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Risk and consequence modelling case study

16 August 2018

The project involved a large residential development set in gardens with community facilities including cafes, restaurants, retail and leisure in a major UK city. Consultancy activities involved advising on DSEAR regulatory compliance in relation to an Energy Centre incorporated into the basement of a multi-use building.

This comprised Computation Fluid Dynamic modelling to evaluate internal ventilation, gas dispersion and explosion using Gexcon FLACS CFD-code. This was followed by a risk assessment including BowTie diagram to communicate the basis of safety satisfying ALARP to various stakeholders.

Tasks

* Creation of a 3D geometrical model of the energy centre building to be implemented within FLACS.
* Ventilation study to assess and challenge the proposed ventilation inside the building and its effect on a dispersed flammable gas cloud, propose modifications to improve efficiency of ventilation as much as possible, reducing location and size of dead spots (limited or no air movement).
* Gas dispersion and explosion modelling studies inside building to assess both the maximum flammable gas cloud sizes and the overpressure inside the building upon explosion, vented or not.
* Gas detection evaluation and optimisation study following potential flammable releases inside the energy centre to avoid the formation of hazardous flammable gas clouds. This work included simulation of a set of accidental gas releases to provide detailed prediction of gas cloud size and dispersion pattern arising from different release scenarios.
* Establish different gas detector layouts, based on different spatial configurations and number of detectors.
* Optional explosion analysis including the performance of a water-based suppression system to evaluate the effects on the explosion development upon the deployment of a water deluge system (activated following gas detection prior to ignition).
* Performance of a semi quantitative risk assessment (using SCRAM methodology) to evaluate the probability and severity of an event and assessment of the layers of protection (Barriers) by way of a BowTie diagram.

Findings

Ventilation
General good ventilation observed particularly around the location of the gas line to help dilute potential leak sources.  Velocity vectors showed a good circulation of air around the boilers heading towards the location of the extraction grill.

Dispersion and Explosion
A series of dispersion scenarios were modelled considering the corresponding gas line pressures for both full bore rupture and small hole sizes releases.

While small hole size leaks are contained around the release location creating small clouds, full bore rupture scenarios (i.e. which are extremely unlikely to occur) create a wider range of potentially hazardous flammable gas clouds.

Unmitigated scenarios were modelled for the different gas cloud sizes obtained from full bore rupture scenarios.

Internal pressures achieved ranged from 2 mbar for the smallest cloud considered (~1 m3) up to 2.5 bar for the largest clouds (~1000 m3).

The effect of water deluge mitigation was modelled inside the energy centre. Different explosion scenarios modelled were tested with the inclusion of a water droplet cloud filling the plant room.

No benefit was observed with the implementation of this type of mitigation system in this particular case. The only effective means of reducing the internal pressures generated from a catastrophic line rupture scenario would be venting by means of explosion pressure relief panels.

Gas Detection
Three different gas detector layouts were considered and its performance analysed against the different dispersion scenarios simulated.

Gas clouds detected at 10% LEL were very small and the potential explosion arising from them generates very low insignificant internal overpressures in the room.
For the layouts considered and dispersion scenarios studied, no difference in gas detection time was observed between 2 and 4 gas detectors.

More detectors only marginally increased the detection efficacy.

Conclusions

General good ventilation was observed, particularly around the location of the gas line.
Dispersion studies considered both full bore and small hole size releases.

Very low frequency loss of containment events could create a more hazardous gas cloud, which could fill the room for a short period of time. Routine maintenance in accordance with accepted best practice expected for a managed facility were deemed sufficient to reduce the probability of a hazardous gas cloud from forming.

Based on more realistic dispersion scenarios (i.e. from smaller hole sizes), a gas detection study was conducted. It was possible to observe that upon detection, a small flammable cloud was formed local to the leak source. If ignited, the resulting overpressures were considered to be insignificant in relation to the volume and design of the overall plant room enclosure.


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