fred cam guide

A user guide to the CAM 2 explosion model in

Shell FRED

By T.M. Cresswell and J.S. Puttock.

Issue Date Author Description

1.0 06 Aug 2001 Tim Cresswell First release

2.0 26 Nov. 2001 Tim Cresswell minor updates for FRED 3.2

Contents i

Contents

Overview of Shell FRED 1 Overview............................................................................................................................. 1 Areas of the FRED screen................................................................................................... 1

Simulation area ..................................................................................................... 1 Main Menu............................................................................................................ 2 Main toolbar.......................................................................................................... 2 Component toolbar ............................................................................................... 2

Running consequence models ............................................................................................. 2 Models available ................................................................................................... 2 Global receivers .................................................................................................... 4

Building a scenario.............................................................................................................. 4 Importing a site map ............................................................................................. 4 Adding a scenario on to the map .......................................................................... 5 Editing a scenario's parameters............................................................................. 5 Consequence calculations ..................................................................................... 5

Overview of CAM 2 7 Overview............................................................................................................................. 7 Experimental data ............................................................................................................... 7

Other FRED Explosion models 11 Overview of the TNT model ............................................................................................. 11 Overview of the TNO model ............................................................................................ 11

CAM Input Parameters 12 The basic properties input page ........................................................................................ 12

Scenario Name.................................................................................................... 12 Selection of fluid................................................................................................. 13

The congested region input page ...................................................................................... 14 Congested region ................................................................................................ 14

The receiver input page..................................................................................................... 18 Source ................................................................................................................. 18 Receiver .............................................................................................................. 18 Orientation .......................................................................................................... 18 Glass window...................................................................................................... 19

CAM hazard calculations 21 Overview........................................................................................................................... 21 Results Summary .............................................................................................................. 21

Scenario .............................................................................................................. 21 Congested region ................................................................................................ 21 Pulse at a distance from edge of the congested region (internal receiver).......... 21 Responses at distances from edge of the congested region ................................ 22

Overpressure decay graph................................................................................................. 22 Map ................................................................................................................................... 22

ii Contents

Receiver.............................................................................................................................22 Summary details..................................................................................................22 Blast wave response............................................................................................23 Effect on humans ................................................................................................23 Effect on equipment............................................................................................23 Glass window breakage pressure ........................................................................23

Performing a CAM assessment 25 Overview...........................................................................................................................25 Assessment of the congestion. ..........................................................................................25

Region Dimensions.............................................................................................25 Blockage grids ....................................................................................................25

Fluid and gas cloud volume ..............................................................................................26 The use of receivers ..........................................................................................................27

Worked examples 29 Basic Properties.................................................................................................................29 Congested region...............................................................................................................29 Receiver.............................................................................................................................30 Results - Summary ............................................................................................................30 Results - Overpressure ......................................................................................................31 Results - Map ....................................................................................................................31 Results - Receiver .............................................................................................................31

Contact details 33 General Enquiries and FRED Support ..............................................................................33 Hazard and risk management consultancy and training ....................................................33

References 35

Glossary of Acronyms 37

Index 39

Shell Global Solutions A user guide to the CAM 2 explosion Model in FRED 3 Overview of Shell FRED 1

Overview of Shell FRED

Overview FRED ( Fire Release Explosion Dispersion) models the consequences of a release of product, both accidental and intentional such as a flare. It is an important enabling tool for a safe, cost-effective approach to plant layout and design, modification and operational procedures. It may also be used in emergency planning and as a screening tool for "effect calculations" in Quantitative Risk Analysis (QRA) studies etc. Typical users of FRED are technical staff in all disciplines who are likely to be involved in plant design safety, accident investigation or safety case discussion and reporting. Users are strongly recommended to attend one of our consequence modelling courses before using FRED. See below for details of how to contact us for information.

FRED contains the latest release and fire modelling packages based largely upon our extensive experimental database. For dispersion modelling FRED contains the latest HGSYSTEM models, with the exception of pool spreading and transient modelling of heavy gas dispersion. For explosion modelling FRED has the state-of-the-art treatment for congested vapour cloud explosions with the CAM 2 model, but for vented explosions or more complex geometries other models would have to be used, e.g. SHELL's SCOPE and EXSIM models.

Areas of the FRED screen The FRED screen is divided into a simulation area, a control menu and two toolbars. A brief overview of these is given below.

Simulation area

The simulation area is part of the screen where the project being evaluated is built up. It contains the "background" site map onto which scenarios are placed.

2 Overview of Shell FRED Shell Global Solutions A user guide to the CAM 2 explosion Model in FRED 3

Main Menu

From the main menu you can access the standard Windows menus and all the features of FRED.

Main toolbar

The main toolbar provides short cuts to the more commonly used FRED features.

Component toolbar

The component toolbar provides the facility to add hazard scenarios to the project being modelled. This is done by selecting the scenario required from the toolbar, shown left, then moving the cursor to the position of the simulation area and clicking again.

Scenarios may be moved to new locations by selecting the scenario then dragging it to its new position. Dragging a scenario with the Ctrl key held down clones the scenario. If you double click on a scenario, its properties screen will appear in which parameters may be defined and results displayed.

Running consequence models

Models available

Fires

, Fire scenarios Tank / Pool fire These fire scenarios enable a "pool fire" to be

modelled. This is normally in some identifiable containment such as a bund, impounding basin or storage tank with a damaged roof.


BLEVE

, BLEVE scenarios Shell / TNO BLEVE The BLEVE scenarios are used to calculate the

overpressure and radiation levels resulting from an accidental BLEVE (Boiling Liquid Expanding Vapour Explosion) of a storage tank.

Temperature rise

Temperature rise scenario

Temperature rise The temperature rise scenario calculates the temperature of an object, which may have a protective

coating, when exposed to fire attack.

Blowdown

Blowdown scenario Blowdown The blowdown scenario can be used to calculate the

time-dependent blowdown of a vessel or pipeline from a single-phase leak.

Release / Gas dispersion / Jet Fires

, Gas jet scenario Gas Jet The gas jet models allow calculation for a gaseous

release (e.g. flare) and radiation contours. Additionally jet stability and noise calculations are possible.

, Pressurised release scenario

Pressurised release These scenarios use the generalised release model for both vapour and liquid releases including flashing where appropriate. Jet fires and plume dispersion are also possible..

Pressure relief valve scenario

PRV The pressure relief valve scenario allows calculation of the radiation and release rates for a range of commercially available pressure relief valves.

LPG 2 phase release scenario

LPG 2 phase The LPG 2 phase model can be used to calculate the flow rate and radiation levels for a release from an LPG storage vessel.

Explosion

CAM explosion scenario CAM The CAM scenario is the vapour cloud explosion

model used for overpressure calculation in congested regions.

TNO Scenario TNO MultiEnergy The TNO scenario calculates the overpressure based

on the TNO Multi-energy method. (Its use is NOT recommended)

TNT scenario TNT The TNT method of explosion overpressure

calculation based upon high-explosive data. (Its use is NOT recommended)

Dispersion

4 Overview of Shell FRED Shell Global Solutions A user guide to the CAM 2 explosion Model in FRED 3

Dense gas jet scenario Dense Gas This scenario provides a user-friendly interface to the

HEGADAS model from the HGSYSTEM suite of programs

, , gaussian dispersion scenario

Gaussian Dispersion Three gaussian dispersion models are available - 1. Continuous dispersion from a point source 2. Instantaneous dispersion from a point source 3. Dispersion from a non boiling pool

Global receivers

Receiver The Global Receiver is used to show the consequences of radiation or overpressure on humans and equipment. The output is only available if a receiver has been placed on the map and selected in the receiver tabbed dialog box for the scenario.

Building a scenario

Importing a site map To import a site map use the Site / Open Map menu command. This presents the dialog input box shown below. Select the background map by highlighting it and pressing the "open" button . In the example below the file "Site.dxf" has been selected.

Once the file has been selected, FRED needs to know the size of the area the map covers so that it can correctly scale any contour results and position scenarios correctly. This is done by setting the region dimensions. The input dialog below is automatically displayed.


Enter the X and Y dimensions. If the maintain aspect ratio is enabled then typing either an X or Y dimension will automatically alter the other input to maintain the relative aspect ratios. The autoscale option will size the bitmap to fill the simulation area.

Adding a scenario on to the map A scenario is added by first selecting it from the component toolbar, and then moving the cross-hairs cursor to the position required on the simulation area. By default the scenario is placed on a snap grid. This automatically moves the scenario to the nearest point on the grid. If precise positioning is required the Site / Position scenario menu option can be used.

Editing a scenario's parameters Once the hazard scenario has been added at the required location, double clicking on it brings up a window that enables both the editing of the scenario's parameters and viewing any calculations. The window is divided into four smaller windows; the top left is used for input and the remaining three output. The information displayed in each of these windows can be changed by selecting the "Tab" at the bottom of the windows in the case of output or across the top for the input window.

Consequence calculations

The apply Button After entering the scenario's parameters, the hazard consequence calculations can be performed by pressing the apply button. The current results being displayed are updated and where appropriate, the links to other models are updated. E.g. for a pressurised release the dispersion and pool calculations are automatically updated if appropriate.

Shell Global Solutions A user guide to the CAM 2 explosion Model in FRED 3 Overview of CAM 2 7

Overview of CAM 2

Overview

Cam Scenario In discussing explosions it is worthwhile making the distinction between "Confinement" and "Congestion". An explosion in a building is confined by the walls of the building, which restrict the escape of the extra volume of gas generated by combustion. Such burning in a completely enclosed building will potentially produce an overpressure of about 8 bar because of this volume generation. Even if there are vents in the walls, high pressures may still be generated because the gas cannot flow fast enough through the vents to relieve the pressure. Explosions in areas which are more than 60% enclosed by a combination of walls or other obstructions are outside of the range of the CAM model and should be modelled using Shell's SCOPE package or other such tools.

Even where there is little confinement, high pressures may be generated by congestion. Consider a vapour cloud ignited in an area containing obstacles such as pipework or process vessels. As the gas pushed forward by the expanding flame encounters the obstacles, the flow is likely to become turbulent. When the flame reaches the turbulent region, it burns faster, generating a faster flow; this faster flow produces stronger turbulence as it reaches more obstacles, in turn increasing the burning rate and so on. The high velocities associated with the rapid burning are sufficient to generate high pressure, i.e. an explosion. This process is known as the Shchelkin mechanism.

Understanding of the physical mechanisms indicates what parameters are important in vapour-cloud explosions. This can guide the development of suitable correlation's to relate features of the plant to possible explosion overpressures.

CAM 2 is based on the original CAM model by Cates[1] which was subsequently improved by Puttock[2][4]. It is a correlation model which aims to predict the peak overpressure generated in a symmetrical unconfined, congested region of plant.

Experimental data CAM 2 is a correlation model, it is based upon experiments from both internal Shell data plus those available in the public domain from TNO and the jointly sponsored MERGE experiments. Two types of experimental data were used,

8 Overview of CAM 2 Shell Global Solutions A user guide to the CAM 2 explosion Model in FRED 3

those where the experiment was totally unconfined and those with a roof present. In all cases the tests consisted of symmetrical congestion.

Experiments with symmetrical congestion, not enclosed.

Experiment

Number of obstacle rows, counting from centre

Average area blockage per row, %

Obstacle diameter mm

Pitch to diameter Ratio

Fluid

MERGE Small 6-10 38 19 5 M,MP,P,E,MO, MOOPO,POO

Medium 5-15 38-52 41-86 3-5 M,MP,P,E,MO, PO

Large 5-10 38-39 86-168 5 M, MP, P

Shell S01 3-6 18-44 27 6-11 M, P, E

S02 3-7 19-26 27 6-17 P, E

S03 4 27 27 6 M, P

S04 3-4 27-32 49 3-6 M, P, E

S06 4-6 27 27 6 M, P, E

Fuels: M methane, P propane, MP methane/propane (ratio 3:1), E ethylene (ethene) subscript O and OO: oxygen enriched, O2 concentration before fuel addition 22.5%, 24% respectively

In the MERGE experiments the congestion comprised a mesh of cylinders oriented in all three co-ordinate directions and intersecting. The congested region was a half-cube on the ground; the horizontal dimensions were 2m for the small scale experiments performed by TNO and 4.5m and 9m for the medium- and large-scale experiments performed by BG Technology. The gases used are listed in the Table above. In the small-scale series, the experiments with the lowest blockage gave low overpressures and low flow velocities; this, combined with the small obstacle diameter would result in a low Reynolds number of the flow past the obstacles. These tests may well have been significantly influenced by viscous effects, and so have been omitted from the data set.

The remaining experiments detailed above are those performed in our Buxton test site, these used uniform rows of parallel cylinders. Parameter variations included, blockage ratio, pitch, obstacle diameter, fuel and stoichiometry.

Experiments with symmetrical congestion and roof.

Experiment Number of obstacle rows, counting from centre

Average area block per row, %

Obstacle diameter mm

Pitch to diameter Ratio

Fluid

TNO DISCOE 4-8 10-50 80 3-6 M, P, E

Shell Global Solutions A user guide to the CAM 2 explosion Model in FRED 3 Overview of CAM 2 9

CECFLOW 4-16 20-55 80 2-6 M, P, E

Shell S05 4-6 18-22 27 6-11 M, P

S10 3-7 20-37 27 6-17 M, P

The DISCOE trials used vertical cylinders arranged in semicircles. A rigid vertical wall was used so that the semicircular experiment simulated what would occur in a full circle. Area blockage and pitch were varied separately.

In both series, area blockage, pitch and fuel were the main parameters varied. In the experiments with ethene where the reported overpressure exceeded 2 bar, the results may well have been affected by localised autoignition, and so these cases have not been used.

Further experiments of this type were performed at our Buxton site. The rig then had a solid roof one metre from the ground, and straight grids of vertical cylinders were used in a square arrangement.

Shell Global Solutions A user guide to the CAM 2 explosion Model in FRED 3 Other FRED Explosion models 11

Other FRED Explosion models

Overview of the TNT model TNT equivalence

scenario

The TNT explosion scenario calculates the overpressure from an explosion given the content of flammables in hydrocarbon gas cloud. It is based upon data from extensive TNT (high explosives) explosions.

It is important to emphasise that this method of predicting overpressure is not directly applicable to vapour-cloud explosions. The pressure decay from a high explosive detonation (as seen in a TNT explosion ) is faster than in the acoustic wave from a vapour-cloud deflagration; so, if the method is arranged to give the correct pressure at one distance, the predictions at all other distances will be incorrect.

This is an outdated methodology that should not be used. It is provided in FRED 3 to facilitate comparison with results from other parties who still apply this methodology, and for historical reference.

Overview of the TNO model TNO Multi-engery

scenario

The TNO multi-energy scenario is a considerable advance on the TNT method described above. It recognises that the volume of the congested region is important in determining the explosion source and the pressure decay curves given in the method which were computed for an initially acoustic pressure wave driven by an idealised piston.

The main difficulty with using the multi-energy method is that it does not provide any means of estimating the pressure generated by the explosion at its source. There is little guidance in the original TNO multi-energy method regarding which curve to use. It used to be conventional to use curve 7, corresponding to 1 bar source overpressure, but it is now known that this is not necessarily conservative. Thus the use of a separate calculation or guidance documents for this purpose is desirable; in their absence, curve 10 may be used

We do not recommend use of this method. It is provided in Shell FRED to facilitate comparison with results from other parties who still apply this methodology.

12 CAM Input Parameters Shell Global Solutions A user guide to the CAM 2 explosion Model in FRED 3

CAM Input Parameters

The basic properties input page

The basic properties input page.

The basic properties input page has two input fields. The Scenario name and fluid type.

Scenario Name The scenario name is used to identify the scenario. It is possible to find the name of a scenario which has already been defined without opening it (i.e. with a double click). "Hover" the mouse pointer over it and the status area of the main FRED window (bottom window frame) displays the scenario name.

Shell Global Solutions A user guide to the CAM 2 explosion Model in FRED 3 CAM Input Parameters 13

Selection of fluid The fluid option is used to set the flammable gas used by the model, for a given geometry more reactive fluids lead to higher overpressure

It should be noted that there are gases that are more reactive than those available in the model, for example hydrogen. If this was released in its pure form, it would present a greater hazard than those above. However, it is often used in mixtures with other gases which can reduce its reactivity.

The behaviour of gas mixtures is still the subject of ongoing study. The reactivity of a mixture does not necessarily vary linearly with the proportions of its constituent parts, so caution is advised. Advice on how to model specific mixtures can be obtained from Shell Global Solutions and data for speciality fluids not in the list above can be generated if required.

Preliminary data from experiments shows that about 10% hydrogen in methane is needed to make the mixture as vigorous as propane, whilst 40% hydrogen in methane behaves like ethene. Mixtures of propane and methane show an initially faster rise in overpressure than linear increase in propane content, but still needs in the order of 10% propane to obtain a pressure which is halfway between the levels of the pure components.

The explosive combustion of aerosols is still an evolving science and advice from Shell Global Solutions should be sought on the best approaches to modelling them.

CAM 2 makes the assumption that the ignition of the gas cloud is from a low energy source, typically an electrical fitting, or an electrostatic discharge, and that the cloud is substantially quiescent prior to ignition taking place. If ignition occurs inside a small, nearly-enclosed area which vents into a congested region close by, then the overpressures generated could be very high and detonation cannot be ruled out. This represents a scenario known as a high energy ignition or "bang box" ignition, and Shell Global Solutions should be contacted for further advice if such a scenario exists, CAM 2 is not appropriate in these circumstances.


The congested region input page

The congested region input page.

The congested region input page allows the user to specify the size and shape of the region being assessed, the level of pipe work complexity and gas cloud volume.

The position of a receiver from the edge of the congested region can also be specified.

Congested region The degree of congestion within a region being assessed can dramatically affect the potential overpressure that can be generated. Generally the more congested and complex an area of plant is the greater the overpressure. Limitations on the entry of grids in the vertical direction are imposed depending upon the aspect ratios of the length to height. This is because a different correlation is used for the "with roof" case which assumes no (or very little) vertical flame travel.

Length / Width / Height. It is assumed that the congested region under consideration is rectangular, with arbitrary orientation. The dimensions of the plant need to be assessed in three perpendicular directions, its length, width and height. These are the total dimensions of the congested area under consideration irrespective of the volume of gas available for combustion. For each of these directions you need to assess the congestion. I.e. the number of grids and their associated blockage ratio, which the model assumes are equally distributed along the length of the plant.

Number of grids

The overpressure is strongly dependent on the number of rows of obstacles passed by the flame. Having defined the dimensions of the plant, it is necessary to characterise the congestion, i.e. vessels, pipes, support or other objects which


are in the path of a flame travelling through the plant. In each of the three perpendicular directions, the congestion is idealised as a number of rows ("grids") of obstacles of specified blockage.

Blockage ratio

The blockage ratio is an area not volume blockage, it is the ratio of the total area of the cross section divided by the blocked area. A blockage ratio of 0.0 is completely unobstructed (i.e. no obstacles), whereas a blockage of 1.0 is completely blocked. (i.e. a solid wall).

All the experiments used in developing the correlation's involved cylindrical obstacles. The drag of obstacles which are sharp edged, for example of square cross-section, is higher; the drag coefficient is typically 2.0, compared with 1.2 for cylinders. The importance of the area blockage of obstacle grids is in its influence on the drag; therefore allowance must be made for the greater effect of sharp obstacles. This can be done approximately with an increase in the blockage ascribed to the sharp obstacles, by dividing the blockage by 0.6. See the Worked examples on page 29.

Obstacle Complexity Obstacles, e.g. vessels and piping, have an effect in a gas explosion in two principal ways. As gas flows past them, they generate turbulence, and turbulent burning velocity is larger than laminar burning velocity. The second effect is that, as the flame burns through a group of obstacles, the flame front becomes distorted, increasing in area. The total burning rate is proportional to the burning velocity multiplied by the flame area, and so flame area enhancements are important.

The obstacles in a real plant environment are typically much more complex than the simple arrays of cylinders used in most idealised experiments. In particular, there is a great range of length scales present. The effect of this is to increase the flame area generation above that pertaining to rows of uniform cylinders. We have performed experiments to demonstrate these effects, using idealised obstacles but with a range of obstacle sizes. We account for these effects by increasing the flame area generation for the higher levels of obstacle complexity. Four complexity levels are defined in semi-quantitative terms as follows:


Figure 1 : Joint Industry Blast and Fire engineering for topsides structures experimental facility. (BFETS phase 2)

Level 1 Idealised arrangements of obstacles of the same diameter, or very few obstacles of significantly different dimension than the dominate obstacle diameter. (Note that, e.g., interconnecting pipes, and fittings on vessels may all count as obstacles).

Level 2 Rather more complex than level 1, for example with two obstacle sizes an order of magnitude apart.

Level 3 Much more like real plant but with much of the detail missing. This is best defined as being similar to the layout of the high density arrangement in the BFETS Phase 2 experiments. (Figure 1)

Level 4 The full complexity of typical congested refinery or offshore plant.

In all realistic circumstances a complexity level of 4 should be used when assessing plant.

Roof / No roof For plant that is confined by a roof, or if a solid platform that separates a plant into vertical sections is present, the "with roof" check box should be used to perform simulations within these areas. A separate correlation based upon data from suitable experiments is used. It should be noted that this correlation would only be expected to apply when the flame travel is essentially in two dimensions, normally horizontal. Thus if the height is greater then half of either of the other two dimensions, there would be significant vertical flame travel for ignition on the ground at the centre. Thus in this case definition of the congestion in the vertical direction is required.


Partially filled. (Cloud volume) If the partially filled check box is not ticked, then the model assumes that the whole of the congested region is filled with a flammable mixture. Checking the partial fill option activates the input box allowing the specification of a smaller gas volume. If the volume entered is greater than that of the plant then the model performs the calculations for the full volume.

Note that the gas expands as it burns, typically by a factor of eight at low pressure. Thus for a lightly congested area, a cloud volume of a little over one eight of the plant volume may give the same result as full fill. For example a hemispherical cloud would double its radius when burnt. For higher overpressures, the expansion is less, and a larger cloud volume is needed to reach the same pressure as full fill.

Angle of congestion from North This input is the angle that the length dimension of the congested region makes with relative to site North. This is needed in order to calculate the distance of any receiver from the edge of the plant.

Receiver distance The receiver distance indicates the position of an "in built receiver" that reports the calculations in the summary page under the headings.

Pulse at a distance from edge of congested region and

Responses at distances from edge of congested region It is important to note that the distance to the receiver is from the edge of the congested region. Compare this with the distance to the "global receiver" described in the next section in which the distance is measured from the centre of the congested region and an allowance is also made for its orientation with respect to north. See the receiver input page for more details.

Plant with one wall

If there is a wall along one side of the plant, an assessment can still be performed, as the wall can be considered as a reflecting surface. Thus source calculations can be performed for a plant twice the size of the actual plant, taking a reflection in the wall.


The receiver input page

The receiver input page enables the selection of the global receiver. A global receiver can be placed on the map at any position, if the map tab is selected a red line is drawn between the selected receiver and scenario.

Source The source radio button enables the selection of the source type the receiver will be exposed to. For the CAM scenario only the explosion radio button is available.

Receiver The receiver section of the input screen enables the selection of the receiver that will be exposed to the hazard. A drop down box is used to select the receiver if more than one has been added to the site map. The dimmed data fields for horizontal distance, height and retreating speed cannot be changed from the scenario as they are parameters of either the receiver itself or its position on the map. To change these, edit the receiver properties, or drag it to a new position.

Orientation The orientation part of the input screen is not relevant to the CAM scenario so is greyed out. It is only applicable to scenarios the cause incident radiation.


Glass window The glass breakage input parameters are only repeated here and can be changed by editing the receiver's parameters. They are used to provide input to the glass breakage model. Its results are displayed when the receiver's tab is selected on the output screen.

Shell Global Solutions A user guide to the CAM 2 explosion Model in FRED 3 CAM hazard calculations 21

CAM hazard calculations

Overview To perform a CAM calculation for a given region of plant enter the parameters associated with the plant and, press the apply button or use the F9 Key. The calculations will be made and the apply button dimmed when complete.

The results of the calculations are made available in 3 of the 4 "split" windows. The tabs across the bottom of these windows are used to select which of the results are to be displayed. Each type of result is discussed below.

Results Summary The results summary provides an overview of the key results output from the model. Each section is described below

Scenario This simply repeats the input data provided by the user relating to the scenario and also includes the fluid details.

Congested region This repeats the input data provided by the user relating to the congested region. The size of the region, the description of the obstacles in terms of blockage and number plus their degree of complexity.

Pulse at a distance from edge of the congested region (internal receiver) This section details the calculations at the distance specified by the user in the "distance from the edge of congest region" input parameter. The results given are:-

The peak overpressure in bar

Pulse duration and its rise time in msec

22 CAM hazard calculations Shell Global Solutions A user guide to the CAM 2 explosion Model in FRED 3

Responses at distances from edge of the congested region This section shows the distance to overpressure levels of 20, 50, 100 and 340 mbars and suggests the likely damage that can be caused by the overpressure. These are :-

Distance to 50% window breaking (0.020 bar)

Distance to glass damage causing injury (0.050 bar)

Distance to repairable damage to buildings (0.100 bar)

Distance to heavy damage buildings/plant (0.340 bar)

This typical overpressure relating to damage is based upon data in Lees.

Overpressure decay graph The graph shows overpressure decay with distance from the edge of the congested region. Please note the scales on both axis are logarithmic. To obtain values from the curve at given points position the cursor above the point on the curve of interest and read both the X and Y values from the lower part of the window frame.

Double clicking on the curve places data points on top of the line. To examine the data numerically use the View / Show data points on plot menu option. This data can be exported into another application by first generating a report then clicking on the "plot raw data" link, which is shown underneath the results graph, and will open an Excel spreadsheet containing the plot data. In the case of logarithmic plots the data is the log of the results.

Map The CAM model does not provide any contour data that can be overlaid onto the map. It does however, show a link to a receiver if one has been selected.

Receiver

Summary details The summary details provides an overview of the receiver parameters that the calculations are being performed on. The following parameters are given :-

Label The name assigned to the receiver by the user.

Scenario The name of the scenario that is acting as the explosion source of the.

Fluid The fluid the scenario is using. Receiver height The height of the receiver above

the ground.

Shell Global Solutions A user guide to the CAM 2 explosion Model in FRED 3 CAM hazard calculations 23

Horizontal distance from scenario The distance from the receiver to the source (scenario))

Blast wave response The blast wave section provides the peak overpressure that would be seen at the receiver in bar. The overpressure is assumed to be an idealised triangular waveform.

Effect on humans The effect on humans provides calculations of the impact of the explosion on a human body at a distance.

The model provides details on the percentage of people that would suffer from eardrum rupture, and death from lung haemorrhage caused by the explosion. These are based on information in Lees[3].

Effect on equipment This section reports the effects the blast wave will have on equipment. A percentage of the frame buildings that will be damaged and typical glass windows found on plant that will be broken are given.

Glass window breakage pressure This section provides more specific calculations for glass breakage where more details of the window in question are available. The values for the parameter below are entered into the receiver and are repeat here.

Window Length The length of the window in metres Window Width The width of the window in metres Glass Thickness The thickness of the glass in metres The overpressure at which this glass window pane will fail is given in bar.

Shell Global Solutions A user guide to the CAM 2 explosion Model in FRED 3 Performing a CAM assessment 25

Performing a CAM assessment

Overview In order to perform a CAM assessment of a refinery or chemical plant, a description of the plant in terms of the parameters required by the CAM model must be made. These can be split up into 2 major categories. Firstly an assessment of the congestion must be gained and second the size of the gas cloud volume.

Assessment of the congestion.

Region Dimensions

The first stage in assessing the plant is to determine the outer dimensions of the plant. These can normally be obtained from plant layout diagrams. The height of plant in question may also be available. If the plant is divided into individual layers, by solid barriers, such as concrete pads, an explosion assessment must be performed for each level, and the "with roof" option turned on. For cases which are completely unconfined vertically it is normal to leave the "with roof" option unchecked. For the case which is somewhere in between these two extremes, where perhaps a very congested pipe rack could potentially form a roof, the impact of using the "with roof" option will depend upon the aspect ratio of the congested region height to length of width. It is suggested that if the vertical blockage is in the order of 80 to 85% then it is likely to restrict the flow significantly rather than cause a flame acceleration, so the "with roof" option would be appropriate.

Blockage grids To assist in determining the congestion there is no substitute for walking around the plant. The normal strategy is to walk into the darkest region in the middle of the plant. Where little daylight reaches is normally the place from which a flame would have to pass the most congestion to reach a vent. From the position where equal congestion must be passed to the outside in all directions, count the number of obstacle layers to the outside (This must be done along the length and width of the plant). Defining what constitutes an obstacle layer is difficult, typically low blockage grids in the order of a few percent, (say less than 10%) should be combined. Single obstacles, even if larger than 10%, are probably better combined into other grids, as the way in which they generate turbulence is different from an evenly distributed grid. All obstacles which are being taken as

26 Performing a CAM assessment Shell Global Solutions A user guide to the CAM 2 explosion Model in FRED 3

one layer should be added in. It is important to include obstacles which are at the intersection of these idealised grids which are in two perpendicular directions in both grids. Grids in the vertical direction are counted from the ground upwards. A certain amount of calculation is desirable, since most people's rough estimate of blockages are too low. The ambiguity in this area illustrates the uncertainty of the method.

The number of grids entered into the model are those along the whole of the length or width of the plant. i.e. if the width is 10m and you pass three grids when going from the centre to the outside, then the number of grids entered for that direction is six.

It is important to check whether some part of the area is excessively confined as an ignition in this region may possibly cause a bang box ignition which can potentially lead to very high overpressures. This cannot be modelled in CAM. More complex explosion prediction tools are required in this situation, e.g. EXSIM.

For the vast majority of plant the obstacle complexity should be taken as 4. However, in some cases this may be reduced to 3 where a significant reduction in the degree of complexity is observed, e.g. some pipe racks consisting of similarly sized pipework with little or no smaller pipe work or fittings. Reducing the complexity below 3 is not recommended as very few real process plant have very little small scale congestion. These values should only be used when comparing idealised experiments.

Fluid and gas cloud volume The fluid available for combustion and its volume is of key importance as this determines the size of the pressure source. The volume may be limited by the inventory of a vessel being considered or by the use of emergency shutdown valves, it is important to remember the model requires the gas volume of a stoichiometric mixture. The model assumes that the cloud is centred around the ignition point which is located in the geometric centre of the plant. It must be remembered that as the combustion of the hydrocarbon takes place, the expansion generated will force the unburnt gas ahead of the flame towards the outside of the plant. This unburnt gas will become turbulent as it passes over any congestion increasing the flame speed and thus overpressure. As the expansion ratio of a typical hydrocarbon is in the order of 8, then for low overpressures slightly over one eighth (12%) of the plant volume is needed to achieve the same source overpressure as a the fully filled case. As the pressure increases, some of this expansion goes to generate pressure rather than flow so a larger cloud is required. Experiments suggest that as the pressure approaches around 4 bar, only approximately 30% of the module is required to be filled with gas to give the same source pressure as the fully filled case.

If the cloud volume specified is greater than that of the complete plant then no increase in pressure over the fully filled case is observed. This is because as the flame exits the congested region it slows down, leading to a resultant reduction in overpressure. (Overpressure is proportional to flame speed squared)

CAM assumes that at the time of ignition the gas cloud is quiescent, i.e. it is not in motion, this may not always be the case for a real release, but it is a restriction of the model. For releases of hydrocarbons that are normally liquid at ambient conditions (i.e. would not normally be considered capable of an explosion), an explosion can still be obtained if they are stored under pressure and accidentally released as aerosol. It has been suggested that explosions of hexane and kerosine aerosols are actually more damaging than similar vapour cloud explosions. In experiments it has been observed that a pressurised liquid hexane release is typically about as reactive as propane vapour. For more details on aerosols or

Shell Global Solutions A user guide to the CAM 2 explosion Model in FRED 3 Performing a CAM assessment 27

speciality fluids not available in the standard fluid list please contact Shell Global Solutions for further advice.

If a release is being modelled in Shell FRED then the AEROPLUME dispersion model, which is linked with the pressurised release scenario can be used to provide a gas cloud size. For the case where the plume does not touch down or does not link to HEGADAS (heavy gas dispersion) or a transition to PGPLUME does not take place, the volume of the plume is available in the file FRED.APR. If link has taken place then this is detailed in the file FRED.APR in the directory containing file that is currently being run.

If a transition to PGPLUME takes place although the cloud does not touch down, it can be adequately considered as a Rugby Ball shape (ellipsoid), and the volume can be calculated as

4/3abc

where a,b and c are the dimensions of the ellispoid in three mutually perpendicular directions.

If the release is into congested plant and the total size of the cloud does not fully fill the plant, then the use of AEROPLUME to calculate the cloud volume is suitable if the total plant volume blockage is less than about 2%. If the volume blockage is greater than this then consideration needs to be given to using more advanced techniques, such as the Shell Global Solutions random walk model.

The use of receivers Within the CAM scenario there are two different types of receivers, one internal and one global. The internal receiver cannot be positioned on the map and its distance is entered directly into the CAM input screen. It is important to note that the distance from the scenario to the internal receiver and the global one are measured from different places. The global receiver is positioned on the map, and its distance from the centre of the scenario is reported. The internal one is defined by its distance from the edge of the congested region.

Shell Global Solutions A user guide to the CAM 2 explosion Model in FRED 3 Worked examples 29

Worked examples

Basic Properties The basic properties input page has two data inputs - Scenario and fluid. The scenario enables a name to be assigned to the CAM model to enable easy recognition if more than one is placed on the site map. For the example name the scenario Example CAM model. The fluid can be selected from one of those in the drop down list. Select Propane gas.

Congested region The first part of a CAM assessment is to determine the size of the congested region. The dimensions of the plant can normally be obtained from plant layout drawings, for example let us assume we are performing an assessment of a congested area that is 49m in length and 29m wide. The height of the congestion is also required so the model can determine the overall volume, this may not be not always be available from layout drawings so either a site visit or elevation plans are required. In this example assume that the congested area is 8m high and does not have a solid roof. Not having a solid roof means that we do not need to check the with roof option.

To determine the number of grids, the easiest way is to walk into the centre of the congested region and then walk to the outer edge through the most congested part of the plant, noting the number of idealised grids that are passed. This must be done along both the length and width of the plant. These numbers are then doubled and entered into FRED. The doubling is because the number of grids is over the whole length or width of the congested region not from the centre to the edge. Alternatively, the number of grids can be determined from plant layout and engineering drawings, however, this is somewhat more difficult. For the example assume that the length has a total of 8 grids of 20% area blockage of round profile obstacles and 3% area blockage of sharp profile obstacles. The sharp obstacles are first converted to equivalent round blockage by dividing by 0.6 before being summed. Thus the total round blockage is :-

20 % + 3% / 0.6 = 25%

In the width there are 4 grids of 32% blockage. The vertical grids can be determined by visual inspection, for this example there are 2 grids of 20% blockage.

The blockage used is based on area and is the blocked area divided by the total area cross sectional area of the plant at that particular point.

30 Worked examples Shell Global Solutions A user guide to the CAM 2 explosion Model in FRED 3

The grid complexity used should always be set to 4, this is representative of real plant.

Set the angle of congested region from north to be zero for the example and life congestion. See the main text for use of this parameter. Place a receiver at 50m.

Let us assume that the total inventory within the congested region would create 3000 cubic metres of propane air mixture. To enter this, first check the partial fill tick box, then enter 3000 into the Volume of flammable gas cloud input.

The input screen should now look like that shown below.

Receiver This input screen enables a receiver to be chosen. Go back to the FRED main window and add a receiver scenario, and name it receiver 1. In the CAM scenario, receiver input tab, select the receiver name from the drop down list Receiver 1. Please note only the explosion radio button is enabled - both the jet and pool sources are disabled.

Results - Summary The top part of the results summary repeats the input parameters, the last two sections provide the calculated results and for the example are repeated below.

Pulse at a distance from edge of congested region

Shell Global Solutions A user guide to the CAM 2 explosion Model in FRED 3 Worked examples 31

Distance = 50 (m) Overpressure = 0.211 (bar) Duration = 51.4 (ms) Rise time = 0.0 (ms)

The example shows a peak overpressure at 50m of 211 mbar with a duration of 51.4 msec. The pressure wave is shocked (has a rise time of 0 msec). The second block of results (below) show the effect of the explosion at a distance away from the blast. The distances quoted are the free field pressure decay to the overpressures indicated.

Responses at distances from edge of congested region

Distance to 50% window breaking (0.020 bar) = 602.2 (m) Distance to glass damage causing injury (0.050 bar) = 268.0 (m) Distance to repairable damage to buildings (0.100 bar) = 125.2(m) Distance to heavy damage buildings/plant (0.340 bar) = 24.5 (m)

Results - Overpressure This results screen shows the pressure decay away from the explosion with distance away from the edge of the congested region. Note the graph is a log - log plot. Direct readings can be taken from the graph by positioning the cursor on the graph and reading the X and Y co-ordinates directly from the bottom part of the window. This can be used to confirm that at 50m the overpressure is about 211mbar.

To obtain more detail, parts of the graph can be zoomed by clicking the mouse and dragging it down wards and to the right over the area to zoom into. If the mouse is clicked and dragged to the left, then the graph will unzoom back to its original size.

Results - Map The map results only show a line to the receiver that has been selected (if any). No calculated results can be displayed overlaid on top of the site map.

Results - Receiver By positioning the receiver at 50m to the north of the CAM scenario, the results as shown below are calculated. Note that these calculations are based upon a 50m distance from the centre of the scenario, so allowances must be made for the size and orientation of the congestion if comparisons with the in-built receiver are going to be made.

Blast wave response

Overpressure = 0.332 (bar)

Effect on humans

Eardrum rupture = 4.1 (%)

Death from lung haemorrhage = 0.0 (%)

Effect on equipment

Effect on equipment = Panelling torn off

32 Worked examples Shell Global Solutions A user guide to the CAM 2 explosion Model in FRED 3

Frame building damage = 94.5 (%)

Glass window breakage = 100.0 (%)

Glass window breakage pressure

Length window = 1 (m)

Width window = 1 (m)

Thickness glass = 0.005 (m)

Overpressure at which glass fails = 0.091 (bar)

Shell Global Solutions A user guide to the CAM 2 explosion Model in FRED 3 Contact details 33

Contact details

General Enquiries and FRED Support

Support provided between 9am and 5pm UK time, Monday to Friday, statutory holidays excluded. Contact Shell Global Solutions (UK)

Tel : +44 (0) 870 908 8809

Fax : +44 (0) 151 373 5058

Email : [email protected]

Web site : www.Shellfred.com

Hazard and risk management consultancy and training Shell Global Solutions

Tel : +44 (0) 151 373 5010

Email : [email protected]

Web site : www.shellshepherd.com

Shell Global Solutions A user guide to the CAM 2 explosion Model in FRED 3 References 35

References

[1] A.T. Cates. (1991) Fuel gas explosion guidelines. Int, Conf. On Fire and

Explosion Hazards, Moreton-in-the-Marsh. Inst. Energy

[2] J.S. Puttock (1995) Fuel gas explosion guidelines - the Congestion Assessment Method. Second European Conference on Major Hazards Onshore and Offshore. 267. IChemE Symposium Series no.139. 1995.

[3] F.P. Lees, The assessment of Major Hazards. A model for fatal injury from burns., Trans I Chem E Vol 72 Part B August 1994, pp127-134

[4] J.S.Puttock, Improvements in guidelines for prediction of vapour-cloud explosions. Presented at Intl. Conf. and workshop on modelling the consequences of Accidental Releases of Hazardous Materials, San Francisco, Sept.-Oct. 1999.

Shell Global Solutions A user guide to the CAM 2 explosion Model in FRED 3 Glossary of Acronyms 37

Glossary of Acronyms

TNT

TriNitro Toluene (High explosive)

HEGADAS

Shells Dense gas dispersion model

PGPLUME

Pasquill Gifford Plume dispersion

BFETS

Blast and Fire Engineering for Topsides Structures

BG

British Gas

BLEVE

Boiling Liquid Expanding Vapour cloud Explosion

LPG

Liquid Petroleum Gas

PRV

Pressure Relief Valve

CAM

Congestion Assessment Method

QRA

Qunatitative Risk Assessment

FRED

Fires Radiation Explosion and Dispersion

EXSIM

EXplosion SIMulator

SCOPE

Shell Code for Overpressure Prediction in gas Explosions.

Shell Global Solutions A user guide to the CAM 2 explosion Model in FRED 3 Index 39

Index

A Angle of congestion from North 17 apply button 5, 21 Assessment of the congestion 25 autoignition 9 autoscale 5

B basic properties 12, 29 Blast wave response 23, 31 BLEVE 3 Blockage ratio 8, 1415 Blowdown 3 burning rate 7, 15

C component toolbar 2, 5 Confinement 7 congested region 78, 11, 1314, 17, 2122, 2527,

2931 Congestion 78, 1417, 2526, 2931, 35 Contact details 33

D Dense Gas 4 DISCOE trials 9

E Editing a scenario's parameters 5 Effect on equipment 23, 31 Effect on humans 23, 31 ethene 89, 13

F fluid 8, 1213, 2122, 2627, 29

G Gas Jet 34 Global receivers 4

H HEGADAS 4, 27 height 14, 16, 18, 22, 25, 29 HGSYSTEM 1, 4

I idealised experiments 15, 26 Importing a site map 4

L length 1415, 17, 23, 2526, 29, 32 LPG 2 phase 3

M main menu 2 main toolbar 2 maintain aspect ratio 5 MERGE experiments 78

N Number of grids 14, 26, 29

O Obstacle Complexity 15, 26 overpressure decay 22

P Partially filled 17 Pressurised release 3, 5, 27 PRV 3

R Roof 2, 79, 14, 16, 25, 29

S sharp obstacles 15, 29 Shchelkin mechanism 7

T TNO MultiEnergy 3 TNT 3, 11

W width 14, 23, 2526, 29, 32

40 Index Shell Global Solutions A user guide to the CAM 2 explosion Model in FRED 3

fred cam guide

Documents

overview of cam

fred explosion models

fred screen

cam input parameters

congested region input

receiver input page

consequence models

basic properties input