Dispersion Models

The dispersion model implemented in the PHAST is called the "Unified Dispersion Model" (UDM) developed by [203]. The UDM calculates a dispersion in the downwind direction (all phases between near-field and far-field dispersion) including possible touchdown, rainout (and subsequent pool formation and re-evaporation). Fig. 4.7 shows the movement of the cloud in the downwind direction for the steady-state/continuous dispersion. The Cartesian coordinate x,y,z correspond to the downwind, cross wind and vertical directions, respectively. The point of release is given by x=0, plume centre line by y=0, and ground level by z=0. In addition to that, the cloud coordinate s as the arc length measured along the plume centre, and Z is the distance from the plume centre-line. In case of continuous dispersion, the coordinate Z indicates the direction perpendicular to the plume centre-line and perpendicular to the-y direction. The angle between the plume centre-line and the horizontal is denoted by 0, and the vertical plume height above the ground by zcld. Thus z and Z are related to each other by z=zcld + Z cos(0).

For instantaneous dispersion, the coordinate Z indicates the vertical distance above the plume centre-line and perpendicular to the-y direction. The angle between the plume centreline and the horizontal is denoted by 0, and the vertical plume height above the ground by zcld. Thus z and Z are related to each other by z = zcld + Z The concentration profiles c for continuous and instantaneous release are given in Appendix E (i.e. Eq. E-8 and E-9).

circular cross-section

<Ry= f^J truncated cross-section

^ semi-elliptic cross-section

Flame Expansion Baker Strehlow
Fig. 4.7 UDM cloud geometry for continuous release [203] 4.5.3 Fire and Explosion Models

PHAST automatically generates the existing fire and explosion models (in section E.3, Appendix E) as long as the material is flammable. However, the right configuration of each model is required. For example, a pool fire only occurs if the flammable material is in liquid form and it is continuously released from a tank or a container in the direction of down-impinging to the ground. Output of the models is presented in the form of graphs and reports. There are several fire and explosion graphs, each of which shows a different type of the fire and explosion results. The following section is to review the types of models available for estimation of the consequences (effects) of accidental explosion and fire accident outcomes.

4.5.3.1 Explosion

When a large amount of hydrogen (liquid or gas) is rapidly released, a vapour cloud forms and disperses in the surrounding air. The release can occur from a storage tank, transport vessel, or pipeline. Event tree diagrams (Fig. 4.5 and 4.6) describe the various failure pathways under which this scenario can occur. If this cloud is ignited before the cloud is diluted below its lower flammability limit (LFL), a vapour cloud explosion (VCE) or flash fire will occur. The main consequence of a VCE is an overpressure that results while the main consequence of a flash fire is a flame contact accompanied by thermal radiation.

There are three methods available in PHAST for calculating the effects of explosions, and the one must be selected to generate an explosion model. The models include TNT, Multi-Energy, and Baker Strehlow. The study uses the TNT model (as the program default) to calculate explosion effects from the hydrogen system. A detailed description of the model is presented in section E-4 (Appendix E).

The explosion may occur either early or delayed (late) explosion (vapour cloud explosion, VCE). Both early explosion and VCE are modelled as two concentric circles with radius R1 and R2 (as shown in Fig. 4.8). However, they have different location of the explosion centre. Explosion centre of the early explosion is on the release point. Meanwhile, for the delayed VCE it is displaced from the release point [203], as shown in Fig. 4.8. The centre of the explosion is taken as the centre of the explosive cloud. The two effect zones correspond to two different explosion damage levels. The lethality is constant with one value inside the central zone and constant with another value in the annulus formed by the inner and outer circles. The lethality in each zone is set according to the vulnerability parameter settings for explosions. There is one outcome representing all weathers and directions. The analytic solution to the number of lethality (N) for this outcome is the sum of the products of the area of each zone, its lethality and the population density.

Fatal effect zone of the explosion (Afatal) is calculated as the sum of the inner effect zone area (Ai) multiplied by its vulnerability fi) and the outer effect zone area (A2) multiplied by its vulnerability f2):

Cloud

Limit

Reli Sol

Cloud

Limit

Reli Sol

Vapor Cloud Explosion

Explosion Damage

Level 2

Fig. 4.8 The effect zone for a vapour cloud explosion [205]

Explosion Damage

Level 2

Fig. 4.8 The effect zone for a vapour cloud explosion [205]

Where

fij2 = vulnerability in inner and outer zone areas

4.5.3.2 Flash Fire

Accidental releases of hydrogen (liquid or gaseous) often result in the formation of a cloud of vapour that is dense relative to ambient conditions. If the cloud encounters an ignition source then a vapour cloud flash fire may result. A flash fire is a non-explosive combustion of a vapour cloud resulting from a release of flammable material into the open air [1]. Major hazards from flash fires are thermal radiation and direct flame contact. Typically, the burning zone is estimated by first performing a dispersion calculation and defining the burning zone from the half-LFL limit back to the release point, even though the vapour concentration might be above the UFL. Turbulence induced combustion mixes this material with air and burns it. The flash fire envelope generated by the program shows the maximum area covered by the flash fire envelope, i.e. the area swept out by the flash fire footprint, through all wind directions.

Flash Fire Envelope
Fig. 4.9 The flammable zone of flash fire from instantaneous release [205]

LFL Fraction Boundary

Half Ellipse used by MPACT

Fig. 4.10 Dispersion of cloud represented by a half-ellipse [205]

Flash fires are treated in different ways depending on the type of release. A flash fire resulting from instantaneous releases is presented as a circular cloud indicating the radius of the LFL fraction (2%) to finish. The circle starts centred at the release point and then proceeds to drift downwind as shown in Fig. 4.9. The flash fire description therefore gives the size and downwind position of the cloud at several time-steps during the time when it is developing to its fullest extent. The full description for each time-step includes: the distance the centre of the cloud has travelled downwind, the radius to the cloud-limit, and the flammable mass of the cloud.

For continuous releases the flash fire effect zone is taken to be the cloud boundary to the LFL fraction represented as an ellipse. There is also the possibility that the ellipse is defined as a 'half-ellipse' rather than the full shape. This approximation is made to economise on storage space and processor time. Fig. 4.10 shows an example where the LFL fraction boundary is described by a half-ellipse.

Two parameters are used to define the dispersion cloud shape; the downwind cloud length (LLFL) and the cloud area (ALFL) within the boundary defined by the LFL fraction. In a full approximate 'semi-ellipse' approximation is applied, where the horizontal and vertical ellipse semi-axis lengths A, B are set using: (i) same flammable length so that A = Llfl and (ii) same flammable area ALFL =0.5*n*A*B, again so that B is defined directly.

4.5.3.3 BLEVE and Fireball

A boiling liquid expanding vapor explosion (BLEVE) may occur when there is a sudden loss of containment containing liquid gas (e.g. LH2). The primary cause is usually an external flame impinging on the shell of a vessel above the liquid level, weakening the container and leading to sudden shell rupture [2]. A pressure relief valve does not protect against this mode of failure, since the shell failure is likely to occur at a pressure below the set pressure of the relief system. It should be noted, however, that a BLEVE can occur due to any mechanism that results in the sudden failure of containment, including impact by an object, corrosion, manufacturing defects, internal overheating, etc. The sudden containment failure allows the superheated liquid to flash, typically increasing its volume over 200 times [2]. This is sufficient to generate a pressure wave and fragments.

Due to the fact that hydrogen is flammable, a fireball may result as well. According to [2], blast overpressure and fragment effects from BLEVEs are usually small compared to fireball effects, although they might be important in the near field. These effects are of interest primarily for the prediction of domino effects on adjacent vessels. The study, however, only considers fireball effects resulting from a BLEVE. The program models a fireball, calculating the shape of the flame, and a wide range of radiation results. Eq. E-14a (Appendix E) used by the program to calculate thermal radiation (kW/m2) resulted from the fireball.

Damage Cirkel After Bleve
Fig. 4.11 The fatal effect zone for a fireball or BLEVE [205]

The dimension used to define a fireball/BLEVE is the radius to the radiation impact of concern. This is defined in terms of 'Thermal Dose Units' ((kW/m2)Ns). The exponent, N, depends on the N constant defined for flammable probit calculations. This measure takes fireball duration into account in calculating the potential fatality effects. The duration calculated by the fireball/BLEVE model used in this calculation is limited to a maximum exposure time parameter. Additional fatality effects due to BLEVE overpressure or vessel fragments are not considered in the study. The zone is centred at the release point. Fig. 4.11 illustrates the approach. The effect distance (z) of the fireball is equal to its downwind radius (x). The fatal effect zone is calculated as the effect zone area (=p^*x2) [in m2] multiplied with its vulnerability level [%].

A jet fire or spray fire is a turbulent diffusion flame resulting from the combustion of a flammable fuel continuously released with some significant momentum in a particular direction [94]. Jet fires can arise from releases of gaseous and liquid pressurized hydrogen. There are two jet fire models available, i.e. API 521 and Shell jet fire, and the model that is selected by the user. The shell method treats the flame as a tilted cone frustum, whereas the API model treats it as a banana-shape plume, i.e. tapered at the end, and bent by the wind. The study uses API model to calculate thermal impacts resulted from jet fires. A detailed description of the API model is presented in section E.4.3 (Appendix E).

Fig. 4.12 The Fatal effect zone for a jet fire [205]

The fatal effect-zone of jet fire is modelled as an ellipse. Three dimensions describe the ellipse as illustrated in Fig. 4.12. Axes "a" and "b" are the major and minor axes of the ellipse, and "d" is relative offset of the ellipse centre from the release point defined as the ratio d=x/a where "x" is the distance from the release point to the ellipse centre. Thus for an ellipse centred at the release point d=0. For an ellipse with the effect zone starting at the same point as the release d=l. Jet fires can be displaced from the release point according to the wind speed and the rainout position, because of the effect of wind speed and also elevation of the flame. "d>1" if the effect zone is displaced from the release point. The effect distance (z) is calculated as the sum of downwind radius (a) and the downwind distance (x) from the release location, or z =a+ x, where x=d*a. The fatal effect zone area (Afatal) is calculated as the effect zone area (Az =p*a*b) [in m2] multiplied with its vulnerability level [in %].

81 Chapter 4 - Risk assessment methods 4.5.3.5 Pool Fire

Pool fire is a turbulent diffusion fire burning above a horizontal pool of vaporising flammable liquid where the liquid has zero or low initial momentum [93, 94]. The program models a pool fire, calculating the shape and intensity of the flame, and a wide range of radiation results. A detailed description of the pool fire model is presented in section E.3.2 (Appendix E). The primary effects of such fires are due to thermal radiation from the flame source. The pool fires, however, tend to be localized in effect and are mainly of concern in establishing the potential for domino effects and employee safety zones, rather than for community risk [2]. Therefore, the pool fire impact was not considered in the risk calculation.

Similar to the jet fire, the fatal effect-zone of pool fire is modelled as an ellipse. Three dimensions describe the ellipse as illustrated in Fig. 4.13. Axes a and b are the major and minor axes of the ellipse, and d is relative offset of the ellipse centre from the release point defined as the ratio x/a where x is the distance from the release point to the ellipse centre. The effect distance and the fatal effect zone of the pool fire are calculated similar to the jet fire (See Section 4.5.5.4)

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