Calculations in Support of IP15: The Area Classification Code for Petroleum Installations
November 2001
By P. T. Roberts OGCH/2 HSE Business Group Shell Global Solutions (UK), Cheshire Innovation Park, P.O. Box 1, Chester CH1 3SH, England Report No. OP.00.47110
Published by The Institute of Petroleum, London A charitable company limited by guarantee
Copyright © 2001 by The Institute of Petroleum, London: A charitable company limited by guarantee. Registered No. 135273, England All rights reserved No part of this book may be reproduced by any means, or transmitted or translated into a machine language without the written permission of the publisher. ISBN 0 85293 339 8 Published by The Institute of Petroleum Further copies can be obtained from Portland Press Ltd. Commerce Way, Whitehall Industrial Estate, Colchester CO2 8HP, UK. Tel: +44 (0) 1206 796 351 email:
[email protected]
CONTENTS
Page
Foreword...................................................................................................................... vii Acknowledgements .................................................................................................... viii Executive summary ....................................................................................................
1
1.
Introduction .....................................................................................................
2
2.
Flammability limits for two phase releases ..................................................
3
3.
Shape factors and hazard radii for pressurised releases ...........................
6
4.
Hazard radii from vents .................................................................................. 10 4.1 Vents from the storage of petroleum products ......................................... 11 4.2 Process vents........................................................................................... 20
5.
Evaporation from pools and sumps .............................................................. 22 5.1 Vapour pressure comparisons of some commonly used Category C fluids...................................................................................... 27
6.
Releases into confined areas......................................................................... 29
7.
Discussion and conclusions.......................................................................... 34
8.
References ....................................................................................................... 35
Appendix A: Methodology................................................................................................................ 37 Appendix B: Preliminary investigation of hazard radii and shape factors for the Revision of IP15: the Area classification code for petroleum installations .......... 40
v
vi
FOREWORD
The Institute of Petroleum commissioned this report to address concern over the effect of a release containing droplets or a mist on the dispersion distances determined by the methodology used in IP publication A Risk-Based Approach to hazardous area classification, 1998. The report also reviews flammability limits, evaporation from pools and releases into confined areas. The aim of this publication is to provide a record of the calculations, methodology and assumptions used to calculate dispersion distances. It provides a traceable scientific basis that will be applied to the 2nd edition of IP publication Model Code of Safe Practice Part 15: Area Classification code for petroleum installations 1st edition, 1990. Although it is believed that the adoption of the recommendations of this report will assist the user, the Institute of Petroleum cannot accept any responsibility, of whatsoever kind, for damage or loss, or alleged damage or loss, arising or otherwise occurring as a result of the application of this report.
vii
ACKNOWLEDGEMENTS
This report was prepared by Dr Peter Roberts and Dr Les Shirvill and was reviewed by members of the Institute of Petroleum’s Area Classification Working Group: Phil Cleaver Howard Crowther Kieran Glynn Alan Tyldesley Mick Wansborough
Advantica Technology Consultant (formerly BP) BP Health and Safety Executive Shell
The Institute wishes to record its appreciation of the work carried out by the members of the group. The Institute also wishes to record its thanks to the Health and Safety Executive for cosponsoring this research.
viii
Calculations in Support of IP 15: The Area Classification Code for Petroleum Installations Executive Summary The Area Classification Code for Petroleum Installations published by the Institute of Petroleum (IP 15) offers guidance on the immediate area of hazard associated with the normal processing and handling of petroleum products and is in very wide use. A major revision of IP 15 is being prepared which aims not only to update the guidance based upon best current practice but also to provide a traceable and scientific basis for the guidance given. This latter is not a trivial task and necessarily depends to a great extent on the methodology developed for assessing the consequences of accidental releases on a large scale - much larger than would arise from normal processing and handling. The quantification of hazard necessarily starts with specifying the type of material and the size of release which is very much unknown in the case of small spills and leaks. Material types have been simulated using 5 example fluid compositions coded (A, B, C, G and Gii) following earlier work to update IP 15. Release rate values used here represent the lower end of the “hazardous release” scale. These should be larger than arise in normal handling and certainly should not be taken as indicative of the magnitude of “acceptable” spills. In all circumstances the potential for spills to occur should be rigorously assessed and a full hazard assessment carried out where necessary. This note contributes a methodology and the physical basis for the deriving several guidance parameters relating to: x the characteristics of two-phase releases compared to single phase releases. x the definition of shape factors for pressurised releases of both heavier than air and lighter than air fluids (fluid categories A, B, C, Gi, Gii). x the flammability limits for the fluids used as examples of categories A, B, C, Gi, Gii. x hazards arising from the evaporation of category C fluids. x releases into confined areas. The major findings arising from this work are summarised below. It has generally been possible to defend the key recommendations of IP 15 as conservative. Where revisions are recommended these are strongly dependent on scenario and fluid type. x The hazard radii for pressurised releases of category B and C fluids should be derived assuming a mechanically generated flammable mist; previously gaseous releases were assumed. The Hazard radii for category B and C fluids are increased relative to previous guidance. x Numeric flammability limits published in Annex D of “A Risk-Based Approach to Hazardous Area Classification” for the category B and C fluids have been updated to take account of the composition of the flammable mist; previously low vapour components were assumed to rain-out and not contribute to the lower flammability limit evaluation. x Shape factors for pressurised releases are revised to take better account of the role of initial jet momentum on the jet trajectory. In particular the lighter than air gases (category Gi and Gii fluids) are found to have qualitatively more similar shape factors to the two-
1
x x
x
x
x x
x
phase category A, B and C releases; previously buoyancy was assumed to dominate their dispersion. Hazard radii for discharges from vents are evaluated. The hazard radius varies from slightly smaller to slightly larger than that in the existing guidance depending on the properties of the vented vapour. The composition of vapour from vents on storage facilities maintained at atmospheric pressure may be variable (in composition, density and flammability) and the user of the new guidance should be aware of the effect of this variability because of the consequence for hazard radii. The example range of venting rate and vent sizes used in the guidance are not wholly consistent with the assumption that the discharge takes place at atmospheric pressure. The relationship between venting rate and pressure of discharge is investigated and a value of 300 mb suggested as a threshold above which the consequences of pressure should be assessed. This is of significance for multicomponent fluids where condensation may occur. The existing guidance for liquid spillages is conservative, judged by the volatility of the model category C fluid. The guidance is applicable to materials with approximately twice the vapour generation rate of category C fluid under the specimen conditions. Relative vapour pressures for some common hydrocarbon compounds are listed. The existing guidance for sumps is conservative, judged by the volatility of the model category C. Vapour generation at the source of spillage of category C fluids is a potential hazard dictated by the spill rate and conditions and not the rate of evaporation of the liquid pool. The new guidance should emphasise the role of release conditions in determining the initial vapour generation from spills of category C fluids. For releases into confined areas the relative size of spillage and building are of key importance. The classification “Adequate ventilation” has been assessed with respect to these parameters.
1. Introduction Shell Global Solutions, on behalf of the Institute of Petroleum (IP), has worked to establish a methodology by which certain guidance parameters in the IP 15 document can be calculated from specific scenarios. The benefit is two-fold. Firstly, the existence of a methodology enables the guidance to be independently verified, secondly, it allows an end user to derive specific fluid and process dependent values in a consistent way when required. The methodology closely follows that used for assessing the consequence of hazardous events. Several of the scenarios adopted to illustrate the effect, say on hazard radius, of changing release scenarios were found to produce events that in practice would require a formal assessment of risk; i.e. they fall outwith the definition of normal processing and handling of petroleum products. None of the discharge rates used in this report should be taken as representing normal or acceptable routine practice. This work took place in two stages. A preliminary investigation was carried out for Shell (UK) in order to verify the hazard radii reported in “A Risk-Based Approach to Hazardous Area Classification”, Institute of Petroleum, November 1998 and to see if the shape factors reported in IP 15 were adequate. x the values of the hazard radius for category B and category C fluids in the Risk-Based Approach Document were too small, and the release scenarios unrealistic. x the shape factors for lighter than air gases (category Gi and Gii fluids), and to a lesser extent, the low vapour pressure category B and C fluids needed to be revised.
2
An abridged version of the report of this preliminary investigation is included as Appendix B to this note. The major findings are restated in the body of this note. The implied changes to key values in IP 15 were significant. The Institute of Petroleum requested that Shell Global Solutions pursue 6 work items to verify and quantify the necessary changes for the revision to IP 15 . These were (in short) Work Item 1: To examine results of the AIChE Release modelling program and other recent data and derive, if possible, an improved estimate of the flammability limit for high flash-point releases (e.g. taking account droplet size, rain-out) that would allow the category C hazard radii to be better assessed. Work Item 2: To describe the method used to calculate the hazard radii; define the shape factors for the fluid releases; state the hazard radii. Work Item 3: Cross check the hazard radii for the process vents. Work Item 4: Liquid Pools due to Spillage (section 5.11) To determine the hazard radii/shape factor for shallow liquid pools. Work item 5: Open Sumps and Interceptors (section 5.12) To determine the hazard radii/shape factor for deep liquid pools by taking the steady state evaporation rate and a steady dispersion calculation. Work item 6: Propose a simple ‘low momentum’ calculation method; implement this method and use it to assess the external hazard for releases of category A and category B fluid inside a building and evaluate the hazard radii. Progress on these work items was reviewed. Specific scenarios were discussed and amended in discussion with the Area Classification Committee of the IP to give the results below. These results are in a form suitable for inclusion in the IP 15 revision. A consequence of changes to some scenarios is that numeric values obtained in the original investigation on behalf of Shell (UK)(Appendix B) are changed. Only values from the main body of this note should be transferred to the new guidance. Where possible publicly available and publicly evaluated hazard assessment models have been used in this work.
2. Flammability Limits for Two Phase Releases. A major advance in hazard assessment has been the development of models capable of describing the dispersion of two-phase liquids. The AEROPLUME model is one example, being part of the HGSYSTEM v3.0 (1995) suite of models developed by Shell for industry consortia as publicly available tools subject to peer review and acting as a standard benchmark in model evaluation exercises. Two-phase releases can arise in two ways: x by the atomisation accompanying the expansion and phase change of material that is liquid under storage conditions of high pressure, and gaseous at atmospheric temperature and pressure.
3
x by the mechanical break-up of a (volatile) liquid into small droplets and their subsequent evaporation. Mists of fine droplets and dust clouds of combustible material can be very highly flammable. Unlike a gaseous mixture that is combustible only within narrow flammability limits each droplet can act as a fuel source surrounded by a plentiful air supply. Mists are optically thick and, once ignition has occurred, heat transfer by radiation very effectively preheats droplets/particles distant from the ignition front. In extreme cases and especially for dust clouds, this preheating is sufficient for auto-ignition to take place causing the cloud to burn throughout its volume. This can be a much more vigorous process than a gas cloud fire where a flame-front passes through the mixture. The flammability hazard arising from a two-phase release depends in a complicated way upon the ease of ignition of the fluid, the droplet size distribution and the concentration of droplets and vapour in air. Unfortunately very little is known about precise flammability criteria for mists arising from “real” releases. The summary guidance based upon a review of available literature (Appendix B, Lees, 1998) is that the potential of a mist to burn should be assessed: take all the droplets present in a volume, evaporate them and see if the resulting vapour and air mixture lies within the known vapour phase flammability limits. The easiest way to evaluate this is to use a mass based flammability limit (kg fuel/m3 air) in place of the standard and familiar volume based limit (m3 fuel/m3 air) used for vapours. This definition of a flammability limit: x has NO effect on hazard distances calculated for gaseous mixtures or two-phase mixtures of very volatile componentsi. x has a profound effect on the hazard distances calculated for the category B and category C example fluids resulting in a substantial INCREASE in hazard radius over previous advice based upon volumetric flammability limit values. Not all of the liquid released from a pressurised source might remain airborne. Thus, loss of fluid to the ground through rain-out may mitigate the hazard associated with two-phase releases with a low volatility component. The question of rain-out has been investigated at length by the Center for Chemical Process Safety of the American Institute of Chemical Engineers by means of a series of experiments and an extended modelling exercise. Unfortunately the problem has not been satisfactorily solved. The combined results of this study are reported by Johnson and Woodward (1999) and expressed in software form as a model, called Release. The Release model does not account for all of the physical processes involved in two-phase releases of low volatility materials and is strongly tuned to account for the results of the experiments that were carried out. These experiments aimed to measure the total liquid deposition from pressurised releases of several materials under a limited and non-ideal set of environmental conditions. An obvious concern is that the validity of the model outside of the range of the calibration data is unknown. The model performance is also poor in several respects.
i
This applies to the category A (two-phase), Gi and Gii (gaseous) example fluids used in the IP 15 revision at the reference atmospheric conditions and release conditions therein.
4
Two versions of the model are supplied on CDROM by AIChE; an original model and a corrected model. These have different functionality. The corrected model is intended for end-use and is referred to hereafter in this report. The Release model was reviewed by AEA Technology for the UK Health and Safety Executive (Ramsdale and Tickle (2000)). We conclude from the AEAT report, together with our less detailed investigation of the model, that: x the Release model would over-predict rain-out by a substantial margin for category C fluids. We also believe that the relationship between over-pressure and rain-out is not robustly developed and may not be extrapolated to conditions outside of the tests. With these reservations in mind, but for completeness, we used the Release model to calculate rain-out for cyclo-hexane using the conditions of release temperature, pressure and hole size used in this study for the IP. Cyclo-hexane is the closest to a category C fluid of those tested. We found that: x mass flow rates calculated by the Release model as a function of hole size and pressure were realistic. x rain-out as a fraction of mass-flow rate was independent of hole size and a function of pressure only. The calculated rain-out values were: Pressure (bar) 5 10 50 100
Rain-out (fraction of mass released) 0.98 0.44 0 0
Table 1 Results of the “Release” model for Cyclo-Hexane. x the rain-out fraction did not depend upon the axial location in the jet.(input parameter ZJ) We do not believe that the values in Table 1 are necessarily correct although they confirm our intuition that pressurised releases should become atomised. Further the consensus of the model reviewers is that Release overestimates the amount of rain-out. For the purposes of this work we therefore assume that: x rain-out of liquid from pressurised releases can be neglected in calculating hazard radii. This is in accord with the concluding remarks (5.2) of the AEAT review which suggest that low volatility materials should conservatively be treated as a mist. There must be a lower limit to the drive pressure for which this is true and further work is needed to establish the proper limits. It does seem credible, from intuition and from Table 1, that the 5 bar pressure releases would rain-out. However, if they do not form a flammable mist they would instead form an initially coherent liquid jet. The throw of a liquid jet can be quite substantial and could credibly extend the same distance as we calculate here for dispersed phase jets. A liquid jet of gasoline, say, would present a contact hazard to electrical equipment on exposed surfaces and also form a liquid pool on the ground that will flow away from the source. The result on ignition would be a pool fire rather than a jet-fire or cloud deflagration. For these reasons we retain the hazard radii for Category C fluid down to the 5 bar condition.
5
It is worth noting that, for flammable as opposed to toxic hazards, the Release model prediction of total rain-out does require qualification as to at what distance from the source the rain-out occurs. Clearly to mitigate the hazard this must be smaller than the calculated hazard radius. For toxic hazards (evaluated at a long distance from the release point) only the total released needs to be known. The revision of the IP guidelines is based on model fluids for the five fluid categories. These first appeared in the Addendum to IP 15: A Risk Based Approach to Hazardous Area Classification. The fluid properties are quoted in Table 2. We note: x The vapour phase flammability limits have been re-calculated and differ from those in the risk-based addendum to IP 15. The derivation of those flammability limits is not known. However, we infer that this is because of a changed assumption that all of the released material might participate in a fire. Certainly the previously published values are consistent with the assumption that only the light ends of the mixture would burn and match those calculated for a pseudo-mixture of hydrocarbons smaller than C8. Stream component (mol %) N2 Nitrogen C1 Methane C2 Ethane C3 Propane C4 Butane C5 Pentane C6 Hexane C7 Heptane C8 Octane C9 Nonane C10 Decane H2O Water Carbon Dioxide Hydrogen Average MW LFL (vol %) LFL (kg/m3)
Fluid Cat. A
Fluid Cat. B
Fluid Cat. C
0.00 0.00 0.00 70.00 30.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
0.00 4.00 0.00 6.00 7.00 9.00 11.00 16.00 22.00 0.00 25.00 0.00 0.00
0.00 0.00 0.00 1.00 1.00 2.00 3.00 3.00 27.00 25.00 38.00 0.00 0.00
Fluid Cat. G (i) 2.00 88.45 4.50 3.00 1.00 1.00 0.00 0.00 0.00 0.00 0.00 0.05 0.00
0.00 48.30
0.00 100.06
0.00 125.03
0.00 18.74
80.00 7.03
2.00 0.039
1.05 0.042
0.86 0.043
4.6 0.034
4.00 0.011
Fluid Cat. G (ii) 2.00 10.00 3.00 3.00 1.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.00
Comp. LFL (vol %) 5.00 3.00 2.10 1.80 1.40 1.20 1.05 0.95 0.85 0.75 -
28.01 16.04 30.07 44.09 58.12 72.15 86.17 100.20 114.23 128.26 142.28 18.02 44.01
4.00
2.02
MW
Boiling point °C -196 -161 -87 -42 -1 36 69 98 126 151 173 100 -78 (sub) -253
Table 2 Composition of the example category A,B,C, G(i) and G(ii) Fluids and their lower flammability limits (LFL)
3. Shape Factors and Hazard Radii for Pressurised Releases The major findings of this study, compared with earlier guidance are that: x Pressurised releases give rise to an approximately spherical hazard zoneii for all categories of fluid, except where the release comes into ground contact where the hazard distance is extended.
ii
In the context of flammable hazards.
6
x Hazard radii for category B and category C fluids are substantially greater than those quoted in the Addendum to IP 15: A Risk-Based Approach to Hazardous Area Classification. x The increase in hazard radius is due to the redefinition of flammability limit and not to substantial differences in modelling changes. The new shape factors are shown in Figure 1. of the release and the hazard radius.
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The shape factor depends upon the height
R1
Source
R1
(a) Releases where H > R1 + 1
H
Source
1m
R2
R1
b) Releases where 1 < H d R1 + 1
Source 1m
R2
c) Releases where H d 1 Figure 1 Shape Factors for Pressurised Releases
8
The key features are: x Releases below a height (H) of 1 m are declared to be influenced by the ground and to have a hazard radius R2. x Releases above 1 m, but at heights below the hazard radius R1 + 1 m are declared to be influenced by the ground if the release is directed downward and passes below 1 m. x Releases at a height above the hazard radius R1 +1 m are declared independent of the ground. The Hazard radii are given in Table 3 for the primary radius R1 and in Table 4 for the ground radius R2. For small releases, giving a dimension R1 not substantially larger than 1 m, the radii are similar. The numerical values given in Table 3 and Table 4 are specific to the example fluids. The release rate for these fluids is only weakly dependent upon small variations in the assumed storage temperature about 20 qC, which is chosen to reflect a daily average UK summer temperature. Other fluids may be more sensitive to temperature changes.
Fluid Category
A
B
C
G(i)
G(ii)
Release pressure (bara) 5* 10 50 100 5 10 50 100 5 10 50 100 5 10 50 100 5 10 50 100
1mm 0.01 0.01 0.03 0.05 0.01 0.02 0.04 0.06 0.01 0.02 0.04 0.06 0.001 0.001 0.007 0.015 0.0004 0.001 0.004 0.007
Release flow rate (kg/s)
Hazard radius R1 (metres)
Release hole diameter
Release hole diameter
2mm 0.04 0.06 0.14 0.2 0.04 0.07 0.15 0.2 0.06 0.1 0.2 0.25 0.002 0.005 0.03 0.06 0.001 0.003 0.02 0.03
5mm 0.3 0.4 0.9 1.20 0.30 0.40 1.0 1.4 0.3 0.4 1.0 1.4 0.02 0.03 0.2 0.4 0.01 0.02 0.1 0.2
10mm 1.0 1.5 3.5 5.0 1.0 1.7 4.0 5.5 1.1 1.7 4.0 6 0.06 0.10 0.7 1.5 0.04 0.07 0.4 0.7
1mm 2 2.5 2.5 2.5 2 2 2 2 2 2.5 2.5 2.5
