Safety of buildings near to process facilities

February 14th, 2014, Published in Articles: EngineerIT

 

Ensuring the safe location and design of occupied buildings (permanent or temporary) is high on the agenda of key stakeholders – industry, regulators, and the workforce and members of the public who have a vested interest in the area.

Fig. 1: Initial stages of an accidental release as modelled by Phast.

Fig. 1: Initial stages of an accidental release as modelled by Phast.

The significant amount of analytical work required to support such an analysis represents a key barrier to conducting such studies in a detailed manner. It is often the case that a range of simplifications are utilised that can result in a less than realistic assessment.

A White Paper by Kehinde Shaba and Colin Hickey of Det NorskeVeritas Software, UK describes the implementation of two of the most widely used vapour cloud explosion (VCE) models, namely the TNO multi-energy (ME) and Baker-Strehlow-Tang (BST) into Phast, a general purpose onshore consequence analysis software package developed by DNV Software. In recent times, much has been done to develop and advance the knowledge gaps in this area.

Yet, a key issue that remains is that the degree of analytical effort required in support of a detailed building evaluation can be significant due to the extensive time and cost required. This is largely due to the often complex, multi-system, large nature of process plant – there is “lots going on” and thus “lots to consider”. It is clearly an unintended consequence of the prevailing process plant design ethos. Adding to the complexity is the intricate nature of the VCE accident sequence. A single loss of containment event can result in numerous outcomes – VCE or otherwise depending on factors such as the prevailing weather conditions, wind direction, degree of interaction with one or more regions of congestion or confinement and the location of ignition sources. Where a VCE outcome is realised, the possible impact to each building of interest has to be evaluated. It is often the case that most of the factors identified above are present in multiples e.g. a range of scenarios, weather conditions, obstructed regions, buildings of concern etc. The wide range of factors that need to be considered (“in isolation” or “as an interaction with other factors”) also has a significant bearing on the level of effort required.

Various techniques are employed to make this task more manageable. The first is “screening exercises”. Whilst there is much to be gained from making effective use of limited resources, screening is generally most useful and less harmful for screening hazards that clearly do not present VCE potential either intrinsically (e.g. degree of flammability) or by virtue of their context (no presence of a congested or confined region), but can be dangerous when used beyond this point (i.e. in grey areas) largely because it becomes more subjective and thus less precise. Whose standards of conservatism are being applied? How conservative is conservative enough? To be used effectively it also requires high levels of expertise and competence.

Another involves “scenario rationalisation”. Much emphasis tends to be placed on establishing a representative set of scenarios that can be considered to be sufficiently reflective of the whole. Again, whilst this is useful, there is a danger that the selected scenarios might not be as representative as thought, leading to an underestimation of the threat level posed.

A third involves using “simplified models” which in some cases are then subject to further simplification. Much of the tediousness is made worse by the fact that the underlying physics of explosions is complex and this is reflected in the equally complex nature of the predictive models which are available. Computational fluid dynamic based models are generally considered to give the best approximations of this complex phenomenon. However, such models are largely considered impractical for day to day work due to the significant resource and time requirements needed and are generally only justifiable for highly uncertain issues. Additionally, the resource demands of this route are such that applying it for a plant wide assessment is generally not feasible. Simplified models are designed to solve this problem by allowing a robust level of characterisation without the added expense. But then again there are varying degrees of simplicity and even the simplified models tend to require a wide range of inputs. The result is further simplification – it is common to place emphasis on the key parameters that are thought have a significant influence. An example is the implementation of the TNO multi energy model into the existing base configurations of Phast (versions 7.01 and below) which utilises the size of the confinement and blast curve number as the sole, key governing parameters. Additionally such models do not often examine the interaction of the flammable cloud with the region of confinement or confinement. Furthermore, such implementations tend to be conservative in their outlook and focus solely on the worst case outcome, a basis not always suitable for risk management. Together, these factors act as impediments to a realistic assessment of VCE potential to occupied buildings.

The paper describes extensively the nature of explosions which is essentially a very rapid release of energy that results in a pressure discontinuityin the atmosphere. This discontinuity – called a “pressure wave” – initiates in the blast sourceand dissipates outwards. The energy associated with the wave is the primary source of destructive energy to objects in its path and is responsible for a large proportion of the damage that occurs from explosions on process facilities

Key vapour cloud explosion models

The most widely used simplified models to characterise – that is estimate the magnitude and duration of the resulting pressure wave as a function of distance from the explosion source – the nature of vapour cloud explosions are the TNO multi-energy (referred to as ME) , the Baker-Strehlow-Tang (referred to as BST) and the congestiona assessment method (CAM). The TNT equivalence model, also a simple model and once used extensively has now been shown to be limited in its ability to adequately characterise VCEs. Consequently, it is not generally recommended for use. For example API standard 752 (API, 2009) explicitly states that the model should not be used. Nevertheless, it still sees widespread application. This is in part driven by its simplicity – only a few inputs are required to the model. That it results in relatively conservative estimates of the peak overpressure (often used as a key indicator of explosion magnitude) is another factor – one that makes it especially appealing as an early screening tool.

Process hazard analysis software tool (Phast)

Phastis a consequence analysis software package developed by DNV software for the modelling of the consequences associated with releases of flammable or toxic materials as would occur on process facilities. Phast’s modelling capabilities are numerous and diverse and allow for the evaluation of a wide range of hazard types in different situations. Its capabilities cover a wide range of release phases (gas, liquid and two-phase) and materials (various flammables and toxic compounds including LNG and CO2) as pure components or in mixtures. In terms of release types, it can model unpressurised and pressurised releases as steady state or as time-dependent releases. At its core is the unified dispersion model (UDM) which enables complete and rigorous modelling of various release types accounting for transition to jet, dense and passive clouds; buoyancy effects, substrate interaction (ground effects); plume lift-off, capping at the mixing/inversion layer, droplet formation and rainout under diverse atmospheric conditions. Phast is an integrated consequence analysis package that models all stages of an accidental release from discharge: including rainout, pool evaporation and spreading to dispersion: from the pool and the discharge orifice as shown in Fig. 1 and effects including toxicity, jet fires, fireballs, pool fires, flash fires and explosions. Major accidents have focussed attention on the need to ensure that occupied buildings are well protected from the often devastating impacts of VCEs arising from process facilities. Hence, it is desirable to have a software tool which allows for a substantive analysis to be undertaken in a rigorous, robust and resource (cost, time) efficient manner without the added risk simplification can introduce.

The paper describes and examines the recent advances in the Phast consequence model designed to address this challenge. It demonstrates that increased rigour can be achieved in an effective manner for any scale study (both large and small) thus negating the need for screening/simplification activities. The advancements to the Phast software platform are a welcome addition to the range of software tools available to facilitate the evaluation of explosion risks to buildings and should help improve the robustness and overall quality of such evaluations resulting ultimately in more effective management of risks to occupied buildings.

The full paper, as presented at the 1st CCPS Asia-Pacific Conference on Process Safety, 4 – 5 September 2013 in Qingdao, China is available on http://goo.gl/Xp1lNC.

 

 

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