE fireball model used in this assessment (7) is similar in principle but different in detail to that used by TNO (8). The assessment methods are compared below.

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APPENDIX 3

ENVIRONMENT GRIDS USED FOR DETERMINATION OF SOCIETAL RISK

GRID 1

GRID 2

Industrial land - 200 persons/km2

Rural land - 20 persons/km2

Housing density A - detached residences Housing density B - semi-detached Housing density C - terraced

3,000 persons/km2

- 10,000 persons/km2

- 15,000 persons/km2

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TABLE 1 Indiv idual Risk - Base Case

Notes

1. Figures in brackets are event frequencies x 106

2. 0 indicates a predicted risk below 1 x lO-9yr-1

3. Outdoors/indoors refers to the individual at risk and includes the assumptions given in Appendices 1 and 2.

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TABLE 2 Frequency of exceeding specified criteria

TABLE 3 Percentage contributions of the main event categories to total individual risk levels.

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TABLE 5 The effect of different delayed ignition probability for plant/pipework release. (Total individual risk for all pipework releases)

TABLE 6 The effect of location (indoors/outdoors) in individual risk

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